Cue music, "Also Sprach Zarathustra".....

A long established year-end tradition – for good or ill – is a review and analysis of the preceding twelve months.  Who am I to fight this trend?  Being that I am a “the glass is not only half-empty, but chipped and cracked down the middle” space policy town crier, be fairly warned as I conclude this year’s blogging with a look back at 2011.

The retirement of the Space Shuttle this past year vindicated T.S. Elliot’s pronouncement about the nature of the end of the world.  The U.S. workhorses that ferried Station pieces and crew to low Earth orbit await their museum berths.  The most heated emotions and debate surrounding this event dealt with the agency’s selection of the final resting places for the working U.S. space access machines.  To the outrage of many, space-oriented places like Houston and Huntsville were cold-shouldered in favor of show business-oriented Los Angeles and New York City.  In the heat of this controversy (so dire that members of Congress from space-economy communities rose from their slumber to pen op-eds mirroring constituent alarm),  few noticed or understood that without a replacement, the country’s capability for humans to access space had been discarded.  As  2011 closes out, construction and assembly of the International Space Station is complete – it is a unique Earth-orbiting platform for ongoing scientific research, accessible for the price of a ride on a Russian Soyuz spacecraft.

This past year was heralded as the opening chapter for a new approach to human spaceflight – the American civil space program was to advance more economically through the use of commercial launch services to LEO.  We’re waiting and watching, with more than a little trepidation, as millions of taxpayer dollars are doled out to “New Space” companies branded “commercial.”  Recent history shows taxpayer-funded, new-technology enterprises have failed spectacularly.  It’s troubling that simultaneously, these space access ventures are making similar claims of soon-to-be superior, cheap alternatives toward solving a pressing national problem.

In other exciting developments, the agency announced their new “mission statement” –  “To reach for new heights and reveal the unknown so that what we do and learn will benefit all humankind.”  Some noted the new statement says nothing about conducting missions and doesn’t mention space.  But it is stirring – a mission statement for an agency without a mission.

After being kicked long and hard by the Congress, NASA finally decided that they should probably go ahead and build a new launch vehicle.  Despite some initial foot-dragging (and the conspicuously ignored presence of an obvious and inexpensive alternative), the agency buckled down and produced a design for a new heavy lift launch vehicle, one that looks remarkably similar to the now-discarded Ares system.  With continued work on the new Multi-Purpose Crew Vehicle, looking remarkably similar to the now-discarded Orion spacecraft, we soon will be ready for new and exciting missions to untrod landscapes in space – perhaps a large rock –in a decade.  Maybe.  Perhaps even for less than its estimated $100 billion cost.

Robotic science missions, the so-called “crown jewels” of the space program, had their own share of difficulties this year.  The Goddard-run James Webb Space Telescope, the second-generation successor to the highly successful Hubble Space Telescope, is coming in late with a price tag of more than $8.7 billion and counting.  Its continued cost growth threatens all NASA space science programs.  JPL’s own giga-project, the $2.5 billion Mars Science Laboratory, was successfully launched and will encounter the planet in about six months, hopefully at very low velocity.  Less costly robotic missions to a variety of destinations continue to return copious amounts of data; whether there will be money to reduce and analyze it all remains uncertain.

The past year was the 50^th anniversary of both Yuri Gagarin’s first flight into space and John F. Kennedy’s announcement of the Moon landing goal – two events separated by type and location but connected in motivation.  It also was the centennial year of the race to, and attainment of, the South Pole – an event with reverberations throughout the ensuing years as a template for national efforts in exploration.  The space program, steeped in the history of global geopolitics and national competition, has sputtered slowly to a stop under that motivational and operational model.  A new paradigm for the space program is needed, one that ensures its long-term viability and stability.

To their own and the nation’s detriment, NASA is trapped by one model when thinking about space.  Missing is the notion of permanence and expansion into space.  A variety of “anyplace-but-the-Moon” destinations for human spaceflight have been mooted and studied in the past year, including near-Earth asteroids, L-points, the tiny, asteroid-like moons of Mars, lunar orbit, and even a human Venus flyby.  All of these imagined missions require knowledge, hardware and technologies that we do not now possess.  All expose human crews to substantial risk through long-term exposure to radiation and microgravity.  None create permanence of human presence or extension of capability in space.  And all travel to destinations offering little scientific and exploratory benefit or variety; their main attraction seems to be the yet-to-be-explained agency imperative to cross them off some “been there” check-list.

Several plans to develop cislunar space through an incremental, step-wise approach have been advanced.  The goal in each is not a flags-and-footprints type of space extravaganza, but the steady expansion of capabilities and reach beyond low Earth orbit.  Such a modus operandi is possible through the development and use of lunar resources —specifically the water ice found in quantity at both poles of the Moon.  In stark contrast to the Apollo template (and regardless of budgetary ups and downs), constant, steady and measurable progress can be realized through the creation of this “transcontinental railroad” in cislunar space.

I note with sadness, the passing of some great space visionaries this year.  John Marburger, former Presidential Science Advisor, was one of the few who truly understood the meaning and purpose of the Vision for Space Exploration.  Lunar and planetary scientists Baruch Blumberg, Bill Muehlberger, Mike Drake, Paul Lowman, Nick Short, Chuck Sonett, and my academic advisor and friend Ron Greeley passed away this year.  Theirs were voices of knowledge and experience and they will be missed.

The year 2011 was an annus horribilis for the national space program.  Here’s to the forthcoming year and hopes for a return of sanity to space policy.

Dark and light streaks on crater walls, Moon. (click to enlarge)

Although the Moon’s gravity is low, only about 0.165 of the Earth, rock and soil move down slope over time.  In geology, such processes are called mass wasting and is one of the principal sources of erosion on the Moon (the other being meteorite bombardment).  Mass wasting includes both gradual, infinitesimally slow soil creep on slopes and rapid, catastrophic mass movements, called landslides.  Long trains of rock debris can form scree slopes, loose fragments lying precariously at the critical angle beyond which they move, the angle of repose.  Because impact craters make steep walls and the larger ones bring up peaks in their centers, most mass wasting on the Moon is found in and around impact craters of all sizes.

As the number of high resolution images taken from the LRO mission continues to proliferate, several interesting and underappreciated lunar surface phenomena are becoming more apparent.  Among the fresh craters of the Moon, we find light and dark steaks on the walls of the ubiquitous craters of the Moon.  Although it is not surprising that material might move or flow down steep slopes on the Moon, the appearance of these flows can be startlingly similar to those seen on other planets, particularly Mars, where such streaks have been cited as evidence for the presence of subsurface water.

The new narrow angle LRO camera can see objects on the surface smaller than one meter (typically, 50 cm per pixel resolution).  These new views have shown us a wide diversity of new features within impact craters and have given us a new appreciation for mass wasting.  Larger crater walls are slumped, with stair step-like wall terraces, concentrically arranged around the crater between rim and floor.  In detail, these terraces show ponds of dark material that seem to collect in low areas.  Most of this material looks like it was once molten but now congealed; it is probably solidified impact melt.  Flows of melt may cascade down and over the walls of fresh craters.

However, many “flows” of both dark and light material on the Moon seem to consist of loose fragments of rock debris lying on steep slopes.  These debris flows show a variety of morphologies, including simple flow shapes, cascades, ponding, and fan-like termini.  Sometimes the dark and light flows intermingle within a single crater while others show only one type.  These debris flows can usually be traced back to outcrops of bedrock in the upper portions of the crater wall.  As the bedrock erodes (usually by meteorite erosion and disaggregation due to the intense fracturing induced by the original impact that formed the crater), it sheds small fragments that train down slope, forming flow-like landforms.

Because crater walls are uneven, undulating surfaces, the rates of down slope movement can vary widely over small distances.  This sometimes results in multiple, overlapping flows of debris.  Factors that control the albedo (reflectivity) of the debris flows are not well understood.  It could be related to composition (for example, dark, iron-rich mare basalt vs. white, anorthositic highland rocks).  Another factor might be particle size; small pebble-sized rock flows could be bright as new, fresh surfaces are constantly exposed.  Flows that contain mixed soil might be darker than normal, as this soil could cover the fragments and reduce its average reflectivity.  But while all these factors may be of significance to one degree or another, the brightness of a streak is not particularly indicative of origin.

Dark streaks on crater walls, Mars. (click to enlarge)

On Mars, many dark streaks are evident on crater walls and, as on the Moon, come in a wide variety of forms and occurrences.  Martian dark streaks have been variously interpreted as being caused by compositional and particle size differences, but the most popular idea is that the dark streaks are wet soil, i.e., they represent areas where liquid water is seeping out from the planet’s subsurface and moistening the surface.  One observation supporting this idea is an apparent correlation of some of the dark streaks with surface temperature, with warmer slopes showing more.  As liquid water is not stable on the martian surface, salt-rich brines (which would have much lower melting points than pure water) have been invoked as the possible liquid phase.

The dark streaks on the crater walls of the Moon call water-related interpretations of similar features on Mars into question.  The nature of down slope movement on Mars is likely to be controlled by even more diverse factors than the lunar case.  For example, large landslides partly cover the floor of the Valles Marineris, the large canyon system on Mars.  These landslides can extend tens of kilometers across the valley floor and the mass flow might have been lubricated by trapped atmospheric gas; this “cushioning” effect occurs within some landslides on the Earth.  Such a process would not occur on the Moon.  The diversity of geological processes on Mars suggests that explanations for dark wall streaks could encompass many more possibilities than simple wetting of the surface.

Although the existence of dark lunar streaks does not negate water-related interpretations of similar features on Mars, they do call attention to the need to keep alternative hypotheses in mind.  For many years (and with some success), planetary geologists have extrapolated landforms and processes (thought to be understood) on Earth, to similar appearing features on the planets.  In the case of the dark streaks, terrestrial water seepages in the desert can be darker than surrounding desiccated terrain.  A wide variety of evidence indicates that water is present in the subsurface on Mars but sometimes other effects such as rock composition or particle size are responsible for the streaks and alternatives to seepage should always be kept in mind.

Professor Ronald Greeley, 1939-2011

Yet another lunar and planetary scientist has departed this world.  My former teacher, dissertation advisor and mentor Professor Ronald Greeley passed away this week at the age of 72.  The news of his death came as a true bolt from the blue – Ron was in apparent good shape, good humor and active in his scientific research.  And so sadly, I say an untimely goodbye to another friend and colleague.

Ron became involved in planetary geology while fulfilling his military service requirement at NASA Ames Research Center.  Ames needed a geologist, and though trained as a paleontologist, Ron was assigned the task of examining images of the Moon to study volcanic landforms.  He quickly became interested in lava tubes (large horizontal conduits that transport lava from the eruptive vent outward as flows).  After an eruption, lava tubes sometimes drain out, leaving behind an empty cave.  Lava tubes can be many kilometers in length and tens of meters in cross section.

Sinuous channels wind their way across the relatively flat smooth surface of the lunar maria.  Some workers noted the similarity of these features to terrestrial lava tubes and postulated that sinuous rilles were remnants of lava tubes and channels on the Moon.  Ron examined this idea in detail by mapping and studying lava tubes on terrestrial volcanoes and by analyzing the images returned by orbiting lunar spacecraft.  He wrote a key paper on Hadley Rille (a large sinuous rille at the base of the Apennine Mountains) the outer ring of Imbrium basin and the largest impact feature on the lunar near side.  This area had been chosen as the landing site for the future Apollo 15 mission and understanding the origin of sinuous rilles was one of the mission objectives.  Ron detailed the evidence that Hadley Rille is a collapsed lava tube.  He noted the rille originated in an elongate, volcanic depression, had slightly raised edges and trended generally down slope to the north.  Parts of the rille were still roofed, raising the possibility that caves could exist on the Moon.  Years later, I had the honor to be a co-author with Ron and Gordon Swann (Principal Investigator of the Apollo 15 Field Geology Experiment) on another paper about Hadley Rille, modifying and extending the model Ron had developed in 1971.

While taking an undergraduate course at ASU in meteoritics, I wrote a term paper on the geology of Hadley Rille.  I was just getting into lunar science and as a big fan of the Apollo 15 mission, I had read Greeley’s paper with interest.   In a strange coincidence, Ron came to ASU that semester to give a talk on planetary geology and I arranged to meet with him after his seminar.   We ended up talking for a couple of hours and he offered me a job for the summer at NASA-Ames.

For a starry-eyed space cadet, this offer was almost too good to be true.  I worked the summer of 1976 on a Mars mapping project for an advanced mission study.  That was the summer of the Viking landings on Mars, and I spent part of my time in Pasadena as a JPL intern.  Ron was a team member of the Orbiter imaging team and arranged for me to work with him and John Guest on certifying the landing site for Viking 2.  It was a memorable and exciting introduction to planetary exploration and I will always be in Ron’s debt for having given me that opportunity.

After studying for my Master’s degree at Brown, I returned to work with Ron at Ames.  When he moved to ASU, I applied there to get my Ph.D.  Ron agreed to take me on and I became one of his first doctoral students.  Ron was a great mentor and a role model for a modern working scientist.  Even as his academic group grew to where he needed to assign work and follow up later with discussion, I was always welcomed into his office to discuss science or other concerns.  Besides showing his students how to do science, Ron also taught us how to survive scientifically.  Science is a social activity.  Navigating the treacherous political shoals of science is a learned and acquired skill and Ron generously passed those valuable lessons on to his students.

One of Ron’s best qualities as an academic mentor was assuming the role of what most graduate students desperately need, yet few ever get – a merciless and persistent editor.  I never learned how to write until I worked for Ron.  Hopefully I would turn in drafts of papers only to have them handed back to me in (almost literally) shreds.  (This was before the days of word processing – we typed our papers and then literally cut-and-pasted the text into some kind of readable form.)  Being told that your prose “stinks” is an infuriating rite of passage if you hope to become an acceptable writer.  Working with Ron all those years convinced me of an uncomfortable truth – there is only one way to learn how to write and that is to write often and be edited heavily.  Many do the first part, but few are fortunate enough to have a good editor for the second.  Of course, I didn’t see it that way at the time; getting a copy of your work covered in red ink is annoying as hell.  But an edit from Ron always improved the text, regardless of what it did to my blood pressure.  Again, I am in his debt.

Ron never let scientific grass grow under his feet.  He developed an interest in the geological effects of wind and was the first to determine the wind speeds needed to start sandstorms on Mars.  He made geological maps of every planet and was involved as an investigator on most of the robotic planetary missions of the last 30 years.  He served the scientific community through numerous committee memberships and chairmanships.  If Ron was asked to study an issue and write a report on it, you could be sure that his report would encompass the best thinking on a subject – lucidly and concisely presented.  He was a superb speaker and presenter of scientific results, always fluent, interesting and engaging.  Beyond science, Ron’s students learned how to write and speak, two critical skills for a working scientist.

In addition to being a good scientist, Ron was a fine man.  He cared deeply for his family and spent as much time with them as he could, taking his lovely and gracious wife Cindy and children Randy and Vanessa with him on many of his national and international travels.  He was a role model for his students both personally and professionally.  If one wants to be remembered as living a productive and valuable life, emulate Ron Greeley.

The lunar feature Ina, an extremely young, unusual depression that may represent a gas eruption site on the Moon. LROC narrow angle camera images.

There are times when seemingly unrelated discoveries about other planets come forward to enlighten us about the history and processes of the Moon. A recent paper, using data from the orbiting MESSENGER mission mapping Mercury, describes a number of newly discovered rimless pits and depressions.  These pits (called hollows by the mission team) are difficult to explain by impact processes and are hypothesized to be the products of outgassing from the planet’s interior.  They are often associated with color anomalies (which implies compositional differences from the surrounding terrain) and frequently found on the floors of impact craters and basins.

Impact craters come in a wide variety of sizes, but within selected size ranges, they all appear more or less similar.  Small craters are nearly perfectly round and bowl-shaped with smooth rims that are raised above the surrounding terrain.  Craters with irregular shapes and no raised rims suggest that processes other than impact might be at work.  It has been suggested that on Mercury, these “hollows” were created by the violent release of volatile substances.  Such a release of gas under pressure accompanies volcanic eruptions called pyroclastic, meaning “fire-broken” (fine liquid rock (magma) fragments spewed into space and cooled during flight).

We’ve known about pyroclastic eruptions on the Moon for many years, evidenced by the green glass of the Apollo 15 site and the orange-black glass from Apollo 17.  Careful search of the images taken from lunar orbit reveal the rimless pits that served as vents for the pyroclastic eruptions that produced these Apollo glasses.  They are distinct from impact craters and often are found on the floors of craters and basins along fractures, the conduit by which volcanic magma travels to the lunar surface.

Sometimes pit craters or “hollows,” found across the surface of the Moon, take unusual form.  The kidney-shaped feature shown above is named Ina; after its discovery in one of the Apollo orbital images, it was informally named the “D-caldera” after its shape and the interpretation that it represented a volcanic collapse feature.  Ina is about 3 km across and consists of a series of small platforms, mounds and holes within a larger irregular depression.  Other similar pits and hollows occur elsewhere on the Moon (e.g., on the floor of Rima Hyginis).  And while not major features, they have been found often enough to bother many lunar scientists, who had no good explanation for their origin.

About five years ago, we got a clue as to the possible origins of these features.  Pete Schultz and associates from Brown University published a paper showing Ina displayed unusual spectral reflectance characteristics.  The slow micrometeorite bombardment of the Moon adds craters to the surface and also makes small iron-rich glass particles that darken and redden the surface.  As these glass particles build up in the soil, a soil is said to “mature.”  Fresh surfaces are more “blue” in color (actually, less red) and become redder with time as the soil matures.  Most lunar features show age or “become mature” on timescales of millions of years.  Ina shows very few impact craters on top of it, meaning that geologically, it is very young.  Moreover, the soils associated with Ina are much bluer than surrounding areas.  Both of these observations suggest that Ina is young with immature surfaces.

How are these features created?  Significant volcanism on the Moon largely stopped at least a couple of billion years ago.  The Brown team thought that the combination of young age, low maturity and unusual morphology suggested a relatively uncommon pit-forming process.  They proposed that the explosive release of volatile substances from the lunar interior would have disrupted the surface, created a chaotic mixture of rock and soil, exposed fresh surfaces (creating the immature spectral signature), and formed a collapse depression caused by the instantaneous removal of mass from below.

Now we can see that the new Mercurian hollows have morphologies displaying spectral anomalies similar to the lunar collapse pits such as Ina.  The new data suggest that Mercury contains significant volatile substances.  These volatiles must be present at some depth, accumulated under high pressure until crustal failure ensues and a massive gas release results in an “eruption.”  This explosive event leaves behind a chaotic, disrupted surface (“immature,” with fresh bedrock and deep regolith “newly” exposed to space).

In the case of Ina on the Moon, its extreme youth is suggested both by the lack of overlying impact craters of almost any size, as well as the sharp preservation of topography in its cliff and pit interior morphology.  This extreme youth may be on the order of thousands to hundreds of thousands of years, not the millions and billions of years that typify most lunar landforms.  Such youth and the widespread distribution of Ina-like collapse pits across the lunar surface implies that outgassing events are occurring on the Moon now; it is highly unlikely that we were just lucky enough to find a singular or unique occurrence.

What might these volatile substances be?  Before the recent lunar missions flew, it was common to declare that water was not a possibility.  However, we recently discovered from study of the lunar samples that water was present in the deep interior of the Moon during the epoch of mare volcanism three billion years ago; water could still be present in the subsurface.  There are many other volatile substances that could be responsible as well, including carbon monoxide, hydrogen sulfide, gaseous sulfur, as well as other more exotic gases.  Because the compositions on Mercury are poorly known, the possibilities for exotic materials there are even more extensive.

The explosive release of gas from the deep interior (without the eruption of magma) appears to be an ongoing lunar process.  This gas release could provide at least a partial answer to two vexing lunar problems: the accumulation of volatiles at the poles of the Moon (discussed in my blogging many times, most recently HERE) and the infamous phenomena of Lunar Transient Phenomena (LTP), described as glowing reddish “clouds” hovering over the lunar surface that mysteriously appear and disappear.  Telescopic observers have reported seeing LTP for many years.  Unfortunately, we have not been able to verify and document these events, largely because they are transient.  Now we have direct morphological evidence for the venting of gas from both planets, making it possible that at least some LTP might be related to gas release from inside the Moon.  Stay tuned – the book of the Moon continues to be rewritten and expanded with new and interesting discoveries.

NOTE: The latest version of the paper Tony Lavoie and I wrote on using lunar resources to create a cislunar space faring system has been published in the Proceedings of the AIAA Space 2011 Conference.  A copy is available for download HERE.

People at an asteroid: What will they do there?

Part II:  Scientific Considerations

In my last post, I examined some of the operational considerations associated with a human mission to a near Earth asteroid and how it contrasted with the simpler, easier operations of lunar return.  Here, I want to consider what we might do at this destination by focusing on the scientific activities and possible return we could expect from such a mission.  Some of the operational constraints mentioned in the previous post will impact the scientific return we expect from a human NEO mission.

Asteroids are the left over debris from the formation of the Solar System.  Solid pieces of refractory (high melting temperature) elements and minerals that make up the rocky planets have their precursors in the asteroids.  We actually have many pieces of these objects now – as meteorites.  The rocks that fall from the sky are overwhelmingly from the small asteroids that orbit the Sun (the exception is that in meteorite collections, some come from larger bodies, including the Moon and Mars).

Moreover, we have flown by almost a dozen small bodies, orbited two, impacted one and “landed” on two others.  Thousands of images and spectra have been obtained for these rocky objects.  The chemical composition of the asteroids Eros and Vesta have been obtained remotely.  We have catalogued the craters, cracks, scarps, grooves and pits that make up the surface features of these objects.  We have seen that some are highly fragmental aggregates of smaller rocks, while others seem to be more solid and denser.  In addition to these spacecraft data, thousands of asteroids have been catalogued, mapped and spectrally characterized from telescopes on the Earth.  We have recognized the compositional variety, the various shapes, spin rates and orbits of these small planetoids.  We now know for certain that the most common type of meteorite (chondrite) is derived from the most spectally common type of asteroid (S-type) as a result from the Hayabusa mission, the world’s first asteroid sample return.

In short, we know quite a bit about the asteroids.  What new knowledge would we gain from a human mission to one?

Although we have (literally) tons of meteorites, extraterrestrial samples without geological context have much less scientific value than those collected from planetary units with regional extent and clear origins.  Many different processes have affected the surfaces of the planets and understanding the precise location and geological setting of a rock is essential to reconstructing the history and processes responsible for its formation and by inference, the history and processes of its host planet.

Most asteroids are made up of primitive, undifferentiated planetary matter.  They have been destroyed and re-assembled by collision and impact over the last 4.5 billion years of Solar System history.  The surface has been ground-up and fragmented by the creation of regolith and some details of this process remain poorly understood.  But in general terms, we pretty much know what asteroids are made of, how they are put together, and what processes operate upon their surfaces.  True enough, the details are not fully understood, but there is no reason to suspect that we are missing a major piece of the asteroid story.  In contrast, planetary bodies such as the Moon have whole epochs and processes that we are just now uncovering – in the case of the Moon, water has been recently found to be present inside, outside and in significant quantity at the poles, relations that have enormous implications for lunar history and about which we were nearly totally ignorant only a couple of years ago.

Most NEOs will be simple ordinary chondrites – we know this because ordinary chondrites make up about 85% of all meteorite falls (an observed fall of a rock from the sky).  This class of meteorite is remarkable, not for its diversity but for its uniformity.  Chondrites are used as a chemical standard in the analysis of planetary rocks and soils to measure the amounts of differentiation or chemical change during geological processing.  In themselves, chondrites do not vary (much) except that they show different degrees of heating subsequent to their formation, but not enough heating to significantly change their chemical composition.

Some NEO asteroids are pieces of bigger objects that experienced chemical and mineral change or differentiation.  Vesta (not a NEO, but a main belt asteroid) has reflection spectra similar to known, evolved meteorites, the eucrite group.  These rocks suggest that some asteroids are small, differentiated planetoids, having volcanic activity that dates from the very beginning of Solar System history.  Moreover, since we have pieces of the Moon and Mars as meteorite fragments, some NEOs may consist of material blasted off these planets.  However, given that most NEOs are inaccessible to human missions, the likelihood that we could visit one of planetary derivation is small (curious that the most interesting of the NEOs appear to be those derived from some bigger (planet-sized) object.)  In broad terms of meteorite science, multiple small samples from a variety of asteroid types are preferable to many bigger samples of a single specimen, exactly the opposite of what a human mission will provide.

What specifically would a crew do during a NEO visit?  An astronaut on a planet typically would explore the surface, map geological relations where possible, collect representative samples of the units and rock types that can be discerned, and collect as much mapping and compositional data as possible to aid in the interpretation of the returned samples.  In the case of a NEO, many of these activities would not be particularly fruitful.  The asteroid is either a pile of rubble or a single huge boulder.  Chondritic meteorites are uniform in composition, so geological setting is not particularly instructive.  We do have questions about the processes of space weathering, the changes that occur in rocks as a result of their exposure to space for varying lengths of time.   Such questions could be addressed by a simple robotic sample collector, as the recently approved OSIRIS mission plans to do.

One question that could be addressed by human visitors to asteroids is their internal make-up and structure.  Some appear to be rubble piles while others are nearly solid – why such different fates in different asteroids?  By using active seismometry (acoustic sounding), a human crew could lay out instruments and sensors to decipher the density profile of an asteroid.  Understanding the internal structure of an asteroid is important for learning how strong such objects are; this could be an important factor in devising mitigation strategies in case we ever have to divert a NEO away from a collision trajectory with the Earth.  As mentioned in my preceding post, the crew had better work quickly – loiter times at the asteroid will probably be short, on the order of a few days at most.

Although we can explore asteroids with human missions, it seems likely that few significant insights into the origins and processes of the early Solar System will result from such exploration.  Such study is already a very active field, using the samples that nature has provided us – the meteorites.  Sample collection from an asteroid will yield more samples of meteorites, only without the melted fusion crusts that passage through the Earth’s atmosphere creates.  In other words, from this mission, scientific progress will be incremental, not revolutionary.

In contrast, because they yield information on geological histories and processes at planet-wide scales, sample collection and return from a large planetary body such as the Moon or Mars could revolutionize our knowledge of these objects in particular and the Solar System in general.  Many years prior to the Moon missions, we had meteorites that showed impact metamorphic effects but the idea of impact-caused mass extinctions of life on Earth only came after we had fully comprehended the impact process recorded in the Apollo samples from the Moon.  The significance of impact-related mineral and chemical features were not appreciated until we had collected samples with geological context to understand what the lunar samples were telling us.

Of course, science being unpredictable, some major surprise that could revolutionize our knowledge may await us on some distant asteroid.  But such surprises doubtless await us in many places throughout the Solar System and the best way to assure ourselves that we will eventually find them is to develop the capability to go anywhere in space at any time.  That means developing and using the resources of space to create new capabilities.  I will consider that in my next post.

Destination: Moon or Asteroid?

Part I:  Operational Considerations

Part III: Resource Utilization Considerations

Lockheed-Martin's Plymouth Rock mission concept

Part I:  Operational Considerations

The current controversy over the direction of our national space program has many dimensions but most of the discourse has focused on the means (government vs. commercial launch vehicles) not the ends (destinations and activities).  Near-Earth objects (NEO, i.e., asteroids) became the next destination for human exploration as an alternative to the Moon when the Augustine committee advocated a “flexible path” in their 2009 report.  The reason for going to an asteroid instead of the Moon was that it costs too much money to develop a lunar lander whereas asteroids, having extremely low surface gravity, don’t require one.  The administration embraced and supported this change in direction and since then, the agency has been studying possible NEO missions and how to conduct them.

On the surface, it might seem that NEO missions answer the requirements for future human destinations.  NEOs are beyond low Earth orbit, they require long transit times and so simulate the duration of future Mars missions, and (wait for it)… we’ve never visited one with people.  However, detailed consideration indicates that NEOs are not the best choice as our next destination in space.  In this post and two additional ones to come, I will consider some of the operational, scientific and resource utilization issues that arise in planning NEO missions and exploration activities and compare them to the lunar alternative.

Most asteroids reside not near the Earth but in a zone between the orbits of Mars and Jupiter, the asteroid belt.  The very strong gravity field of Jupiter will sometimes perturb the orbits of these rocky bodies and hurl them into the inner Solar System, where they usually hit the Sun or one of the inner planets.  Between those two events, they orbit the Sun, sometimes coming close to the Earth.  Such asteroids are called near-Earth objects and can be any of a variety of different types of asteroids.  Typically, they are small, on the order of tens of meters to a few kilometers in size.  As such, they do not have significant gravity fields of their own, so missions to them do not “land” on an alien world, but rather rendezvous and station-keep with it in deep space.  Think “formation flying” with the International Space Station (ISS) without the option to dock.

The moniker “near Earth” is a relative descriptor.  These objects orbit the Sun just as the Earth does and vary in distance to the Earth from a few million km to hundreds of millions of km, depending upon the time of year.  Getting to one has nothing to do with getting to another, so multiple NEO destinations in one trip are unlikely.  Because the distance to a NEO varies widely, we cannot just go to one whenever we choose – launch windows open at certain times of the year and because the NEO is in its own orbit, these windows occur infrequently and are of very short duration, usually a few days.  Moreover, due to the distances between Earth and the NEO, radio communications will not be instantaneous, with varying time-lags of tens of seconds to several minutes between transmission and reception.  Thus, the crew must be autonomous during operations.

Although there are several thousand NEOs, few of them are possible destinations for human missions.  This is a consequence of two factors.  First, space is very big and even several thousand rocks spread out over several billion cubic kilometers of empty space results in a very low density of objects.  Second, many of these objects are unreachable, requiring too much velocity change (“delta-v”) from an Earth departure stage; this can be a result of either too high of an orbital inclination (out of the plane of the Earth’s orbit) or an orbit that is too eccentric (all orbits are elliptical).  These factors result in reducing the field of possible destinations from thousands to a dozen or so at best.  Moreover, the few NEOs that can be reached are all very small, from a few meters to perhaps a km or two in size.  Not much exploratory area there, especially after a months-long trip in deep space.

That’s another consideration – transit time.  Not only are there few targets, it takes months to reach one of them.  Long transit time is sold as a benefit by asteroid advocates:  because a trip to Mars will take months, a NEO mission will allow us to test out the systems for Mars missions.  But such systems do not yet exist.  On a human mission to a NEO, the crew is beyond help from Earth, except for radioed instructions and sympathy.  A human NEO mission will have to be self-sufficient to a degree that does not now exist.  Parts on the ISS fail all the time, but because it is only 400 km above the Earth, it is relatively straightforward to send replacement parts up on the next supply mission (unless your supply fleet is grounded, as currently it has been).  On a NEO mission, a broken system must be both fixable and fixed by the crew.  Even seemingly annoying malfunctions can become critical.  As ISS astronaut Don Pettit puts it, “If your toilet breaks, you’re dead.”

Crew exposure is another consequence of long flight times, in this case to the radiation environment of interplanetary space.  This hazard comes in two flavors – solar flares and galactic cosmic rays.  Solar flares are massive eruptions of high-energy particles from the Sun, occurring at irregular intervals.  We must carry some type of high-mass shielding to protect the crew from this deadly radiation.  Because we cannot predict when a flare might occur, this massive solar “storm shelter” must be carried wherever we go in the Solar System (because Apollo missions were only a few days long, the crew simply accepted the risk of possible death from a solar flare).  Cosmic rays are much less intense, but constant.  The normal ones are relatively harmless, but high-energy versions (heavy nuclei from ancient supernovae) can cause serious tissue damage.  Although crew can be partly shielded from this hazard, they are never totally protected from it.  Astronauts in low Earth orbit are largely protected from radiation because they orbit beneath the van Allen radiation belts, which protect life on the Earth.  On the Moon, we can use regolith to shield crew but for now, such mass is not available to astronauts traveling in deep space.

When the crew finally arrives at their destination, more difficulties await.  Most NEOs spin very rapidly, with rotation periods on the order of a few hours at most.  This means that the object is approachable only near its polar area.  But because these rocks are irregularly shaped, rotation is not the smooth, regular spin of a planet, but more like that of a wobbling toy top.  If material is disturbed on the surface, the rapid spin of the asteroid will launch the debris into space, creating a possible collision hazard to the human vehicle and crew.  The lack of gravity means that “walking” on the surface of the asteroid is not possible; crew will “float” above the surface of the object and just as occurs in Earth orbit, each touch of the object (action) will result in a propulsive maneuver away from the surface (reaction).

We need to learn how to work quickly at the asteroid because we don’t have much time there.  Loiter times near the asteroid for most opportunities are on the order of a few days.  Why so short?  Because the crew wants to be able to come home.  Both NEO and Earth continue to orbit the Sun and we need to make sure that the Earth is in the right place when we arrive back at its orbit.  So in effect, we will spend months traveling there, in a vehicle with the habitable volume of a large walk-in closet (OK, two walk-in closets maybe), a short time at the destination and then months for the trip home.  Is it worth it?  That will be the subject of my next post.

Destination:  Moon or Asteroid?

Part II:  Science Considerations

Part III: Resource Utilization Considerations

Did two sub-moons collide to form our Moon? From Jutzi and Asphaug, Nature 476, 4 August 2011.

A recent paper suggests that early in the history of the Solar System, two sub-moons collided to create Earth’s present-day Moon.  Several people have asked for my opinion on this new concept, so I will examine how this result was obtained, along with some general remarks on the nature of modern scientific research.

Over 25 years ago, a popular model for the origin of the Moon emerged at a special conference on the Moon held in Kona, Hawaii.  Whenever I mention that we had a conference in Hawaii, snickering about exotic travel boondoggles invariably follows, but you should note that at this particular conference, it was hard to get attendees out of the meeting room – the tension and excitement of a new and revolutionary discovery was that great.  The collective understanding of the then-current models of lunar origin was that they were all inadequate in one way or another.  But at Kona, a “new idea” was advocated – that a giant impact sprayed material into orbit around the Earth and that debris coalesced into the Moon.   This concept was supported by nearly all attendees and affectionately became known as the “Big Whack” model.  It seemed to satisfy most of the important physical and chemical constraints on lunar origin.  Subsequent work elaborated on the details concerning this model, but its salient features were pretty well defined at Kona in 1984.

The Big Whack has subsequently entered the realm of “settled science” in regard to lunar origin, although some dissenters remain.  But a “consensus” of working lunar scientists seemed satisfied that the origin of the Moon had become a “solved problem.”  Much of the detailed information on such a planetary scale collision comes from computer modeling, in which the basic physical parameters such as size of the two bodies, impact speed, angle of encounter, and composition in broad terms are specified as input variables.  The output of the computer model tells us how much material was vaporized, melted and ejected, and how fast the ejecta was squirted out and where it was deposited.  As you might expect, these calculations are extremely involved, requiring advanced supercomputers working day and night for weeks to churn out the results.

Some scientists tend to be skeptical of purely computational results.  In computer modeling, results are only as good as the input values and assumptions, the realism of the model, the inevitable simplification necessary to make the model fit into the computer and how carefully and thoughtfully the results are interpreted.  After the first few Big Whack computer models were run and presented at scientific conferences, various lunar workers would advance questions or problems that weren’t well explained by the existing models.  The models were tweaked to accommodate the difficulties.  In fact, it seemed that the models were amenable to endless tweaking.  If a tweak couldn’t be found, the observation was questioned or deemed irrelevant.  Models should be flexible enough to explain data outliers and the odd inconvenient fact, but they should also make predictions that can be tested by experiment or observation.  A model that is infinitely flexible ultimately is scientifically worthless.

So in regard to the origin of the Moon, we find ourselves with a solved problem for which a strong consensus of the experts exists.  Big Whack skeptics either have poor or irrelevant observations or are right-brained, qualitative geoscientists incapable of understanding complex planetary “physics.”

Which brings us back to Two Moon Junction.  The recent study suggesting that the Moon is the product of the collision of two sub-moons is an outgrowth of the same type of computer modeling done on problems in planetary accretion, including the Big Whack.  What’s unusual in the new scenario is that the two objects are relatively small to begin with (not Earth-sized, but a few hundreds of kilometers across) and collide at relatively low velocities, less than 2 km/sec.  The result of these unusual conditions, it is claimed, is that the impactor “plastered” itself onto the larger object, without forming a crater.  This “spackling” of matter adds an anomalously thick crust to the far side of the Moon and shoves semi-molten, late-stage liquids around to the near side, simultaneously accounting for two major lunar conundrums – the thicker far side crust and the concentration of KREEP (potassium, rare earths, and phosphorus) on the western near side of the Moon.

Sounds pretty good, eh?  Well, there are some issues with it.  The idea that a low velocity impact does not make a crater is counter-indicated by the existence of secondary impact craters on the Moon.  Secondary craters are made when blocks and clouds of debris ejected from an impact crater land on the Moon and dig up new craters, either as isolated single holes or as chains and clusters of multiple craters.  Since these features are formed by material thrown from the Moon’s surface, they cannot have been created at speeds greater than lunar orbital velocity (about 1600 m/sec).  Since the ballistic range for most secondaries is typically less than a few tens of kilometers from the primary, most were formed by impacts at much lower speeds, typically less than 1 km/sec.  Moreover, the addition of the far side crust as a sedimentary layer does not jibe with the observation that the lunar crust is a laterally contiguous global layer, composed everywhere of similar rocks (but varying in proportion).  The authors of the study acknowledge this is an issue, but suggest that the two sub-moons would have already formed their own crusts, probably of the same composition since they come from the same region of the Solar System.  This explanation appears rather ad hoc and elastic to me, an example of the “flexibility” for which computer models are renowned.

The Big Splat has not yet been embraced by most of the lunar science community, but will doubtless be examined and considered by many.  At this stage, it remains a model and not a description of reality, but rather, the description of a possible reality.  The distinction is important.  Neither the “votes” of the lunar science community nor the “elegance” of the model are relevant in terms of its validity.  The authors describe some possible tests of their model in the paper, but these seem to me neither particularly conclusive nor easy to accomplish.

So were there originally two moons over Miami (or rather, where Miami would one day exist)?  Maybe.  But the fact that someone can make a computer model of a complex process is not proof of its reality.  In this and similar cases, the burden is on its proponents to offer experimental tests or observations to prove their case.  In the mean time, nothing is settled and consensus is irrelevant.

Top: Map of thorium concentrations near Compton crater on the lunar far side. Bottom: LRO view of the felsic highland volcano. After Jolliff et al. (2011), Nature Geoscience 4, 566.

The flood of new data from the Moon continues to enlighten and puzzle lunar scientists.  Members of the Lunar Reconnaissance Orbiter Camera team have noticed an unusual landform on the far side of the Moon that was as unexpected as it might be significant.

We’ve known for many years that early in its history, the Moon was volcanically active.  The dark, smooth maria of the Moon is made up of lava flows, individually erupted over at least a billion year time span and possibly for much longer.  In total volume, the mare lavas make up only a percent or so of the crust, so the Moon is not the volcanic cauldron that Io, the large moon of Jupiter, appears to be.  But these lavas indicate that the early Moon was hot and that melted material spilled onto its surface in the past.

The volcanic rocks of the Moon are what geologists call mafic, meaning that they are enriched in iron and magnesium.  Mafic lavas (basalt) are commonly found as plains and low relief shield volcanoes, such as the Hawaiian islands.  On Earth, volcanic rocks can be mafic (and in fact, are the most abundant rocks on Earth, comprising the ocean floor bedrock) or they can be felsic, meaning enriched in silica (SiO₂) and depleted in iron.  On Earth, felsic rocks occur in mid-continental volcanoes and the stratovolcanoes that are found along the margins of the giant tectonic plates that make up Earth’s outer, rigid layer (lithosphere).  Felsic lavas are often associated with explosive, violent eruptions, such as the Mt. St. Helens eruption of 1980.

All of the volcanic rocks returned from the Moon by the Apollo astronauts are mafic.  Most of them are basalts – lava erupted as quiet, fissure-fed sheets.  A few of the samples are tiny glass beads of mafic composition, erupted when low viscosity (runny) fluid lava squirted into space as a spray of lava called a fire-fountain.  Sprayed droplets of lava cool in ballistic flight and land on the Moon as a uniform deposit of tiny (40 micron diameter) glass beads, forming a lunar ash bed.  Although we did find tiny fragments of felsic material in some of the complex breccias from the highlands of the Moon, no felsic lavas or ash were collected on Apollo.

In April 1972, the Apollo 16 mission was sent to the Descartes highlands on the near side of the Moon.  Pre-mission mapping and studies indicated to the geology team that the plains and mountains of Descartes are felsic volcanoes, having the morphology of lava domes and ash flows on Earth.  The Apollo 16 crew was given intensive instruction in the recognition and mapping of volcanic units on the Earth, so they would recognize the abundant felsic volcanics thought to make up the Descartes highlands.

John Young and Charlie Duke put their geological training to good use when they landed on the Moon.  Not only were the rocks at Descartes not silica-rich volcanics, they weren’t even volcanic!  The crew immediately recognized that all the rocks they found were breccias – aggregates of many rocks assembled by impact.  As Command Module Pilot Ken Mattingly wryly noted, “Well, it’s back to the drawing boards – or wherever geologists go!”  (They usually go for a beer.)

After that sobering experience, lunar geologists were hesitant to map felsic volcanoes on the Moon again.  In fact, the pendulum swung away from such a process ever having occurred on the Moon at all.  Nevertheless, we continued to note small geological anomalies around the Moon, hills and domes that are difficult to explain as impact features.  Additionally, some of these small landforms apparently have unusual composition as they have unique spectral properties, being anomalously “redder” (i.e., higher reflectance at longer visible wavelengths) than surrounding terrain.  These features were imaginatively named “red spots.”  Although we could determine that lunar red spots were compositionally distinct, we did not know exactly what those compositions were.  Now, with new data from the orbiting lunar missions, the mystery of the red spots is finally solved.

The red spots are small volcanoes made up of felsic rocks.  We know from data returned by the DIVINER thermal imaging spectrometer on Lunar Reconnaissance Orbiter that these landforms are rich in silica.  From the Lunar Prospector gamma-ray data, we have determined that they are also enriched in the element thorium, a key indicator of chemically evolved rock types.  Finally, data from both Earth-based telescopes and from the Moon Mineralogy Mapper on the Chandrayaan-1 mission, show that some of the red spots are made of almost pure glass.  On Earth, silica-rich volcanic glass forms a deposit called obsidian; its crystallized form is rhyolite.  New, remotely sensed compositional data show that the lunar red spots are felsic domes of obsidian and rhyolite.  Red spots occur mostly on the western near side of the Moon, the area in and around Oceanus Procellarum.  The new finding of an isolated felsic volcano on the far side of the Moon indicates that such eruptions were a global phenomenon.

These features are not volumetrically major and occur as small geological oddities set within the predominantly mafic, basaltic volcanic terrain of the lunar surface.  Their presence was not predicted by the prevailing model of lunar volcanism.  After the fiasco of the mistaken Apollo 16 prediction, geologists were hesitant to pronounce any dome on the Moon to be a felsic volcano.  Suitably chastened, they re-interpreted those dome-like landforms of the highlands to be by-products of basin-forming impacts.  We see by the existence of these features that in some cases, the volcanic interpretation is viable.  This information adds to our understanding of the incredibly rich and complex geological story of the Moon.

One hundred years ago: Robert Falcon Scott and the crew of the Terra Nova enjoy a celebratory dinner, Midwinter's Day, Antarctica, 1911

“Now is the winter of our discontent” – Richard III, Act 1, scene 1

There is a good piece in today’s Telegraph UK by David Robson of a fateful one-hundredth anniversary – the Midwinter Dinner — June 22, 1911 held in Robert Falcon Scott’s Ross Island hut.  A year earlier, Scott and the crew of the Terra Nova had set off for the Antarctic and the south pole.  It was a carefully planned and perilously financed expedition, a classic journey of the “golden age” of polar exploration.  At the time, Scott had no idea that Roald Amundsen, the famous Norwegian polar explorer, had turned his north pole-bound Fram due south and unknown to Scott and his men, was at that moment camped on the opposite side of the Ross Sea, carefully planning a summer dash to the south pole.

Of what relevance is this story to space and the Moon?  To me, it encompasses and restates several themes I have developed on this blog about the nature of exploration and sustainable presence in a hostile environment.  The theme of the Telegraph article is that Scott’s expedition was all about science.  His team included geographers, geologists, biologists and meteorologists.  They collected specimens, documented phenomena, made observations, and conducted experiments.  Scott’s expedition was organized like a carefully planned military campaign.  Although conducted under the command structure of the Royal Navy, it was a civilian expedition, funded by subscription.  No tax money was used and financing was always a major headache for Scott.

A theme running through Robson’s article has been a recurring motif in polar literature for many years – that while Scott and his team were honorable scientists, conducting true “exploration,” Amundsen and his men were publicity-seeking interlopers, cads and bounders who treacherously misled the noble and long-suffering Scott about their true intentions, and who then had the cheek to actually race ahead to beat Scott to the south pole.  This theme has long been a part of British polar exploration literature – the sting of Amundsen’s victory in the race to the south pole still hurts.  A book and television series on the polar race published over 20 years ago attempted to deconstruct this myth and was roundly blasted in the British press at the time.

But the Telegraph piece contains a fundamental contradiction.  It takes great pains to show Scott’s expedition as a scientific, scholar’s investigation, as opposed to the “PR stunt” of Amundsen’s polar dash.  If this is true, then of what importance was priority in attainment of the south pole anyway?  The pole is merely one more data point on a string of measurement stations.  Scott’s purpose was science, not stunts.  He led a carefully planned and documented expedition to unravel the secrets of the Antarctic.  By arriving at the pole after Amundsen, what could it matter?  He still had his fossils, rock samples and observations, did he not?

Obviously there was much more at stake than admitted, both then and now.  The great age of polar exploration was not about science, any more than Apollo to the Moon was about our first visit to another world.  Large public spectacles like polar exploration were both theater and geopolitical struggles.  In the decades leading to the Scott and Amundsen efforts, many had tried (and failed) to take the north pole.  An entire subculture of polar explorers had developed, each group knowing of the other groups’ efforts in the desperate competition to be the first to stand on top of the world.  Establishing priority became an obsession with many and proof was difficult to obtain (the Frederick Cook-Robert Peary controversy over who was the first at the north pole continues to this day).

Both Scott and Amundsen lived in this milieu.  But they were also Edwardian gentlemen and sporting conduct was natural and expected behavior.  Amundsen’s “sin” was that he discarded the fig leaf of “science” and exposed to public view the raw power politics involved in exploration.  In the words of the President of the Royal Geographic Society Leonard Darwin (son of Charles), Amundsen had not “played the game.”

The idea that exploration is for scientific purposes stems largely from this golden age of polar exploration.  In part, the conflation developed because of the need for Britain to attribute a noble and uplifting rationale to Scott’s polar trip.  His tragic death on the way back from the south pole was made especially bitter by the loss of priority – when Scott arrived at the pole, he found that Amundsen had beaten him there.  One way to make this unpleasant pill more palatable was to assign noble motives to Scott and base ones to Amundsen.  Hence, a mythos developed, sanctifying Scott as a martyr for science and depicting Amundsen as a crass interloper.  An unnoticed side-effect of this storyline was the simultaneous sanctification of science as the rationale for exploration.  This attitude is typified by a comment from an astronomer in the early days of implementation of the Vision for Space Exploration in 2004 that “exploration without science is tourism.”  Scott’s hagiographer could not have put it better.

But this concept, developed one hundred years ago to salve the outrage and hurt feelings of a disappointed nation, does not serve us well as we contemplate the exploration of our Solar System.  Exploration traditionally has a much broader meaning.  Columbus, Balboa and Magellan did not undertake their expeditions for science.  They sought wealth and power; they envisioned new lands for settlement and the spread of their own culture.  In short, the view of  “exploration” prior to being redefined during the golden age of polar exploration had little to do with science and much to do with wealth creation, power projection and settlement.

Science is great and knowledge always has both practical and intangible value, but it is a small part of the motivation for exploration.  The Antarctic is a continent for science but only by mutual agreement of the international community.  The riches of Antarctica remained locked up as scientists hunt its surface for fallen asteroids and evidence for global warming.  Some think this is a template for space exploration; others find such an idea anathema.  Science stagnates when exploration stalls.  If we were exploring the Moon, scientists would find a bounty of extraterrestrial samples and have an unparalleled opportunity to study the record of Earth’s climate locked in eons of undisturbed solar wind in the lunar regolith.  Once humanity and technology are able to utilize the Moon’s resources to break the tyranny of the rocket equation, the vast riches of our Solar System will open to explorers, entrepreneurs, settlers, and scientists alike.

We explore for many reasons. There are many valid and important national interests of which science is but one.  Scott understood this; hence, his disappointment at his own failure to reach the pole first.  As we prepare to leave the Earth on a more permanent basis, it is well to look back at this curious and (I would say) singular interval in history – a time (so we are told) when science became the rationale for exploration.  It wasn’t true then and isn’t true now.

Related side-note:  Videos of my Space Pioneer Award talk at the recent 2011 International Space Development Conference in Hunstville AL have been posted in two parts, HERE and HERE.  This talk touches on several of the themes I mention above.  The slides from my talk are available for download HERE.

The solar corona is a stream of energetic particles flowing from the Sun, the solar wind

A recent article about the role of global magnetic fields in the loss of planetary volatiles caught my attention.  The article addresses planetary climate issues as they relate to Earth, Mars and Venus, but what struck me was this statement:

We don’t have a direct record of the sun’s history, but astronomers can study other stars that are similar to our sun at an earlier age.

But in fact, we do have an excellent historical record of the Sun’s history – preserved on our nearby Moon.

The Sun constantly emits streams of high-energy particles, consisting mostly of hydrogen atoms and ions (protons).  This stream, called the solar wind, has been monitored and studied since satellites were first launched.  High-energy solar wind flows around the protective bubble of Earth’s global magnetic field and into interplanetary space.  Some of these charged particles become trapped between magnetic lines of force, creating spectacular displays of aurora, known as the “northern lights.”

The Moon does not have a global magnetic field, so its surface is directly exposed to the solar wind.  These charged particles and neutral atoms impinge directly upon the surface, where some of its atoms are retained on the grains, thus creating a recoverable record of matter from the Sun. The antiquity of the lunar surface means a preserved solar record extending back at least several billion years – the average age of the surface units of the Moon.

We have measured solar wind gas implanted onto the dust grains of the Moon using the Apollo samples and have a good indication that this record contains some significant information.  One curious and obscure relation in the solar wind record recovered from the Moon suggests that the ratio of some isotopes of nitrogen (specifically, the ¹⁵N/¹⁴N ratio) has increased over the last couple of billion years.  This increase is not predicted in current models of stellar evolution; the current interpretation is that it reflects the addition of a meteoritic component, but changes in the solar output have not been ruled out.  So the Sun may be evolving and changing in ways we do not fully understand.

The Sun literally is responsible for our existence – without it, life on Earth would not be possible.  Media coverage of climate change tends to ignore the critical fact that the primary driver of climate on Earth and all terrestrial planets is the Sun.  Before we can understand how and why climate changes on Earth (and it has repeatedly throughout geological history), we must understand what historic role the Sun has played in this complex exchange.

At any given time, only the uppermost few millimeters of the Moon’s regolith is exposed to the Sun.  Because the regolith is continually excavated, buried, mixed and turned over by the bombardment of meteorites, we have a very complex record to decipher.  Such a chaotic, random process would seem poised to destroy exactly the very information we need to access and study, similar to the destruction of scientific clues about past climatic conditions on our geologically dynamic Earth.  But our nearest neighbor has provided a process that preserves the solar record in ancient regoliths, whereby the solar record is isolated and sequestered for very long periods of time.

The dark maria of the Moon is made up of a myriad of individual lava flows, erupted sporadically but continuously, since 3.9 billion years ago, possibly to as recently as less than 1 billion years ago. These fresh surfaces are readily exposed to the solar wind, which implants its atoms onto the dust grains.  From the moment the lava flow cools, this fresh surface is slowly ground up and broken apart by meteorite impact (regolith formation). Then, as new lava flows are extruded, they cover the pre-existing surface regolith, forever sealing it off, along with its preserved solar record, from active surface processes.  Thus, thousands of individual lava flows in the maria have buried and preserved millions of ancient regolith deposits, all potentially available for study, allowing us to see not only the output of the current Sun, but the solar wind record of some ancient Sun as well.

Thus, the dusty regolith of the Moon acts like a tape recorder, detailing the output of the Sun throughout time.  How might we find and access these ancient regoliths and read the preserved solar record?  These deposits are accessible wherever there is an exposed section of bedrock in the lunar maria.  On the Moon, such exposure occurs within the walls of craters, sinuous rilles and other structural depressions.  Not only did we photograph such exposures during the Apollo missions, we may already have sampled a “fossil regolith” from a unit that is more than 3.84 billion years old.  Finding and sampling more of these buried units will allow us to reconstruct the output and history of the Sun over the course of at least the last 4 billion years.

Here is yet another reason to return to the Moon: to understand the history of our Sun, the primary driver of climate and life on Earth.  It is ironic that many people who are most ardent in their concern about Earth’s changing climate disparage lunar return because “we’ve been there.”  By dismissing the Moon, they are missing one of the most important chapters necessary in understanding the grand story of the past, present and probable future of the Earth and the Solar System.  That chapter – holding vital answers necessary for an informed debate about our constantly changing climate – patiently waits for us on the Moon.

Marius Hills on the Moon (left) and Marion Island, Indian Ocean, Earth (right) -- Twins?

Come home with your shield, or on it – Spartan women to their husbands, marching off to war.

From the giant Olympus Mons shield on Mars (600 kilometers across and 27 km high) to the large volcanoes of Venus, shield-building was thought to be a common expression of volcanism on all rocky Solar System bodies; the Moon appeared to be a conspicuous exception.  In geology, a shield volcano is a volcanic construct with a broad, low profile made up primarily of thin lava flows with little ash deposits.  Earth’s shield volcanoes range in size from a few to more than 200 km for the Big Island of Hawaii, the extent of its base on the sea floor beneath the surface of the Pacific Ocean.

Our understanding of lunar volcanism has been informed and shaped both by images and samples.  The large-scale shield volcanoes so prominent on Mars, Venus and Earth were believed to be absent on the Moon.  Before the Apollo 11 astronauts visited Mare Tranquillitatis in 1969, we understood that the dark maria of the Moon were volcanic lava plains.  Orbital images showed us a landscape of domes, small cones, sinuous lava channels (rilles) and collapse pits – surface features created by volcanic activity.  Many of these small volcanic features tend to be clustered in provinces concentrated on the western near side.

Rocks from the maria are basalts, the most common type of igneous rock in the Solar System.  They are rich in iron and magnesium and poor in silica.  On Earth, when such rocks are molten, the resulting magma has a very low viscosity (i.e., they are very fluid, spreading onto flat surfaces in thin sheets).  We understand lunar lavas to be similarly fluid, having erupted in thin sheet-like flows onto the airless surface of the Moon.  The maria formed as this geologic process of massive high-volume eruptions built up stacks from the thin, fluid flows which extend for hundreds of kilometers.  Scattered within the ancient maria are numerous small volcanic constructs, previously believed to be the only manifestation of central-vent volcanism on the Moon.

When the Moon’s topography was mapped with laser altimetry (first by Clementine in 1994, then at greater resolution by the Japanese Kaguya spacecraft and NASA’s Lunar Reconnaissance Orbiter mission), it showed clusters of many small volcanoes occurring on topographic highs that are quasi-circular, with low relief and shield-shaped.  Pat McGovern, Walter Kiefer (colleagues at the Lunar and Planetary Institute) and I were intrigued by this correspondence.  We studied these areas by mapping volcanic features, integrating the new topographic data, and examining their gravity signatures (the amount the local gravitational attraction is enhanced or depleted from normal).

We found that these large shield-shaped topographic swells are made of basaltic lava and display concentrations of volcanic features.  Such a structure found on Venus or Mars would be classified as a shield volcano; therefore, we interpret these features on the Moon as shield volcanoes.  We have found seven of these large structures on the Moon, ranging in size from 66 to almost 400 kilometers in diameter and from 600 to over 3200 meters in height.  Such sizes and shapes are very similar to large shields on Earth, Venus and Mars.  The average slopes on these volcanoes are very low, typically less than a few degrees, as would be expected for structures made from very fluid lava.  These lunar shields display abundant volcanic features, including domes and cones, sinuous rilles (lava channels and tubes) and collapse features – all common morphologies in terrestrial shield volcanoes.

Topographic map of the Marius Hills shield on the Moon from LOLA laser altimetry. A broad topographic swell with many small cones and domes on it.

Although we believe these features are shield volcanoes, this new interpretation is not without some difficulties.  Unlike most shield volcanoes on the other planets, none of the lunar shields has a central collapse pit (caldera).  However, many shields – especially those on Venus – likewise do not show central calderas.  Additionally, while evidence for some lunar shields such as the Marius Hills is pretty convincing (e.g., shield shape, high gravity signature indicating dense stacks of lava), the evidence for others is not as clear.  The largest feature we identified, the Cauchy shield, possesses the correct topographic shape and has numerous small cones, rilles, and vents on it, but remote sensing data suggest that the lava thickness in eastern Mare Tranquillitatis is relatively thin, which might mean that Cauchy is not a thick stack of lava as Marius appears to be.  We still think that Cauchy is a shield volcano, but acknowledge that our interpretation is tentative and we will continue studying these enigmatic features to better understand their history.

But the real story here is not whether these features are true shield volcanoes or not, but rather, how the advent of new, high-precision data (high resolution topography) can cause scientists to reexamine areas and processes long thought understood and perhaps come to surprisingly different interpretations.  We are currently in the midst of a revolution in lunar science.  The 42^nd Lunar and Planetary Science Conference held this month in Houston highlighted new scientific findings about the history and processes of the Moon.   New, high-quality data coming from an international flotilla of lunar orbital mappers – Chandrayaan, Kaguya, Chang’E and LRO – has scientists seriously reconsidering our current understanding of the processes, history, resources and potential of the Moon.

New interpretation of the lunar interior (from Weber et al., 2011, Science 331, 309-312)

A recently published science paper presented results of a re-analysis of seismic (moonquake) data sent to the Earth from a network emplaced by the Apollo astronauts 40 years ago.  The scientists processing the old data found that the Moon may have more than a simple core – it may have a layered, partly liquid metallic core.

Why is this important?  Scientists have known for many years that the Earth has a layered interior structure.  The outermost layer, called the crust, is the only part of the Earth directly accessible to us for study.  The crust varies in thickness, ranging from a few kilometers in the ocean basins to over 20 km in continental areas.  The next zone down is called the mantle.  The mantle is very thick – almost 3000 km.  It is made up of a dense, iron- and magnesium-rich rock type called peridotite.  Partial melting in the mantle is the source of basaltic magma that erupts to make up the floors of ocean basins worldwide.  The innermost part of the Earth is the core, comprised mostly of metallic iron and nickel, and over 3000 km in radius.  The outer layer of the core is liquid, but the enormous pressure that contains the inner core keeps it solid.

The Earth’s core is electrically conducting as the rotation of the Earth induces currents within it.  It is thought that these electrical currents are responsible for the dynamo that generates the magnetic field of the Earth.  Because most of the Earth’s iron is contained in the core, we know that in bulk composition, the Earth is made from chondrites, the same stony material found as primitive meteorites in space. Thus, understanding the core is relevant to the origin of its magnetic field and the internal structure and bulk composition of the Earth.

For these reasons, we are interested in the possibility of a core within the Moon.  Even before we went to the Moon, we understood that an internal structure similar to Earth was not likely.  A property called moment of inertia told us in broad terms that, unlike the layered structure of Earth, the Moon was more or less homogeneous inside.  The moment of inertia indicated that any core inside the Moon must be smaller than a couple of hundred kilometers at most (the Moon’s radius is 1740 km).

Seismometers, deployed on the Moon as part of a surface network during the Apollo missions, operated for over seven years collecting data on tremors within the Moon.  Because certain rocks have known physical properties (e.g., density), we use the velocity of seismic waves in an indirect way to infer the presence of these rock types and physical structure.  From our initial analyses of these data, we determined that the Moon had a fairly thick crust (from 50-80 km, more than twice the thickness of Earth’s crust) and a very thick mantle, almost the remainder of the lunar radius.

The question of the existence of a lunar core remained uncertain.  One moonquake resulting from a fairly large impact on the far side of the Moon  a couple of years after the Apollo missions had ended produced a signal that suggested the presence of a small core (less than 400 km radius).  Moreover, because seismic waves come in two varieties – P-waves, or compression (or sound) waves and S-waves (shear waves, which cannot propagate through liquids) – the partial suppression of S-waves through the center of the Moon during this event suggested that the lunar core might be partly liquid.

But this result was so uncertain that few lunar scientists actually believed it.  They proceeded to try and constrain the dimensions and composition of a lunar core through other means.  A core may be important in the generation of an early global magnetic field that some of the lunar samples seems to indicate (the current Moon has no global field).  By carefully measuring the ways in which the magnetic field of the Sun and Earth is modified when the Moon passes through it (as it does during its orbit around the Earth), it was thought that it might be possible to “sense” the presence of a lunar core by measuring these deviations.  Results indicated that the core of the Moon had to be small (less than 400 km in radius) and probably made of iron sulfide (FeS).

After seven years of operation, the Apollo seismic net was turned off to save money.  Up until it was turned off, we had received a large amount of data but processing it was extremely difficult.  The Apollo instruments, although sensitive, were very noisy and not well coupled to bedrock as are seismometers on Earth.   Fortunately, faster and more capable computers, along with new techniques to process and analyze noisy data, were developed.  And a new generation of scientists came forward to re-examine the old seismic data to see if anything could be discerned from it.

The new results are surprisingly detailed.  Not only do these researchers think they have detected a core inside the Moon, but a core with three separate layers – an inner solid core and outer core, very similar in structure to that of the Earth, but with the added wrinkle of a partly molten outermost layer.  The entire core is almost 500 km in radius, slightly larger than the diameter inferred from deep magnetic sounding.

The presence of currently molten core inside the Moon is rather startling; even the earlier idea about a partly molten zone was viewed askance by most lunar students.  But this new idea has revived concepts about a magnetic core dynamo inside the Moon, generating a global field early in lunar history.  Such a dynamo might explain a lot about the remnant magnetic fields measured in some of the returned lunar rocks.  But there is no obvious reason why such a field would suddenly stop being generated.

Even though the old Apollo network data may still be mined for information, to fully understand lunar structure and history we must emplace a long-lived, global network of new instruments to fully characterize the interior of the Moon.  Although studies are underway to determine how this might be accomplished, deployment of such a network is difficult to achieve by robotic spacecraft alone and long life on the Moon may require a nuclear power supply.  Each and every time we start believing that we understand our Moon, a new discovery raises even more questions.

LROC composite image of the south pole, showing quasi-permanent sunlit areas (LROC Ariz. State Univ.)

A new image released this week by the Lunar Reconnaissance Orbiter Camera Team shows the lighting conditions of the south pole of the Moon.  This new data supports the conclusions of many previous studies that areas exist on the Moon that are illuminated by the sun for more than one-half the lunar day (the time it takes the Moon to rotate once on its axis, a bit more than 29 Earth days or about 708 hours).

Why do such areas exist and why are they important?  Most locations on the Moon experience a day/night cycle, albeit one of an Earth month duration.  But unlike the Earth, the spin axis of the Moon is nearly perpendicular (off from the vertical by 1.5°) to the plane of its orbit around the Sun (the Moon orbits the Earth, but as the Earth orbits the Sun, the Moon can be said to do the same).  This means that at the poles, the Sun is always close to the horizon.  As the Moon slowly rotates during the course of a lunar day, the Sun tracks a 360° circle around the pole, sometimes just above the horizon, sometimes dipping just below it.

Or rather, it would do that if the Moon were a smooth sphere.  But as we all know, the Moon is not smooth – deep craters and basin make rims, peaks and holes that complicate the picture.  The deep interiors of craters may never see any sunlight at all.  These areas are extremely cold; we’ve learned from new orbital data that some of these cold traps are only a couple of tens of degrees above absolute zero.  It is for this reason that we find water ice and other volatiles near the poles – they are stable in the permanently dark, cold areas here.

On the other hand, if some bit of terrain near the pole is topographically high, it may stick up into the sunlight for a much longer time than other spots on the Moon.  This concept was first postulated in 1837 by German astronomers Wilhelm Beer and Johann Mädler and popularized in 1879 by French astronomer Camille Flammarion, who dubbed these areas pics de lumière éternelle (peaks of eternal light).  If such an area could be found near one of the lunar poles, the only time it would not be in sunlight would be during a lunar eclipse, which occur infrequently and last only a few hours.

We got our first good look at the lunar poles in 1994 with the global mapping obtained by the Clementine spacecraft.  Although Clementine only orbited the Moon for 71 days, we were able to determine that no peaks of “eternal light” existed at the south pole.  However, we did find small areas near the south pole that are lit more than 70% of the lunar day, and this was during the southern “winter” season (the 1.5° obliquity of the Moon provides some small seasonal variation).  We also found locations that are lit 100% of the day at the north pole.  These images were taken during mid-summer, when the north pole receives maximum solar illumination.

Lighting at the poles is primarily dependent on local topographic relief.  Because Clementine did not get laser topography for latitudes greater than 70°, we had a poor understanding of polar topography until the Japanese Kaguya mission flew in 2008.  The Kaguya spacecraft made a detailed laser altimetry map of the entire Moon, including both poles.  From this precision topographic data, we made a simulated relief model of the poles and illuminated it as the real Moon would be illuminated by the Sun over the course of a year.  Our new results suggest at least four areas near the south pole are in sunlight for large fractions of the lunar day.  One location (B) is illuminated more than 82% of the lunar day and is only 10 km from another point (A) that is lit 81% of the day.  Moreover, these two points are complementary in that the dark times at one corresponds to sunlit times at the other.  The four topographically high sunlight points are collectively illuminated 100% of the time during the lunar seasons.

The new composite image from LROC confirms the inferences from the illumination model we devised from the Kaguya altimetry.  The four high points (A-D) correspond to bright zones on the illumination map (see image above), indicating that they are sunlit most of the time.  These areas of “quasi-permanent” sunlight are the closest things we have found to correspond to Flammarion’s imagined pics de lumière éternelle. Although not “eternal” in the original sense, they are sunlit for extended periods, well beyond the typical lunar day-night cycle.

What is the significance of such features?  Permanently lit areas of the Moon are important for future habitation and use of the Moon for two principal reasons.  First, these sunlit areas are prime locations for the establishment of solar photovoltaic arrays.  The constant sunlight here means continuous generation of electrical power using solar panels.  This solves one of the most difficult problems of lunar habitation, survival during the 354-hour lunar night.  Prior to the discovery of the quasi-permanently lit areas, we imagined that the only feasible power source to survive this long night was nuclear reactors.  Such a power system does not exist and would require several tens of billions of dollars to develop.  So sunlit zones allow us to go to the Moon and stay there without this expense and technology development.

The second advantage of a sunlit area is that it is thermally benign.  The surface temperatures at the lunar equator and mid-latitudes depend almost entirely upon incident solar illumination and range from less than -150° to over 100° C, a 250° temperature-swing over the course of a day.  In contrast, the surface temperature of these quasi-permanent lit areas is nearly constant – a nice, toasty -50° ± 10° C.  This simplifies the thermal design of surface habitats and equipment and greatly relieves the energy required for thermal control at an outpost.

The sunlit areas of the poles occur in close proximity to high concentrations of water ice and other volatiles at the poles of the Moon.  Their presence indicates the lunar poles are the best places we have found off-planet for human habitation.  Constant sunlight, benign temperatures, near the water and a great view – that’s prime real estate.

Ralph Baldwin (photo by J. Wood from U. B. Marvin, 2003)

I learned that a titan of lunar science passed away last month.  Dr. Ralph Belknap Baldwin (1912-2010) was a rare specimen – a gentleman scholar, businessman and pioneering student of the Moon.  Beyond the impact of his books and papers, he influenced space history in several profound ways.

Baldwin, an astronomer by training, noticed the spectacular images of the Moon displayed in Chicago’s Adler Planetarium while giving public lectures there.  Those images showed a system of grooves and lineations that seemed to radiate from the center of Mare Imbrium (a large dark region on the lunar near side).  Upon investigation of the literature, Baldwin was surprised to find no good explanation for these patterns.  In fact, to astronomers, the Moon was an unattractive target for their attentions.  The biggest influence the Moon had on their work was that its bright, reflected light shining in the night sky often ruined observations of the faint star field that lay beyond.

The prevailing wisdom in the 1940s held that lunar craters were primarily produced by volcanism and Baldwin quickly discovered that no one was particularly interested in his new observation.  However, he persisted wondering what could have produced such a global pattern. He noted that the largest volcanic eruption in recorded history on Earth (Tambora in 1815) had left a crater a mere 4 km in diameter.  He reasoned that such a process – especially on the smaller and presumably cooler Moon – could not possibly have produced the “crater” the size of the Imbrium basin, a depression over 1000 km in diameter.  In his mind, the only alternative was that it formed by the impact of a large asteroid or comet.

Today we take the process of impact for granted but it was quite controversial before the space age.  Geologists hated the concept of impact because it smacked of catastrophism (the doctrine that singular, large-scale events could create landforms) and as that battle of ideas had been fought for over a hundred years, they resisted the idea that catastrophes may happen.  Astronomers ignored the concept completely, feeling that anything dealing with the planets (and certainly the Earth and Moon) was outside their purview.  So by championing the impact origin of lunar craters and attributing scientific importance to the Moon, Baldwin gained the distinction of being a double heretic.

Baldwin used his collected observations of the Moon, terrestrial impact craters such as Meteor crater in Arizona, and aerial images of bomb craters (created during the recently ended war) to argue that the vast bulk of lunar craters were formed by the collision of solid objects with the Moon’s surface.  He believed that impacts were explosive events that threw enormous amounts of debris across the surface of the Moon and, in the case of Imbrium, created the radial “sculpture” that dominates portions of the near side.  His book, The Face of the Moon, published by the University of Chicago Press in 1949 was widely ignored by most but not all.

During a party, Nobel laureate Harold Urey, a chemist who had worked on the Manhattan Project, picked up Baldwin’s book.  He consumed it in one sitting.  The idea of impact fascinated him and he became convinced the Moon was the critical object to understanding the origin of the Solar System.  As Urey’s opinion was considered important, the scientific stature of the Moon rose.  Researchers like Eugene Shoemaker, a young geologist with dreams of flying to the Moon, combed the literature for information about it.   The Face of the Moon was one of the few geologically insightful works available at that time.  Shoemaker went on to become a renowned lunar scientist and cratering expert, instrumental in assuring that astronauts going to the Moon conducted lunar field geology.  Although he never made the journey to the Moon in his lifetime  (some of his ashes were sent to the Moon on the Lunar Prospector mission in 1998), he trained the Apollo astronauts how to observe and collect samples on the Moon – samples that demonstrated the importance of impact on our history.

Ralph Baldwin (whose day job was Vice President of Oliver Machinery Corporation of Grand Rapids Michigan, the family business) was a gentlemen scholar who studied the Moon simply because he loved it.  His two principal books (he wrote an extension and revision of his book in 1963 entitled The Measure of the Moon) got the lunar story correct.  Almost all lunar craters were formed by impact.  The largest craters on the Moon are the gigantic, multi-ringed basins that contain the dark maria of the Moon.  The maria are volcanic lava flows, unrelated to the basins that contain them and erupted on the Moon long after the basins were created by impact.  The Moon’s surface features are very old; we don’t find large numbers of craters on the Earth because they formed mostly during the early history of the planets and have been erased from the Earth’s surface by active geological processes.  Baldwin had found his niche, achieving an important understanding of the Moon at a time when most of the scientific community was preoccupied with virtually any problem except the Moon.

I was fortunate to meet Ralph Baldwin in 1981 at a special conference on the origin of multi-ring basins (my dissertation topic).  I remember standing in front of a wall-sized enlargement of the lunar near side with him, looking at basin rings.  He was surprised and pleased to find out that I agreed with him on the existence of a very old, vaguely expressed basin in the central highlands.  We talked mostly about the Nectaris basin, a key feature in lunar history.  A couple of years later, while I working at the U. S. Geological Survey in Flagstaff, my boss Larry Soderblom gave me a paper by Baldwin on the topography of the Nectaris basin that he had been sent to review.  Baldwin believed that basins had relaxed through viscous flow of their topographic relief.  Since rocks have finite physical strength, given enough time, they will deform plastically.  High peaks or deep holes slowly will be erased as the rocks flow and relax under their own weight.  Baldwin had collected data that (he thought) demonstrated this effect occurred in lunar basins.  While this is undoubtedly possible, the degree to which it occurs on the Moon is still fiercely debated.

I read Baldwin’s paper in detail and made extensive notes.  I collected new measurements of the Nectaris basin using topographic maps that Baldwin apparently did not have.  I wrote all this up in a ten-page memo and sent it to Larry for his use in reviewing Baldwin’s paper.  To my initial horror, Larry simply sent my comments directly to Baldwin.  My fear was that he would be offended by an upstart student, questioning his data and conclusions.  To my delight and surprise, Baldwin phoned to thank me for my review efforts and asked for copies of the newly made topographic maps.  He later wrote me a very courteous and kind letter, thanking me for all of my review efforts and data collection on his behalf.  To me, this episode demonstrated the mark of the true scientist, one who is willing to consider valid criticism from wherever it may come.

As it turned out, Baldwin retained his original hypothesis, regardless of my own (and others) criticisms.  It is very human to hold to one’s own ideas.   Scientists must strive to discard useless or wrong concepts as new data or insight becomes available (something we regularly do, though sometimes reluctantly).  I too have felt the tendency to hold on to an idea, even when overwhelming new evidence shows it to be wrong, or at least, incomplete.  As in many other fields of endeavor, scientific research is a very human experience.  Scientists get intensely involved in their research and personally invested their ideas – more Vincent Van Gogh than Mr. Spock.

With little background in geology, except that which he taught himself, Baldwin deciphered most of the geological story of the Moon.  Moreover, his scientific work was done in his spare time and largely alone, as opportunities to discuss his work with other interested parties was rare. It is astounding to leaf through the 1949 The Face of the Moon today and realize how his insight – much of it completely intuitive – is still pertinent.  One of my personal favorites:  Baldwin suggests that extinction events in the fossil record might be caused by the impact of large meteorites and comets (page 155).  This suggestion, a throw-away line in a chapter about the frequency of large body impact, was proven correct in 1980 with the discovery of a large impact at the end of the Cretaceous, causing the extinction of dinosaurs and many other fossil families.

After all is said and done, I’ll let my own mentor in lunar science, Don Wilhelms, have the last word.  In the dedication of his excellent history of lunar science, To a Rocky Moon: A Geologist’s History of Lunar Exploration, he wrote:

“Dedicated to the amazing Ralph Baldwin, who got so much so right so early.”

Comet Hartley 2 as seen from the EPOXI spacecraft this week. Comets are one source of water for lunar polar ice.

Although the discovery of ice on the Moon comes from a wide variety of different measurements, they are all “remote sensing.”  We have not yet landed near these deposits and examined them up close.  Thus, we do not know the physical nature of lunar polar ice.  Having spent the last couple of weeks at several meetings in which this became an issue, I’ve been thinking about the nature of lunar ice.  What is lunar polar ice?  Is it a smooth pond of solid ice?  Perhaps it is broken-up blocks and slabs of tough, compacted ice chunks.  Maybe it’s a porous, void-filled snow-like aggregation of microscopic ice pieces.  The question of the nature of lunar ice is not academic.  If we plan to go to the Moon to harvest this ice to support human presence and space transportation, we must understand the physical nature of the deposits.

Although the details are probably complex, the concept of lunar ice deposition is simple.  Water ice is stable in the cold, dark areas near the lunar poles.  Any water that is made or deposited on the Moon is not stable in sunlit areas, and so will migrate across the surface.  If it gets into one of these polar cold traps, it is there forever – no known process exists to remove it.  Thus, even though the addition of water is extremely slow, over very long periods of time, a substantial amount of water may accumulate there.

But what is the physical nature of these deposits?  Our expectations derived from experience here on Earth are probably misleading.  We deal daily with water in liquid form and ice on Earth is usually made by the freezing of liquid water.  This results in crystalline ice, as water molecules in solid form assume an ordered, tightly bonded lattice structure.  As everyone knows, this material is both hard and tough and greatly resists attempts to break or gather it using normal digging tools.  Water also can crystallize directly from vapor form into a solid as frost, usually found as an extremely thin coating that is very soft and easily scraped and removed from the object on which it forms.

Water that freezes within soil forms a tough, indurated deposit that can be quite difficult to dig or excavate.  In the polar regions of the Earth, this material is frozen solid year-round and is called permafrost.  Permafrost is extremely hard and difficult to excavate.  Buildings in arctic regions require heavy equipment to dig and move the permafrost, including the use of explosives to break up the rock-hard frozen soil.  If lunar ice is like this stuff, it will be extremely difficult to dig up and mine.

In contrast, snow is soft and easy to excavate.  Snow is created when precipitation droplets (rain) freeze before they land on the ground.  Typically, airborne dust particles will nucleate these droplets.  Small drops have time to crystallize into magnificent ice crystals which famously, are each unique and individual.  Sometimes larger water drops freeze quickly in flight and form ice blobs which may land on the ground as hail.  In any event, if this material accumulates on the ground, we have a porous, weakly bonded deposit that is easily scooped up, usually by cursing inhabitants wielding large flat shovels.

Neither of these two accumulation scenarios occur on the Moon.  We don’t know whether lunar ice is deposited (e.g., by comets hitting the Moon) or made (e.g., by solar wind hydrogen reacting with mineral surfaces).  But however it is deposited, the water exists as individual molecules in gaseous form.  Although this water is found all over the Moon, it is not stable everywhere.  The molecules hop around the surface randomly, not slowing down until they land at a cooler locality and don’t stop until they reach a cold trap.  The Moon loses most of these water molecules by a variety of mechanisms, including escape, disassociation and combination with minerals.  The lucky few that reach a polar cold trap are there forever.

So what form do lunar ice deposits take?  They are not now and never have been in liquid form, so crystallization into dense, “ponds” of ice is not likely.  This lack of history as a liquid also means that “permafrost” (at least as we understand that term from terrestrial experience) is not likely either.  Both of these ice forms ultimately require a freeze-thaw cycle, even if the time frame for such a cycle is hundreds of years.  The lunar cold traps are cold now and have been for billions of years.  And for this length of time, they have been gathering water molecules, sometimes at very high rates of accumulation (as when a comet strikes the Moon nearby) but usually at very slow, steady rates of accumulation.

Lunar ice probably is very porous, or at least “solid” but weakly bound together.  The tight bonding of crystalline ice is made during the transition from liquid to solid during freezing.  This doesn’t happen on the Moon; the water is added to the surface through direct ballistic deposition as individual molecules.  In addition to the accumulation of water in the form of extremely tenuous vapor, dust and soil particles may interact with the water, creating a deposit with variable strength and water content.   Even this material is likely to be loosely bound, as this mixing occurs at low temperatures and the water does not have a chance to re-crystallize, the usual reason for the steel-like hardness of permafrost.  In astrophysics, a fine-grained, loosely bound structure is referred to as “fairy castle structure.”

Do we have any evidence that this guess may be correct?  We have only a few indirect clues at present.  The ejecta plume observed during the impact of the LCROSS upper stage was unusually narrow.  The science team suggested that this was a result of impact into an unusually low density soil; the term they used to describe it was “fluffy.”  In addition to the high CPR fill of anomalous craters seen in the Mini-RF radar images of the poles (which we interpret as ice), we also observe anomalously low CPR in the areas surrounding the anomalous craters.  Extremely low CPR implies fine-grained, lower than  average density deposits with few rocks.  Yet because polar ice is geologically young (less than a couple of billion years), if there were rock-hard, crystalline ice in abundance, we might expect a higher than average radar CPR, caused by abundant angular blocks excavated by impacts.  Such a signal is not observed.

Admittedly, the evidence for this story is very weak.  To determine the true physical and chemical nature of lunar polar ice, we must examine and study it in detail from a suitably equipped surface rover.  Such a mission has been repeatedly proposed and I note that it is one of the proposed mission studies in the National Academy’s Planetary Exploration Decadal Survey.  For a resource that may change the rules of spaceflight, determining its properties should be a high priority for exploration.