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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