A watery double-planet: Luna and Terra

Science Magazine recently published a paper that reports that minute quantities of water contained in lunar volcanic glass appear to be identical in isotopic composition to terrestrial water.  According to subsequent press reports, this finding revolutionizes our understanding of the origin of Earth and Moon.  But does it?

Water is a simple molecule, made up of two hydrogen atoms and one oxygen atom.  However, these atoms are not all made the same – they always contain the same number of protons and electrons but the number of neutrons they contain varies.  In particular, some naturally occurring hydrogen contains an extra neutron and hence has twice the mass of normal hydrogen.  This “heavy hydrogen” (called deuterium, for its atomic weight of two) is much less abundant than its lighter version.  Planetary scientists use the amounts of deuterium, relative to normal hydrogen, as a measure of the provenance of the material, i.e., where it formed relative to the Sun. 

Ultimately, substances that have identical deuterium/hydrogen ratios are presumed to have come from the same source.  We have reason to believe this ratio increases systematically outward from the Sun, depending upon where in the early “solar nebula” the material condensed and its subsequent geological processing.  Oxygen (the other element in water) also has an isotopic variation; normal oxygen has 16 protons in its nucleus, but the other isotopes of oxygen can have an additional neutron or two.  As with hydrogen, the variation in the ratios of normal to “heavy” oxygen is thought to be indicative of where the material comes from.

Of course nothing is ever quite so simple and straightforward.  Subsequent processing, such as interaction with cosmic rays, can sometimes alter the composition of samples but if these effects can be accounted for and eliminated, isotopic composition can be used as a tool to map the ultimate sources of Solar System debris.  This has been done with many different elements and compounds, but oxygen and hydrogen are very volatile and thus, sensitive indicators of the thermal environment in which they formed. 

When the isotopic composition of an element like oxygen is plotted for the various groups of Solar System materials – meteorites, lunar, martian and terrestrial samples – they all form distinct groups, indicating that the source reservoirs of these materials formed in different locations of the nebula.  The most primitive type of meteorite – carbonaceous chondrite – appears to have formed at the farthest distance from the Sun.  These rocks are thought to have originated within once icy bodies, the cores of objects known as comets.  Comets form in the outer Solar System where low temperature substances are abundant and are occasionally perturbed by gravity to enter the inner Solar System, i.e., inside the orbit of Jupiter.  Once there, they are heated by the Sun and their most volatile components are sublimed away; after multiple passes through the inner planet zone, only a small fraction of this primitive material remains.

The new findings indicate that the isotopic composition of the hydrogen in water in the mantle (deep interior) of the Moon is nearly identical to that in the water of Earth’s mantle, and both appear to have come from carbonaceous chondrite (most primitive) meteorites.  When compared to a variety of data from other Solar System objects (including the giant planets, icy outer planet satellites and meteorite groups) the Earth-Moon system is compositionally distinct and identical, indicating that, whatever our origins, the description of Earth and Moon as a double-planet is even more appropriate than we had thought.

What does this mean for lunar origin and what does it say about the water at the Moon’s poles?  The bulk composition of the Moon has long been recognized as a key constraint on models of lunar origin.  A basic question is whether the Moon is made of the same material as the Earth or not.  The new results indicate that it is and as such, is another contributory piece of evidence that the materials of the Earth and Moon were brewed in the same pot.  Interestingly, this pot of material is distinct from virtually every other Solar System object (as near as we can tell based on limited information from the other planets).  Whatever process formed the Moon, it involved objects that were created more or less in this neighborhood of the Solar System.  The new results also suggest that both Earth and Moon had a significant component of water early in its history.  Earlier studies had suggested that the terrestrial hydrosphere was a late addition, a veneer of cometary debris from deep space that was added to the Earth late in its history.  We now know that this water was incorporated into the Earth very early, possibly from the beginning of accretion.  The Moon shares this trait – and the same source of water.

So is the giant impact model of lunar origin still viable?  The existence of water in the lunar interior is not a prediction of the giant impact model but as has happened previously, the model will probably be modified to accommodate the new findings.  We have a tendency to imagine (and desire) simple systems in chemical and thermal equilibrium, in which materials and energy behave in a straightforward, predictable manner.  But this event (if it occurred) was a singular one, possibly involving complex, chaotic behavior.  Thus, some of the difficulties created by the new data will probably be explained away.  A hypothesis elastic enough to be stretched to fit any new discordant observation isn’t particularly useful and certainly isn’t scientific.

How does this affect our thinking about the water ice trapped at the Moon’s poles?  As we continue to find that the interior of the early Moon was more water rich than previously thought, we must add lunar water to the long list of possible sources for polar-trapped water.  (As a reminder, the previous idea was that polar water was derived from external sources – the Sun via the solar wind hydrogen, water-bearing meteorites and comets).  Could at least some of the water at the poles be of lunar origin?  One problem that we still don’t understand is the geological age of the polar cold traps – they exist because the spin axis of the Moon is normal to the ecliptic plane.  How long has the Moon been in this orientation?  We suspect that the Moon has been stable for at least the last 2 billion years but water is being found in volcanic glass over 3 billion years old and thus, released before the current polar cold traps existed.  So at least for now, it seems that the Moon’s own water is an unlikely contributor to the ice at its poles.  But that story could change too.

The Moon’s surprisingly complex and interesting history continues to confound the experts.  We may have already “been there” but we still don’t fully understand the Moon’s story and true potential.

Map of the thickness of the Moon's crust from GRAIL mission gravity data. Mean thickness are estimated to be 34-43 km.

Imagine a system of molten silicate material, where low-density minerals float and higher density minerals sink.  Minerals rich in iron and magnesium (such as olivine and pyroxene) will settle toward the bottom of the magma body while those rich in the elements aluminum and calcium (such as plagioclase feldspar) will float.  Just such a scenario – on a global basis – is thought to have created the crust of the Moon.

Before Apollo, many believed that the Moon was a primitive, undifferentiated lump of cosmic debris.  By studying the samples returned by Apollo 11, scientists identified small fragments of white, plagioclase-rich rocks (anorthosite).  There are no known magma compositions corresponding to this rock type – anorthosite is created by removing low-density plagioclase from a crystallizing system and concentrating it by floatation.  From the evidence of fragments in the lunar soil, large amounts of anorthosite were inferred to be present in the nearby highlands of the Moon.  As the highlands make up more than 85% of the surface of the Moon, it was postulated that the crust of the Moon formed early in its history by global melting, an episode termed the “magma ocean.”

Expecting only minor volcanic activity and perhaps a local igneous intrusion, the concept of a global ocean of magma was surprising to most scientists.  Given its small size and consequent paucity of radioactive heat-producing elements, the idea that most of the Moon might have melted and differentiated was astounding.  The existence of an early magma ocean, which implied high-energy processes, provided us with clues to lunar origin.  Once it was recognized that the Moon had a crust, it was important to gain an understanding of its composition and physical nature.

On subsequent missions, Apollo astronauts were tasked with laying out a series of seismic stations across the near side.  These stations allowed us to measure “moonquakes” – both natural events as well as those created artificially by slamming spent rocket stages and satellites into the Moon.  Seismic recording allowed us to infer the speed at which seismic waves traveled through the lunar interior.  These estimated speeds indicated densities that implied composition, allowing us to deduce the probable chemical and mineral composition of the lunar interior.

The Apollo seismic network indicated that the crust of the Moon was about 50-60 km thick in the central near side, a surprisingly large value, especially compared to the thickness of the crust of the Earth (which varies from as thin as 5-10 km under the ocean basins to over 30 km in continental areas).  Such a thick crust for the Moon led to the postulation of a global magma ocean, as so much anorthosite could only be produced under the conditions of near global melting.  Subsequent studies incorporating gravity data from Lunar Orbiter and other missions suggested that the lunar crust is variable in thickness, with values exceeding 100 km in some regions of the far side highlands.

Re-analysis of the Apollo seismic data gave the first indication that those values might be overestimated.  Using modern techniques on these old data, new analysis revealed that the crust might be thinner than we had originally thought, on the order of 40-50 km thick.  This lower value of crustal thickness had some implications for estimating the bulk chemical composition of the Moon, but because it was considered to be a relatively minor adjustment, it caused no major difficulties for the rest of lunar science.

However, the recent GRAIL mission to the Moon (using high precision gravity mapping) ascertained the thickness of its crust to be 34-43 km.  Why should this new value worry some scientists?  Because we are now entering realms in which the new estimates of crustal thickness create consistency problems for other aspects of lunar science.  A crust as thin as 35 km on the near side of the Moon implies that the largest impacts – the multi-ring basins – should have excavated considerable amounts of material from the layer below the crust, the mantle of the Moon.  One might object that, as this region of the interior is inaccessible, we don’t know what the mantle would look like.  But in fact, the density constraints imposed by the seismic and gravity data dictate that it must be a rock type rich in iron and magnesium, made up mostly of the minerals olivine and pyroxene.  Such rocks are not unknown in the lunar collections, but they possess chemical and mineralogical characteristics indicating their origins at much shallower (crustal) depths.  In other words, there does not appear to be any material from the lunar mantle in the Apollo collections.  Given our obviously incomplete sampling of the Moon, should this be a problem?

Several Apollo landing sites (e.g., Apollo 14 and 15) were specifically chosen to maximize the chances of sampling ejecta from the enormous 1100 km diameter Imbrium basin (one of the biggest impact features on the Moon).  Virtually any reconstruction of the dimensions of the excavation cavity of this basin indicates that it should have dug up material from tens of kilometers depth, much deeper than the new value of crustal thickness implied by the GRAIL data.  So where is this debris from the mantle of the Moon?  True enough, it is possible that it may have been missed during the limited exploration time available to the Apollo crews, but the astronauts were trained to recognize such rocks and none were found.  Additionally, because we can map rock types by remote sensing (both from spacecraft and from Earth), we have an understanding of the regional distribution of rocks around these large impact features.  Despite a 30-year, exhaustively detailed search of the Imbrium impact basin (an area larger than Texas), we have found no convincing evidence for mantle material on the surface of the Moon.

So where does this leave us? In science, new data can solve some problems but at the same time, it may also create new ones.  Modern analyses of the old seismic data and new information on the Moon’s gravity field both suggest a relatively thin crust, with mantle material being very close to the surface (a few km) in some areas.  On the other hand, none of the ubiquitous impact basins and large craters of the Moon show evidence for mantle material in their ejecta, either in the Apollo collections or in remote sensing data.  Could our understanding of impact mechanics be completely wrong?  Or are we misunderstanding the new gravity data?  How could an event that formed an impact crater thousands of kilometers across excavate only a few kilometers deep?

Shackleton crater, Moon. Clockwise from top left: topography from laser altimetry, image from SMART-1 mission, lighting map (brighter is longer periods of illumination) from the LRO Camera, Mini-RF CPR image draped over shaded relief map. The crater is about 20 km in diameter.

Though unremarkable in appearance compared to the roughly 4,000 craters on the Moon in its size range, the 20 km diameter crater Shackleton has been the source of relentless scientific controversy for the past 20 years.  Shackleton is located at the south pole of the Moon; indeed, its near side rim is the precise location of the geographic pole itself.   Its location makes observation by Earth-based telescopes difficult and it was not well photographed by the Lunar Orbiter series (our principal source of lunar images) of the 1960s.  That all changed in 1994 with the flight of the joint DoD-NASA mission to the Moon, Clementine.

Clementine carried cameras that globally imaged the Moon in eleven visible and near-infrared wavelengths.  In addition, it mapped the surface and lighting of the poles of the Moon at uniform resolution over the course of almost three lunar days (74 Earth days).  When the Science Team first saw the south polar mosaic, the extent of darkness in the map was striking.  Because the Moon’s spin axis is close to perpendicular to the ecliptic plane, the Sun is always at the horizon at the lunar poles.  Instead of rising and setting, the Sun circles around the poles at or near the horizon.  Because of this grazing incidence, an area in a topographic depression may be in permanent shadow.  And so it appeared for Shackleton crater in the Clementine data, setting off bells in the heads of the Science Team.

A key controversy of the post-Apollo era was whether the lunar poles might contain water or not.  Although the Apollo samples had been studied and found to be “bone-dry,” we had not been to the poles on any Apollo mission.  We knew that any shadowed areas had to be extremely cold as well as permanently dark.  As water-bearing debris in the form of asteroids and comets constantly strike the Moon, it was thought that some of that water might get into a polar “cold trap” and would be kept there (essentially) forever – billions of years of impacting cosmic “debris” can add up.

Clementine was not configured to measure the presence of water, but a cleverly improvised experiment used the spacecraft’s data transmitter to beam radio waves into the dark regions near the poles and listen to their reflected echoes on the enormous (70 m) dish antenna of NASA’s Deep Space Network.  Interestingly, the reflections indicated an enhancement of “same sense” polarization within the (very large) resolution cell that contained Shackleton crater.  A collect of data from a nearby sunlit area (taken as an experimental control) did not show this peak.  The Clementine team interpreted the RF peak as evidence for the presence of a few percent water ice within the dark, cold interior of Shackleton crater.  The media quickly spread the startling news about water on our “bone-dry” Moon.

Such a controversial conclusion did not go unchallenged.  Some in the radar community argued that abundant wavelength-sized rocks on the surface were the source of the enhanced same sense reflection.  Since the lunar surface is indeed rocky, this interpretation could not be ruled out.  Then a few years later, the Lunar Prospector (LP) mission found an enhancement of hydrogen concentration at both poles of the Moon; as hydrogen is a major constituent of water, the idea ice exists in the dark areas gained credence and has lead to a decade-long scientific search (using a variety of techniques) for lunar polar ice.  Though many areas near the poles were studied in detail, attention continued to be drawn back to Shackleton and the area near the south pole.

From studying Clementine images, we discovered that part of the rim crest of Shackleton is one of the most sunlit areas on the Moon.  Now we had a double-attraction: constant sunlight with water ice nearby.  At a press briefing in 1996, I called this area of water and sunlight “the most valuable piece of real estate in the Solar System.” Nothing found subsequently has changed my mind on that judgment.

So what have we learned about Shackleton lately?  Many different, new sensors have flown to the Moon in the last few years, including radar, ultraviolet (UV) imaging, laser reflections, and low-light level imaging.  And yet again, Shackleton crater continues to confound us with contradictory evidence, both for and against the presence of water ice in its interior.

In 2009, the question regarding the presence of water ice somewhere near the lunar south pole was answered when the LCROSS impactor threw up a cloud of water vapor and ice particles during its collision with the floor of the nearby crater Cabaeus.  Spectral mapping instruments on three different spacecraft (Chandrayaan-1, Cassini, and EPOXI) documented the presence of adsorbed water on the lunar surface, increasing in concentration with latitude toward both poles.  A small impact probe flown by India (MIP) passed through a water vapor zone in the exosphere just above the lunar south pole.  And radar images from Mini-RF, our radar imaging experiment on both Chandrayaan-1 and Lunar Reconnaissance Orbiter (LRO), found evidence of high same sense reflections (just as Clementine had suggested in 1994) within the interior of Shackleton crater.  These new lines of supporting evidence were countered by Japanese researchers, whose Kaguya spacecraft imaged the interior of the crater and found morphology similar to other lunar craters in the same size-class.  But no one had ever claimed that the interior of Shackleton was a skating rink of pure ice – the lunar polar ice is partly covered by waterless dust and mixed with an unknown amount of dry regolith.

Interpretation of the new data continues to vex us.  The LOLA (laser altimeter) team on LRO recently published a paper that documents the high reflectivity (at 1 micron wavelength) of the walls of Shackleton.  Although the team’s favored interpretation is that this is caused by a constant exposure of fresh material on a steep slope, they also note that it is consistent with the presence of water ice on the walls of the crater.  In addition, a team analyzing neutron spectrometer data from both LP and LRO found evidence in the fast neutron data (never before analyzed) that water in the interior of Shackleton is a possible explanation for its signal.  Detailed analysis of the Mini-RF data for Shackleton corrected for its steep wall slopes and found that the presence of 5-10 wt.% water there provides the best model fit to the observed data.  Newly obtained UV images from LRO show the existence of water frost in the interiors of the craters Haworth and Shackleton, and the neutron detector on LRO shows enhanced hydrogen within both Shoemaker and Shackleton craters.  The Japanese team from Kaguya continue to insist that the no-ice interpretation is the correct one.

So we are left with a mystery.  Some evidence is pro-ice and some is contra-ice.  I find it interesting that for most of the investigators, new data does not necessarily change any minds, but tends to be interpreted in a way most favorable to their previously published ideas.  This should not be terribly surprising; the people who have argued for some specific interpretation presumably did so for good reasons and desire hard and clear-cut evidence to the contrary before abandoning a previously held position, one no doubt reached after much thought and soul-searching.

The way to unravel the water-ice mystery is to go to the surface of the lunar south pole (or both poles) and measure the composition of the surfaces in question.  Getting a definitive answer about the nature of lunar water would be game changing.   Some say the bigger mystery is:  Why hasn’t the United States sent a rover to the south pole of the Moon to take a closer look?

Sedimentary rock on Mars as viewed and analyzed by the Curiosity rover (NASA).

The news of the day is abuzz with the new and astounding discoveries from the Curiosity rover that Mars once had an environment conducive to life.  Once it was warmer, wetter, more hospitable.  Water flowed over its surface.  The chemicals necessary for life’s emergence and development are present on Mars, suggesting that life may have arisen there in the distant past.  So why do I have this sense of déjà vu?  Perhaps because this new “result” gets trumpeted anew every few years.

The fixation on the possibility of martian life has been a constant throughout the history of the space program, starting before the first planetary mission to Mars in 1965 (Mariner 4) and then waxing and waning in likelihood every few years.  Mariner 4 showed us a moon-like Mars, with a rough, cratered surface and thin cold atmosphere.  The stock for martian life fell accordingly.  A few years later, the twin probes Mariners 6 and 7 flew by Mars, again returning pictures of a cratered surface, but with hints of the presence of unusual terrain, possibly the result of subsurface ice.  The stock of the life story rose slightly, but the barren cold desert of the martian surface was hardly a Garden of Eden.

A big breakthrough came with the flight of Mariner 9 in 1971.  To the astonishment of most planetary scientists, it revealed a world of giant volcanoes, canyons much larger than the Grand Canyon on Earth, and amazingly, channels that looked as though they were carved by running water.  The idea of life on Mars – at least in the distant past – gained credence and served as a springboard for the Viking missions of 1976, America’s bicentennial year.  These two missions consisted of both a lander and an orbiter and were specifically designed to test the surface of Mars for the possibility of life.  Both landers returned results that were immediately interpreted as negative (although there was some dissent); the surface materials of Mars had a very reactive chemistry, but no organic material was found in the soil, even at concentration levels measured in parts-per-billion.  Thus, we had the conundrum of abundant landform evidence for an early, warm and wet climate yet chemical evidence for an almost sterilizing environment at present.  If Mars had life, it must have been present only in the distant past.  The results from Viking were considered so definitive that no mission was sent to Mars for over 20 years.

What precipitated the new flurry of interest in Mars about twenty years ago was the finding that, astonishingly enough, we have samples in our possession from Mars in the form of meteorites, the so-called “SNC meteorites” (the initials of Shergotty, Nakhla, and Chassigny, the first three meteorites recognized to be of martian origin).  It had been thought that the preservation of rocks intact during ejection from the planet at escape velocities and greater was not possible, but in this case, observations trumped theory.  Even more amazing, it was claimed that in one of these putative martian rocks, small features within it were actually fossils of ancient bacteria.  Although highly controversial then (and now), this finding was given widespread publicity (including even a Rose Garden Presidential statement) and the agency used this discovery to sell a program to send a series of probes to Mars at every two-year opportunity for the next decade.

This fleet of orbiters and landers returned an abundance of new, high-quality data on the martian surface, its composition, the locations of water and its environment.  Each mission confirmed that water had once been present on Mars.  Each mission confirmed that at present, the surface was not conducive to life.  Each lander went to a site that was thought to have been more promising for the development of life than the ones that preceded it.  As the years rolled on, each “new discovery” of the former presence of water and favorable environmental conditions on Mars became something of a joke among my colleagues in the planetary science business – how many times can you claim the discovery of something already known?

Lest you think that I am simply expressing my lunar parochialism, I note that this same media phenomenon occurs in regard to the existence of water ice at the poles of the Moon.  The theoretical possibility of ice on the Moon had been known for many years.  We first found direct evidence for it in 1996 with an improvised radio experiment on the Clementine mission.  Subsequent studies from Earth and a variety of other space missions caused the stock for lunar polar water to rise and fall, depending on who issued the latest press release for their published work.  Finally, the collision of the LCROSS impactor in 2010 removed all doubt – there was and is ice there, at least at the south pole and in quantities greater than could be reasonably expected to have resulted simply from solar wind deposition.  Yet each new finding was announced as a new “discovery” in the press.  So this media frenzy is not simply related to Mars mania or even to the over-preoccupation with finding life elsewhere.

The basic fact is that most in the news business do not understand (or at least, do not fully appreciate) the incremental, cumulative nature of modern science.  It is seldom indeed when a single experiment or observation causes a scientific revolution.  Moreover, it is equally seldom that a breakthough comes from one person or even one research team.  Science is a complex, interdisciplinary effort.  It makes progress, but slowly and in a manner that includes both leaps forward and (sometimes) backward.  Only over long periods of time (decades and greater) is it apparent what the key observation or measurement is and how it fits into a pattern of understanding.  Each new mission result adds knowledge, sometimes in great leaps and sometimes in increments so tiny that one can question whether anything new is being learned at all.  But even a repeated observation has value in science – in fact, if an observation is not repeatable, it is not a valid scientific observation.

The new inferences from Curiosity suggest a more benign and hospitable environment for life, but few working Mars investigators doubted that such existed in the past.  Even if it did not, we have found in the past few decades that even extreme environments on the Earth can support certain types of microbial life.  So the new results broaden and deepen our understanding of martian surface properties and processes, they do not revolutionize them.  That’s just how science normally works.  If some scientists tend to oversell their results, well, they’re only human.

A Soviet Luna spacecraft lifts off from the Moon after collecting a drill sample for return to Earth (early to mid-1970s).

Samples are currently making news for NASA’s planetary exploration program.  Last August, the rover Curiosity, equipped with a package of laboratory instruments, landed on Mars.  On February 9^th the rover’s robotic arm drilled its first hole in a rock selected by scientists.  In their attempt to gain more information about Mars, scientists will use the rover’s science package to remotely analyze these samples on the martian surface.  The results will give them some fairly detailed knowledge on the chemical and mineral make up of these rocks.  But what else can we possibly learn from samples?

Geologists in general and planetary scientists in particular often emphasize that “such and such” cannot be known for certain “until we obtain samples” of some planetary surface or outcrop.  What is this obsession with samples?  Why do (some) scientists value them so highly and exactly what do they tell us?  Answers to this question (for there is not a single, simple one) are more involved than you might think.

With today’s technology providing us with only the most rudimentary information, sample analyses made remotely on a distant planetary surface is limited.  Some of the things we want to know, such as the formation age of rocks, can only be discovered with high precision, careful laboratory work.  That’s a tall order for remote systems.  For example, one of the most common techniques used to “date” a rock’s age requires the separation of individual minerals that make up the rock.  Next, the ratio of minute trace elements and their isotopes in each grain must be determined.  Assuming that the rock has not been disturbed by heating or a crater shock event, this information can be used to infer an age of formation.  If we can convince ourselves that the rock being studied is representative of some larger unit of regional significance, we can use this information to reconstruct the geological history of the region and eventually, the entire alien world.  So sample analysis is an important aspect of geological exploration.

As I have written previously, we used images to geologically map the entire Moon, noting its crater, basin and mare deposits, and their relative sequence of formation.  When the first landing missions were sent to the Moon, great emphasis was placed on obtaining representative samples of each landing site.  It was thought that such samples could be studied in detail in Earth laboratories and then extrapolated to the larger regional units shown on the geologic maps.  With few exceptions, this approach worked pretty well.  As we moved from the landing sites on the maria (ancient lava flows) into the complex highlands, the “context” of the samples – their relation to observed regional landforms or events – became more obscure.  A lunar highland rock is typically a complex mixture of earlier rocks, sometimes showing evidence for several generations of mixture, re-fragmentation, and re-assembly.  Loose samples lying on the surface were collected from the highlands, none of them were sampled “in place” (i.e., from bedrock).  Although this is also true of the rocks from the maria, we observed bedrock “in place” at most of the mare sites and may have actually collected at least one sample from lava bedrock at the edge of Hadley Rille near the Apollo 15 site.

None of the highland samples possess the same degree of contextual certainty as the mare samples.  This fact, coupled with their individual complexity, sometimes leads to consternation over exactly what the samples are telling us.  It doesn’t help that the Moon’s early history was itself very complex, with magmas solidifying, lavas erupting, volcanic ash hurled into space and laid down in bedded deposits.  On top of all those processes were cratering events that mixed and reassembled everything into a complex geologic puzzle, a virtual stew of processes and compositions that hold clues to billions of years of the Moon’s (and Earth’s) history.  Nonetheless, we can still perceive most of the story of the Moon’s history, enough at this point to tell us that without those lunar samples in hand, we would be well and truly ignorant of even its most important events and basic processes.  The fixation with sample return stems from the science community’s belief that with just a few more carefully selected samples from some key units, all that is now dark will be made light.

There may be severe consequences to the science community’s insistence on the primacy of sample return.  The most recent “decadal survey,” the ten-year community study that gives NASA our wish lists for missions and exploration, made a sample return from Mars the centerpiece and sine qua non of future robotic missions.  The NRC report was so emphatic in its insistence that it might be paraphrased as saying, in effect, “Give us a Mars sample or give us death!” (with apologies to Patrick Henry).  Alas, that formulation may be more apt than anyone desired, as proposed out year budgets for the next five years of NASA funding cuts planetary exploration by almost 30% – a landscape of shifting priorities that raises questions and uncertainty for the future.

Robotic sample return missions to large bodies like the Moon or Mars are expensive because they consist of multiple spacecraft – a lander, which softly places the spacecraft on the surface, a device (such as a rover) to collect and store the samples and an ascent vehicle to bring the sample back to Earth.  While none of these functions individually are exceedingly difficult to achieve, all of them (done correctly and in proper sequence) add up to a substantially difficult, complex mission profile.

In the space business (as with most endeavors), more difficult and complex means that more money is required.  Moonrise, a proposed robotic mission to return about a kilogram of sample from the far side of the Moon, was projected to cost around one billion dollars.  A Mars sample return mission consisted of three separate missions: one to land, collect and store the samples, another one to retrieve those samples and place them into orbit around Mars, and a final mission to return the samples to Earth.  With each step costing up to several billion dollars, such a technically challenging Mars sample return mission would be unaffordable.

Although samples have many advantages over remote measurements, those benefits must be weighed against the cost and difficulty of obtaining them.  Perhaps the complete extent of what can be accomplished remotely has yet to be fully explored.  As mentioned above, absolute ages are key information that we get from samples.  Several dating techniques could be adapted to a remote instrument; these methods may not be the most precise imaginable, but they might be of adequate precision to answer the most critical questions.  On the Moon, we do not know the absolute age of the youngest lava flows in the maria; age estimates range from as old as ~ 3 billion years to as young as less than 1 billion years.  In such a case, a measurement with 10-20% precision is  adequate to resolve the first-order question:  When did lunar volcanism cease?  In addition, such a result would enable us to calibrate the cratering curve for this part of lunar history, a function that is widely used to infer absolute ages throughout the Solar System.  A solid result obtained from a robotic lander – even such a relatively imprecise one – would have important implications for lunar volcanic processes, thermal history, impact flux, and bulk composition.

Complex robotic operations in space are always dicey, especially when attempting something for the first time.  Samples are a key part of a planetary scientist’s toolbox but their acquisition is difficult, time-consuming and expensive.  Samples from robotic missions are more likely to have ambiguous context, thus rendering less scientific value.  Scientifically useful sample collection may remain problematic until people can physically go to exotic places in space and fully use their complex cognitive skills.  This trade-off between cost and capability must be carefully considered when weighing future exploration alternatives and desired outcomes.

Previous relevant posts:

Humans and field work

Lunar robotic sample return

Mars sample return

Eugene Shoemaker with some of the first geologic maps of the Moon, Flagstaff AZ, mid-1960's

Many people are surprised when they learn that well before the first landing of Apollo in 1969, we already understood the geological history of the Moon.  The idea that such a thing was even possible drew considerable skepticism during early preparations for landing on the Moon.  The principles for the remote mapping of the geology of the Moon came from several closely related but distinct threads.  Eugene M. Shoemaker, a geologist with the U. S. Geological Survey (USGS) who founded the Branch of Astrogeology, laid out the methodology in broad outline from and through the systematic study of lunar surface images in the early 1960s.

One of the basic principles of geology is that younger rocks lie on top of (or intrude into) older rocks.  Interestingly, this relationship can be discerned from a photograph.  In the case of the Moon, images show the dark smooth plains of the maria (lava) and the rough, cratered highlands.  Some craters were found on top of the dark mare plains, while others were filled with mare.  Clearly, the craters on top of the mare formed after those plains existed and were thus younger than the maria.  On the other hand, dark mare that fills a crater must have formed after that crater existed and so in this case, the crater was older.

By following these simple relations over large areas, it is possible to determine the relative ages of mare and craters, both among themselves and to each other.  But such information is trivial unless we can relate these individual ages to some unit or event of regional significance.  In principle, if such a relationship can be defined we can extend relative age assignments over large areas, ultimately on a global basis.

The first effort to map the geology of the Moon was by the USGS, but not by the then-newly created Astrogeology Branch.  Branch of Military Geology scientists Arnold Mason and Robert Hackman produced the “Engineer’s Special Study of the Moon” in 1960.  This special one-off product documented the principal terrain types of the Moon (maria and highlands) and ordered features into three categories of relative age: post-mare craters (youngest), maria, and highlands (oldest).  Additionally, the map showed the distribution of linear features, presumed to be faults (fractures along which movement has occurred), and mare ridges (presumed to be folds) over the near side.  In this sense, the Engineer Special Study was a geological map because it showed the spatial distribution of rock types, their relative ages, and the inferred structure of the lunar surface.  This map was accompanied by a detailed text chart, which showed a region-by-region evaluation of the terrain and construction challenges for each area.  But a critical element was still missing.

On Earth, the geologist recognizes the rocks in the field, maps their locations and orientation, and documents the structure of the area under study.  But a key part of this work is to figure out where a particular area fits in the global column of geologic units.  On Earth, by documenting the slow, gradual nature of geological processes the stratigraphic column was developed slowly over the course of about a hundred years.  The terrestrial stratigraphic column also provided key evidence needed to show the gradual transition of life forms from simple invertebrate organisms in the earliest rocks, to the complex and varied life forms in succeeding strata.  With the development of a global stratigraphic system and accompanying geologic time scale for the Earth, a framework for understanding the history and processes of the Earth was created.

Gene Shoemaker recognized the need for an organized stratigraphy to aid in our understanding of the Moon.  He wanted to understand the Moon’s evolution and age, but also to correlate events on the Moon with events in Earth history.  He recognized that a major step forward to such an end was to define a formal stratigraphic system for the Moon – a clear succession of rock types with key regional units defining the system boundaries.  He began mapping the area around the crater Copernicus, which lies on the central near side of the Moon, recognizing that the rocks exposed there (from what had been discerned from images) represented all the distinct phases of lunar history.

Image of the area near Copernicus, showing how the relative ages of geologic features are determined from an image.

The basic sequence is easy to follow.  The oldest rocks (1) are those that form the highland units of the large, circular Imbrium impact basin.  These units are the mountains that make up the rim of the basin as well as the regional highlands around Copernicus, which are ejecta from the basin forming event.  Partial flooding by the dark, smooth maria followed (2), including both dark, ash-like materials and smooth flood-like plains (interpreted even then as flows of basalt, the most common volcanic rock type on Earth).  These eruptions were followed by the formation of impact craters, of which two kinds could be recognized:  an older group (3) that had slightly eroded and lost their bright rays (such as Eratosthenes) and a younger group (4) that preserved the bright rays and showed a fresh, unmodified form (such as Copernicus.)

Shoemaker used these rock units to define the lunar time-stratigraphic systems:  the Imbrian, Procellarian, Eratosthenian and Copernican Systems were each assigned to represent an archetypical deposition event.  Rocks that existed before the formation of the Imbrium basin were assigned to an informal category, the pre-Imbrian.  Thus, Shoemaker created a geologic map that not only showed the distribution of rock units and the structure of a given area, but also classified these rock types into a stratigraphic column for the Moon, one that (because of the enormous extent of the Imbrium basin) could be applied to areas across the lunar near side.  With slight modification (the “Procellarian” System is no longer used and the pre-Imbrian has been subdivided into the Nectarian System and pre-Nectarian), this classification scheme subsequently has been applied to the entire Moon.

Shoemaker’s work on geologic mapping of the Moon gave us the ability to immediately put the lunar samples returned by Apollo into a regional and global context.  We found that most lunar events occurred very early in its history, with intense geological activity in the first 1-2 billion years and little activity since.  Thus, the Moon’s geological record perfectly complemented that of the Earth, whose traces of earliest activity have been erased over time by the active processes of erosion and plate tectonics.

The first geologic quadrangle map of the Moon showing rock units (basin, crater, and mare materials), structures, and their stratigraphic arrangement.

The 1960 Copernicus Prototype Chart LPC-58, the first true geological map of the Moon, was not formally published by the USGS, though a modified and updated version was published later in that decade.  By then, Gene had picked up a couple of co-authors for his effort, including one Harrison Hagan Schmitt (a young geologist with the USGS in the early 1960s), who in 1972 ultimately got the chance on the Apollo 17 mission to do what Gene Shoemaker originally got into the space business to do – check the interpretations of the remote lunar geologic mapping by doing field work on the Moon.

Click here to view Shoemaker’s LPC-58 geological map at full resolution.

President John F. Kennedy is briefed by Wernher Von Braun (partly hidden) and Hugh Dryden (speaking to the President) at Pad B, Complex 37, Cape Canaveral, Florida, November 1963. (John F. Kennedy Presidential Library)

As a memorial to honor Neil Armstrong’s contributions to aeronautics and astronautics, a bill (HR 6612) was recently introduced by Congressman Kevin McCarthy and passed by the House of Representatives to change the name of the NASA Dryden Flight Research Center (a field center proximate to Edwards Air Force Base in the Mojave desert north of Los Angeles) to the Neil A. Armstrong Flight Research Center.  While I take a back seat to no one in regard to my respect and admiration for Neil and his life of accomplishment, I think that this effort is both mistaken and inappropriate.

Who was this Dryden guy anyway?  Hugh L. Dryden was an American aeronautical engineer who became the last head of the National Advisory Committee for Aeronautics (NACA)* in 1947 and the first Deputy Administrator of the National Aeronautics and Space Administration (NASA) in 1958.  Dryden had a long research career in the complexities of airflow and the boundary layer, critical subjects in the science of aerodynamics.  Dryden’s published work in this field became standard texts for upcoming aeronautical engineers and aircraft designers.  Dryden, a quiet man whose life story is filled with notable achievements and roles, took the lead in establishing the National Academy of Engineering, the sister entity of the National Academy of Science.

In 1958, an act of Congress established NASA which absorbed the NACA and its aeronautical research facilities, including the field centers of Langley Aeronautical Laboratory near Hampton VA, Lewis (now Glenn) Research Center in Cleveland OH, and Ames Research Center next to Moffett Field in CA.  President Dwight D. Eisenhower tapped T. Keith Glennan to be NASA’s first Administrator.  Hugh Dryden was asked to join the new agency as its first Deputy.  In his new role, Dryden was a key link to the immediate past, providing both institutional memory and continuity of service.  The NACA had been involved in space research, including the X-15 project, a rocket-powered, piloted aircraft capable of supersonic transport to the outer fringes of the atmosphere.  Neil Armstrong, a NACA test pilot, flew seven X-15 missions before his career as a NASA Gemini and Apollo astronaut.

Dryden and the NACA worked with the U.S. Air Force on the MISS (Man-In-Space-Soonest) project, which ultimately became Project Mercury, our first human spaceflight program.  This program was being developed and managed out of Langley Aeronautical Laboratory, a NACA facility.  The Space Task Group at Langley was headed by Bob Gilruth (later center director of Johnson Space Center), with Max Faget as one of his young, bright engineers grappling with the problems of hypersonic and orbital flight.

Hugh Dryden performed admirably the job of technocrat and manager during these early, exciting years, but perhaps his biggest contribution to space history is barely known.  The fate of Project Mercury was unknown in early 1961.  Recently sworn in as the 35^th President of the United States, John F. Kennedy seemed supportive of bold new technical endeavors but had been largely silent on his plans, if any, for the civil space program.  Although Kennedy made much about a supposed “missile gap” with the Soviet Union, this policy discussion was focused entirely on our parity in ICBM deployment (or rather, the alleged lack thereof).

This all changed in April of that fateful year.  The Soviets launched Yuri Gagarin on his single orbit flight, once again beating America to the punch by putting the first man in space.  In the same month, the United States suffered a humiliating military and diplomatic setback with the very public failure of an American-instigated invasion of Cuba at the Bay of Pigs.  The new President eagerly sought a high-visibility field of endeavor (preferably technological) in which America could demonstrate its superiority over the USSR.  Initially, the desalination of seawater was a leading candidate among the many projects Kennedy considered.  However, at the height of the Cold War, that challenge didn’t quite fill the bill.

On April 14, two days after Gagarin orbited the Earth, Kennedy met with his new NASA Administrator James Webb and his deputy, the holdover from the Eisenhower Administration, Hugh Dryden.  During this meeting, Dryden pointed out that while the Soviets could beat America to many different space “firsts,” a near-term human landing on the Moon was out of reach for both nations – that while declaring a “contest” with the Soviets on virtually any space goal ran the risk of America losing, odds were even for the first manned lunar landing.  America could not go to the Moon now, but likely we could within a few years.  Thus, if space was to be the chosen field for a superpower contest, Dryden believed the goal of a human lunar landing was the challenge we could win.

Kennedy received a detailed memorandum outlining all his space options from Vice President Lyndon Johnson on April 29, 1961, but Dryden had already forcefully made his case for a lunar landing to the President two weeks earlier.  It is often thought that Wernher von Braun was the one who convinced Kennedy that the Moon was the proper goal for Apollo, but Dryden had digested and presented von Braun’s technical arguments in policy terms that Kennedy could understand.  In the public’s mind, von Braun was “Dr. Space,” largely because of his work with Walt Disney in the 1950s popularizing the idea of space travel.  But it was Hugh Dryden who helped turn the dream of landing people on the Moon into a political commitment from the President and ultimately, a reality.

Hugh Dryden remained the Deputy Administrator of NASA until his untimely death in 1965.  He has been honored with a crater named for him on the Moon and as the namesake of the NASA Dryden Flight Research Center, an entirely appropriate memorial given his contributions to aeronautics and his key role in the establishment of the Apollo program.  He was at the right place (the White House) with the right President (Kennedy) at the right time (when America needed a challenging yet achievable space goal).  His life was one of service and excellence.  I think it does a disservice to the memory of Hugh Dryden to re-name the Dryden Flight Research Center and what’s more, I believe that Neil – the consummate gentleman – would also view HR 6612, the congressional bill passed to drop Dryden’s name and insert his in its stead, as unnecessary and wrong-headed.

I certainly agree that we should name a major facility for Neil Armstrong.   May I suggest that the first manned lunar outpost be named for Neil Armstrong – the first man to set foot on the Moon.

* Pronounced by saying each individual letter: “N-A-C-A,” not as a single word, as we do for its successor agency, NASA.

——

Note Added Jan. 7, 2013:   I have been reminded that the NASA Authorization Act of 2008 had already designated the American portion of the then-planned international lunar outpost as the “Neil A. Armstrong Lunar Outpost” (sec. 404 b).  Thanks to both Bill Mellberg and Joel Raupe for jogging my memory on this.

Map of the Moon by Grimaldi and Riccioli, 1651. Most of the names on this map are still in use today.

The Moon is remarkable for the variety and unusual nature of the names of its surface features.  The dark, smooth maria are named for weather or states of mind (Sea of Rains, Sea of Tranquility) while many of the abundant craters of the Moon are named for famous scientists, philosophers, mathematicians and explorers.  Before the advent of the space age, only the near side of the Moon was visible, although most scientists believed that the far side probably looked exactly like the one facing Earth. (How wrong they were!)  Naturally, once we had the ability to see uncharted lunar territory, a new era of name assignment commenced.  But even now, many lunar craters and features await something more than mere coordinates.

The drawings by Galileo of the Moon in 1610 show craters and mountain ranges but he did not assign names to them.  As telescopes improved, revealing finer surface details, several maps appeared with names bestowed by their astronomer authors to flatter patrons or express their nationalism.  Most of those early names have been forgotten to history.  In 1651, an influential map by Jesuit astronomers Grimaldi and Riccioli became the foundation for the official naming reference guide that we use today.

With the flight of the Luna 3 probe in 1959, the Soviet Union was the first nation to image the far side of the Moon.  To the surprise of most, large regions of maria (so prominent on the near side) were mostly missing from the far side.  Although the first images were of very low quality, the Soviets couldn’t resist the urge to name newly discovered features for a variety of Russian heroes and place names, such as Tsiolkovsky and the Sea of Moscow.  Some new “features” were misidentified because of the low resolution – the name “Soviet Mountains” (no longer used) was given to a bright linear streak across the far side globe (a feature that turned out to be a long ray from the fresh crater Giordano Bruno and not a mountain range).

Over subsequent years, as both American and Soviet spacecraft filled in the far side coverage with increasingly higher quality images, most major far side craters received names of various scientists and engineers.   From around the world, a mixed bag of names were submitted to the International Astronomical Union (IAU – the body of scientists who authorize the names of planetary surface features) for consideration and approval.  Although some were historically significant, many were people with whom few were familiar.

Though NASA does not have the authority to assign names to features on the Moon, an informal practice of naming landmarks was common during the Apollo missions.  Names were given to the small craters and mountains near each landing site (e.g., Shorty, St. George, Stone Mountain) but official names were used as well (e.g., Hadley Rille).  NASA adopts informal names for the same reason that names are given to geographical features on Earth – as shorthand to refer to landmarks and other mapped features.  The most recent illustration of this practice occurred on December 17, 2012 when NASA named the location where the deliberately de-orbited GRAIL spacecraft crashed onto the Moon near the crater Goldschmidt (73°N, 4°W) the Sally K. Ride Impact Site.  Sally thus joins other women of science and note who have lunar features named for them – Hypatia, Caroline Herschel and Marie Curie, among others.  Most of the informal names assigned during Apollo were later given “official” status by the IAU.

The Apollo basin (a 540 km diameter crater on the southwestern far side) was named to honor the Apollo missions – the only crater on the Moon so designated.  Within a few years of their missions, smaller craters were named for the living crews of Apollo 8 (Borman, Lovell and Anders) and Apollo 11 (Armstrong, Aldrin and Collins).  Also located around the Apollo basin are craters named for deceased astronauts and NASA employees, including the lost crews of Apollo 1 and the lost crews of the final missions of the Challenger and Columbia Space Shuttles.  It is appropriate that some feature honors humanity’s first efforts to reach the Moon, as well as others who gave their lives pioneering space.  In a similar vein, craters near the poles of the Moon tend to be named for famous polar scientists and explorers, such as Nansen, Shackleton, and Amundsen.

Other than these exceptions, the location of specifically named craters has little rhyme or reason.  Neither scientific prominence nor contribution guarantees any crater-endowed immortality.  Copernicus and Archimedes are rightly honored with spectacular craters named for them.  But Galileo and Newton (titans in the history of science) are fobbed off with insignificant or barely detectable features.  One of the most prominent craters on the Moon is named for the astronomer Tycho Brahe, an eccentric who spent most of his career trying to validate a variant of the Earth-centered, Ptolemaic model of the Solar System (Ptolemy also has a prominent crater in the center of the near side named for him).  It’s not clear why Riccioli assigned the names he did to these craters, though he cannot be blamed for giving Newton short shrift, as the future Sir Isaac was only nine years old when the Grimaldi and Riccioli map was published.

It is possible to both suggest a name and to propose a crater for that name, though the IAU is not obliged to accept either.  Often, a suggested name is approved but assigned to a different crater.  Currently, the guidelines for submission and assignment of new names for lunar craters are: 1) a scientist or explorer who has made some significant contribution, preferably to the study of the Moon and planets; 2) deceased for at least three years before a crater name becomes official; 3) it cannot duplicate any existing lunar name.

In 2005, I proposed the name Ryder (to honor my colleague Graham Ryder, a lunar scientist who passed away in 2002) and suggested a small, bright crater on the far side to carry his name.  Both suggestions were adopted.  We have since found that Ryder crater is actually quite a geologically spectacular feature (Graham would be proud of his namesake).  In a truly singular event, the crater Shoemaker (named in 2000 and located near the south pole of the Moon) actually contains some of Gene Shoemaker’s remains – a small portion of his ashes was carried aboard the Lunar Prospector spacecraft in 1998.  At the conclusion of that mission, the vehicle was crashed into the south polar crater that was subsequently named for him.

We don’t know what the IAU will do concerning the designation of the Sally K. Ride Impact Site but as history suggests, granting of official status is not guaranteed.  No matter – we will continue to assign names to features as needed and the IAU will do what they do.  In the early 1970s, the IAU (by fiat) abolished the famous Mädler nomenclature system (wherein a small, nearby crater is given the name of a large neighbor plus a letter, such as Copernicus H).  Most working lunar scientists stubbornly refused to accept this decision and continued using the old crater names.  After 30 years of bureaucratic intractability, the IAU finally surrendered and formally adopted the Mädler system.

Official or not, with the passage of time, named lunar landmarks will become familiar to those visiting and working on our nearest neighbor.  Perhaps interesting monikers will be attached by those locals, as is done here on Earth when we assign nicknames to places – like the Big Apple, the Windy City, the Big Easy and the City by the Bay.

Just published:  The Clementine Atlas of the Moon, Revised Edition, an updated atlas and reference guide to lunar features, by Ben Bussey and yours truly.

Map of the gravity gradients of the Moon as recorded by the GRAIL spacecraft (NASA)

The NASA mission GRAIL (Gravity Recovery And Interior Laboratory) has been orbiting the Moon since last spring.  The mission consists of two identical small spacecraft (dubbed Ebb and Flow) that very carefully keep track of their relative position from each other.  By tracking both of these spacecraft with high precision from Earth, we can monitor any small variations (caused by variations in the Moon’s gravity field) away from their predicted orbital paths.  If the satellite is flying over an area on the Moon with less material than normal (for example, over a deep crater, a hole in the Moon’s crust), it will be less attracted to the Moon because of this mass deficiency and will therefore fly away from the Moon.  If, on the other hand, it flies over an area of excess mass, such as a thick stack of dense lava flows, the excess mass pulls the satellite slightly toward it, increasing its speed and pulling it downwards.  As Ebb and Flow orbit the Moon, they conduct a delicate “dance.”  These movements are caused by variations in the Moon’s gravity (largely a reflection of variations in the density of its crustal rocks).  When combined with the high-resolution, precision topography of the Moon (currently being gathered by the Lunar Reconnaissance Orbiter), we are able to reconstruct the structure and thickness of the lunar crust from orbit.

GRAIL has unveiled a new global gravity data set, very high in resolution and precision and greater than ten times better than our previous version of the global gravity from the Japanese mission SELENE (or Kaguya).  One interesting result shows unusual structure – long, quasi-linear gravity features appear in a variety of locations associated with lunar impact basins.  Basins are very large craters that formed during asteroid collisions prior to 3.8 billion years ago.  Some of these linear features extend on great circles across the lunar globe for distances of more than 500 km. These results suggest that solidified intrusions of once-molten rock may form a dense, criss-crossing network within the upper crust.

In order to understand the significance of these gravity features, it is necessary to understand some elementary facts about planetary geology.  Planets generate heat and this heat must be dissipated.  Typically, the heat generated from both the original energy release during formation (accretion) and from the decay of radioactive elements (e.g., uranium) melts the interiors of planets, forming bodies of liquid rock called magma.  This magma is usually less dense than the rocks from which it forms and thus, rises upwards towards the surface.  Sometimes, the molten rock cannot ascend any higher from the deep locations where it comes from and freezes in place – geologists call this type of frozen rock body an intrusion, because it intrudes into pre-existing rock as a liquid and then solidifies by crystallizing.  When magma actually reaches the surface of a planet, it can erupt onto its surface as lava; this activity is called extrusive because the molten rock extrudes onto the surface and then solidifies as lava flows.

Clearly, all erupting lava must have at one time been an intrusive magma body, at least during the time it was ascending upwards toward the surface.  Although many magma bodies reach the surface and create lava flows (such as the dark, smooth maria of the lunar lowlands), sometimes this magma cannot reach the surface and freezes in place within the crust as a linear or tabular body.  Such features (called dikes) are an essential part of the underground, igneous plumbing of volcanoes on all of the terrestrial planets.  We knew that they must have formed on the Moon because we saw the evidence of vents and structures in the maria that are the surface expression of such features.

The Crisium basin; gravity gradient map at left, topographic map at right. Linear feature at far left extends over 300 km (NASA)

For the first time, the new GRAIL data show us direct evidence for these buried igneous dikes within the lunar crust.  One particularly prominent dike occurs near the Crisium basin, on the eastern near side of the Moon.  This dike extends over 1000 km in a quasi-radial direction northwest of the Crisium rim, disappearing beneath the mare lavas of that basin.  The fact that it is not clearly aligned with the basin structure suggests that it may predate it; we estimate that Crisium basin is older than 3.9 billion years.  This long linear feature may have been formed when molten magma from the deep interior of the Moon oozed its way toward the surface, before “freezing” at some intermediate level.  Its presence, evident now only by a faint gravity signature (those denser areas “tugging” on the GRAIL satellites “Ebb” and “Flow”), is a tell-tale remnant of its existence deep inside the Moon’s crust.

Many other linear and circular features are evident in the gravity gradient map produced by GRAIL.  Most of these seem to be associated with the large basins of the lunar highlands, the largest impact craters on the Moon. These features both excavate large amounts of crustal material during formation, and serve as topographic lows and structural traps for the accumulation of subsequent erupted lavas.  The gradient structures show a complex network of density patterns in the shallow subsurface of the Moon; this area is a morass of crushed rock, fractures, large faults and collapse features.  The entire outer portion of the lunar crust has been shattered and broken by an impact barrage of almost unimaginable violence.  The crust has since been partly annealed together by heat, re-fractured by additional impacts, intruded by large bodies of molten rock, resurfaced by the eruption of lavas from the deep interior, and finally has had its outermost surface pulverized into a fine powder by the micrometeorite bombardment.

The Moon may look like a silent, dead world but its past (which is Earth’s past) is testament to an early history of extreme violence and chaos.  The results from the GRAIL mission are helping us understand this complex story.

Composite image of the north pole of Mercury. Red are the areas of permanent shadow; yellow delineates radar bright deposits mapped from Earth. Data are plotted on a photomosaic of MESSENGER images. NASA

Mercury – the planet, not the element – was in the news this past week.  For some time, we had suspected that the poles of Mercury might harbor deposits of water ice.  This – on a planet so close to the Sun that the surface temperature at the equator is hot enough to melt lead!

Yet like the Moon, Mercury’s spin axis is perpendicular to the plane in which it orbits the Sun.  This means that large craters near Mercury’s poles lie in permanent shadow (“shivering” around -170° C), unaffected by the Sun’s searing heat (equivalent to more than eleven times the solar flux we get on Earth).  As on the Moon, these permanently shadowed areas get heat from only two sources – the 3 K background heat of space, created during the Big Bang some 15 billion years ago, and whatever heat is being generated now from the deep interior (a quantity that geophysicists call the heat flow of a planet).

Large planets (like Earth) generate heat mostly from the decay of radioactive elements deep inside them.  This heat is lost largely through the phenomenon of volcanism, in which melted rock from the interior is erupted onto a planet’s surface as lava and ash.  Smaller planets and moons likewise experience this heating and volcanism, but because they are have lower overall contents of heat-producing elements, their volcanic episodes occurred in the distant past.  Much of the heat of these smaller planets has been largely dissipated.  Thus, on Mercury, we suspect that the overall heat flow is very low, resulting in extremely cold temperatures on the floors of its permanently shaded polar craters.

For many years, astronomers have studied Mercury with radio telescopes from Earth (using radar to make images of its surface).  Because the orbital inclination of Mercury is relatively high (about 7°), we can get a fairly good look into the interiors of the polar craters.  Interestingly, even though Mercury is much farther away than the Moon, we can see more of the mercurian polar areas because of this relatively high orbital inclination (the Moon’s orbital plane is inclined only 5°).  These radar pictures showed an amazing and unexpected feature – the dark areas are filled with material that is highly reflective at radio frequencies, properties similar to the surfaces of the icy moons of Jupiter (Europa, Ganymede and Callisto).

These results were so unexpected and startling that debate raged for many years whether these deposits really were what they appeared to be: water ice.  Facts are stubborn things and few materials have radio properties similar to ice.  Some suggested that sulfur might be an alternative explanation, but provided little evidence for such behavior.  Moreover, another moon of Jupiter, Io, which has a surface largely composed of sulfur, does not show the radar brightness or “glint” seen on the other, ice-rich Jovian moons.

The debate on the nature of the Mercury polar deposits has now been settled with the release of new data from the MESSENGER mission.  Launched on August 3, 2004, with insertion into obit around the planet on March 18, 2011, the spacecraft has been taking pictures and making measurements of Mercury for the last two years.  We have mapped the extent of darkness near the poles, measured the temperatures of the surface inside these regions, and detected the presence of significant amounts of hydrogen there.  All of these results are strongly supportive of the water ice interpretation.

The existence of ice near the poles of Mercury supports the case for water ice on our own Moon, although there are some significant differences between the two occurrences.  Like Mercury, the Moon’s spin axis is nearly perpendicular to the plane of its orbit around the Sun.  The similarity of the terrain of both bodies results in deep holes that hide large expanses of terrain from the glare and heat of the Sun.  Both objects have been volcanically active in the past, but not today, meaning that the average rates of heat flow on both are low.  These properties result in the creation of polar “cold traps” in which any entering volatile substance (such as water molecules) cannot escape.

The solid bodies of the inner Solar System are constantly hit by debris from comets and asteroids.  This material contains water, both in free form and bound within hydrous minerals.  On smaller objects (like the Moon and Mercury), most of this water is lost to space, but we suspected that some of it might be retained within these dark cold traps near the poles.  Now we know that such a process does occur.

Differences between the Moon and Mercury result in differing amounts and settings for their polar deposits.  Being much closer to the Sun, one might expect Mercury to contain less water ice, but a variety of evidence suggests that the opposite is the case.  The polar ice of Mercury appears to be greater in extent and thickness than comparable deposits on the Moon.  This probably results from two factors.  First, Mercury is a bigger object, with a surface gravity about twice that of the Moon.  Thus, it is more difficult for water to “escape” from Mercury.  Second, the closeness of Mercury to the Sun (the edge of biggest gravity well of the Solar System) results in a higher flux of cometary impacts there than experienced in the Earth-Moon system.  So more water is being added to Mercury, where it is more easily retained.

Nonetheless, both Moon and Mercury have similar polar environments and processes.  The long debate – a scientific controversy for over 50 years – about water at the poles of these objects has been resolved.  The next steps will be to characterize these deposits in situ using a soft lander and selected instruments to measure the amounts, states and distributions of water in the polar areas.  Because of the great difficulty in even getting into orbit around Mercury (let alone landing there), doing this first on the Moon will mostly likely happen first.  So, here again is another rationale for sending a robotic surveying lander and rover mission to the poles of the Moon – in addition to characterizing these areas for our future presence there, by inference, we will also learn about the polar processes on and environment of Mercury.

A planetary “two-fer.”  Let’s get on with it.

Future industrial activity on the Moon -- science fiction? (Artwork by Pat Rawlings)

Space missions are commonly thought of as the ultimate in “high tech.”  After all, rockets blast off into the wild blue yonder, accelerate their payloads to hypersonic and orbital speeds and then operate in zero gravity in the ice-cold, black sky of space.  It requires our best technology to pull off this modern miracle and even then, things can go wrong.  Why would anyone believe that with high technology, sometimes less can be more – that we’re missing a bet by not utilizing current technology.   Like the intellectual tug of war involving man vs. machine, there also is a tug of war between proven technology and high-tech.  Creating these barriers and distinctions is nonsensical.  We need it all.  And we can have it all.

Point in question – in situ resource utilization (ISRU), which is the general term given to the concept of learning how to use the materials and energy we find in space.  The idea of learning how to “live off the land” in space has been around for a long, long time.  Countless papers have been written discussing the theory and practice of this operational approach.  Yet to date, the only resource we have actually used in space is the conversion of sunlight into electricity via arrays of photovoltaic cells.  Such power generation is clearly “mature” from a technical viewpoint, but it had to be demonstrated in actual spaceflight before it became considered as such (the earliest satellites were powered by batteries).

The reason we have not used ISRU is because we’ve spent the last 30 years in low Earth orbit, without access to the material resources of space.  Many ideas have been proposed to use the material resources of the Moon.  A big advantage of doing so is that much less mass needs to be transported from Earth.  The propellant needed to transport a unit of mass from the Earth to the Moon keeps us hobbled to the tyranny of the rocket equation – a constant roadblock to progress.  If it takes several thousand dollars to launch one pound into Earth orbit, multiply that amount times ten to get the cost to put a pound of mass on the Moon.

In the space business, new technologies tend to be viewed with a jaundiced eye.  Aerospace engineers in particular are typically very conservative when it comes to integrating new technology into spacecraft and mission designs, largely on the basis that if we are not careful, missions can fail in a spectacularly dreadful fashion.  To determine if a technology is ready for prime time, NASA developed the Technology Readiness Level (TRL) scale, a nine-step list of criteria that managers use to evaluate and classify how mature a technical concept is and whether the new technology is mission ready.

Resource utilization has a very low TRL level – usually TRL 4 or lower.  Thus, many engineers don’t think of ISRU as a viable technique to implement on a real mission.  It seems too “far out” (more science fiction than science).  Believing that a technology is too immature for use can become a self-fulfilling prophecy, a “Catch-22” for spaceflight:  a technology is too immature for flight because it’s never flown and it’s never flown because it’s too immature.  This prejudice is widespread among many “old hands” in the space business, who wield TRL quite effectively in order to keep new and innovative ideas stuffed in the closet and off flight manifests.

In truth, the idea that the processing and use of off-planet resources is “high technology” is exactly backwards – most of the ideas proposed for ISRU are some of the simplest and oldest technologies known to man.  One of the first ideas advanced for using resources on the Moon involve building things out of bulk regolith  (rocks and soil of the lunar surface).  This is certainly not high-tech; the use of building aggregate dates back to ancient times, reaching a high level of sophistication under the Romans, who over 2000 years ago built what is still the largest free-supported concrete dome in the world (the Pantheon).  The Coliseum was made of concrete faced by marble.  The Romans also built a complex network of roads, some which remain in use to this day; paving and grading is one of the oldest and most straightforward technologies known.  Odd as it may seem, sand and gravel building material is the largest source of wealth from a terrestrial resource – the biggest economic material resource on Earth.

Recently, interest has focused on the harvesting and use of water, found as ice deposits, at the poles of the Moon.  Digging up ice-laden soil and heating it to extract water is very old, dating back to at least prehistoric times.  This water could contain other substances, including possibly toxic amounts of some exotic elements, such as silver and mercury.  No problem – we understand fractional distillation, a medieval separation technique based on the differing boiling temperatures of various substances.  Again, this concept is not particularly high-tech as only a heater and a cooling column is needed (basically the configuration of an oil refinery).  Some workers have suggested that lunar regolith could be mined for metals, which can then be used to manufacture both large construction pieces and complex equipment.  Extracting metal from rocks and minerals is likewise very old, developed by the ancients and simply improved in efficiency over time.  Processes like carbothermal reduction have been used for hundreds of years. The reactions and yields are well known, and the machinery needed to create a processing stream is simple and easy to operate.

In short, the means needed to extract and use the material wealth of the Moon and other extraterrestrial bodies is technology that is centuries old.  Even advanced chemical processing was largely completely developed by the 19^th Century in both Europe and America.  The “new” aspects of ISRU technology revolve around the use of computers to control and regulate the processing stream.  Such control is already used in many industries on Earth, including the new and potentially revolutionary technique of three-dimensional printing.  A key aspect of the old “Faster-Cheaper-Better” idea (one NASA never really embraced) was to push the envelope by relying more on “off-the-wall” ideas, whereby more innovation on more flights would lead to greater capability over time.

Nothing that we plan to do on the Moon involves magic, alchemy or extremely high technology.  Like most new fields of endeavor, we can start small and build capability over time.  The TRL concept was designed as a guideline.  It was not intended as a weapon eliminating possibly game-changing techniques from consideration or to carve out funding territories.  Attitudes toward TRL must change at all levels, from the lowly subsystem to the complete, end-to-end architectural plan.  A critical first step toward true space utilization and for understanding and controlling our destiny there is to recognize and take advantage of the leverage one gets from lunar (and in time planetary) resource utilization.

LRO topographic map of the Moon, showing the approximate outline of the "Procellarum" basin on the near side (left) and the South Pole-Aitken basin on the far side (right). One's real, the other isn't.

Once upon a time, back in the Dark ages when I was a young student of lunar science, an idea was advanced that Oceanus Procellarum (the largest dark maria on the near side of the Moon) was the site of an ancient, almost obliterated impact basin.  This “Procellarum basin” (then called the “Gargantuan” basin – superlatives fail us sometimes) has been invoked to explain any and every observed aspect of lunar geology, from the distribution of the dark mare lavas, the near/far side dichotomy, the thickness of the crust, the composition of highland rocks, and the relative amounts of radioactively generated heat flow in the Moon.  Such a useful concept to explain so much!

The acceptance by lunar scientists of a Procellarum basin has waxed and waned over the years.  Originally proposed by Peter Cadogan in 1974, the presence of a large, ancient impact basin covering most of the western near side of this part of the Moon, was advanced to explain the unusually high concentration of the chemical component called KREEP – (K) potassium, (REE) rare earth elements, and (P) phosphorus.  Subsequently, Ewen Whitaker (noted cartographer of the Moon) carefully mapped landforms, such as ridges and massifs (mountains) over this area, which purportedly showed that the patterns were best explained by a three-ring basin – 3200 km across, centered on the western near side.  Whitaker named this feature the “Procellarum basin” after the largest mare region that filled it.  Lunar geologist Don Wilhelms fully embraced this interpretation in his classic book The Geologic History of the Moon, making the Procellarum basin the prime cause for the distribution of geologic units on the Moon.

Yet doubts persisted.  In 1985, Peter Schultz and I suggested that the quasi-concentric arrangements mapped by Whitaker, were related to the Imbrium basin (not to an earlier, underlying mega-basin) on the basis of the ring pattern of this putative feature.  We also pointed out that the patterns of rock compositions supposedly explained by a Procellarum basin were not consistent everywhere, at least casting doubt on the predictive power of the basin’s presence.  The 1994 Clementine mission gave us our first global topographic map of the Moon.  Interestingly, that map dramatically revealed the presence of a circular mega-basin on the far side of the Moon – the enormous 2600 km-diameter South Pole-Aitken basin.  The Procellarum region was also shown to be a low region, but it is not circular (more horseshoe-shaped) and is not as clearly defined as Whitaker’s ring structure suggested.  The stock in the existence of Procellarum basin declined.

But some ideas in lunar science never really go away.  Since that time, several attempts have been made to resurrect the basin.  The latest effort, just published in Nature Geoscience, comes from mineralogical mapping data obtained from the Japanese Kaguya (SELENE) mission.  The authors of this study claim that orthopyroxene (a magnesium-silicate mineral) is distributed on the Moon in association with its largest basins – South Pole-Aitken and Imbrium.  However, in addition to those occurrences, additional outcrops occur in the highlands adjacent to Oceanus Procellarum.  Therefore, these rocks were made during the slow cooling of an enormous impact melt sheet created by the impact which formed the Procellarum basin.

The logic here seems weak.  It has not been established that orthopyroxene only forms from the slow cooling of an impact melt sheet.  When this mineral occurs with the most abundant mineral of the lunar highlands (plagioclase), it makes up a rock type called norite.  Norite is very abundant on the Moon.  It is the dominant rock type at the Apollo 14, 15 and 17 landing sites and occurs elsewhere on the Moon in quantity.  It is particularly prevalent around the edges of the Imbrium basin and one could argue that norite is a characteristic of that basin and the presence of Procellarum basin to explain its occurrence is unnecessary.  Likewise, the existence here of a large differentiated impact melt sheet is inferred from analogy to a terrestrial example, the Sudbury igneous complex, but even in this case, the impact origin of the terrestrial igneous body is not universally accepted.

Evidence for the existence of Procellarum basin must be sought in its topography.  The clarity and preservation of the far side’s South Pole-Aitken basin in the topographic data is surprising.  This feature is one of the oldest on the Moon, yet it preserves relief of over 12 km (the depth one would expect of a fresh feature).  One might expect such an old feature to be indistinct at best, making the discovery of its large relief one of the surprises of the Clementine mission.  At the same time, Procellarum is a vast irregular depression averaging less than 3-4 km deep; its lack of topographic expression is more in line with what one might expect for the oldest basin on the Moon.  However, unlike all other lunar basins, a topographic bulge 2-3 km high occurs near the center of this feature (near the crater Copernicus).  No other basin on the Moon (or on any other planet) contains interior topography higher than the elevation of its topographic rim; at SPA, all of the terrain within the 2600 km diameter rim crest is lower than its rim.  The unusual relation of a bulge within Procellarum does not support the concept that it is an impact basin.  It seems more likely that it is either a feature of internal origin (possibly related to early melting episodes) or a coalescence of several overlapping impact craters and basins.

As we search for the truth, Procellarum basin may well crop up again.  But for today and contrary to the current space press, the new results do not uniquely point to the existence of a large basin here.  In fact, the observations tend to support previous ideas that it is the smaller, overlying Imbrium basin that is associated with a large regional ejecta blanket of roughly noritic composition.

The Sun exudes a constant stream of hydrogen, called the "solar wind."

New data returned from a fleet of orbiting satellites changes our perceptions of the history and processes of the Moon.  Concentrated at both lunar poles, and to date the most striking discovery, is the documentation of the presence of large amounts of water.  Though this water has been confirmed by several differing techniques (from multiple missions), we remain uncertain about its source.  Two principal origins have been proposed: 1) water added by the in-fall of water-bearing meteorites and comets during the impact bombardment of the Moon; and 2) the manufacture of water from hydrogen implanted in the lunar soil by the wind from the Sun.

A recent discovery may shed some new light on the origin of lunar water.  Researchers conducting detailed examination of tiny fragments of glass in soil returned by the Apollo astronauts found the molecule hydroxyl (OH) present in the glass.  Interestingly, the isotopic composition of these OH molecules indicates the bulk of the hydrogen comes from the Sun, not from cometary and asteroidal impacts.

The Moon has no atmosphere and no global magnetic field.  As a result, the solar wind – the stream of atoms and molecules constantly emitted by the Sun – directly impinges upon the lunar surface.  Most of this solar wind consists of hydrogen, either in the form of neutral atoms or positively charged ions (i.e., protons).   After it encounters the Moon, this spray of hydrogen has a complex fate, with at least some of it being implanted into the lunar dust.  In a process called adsorption, many of the hydrogen atoms stick to the surfaces of the dust grains.  The amount of adsorbed hydrogen varies by position and chemical composition around the Moon, but it can be present in quantities ranging from less than 10 to over 100 parts per million (ppm).

Impact glass is a major component of lunar regolith – up to 60% by weight of the soil at some landing sites.  The constant bombardment of the lunar surface by microscopic meteorites crushes and grinds up the surface rock, continually mixing the outer layer of the Moon.  When a micrometeorite strikes a rock, it forms a micro-crater (wholly melting the surface beneath this pit) and creates a clear, chemically homogeneous glass particle.  However, when a micrometeorite strikes lunar soil instead of rock, its energy is converted mostly into heat.  This flash heating creates a mixture of melt and mineral debris called agglutinate glass.

The new work details results of analyses of agglutinates returned from several lunar landing sites.  Their study measured both the amounts of hydroxyl present and its isotopic composition.  A normal atom of hydrogen is a single proton and an electron.  But in a rare form of hydrogen, called deuterium, the nucleus contains both a proton and a neutron.  The ratio of this form of “heavy hydrogen” to “normal” hydrogen is unique for different materials throughout the Solar System.  By tracking the D/H ratio in the sample, one can assign a source origin to the measured hydrogen.

When the lunar agglutinate glasses were studied, it was found that their D/H ratios indicated that most of the hydrogen in the hydroxyl molecules came from the Sun and not from cometary or meteoritic sources.  However, the source of the hydrogen is not completely solar, as the D/H ratios suggest some mixing with a subordinate component of either lunar or cometary origin.  The authors of this study suggest that the hydroxyl found on the Moon was created when a small impact flash heated the soil, releasing the adsorbed hydrogen and chemically reducing the metallic oxides in the soil into native metal (found as extremely tiny grains on the surfaces of the agglutinates) and hydroxyl molecules.  Multiplied by billions, such a process could account for the generation of water on the lunar surface.  Subsequent migration of these molecules toward cooler-than-average areas of the Moon (i.e., the higher latitudes, up to and including the poles) may have created the polar ice deposits found by numerous techniques.  In the view of the authors of this study, lunar water comes mostly (but not entirely) from the Sun.  This constant process, occurring on the sunlit hemisphere of the Moon, could create an enormous reservoir of hydroxyl molecules (in motion due to their thermal instability), slowly but constantly moving toward the poles.

If such a process occurs on the Moon, one might expect the accumulation of water in every location where water is stable (i.e., within every permanently dark and cold region near both poles).  But it appears that ice at the poles is not uniformly distributed, occurring in high concentration in some areas while absent in others.  This pattern suggests that the source of polar water might be controlled by a non-equillibrium process, such as episodic bombardment by asteroids and comets.  In fact, both solar wind-produced and cometary water may be present at the poles, but until the ice there is actually analyzed for its D/H content, we cannot be certain of its origin.  Such a measurement does not require the return of a polar ice sample to the Earth.  It could be made remotely in situ on the Moon with a properly instrumented robotic spacecraft.

It is important to emphasize that although the quantities of water generated by this process are potentially very large, the hydroxyl in agglutinate glass should not be considered an economic resource.  These molecules occur globally but at very low levels of concentration (tens of ppm).  Even if this water is the primary and ultimate source reservoir of lunar water, the migration of the molecules and their subsequent collection by the cold traps near the poles serve as a concentrating mechanism, where ice accumulates in large quantities, confined within small areas — the classic definition of an ore body.

What a change has occured in the mindset the lunar science community in the past few years!  From a bone-dry lump of rock in space to a complex, still mysterious body with a dynamic hydrological cycle.  It’s clear that many more discoveries about our Moon and its resources have yet to be revealed.  The more we learn about the Moon, the greater the range of processes we must account for and the more subtle and complex its history becomes.

"True" color (left) and "false" color (right) images of the near side of the Moon from Clementine. "Blue" units in Mare Tranquillitatis (right middle of false color image) are ilmenite-rich lavas.

The color of the Moon has been studied for years.  Lunar color is a subtle, yet fascinating phenomenon.  Just when it seemed that we had an explanation, complications would arise.  We now think we have a reasonable explanation for it.  So, why is the Moon gray?  Or to ask the question “scientifically”— What factors account for the range of spectral reflectance seen on the Moon?

Early Apollo astronauts were very impressed with the Moon’s lack of color.  During Apollo 8 (first mission to orbit the Moon in 1968) Jim Lovell remarked, “The Moon is basically gray – no color.”   The Apollo 10 crew was struck by the numerous brownish hues exhibited by the Moon – from a bright tan to a dark, chocolate brown.  When the first astronauts landed and walked on the Moon (Apollo 11), they had an even closer view.  Buzz Aldrin mentioned that although the surface color was basically gray, he could see interesting colors within some rocks outside the LM window.  During the EVA, Aldrin mentioned to Neil Armstrong that he had seen “some purple rocks.”  Purple? — perhaps so.

The Apollo 15 crew was surprised on their 1971 mission to catch a fleeting glimpse of green on the surface (in film shot earlier by crews on the lunar surface, color was too subtle to be seen). When they raised the sun visors of their helmets to again see that the soil was gray, the disappointment in their voices was palpable.  But then, at the very next station, they again saw a flash of green and this time, it was still green when the visors were raised.  Despite the predictable remarks about “green cheese,” this lunar material – consisting of volcanic glass erupted from deep (> 400 km depth) within the Moon under high pressure – was still green when brought back to Earth.

During their second lunar traverse in 1972, the crew of Apollo 17 found orange soil at Shorty crater.  Also volcanic glass, this soil is made up of tiny (~50 micron) beads of orange glass, again erupted from great depth.  It is orange (as opposed to the Apollo 15 green glass) because of its relatively high titanium content.  It is mixed with black glass beads, of identical composition, but in this case, partly crystallized.  Subsequent study of the Apollo samples have found volcanic glass fragments in almost every color in the spectrum, from red to yellow and brown in addition to the two described above.

True colors of some selected lunar samples. Top left - green glass pyroclastics from the Apollo 15 landing site. Top right - orange and black glass from Apollo 17. Bottom left -- troctolite showing yellow-brown olivine crystals. Bottom right - brownish crystals of orthopyroxene in Apollo 17 norite sample.

At this point, it is tempting to ascribe lunar color seen at a distance to the intimate mixing of a variety of colors present at fine scale.  But this is not quite correct.  Most returned lunar samples are also gray, ranging from a very dark charcoal to a light, almost white-gray shade.  Minor variations can be seen as a result of the presence of certain minerals.  In particular, the mineral olivine (an Mg- and Fe-rich silicate) is abundant in the lunar crust and is often green or a brownish yellow.  Ilmenite (and iron- and titanium oxide) is bluish-black and probably the source of the  “purple” Aldrin saw in some rocks during the Apollo 11 EVA.  Moreover, the astronauts could sometimes see significant color units from space.  After his surface visit, Apollo 17 astronaut Jack Schmitt (in orbit) saw orange material, excavated by small craters on the southwestern rim of the Serenitatis basin.  He suggested that this material might be related to the orange soil collected at the landing site a few days earlier.

Interestingly, one can detect subtle color differences on the Moon with telescopes and from spacecraft.  Although the Moon appears gray at first glance, one notices different hues of gray in certain places.  The dark Mare Tranquillitatis on the eastern near side is a noticeably darker and “bluish-gray” compared to the dark mare plains just to the north in Mare Serenitatis.  Part of the reason the Moon looks whitish-gray in the sky can be attributed to the fact that it is the brightest object in the night sky – dazzling the eye when first looked at (either with your naked eye or through a telescope).  Spacecraft views also reveal color differences.  It is common practice for lunar scientists to work with “false color” composite images, where color variations are “stretched” to extreme degrees to exaggerate differences in order to make them easier to work with.  The typical “false color” version of the near side of the Moon shows brilliantly colored “blue” and “red” maria; these color units do not coincide with mare-highland boundaries.  The received wisdom is that the different color units in the lunar maria represent lava flows of differing composition. That some lavas are enriched in titanium was a major finding from the Apollo sample studies.  Interestingly, these high-titanium lavas come from “blue” regions in the maria.  Initially, this was only an empirical correlation but we now know that it is the presence of ilmenite (the iron-, titanium-rich oxide) in these basalts that makes them “blue.”

It should be noted that color differences on the Moon are extremely subtle, requiring intensive image processing to display them clearly.  Typically, color differences on the Moon are less than about one percent or so.  We are able to see these differences with a careful look, but mapping the detailed boundaries of individual lava flows requires image processing to make the “false color” composites.

Lunar soil from the Apollo 11 landing site. Mostly gray, the fine material shows splashes of colors, including green, red and brown. Image by Randy Korotev, Washington Univ.

The “true” color of the Moon is a brownish (i.e., reddish) gray, but overall, the surface is fairly neutral in tone.  If the Earth had no atmosphere, hydrosphere or biosphere, it too would be largely a brownish-gray, as its crust is made up (more or less) of the same silicate and oxide minerals as the Moon (in slightly different proportions).  It is the weathering effects of air and water and biological activity at the Earth’s surface that makes it so colorful.  The Moon – having none of these processes – displays the “true color” of the rocky planets of the Solar System.  The dominant mineral in the lunar crust is plagioclase, a calcium/aluminum-rich silicate mineral.  Plagioclase is gray.  Thus, the dusty surface of the Moon, derived from plagioclase-rich rocks, is likewise gray.  When we talk about “red” and “blue” in lunar terms (as in “blue mare basalts”), we mean bluer, or less reddish, than comparable mare deposits elsewhere on the Moon.  So in reality, lunar color differences are really just varying degrees of reddish gray, some more so than others.

And what of the blue Moon?  As Conan the Barbarian might say, “But that is another story…..”

Computer simulation of the "hit-and-run" impact scenario for lunar origin. Sequence progresses in time from bottom right to top left. From Reufer et al. (2012) Icarus 221, 296.

The origin of the Moon is a long-standing problem in planetary science.  Reconstructing complex events in the distant past is difficult and requires both knowledge and imagination.  The facts to be explained are relatively straightforward.  The Moon’s overall density (about 3.3 grams per cubic centimeter) and bulk chemical composition are about the same as that of the mantle of the Earth, suggesting a possible relationship between the two.  The idea that Earth and Moon are compositionally related is supported by the ratio of isotopes of oxygen in the lunar samples, which indicate that Earth and Moon are made from matter derived from the same region of the solar nebula (material that is compositionally distinct from that making up the various meteorite groups).  Finally, the Earth and Moon collectively have a very high angular momentum, mostly as a consequence of the high spin rate of Earth and the relatively large mass of our Moon compared to its primary planet.

Prior to the Apollo missions, three different models (capture, fission, binary accretion) vied for acceptance among the lunar science community.  The capture model proposed that the Moon formed elsewhere in the Solar System before a close, chance encounter resulted in the Earth capturing the Moon into orbit.  The fission model proposed that a large mass of molten material spun off a rapidly spinning early Earth, was thrown into orbit and over time, coalesced into the Moon.  The binary accretion model suggested that Earth and Moon assembled themselves independently as two distinct and separate bodies from the beginning.  None of these models seemed able to account for all the “constraints” mentioned above, but no one had any better ideas.

About 30 years ago, the problem of lunar origin was widely considered “solved” with the general acceptance of the Giant Impact model.  In this concept, four and a half billion years ago, the proto-Earth shared its orbit around the Sun with an object about the size of the planet Mars (dubbed Theia, in Greek mythology, the titan who gave birth to Selene, goddess of the Moon).  A chance encounter between these two planetoids resulted in their merging as the Earth-Moon system.  It was thought that a grazing (low angle) impact would serve to both spin up the Terra-Luna system, resulting in its relatively high angular momentum, and hurl vaporized mantle material from Theia into orbit around the Earth.  The disk of orbiting debris quickly coalesced into the Moon and this rapid accumulation resulted in the release of large amounts of heat, which proceeded to melt at least the outer few hundred kilometers of the Moon, creating an “ocean” of molten rock, or magma.

The Giant Impact model seemed to nicely account for most of the properties of the Moon.  But like many big ideas in science, the closer and longer we look at it, the more issues seem to arise.  It was long assumed that the Moon was made of material derived mostly from mantle of the impacting planet (Theia); in this view, the Giant Impact was really just a variant of the capture model.  As such, it did not explain either the chemical similarity of the Moon to the mantle of the Earth, nor their identical oxygen isotope compositions.  This objection was usually brushed away with the admonition that complications might be expected from planet-scale impacts.

A new set of computer models has looked at the consequences of a slightly more head-on planetary collision.  In contrast to the traditional oblique (few degrees) off-center Big Whack, researchers modeled the effects of an impact at about 30° incidence and relatively high velocity (about 1.3 times escape velocity, or roughly 14 km/sec).  They find that in this case, most of the material from which the Moon forms comes not from the impactor Theia, but from the mantle of the Earth.  This result might better explain the compositional attributes of the Earth-Moon system.  In fact, several models were run (slightly varying these conditions) and while none perfectly fit the chemical and dynamical constraints, this one matched them most closely.

While this modeling was underway, another group was analyzing the composition of isotopes of titanium in samples from the Earth, the Moon and meteorites.  The work has established that the chemical fingerprints that relate Earth and Moon are not merely close – they are virtually identical (to the best precision of the measurements).  The authors of this study claim that this result creates problems for the Giant Impact model, as that idea had called for most of the Moon to be derived from the mantle of the impacting planet Theia.  However, with the results of the new computer models of giant impacts discussed above demonstrating that the parameters of the collision can be adjusted to match the constraints on lunar origin, perhaps this is not such a problem for the Giant Impact model after all.

These developments should probably give lunar scientists pause.  After all, the Giant Impact model became popular because the earlier, traditional three models (capture, fission, binary accretion) were all inadequate and their boundaries and defining parameters had to be adjusted to permit their (barely acceptable) viability.  In other words, the models were stretched to fit any inconvenient facts or problem observations.  Now it appears that the same thing is happening to the new, “explains-it-all” Giant Impact model.  A scientific idea that can be stretched to fit any observable fact is not very useful as an explanatory principle – it is simply a glorified “Just So” story.  The late Karl Popper argued that often in science, an idea cannot be shown to be true, but it can always be shown to be wrong – that is, “falsified.”  If a hypothesis cannot be falsified, Popper argued, then it was not scientific. We need a mechanism in science to enable us to dismiss useless or irrelevant concepts and falsification is one way to do that.

So where does such philosophy leave the origin of the Moon?  Perhaps more knowledge and imagination is needed before we can pronounce lunar genesis a “solved problem.”