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Lunch from the lunar dirt (Photo courtesy of Dr. Jeff Taylor, Univ. Hawaii)

We often hear the Moon described as a lifeless desert, a barren rock in space where nothing can survive.  Although the Moon is certainly different from the Earth, it is hardly barren.  From the 1970’s through the 1990’s (largely before we knew about the presence of water and other volatiles in the lunar polar regions) the late, lunar scientist Dr. Larry Haskin set forth some basic facts about the chemical composition of the Moon.  Larry was a chemist by training and his view was that the Moon has all that we need – just not in the form in which we need it.

Larry wrote a very interesting paper for the 1988 Second Symposium on Lunar Bases.  Over the years, I heard him give several different versions of this talk.  Initially, he called it “Wine and Cheese from the Lunar Desert” but after deciding that he didn’t want to drive away or offend any teetotalers in his audience, he changed it, first to “Cola and Cheese” and then “Water and Cheese from the Lunar Desert.”  Although the liquid varied, the cheese stayed.

Haskin’s argument is very simple.  Take a cubic volume of soil (about 1 meter in dimension) from anywhere on the Moon.  In that volume of soil (weight about 1600 kg), there is enough hydrogen, carbon, and nitrogen – the principal volatile elements implanted by the solar wind – to make lunch for two.  Larry’s menu was modest, but satisfying:  two cheese sandwiches, two glasses of wine (or cola, with real sugar), and two plums.  Chemical atoms needed to make up this meal are all present in that relatively small volume of soil; they are just not arranged in the form that we need them.  But the task is possible, given time and energy.

Because the Moon has no atmosphere and no global magnetic field, the highly energetic stream of particles from the Sun (the solar wind) implants its atoms directly onto the dust grains of the soil.  This material is mostly hydrogen and helium, but other light atoms such as carbon, nitrogen and other noble gases are also present.  These volatile elements seem to correlate with a property called “maturity” which means the amount of time a soil has been exposed to the space environment.  The amount of solar wind gas also correlates inversely with grain size – the finest fraction of the soil contains the most solar wind.  Another unusual correlation is with titanium; the highest quantities of solar wind hydrogen are found in very high titanium soils.  It’s not clear why this should be true, although it is postulated that the crystal structure of ilmenite (an iron- and titanium-oxide mineral) acts as a “sponge” for solar wind atoms.

Given these properties, the best soil on the Moon to process and extract these important volatile substances would be very fine-grained, high-titanium soils.  In fact, this soil occurs as the dark pyroclastic ash that sometimes covers mare and highlands areas on the Moon.  They are very fine-grained (typical mean grain sizes of a few tens of microns) and some are rich in titanium.  The tiny black glass beads returned by the Apollo 17 mission have up to 13 wt.% titanium dioxide (among the highest found on the Moon).  However, these Apollo 17 samples were buried by a landslide for millions of years so we do not know how much volatile material a mature, exposed surface ash deposit might contain.  A robotic mission to such an area to measure the amount of solar wind gas could answer these questions.

Extracting the volatiles from soil is very simple: just heat the soil to about 700° C.  Although simple in concept, in practice this may be a very difficult job.  We need to find a way to process the lunar soil in a continuous stream.  Batch processing is much less efficient and expensive.  Soil roasters that continuously roam the surface, heating the soil using solar thermal power and collecting and storing the emitted gas, is likely to be at least part of the ultimate solution.

During recent discussions about using lunar polar ice, some expressed concern that we would too rapidly devour what they perceive to be a limited resource.  Although the Moon has hundreds of millions of tones of water around the poles, ultimately, we will need to learn how to use the lower grade ore present elsewhere around the globe.  In this case, it will be the bountiful lunar regolith – the meters-thick outer layer of the Moon.  This resource can truly last a lifetime – the lifetime of humanity in space.  Wine and cheese (or beer or cola and cheese, if you prefer) is there for the taking and the making.  We are limited not by the intrinsic resources of the Moon but only by our own imaginations.

Something else to be thankful for this season—a Moon that has what we need to survive and thrive in space.

An ice rainbow seen in cirrus clouds on Earth. (UCSB Dept. Geography)

Five weeks ago a crater from the LCROSS impact formed on the Moon.  The pre-impact build-up had been sensational, but the actual event was largely invisible to observers on Earth. It was a different story on the Moon.  The slowly growing impact ejecta curtain threw water ice particles and vapor far out into space.  When the crater formed, flying ice particles could have refracted the glare of unfiltered sunlight into an “ice rainbow,” similar to those seen through very high altitude clouds on Earth.  For a very brief time, a rainbow might have been visible to an observer standing on the lunar surface.  And like its namesake, this rainbow is a promise – a promise that the Moon is habitable.  It is an invitation to humanity to extend man’s domain to our nearest planetary neighbor.

The LCROSS science team’s initial analysis of ejected impact plume data found evidence for water.  It appears that several other species, particularly some carbon substances also found in the cores of comets, may be present.  The new results suggest that some lunar polar volatiles may have their origins from outside the Moon, deposited there over millions of years by the impact of comets and asteroids.

Over the last 50 years, the idea of water ice at the lunar poles has generated as much angst as excitement within the scientific community.  Ice on the Moon was suggested by Watson, Murray and Brown in 1960.  They recognized that, regardless of the fate of such substances elsewhere on the Moon, the dark, cold floors of polar craters might retain volatile substances.  Rock and soil samples returned by the Apollo missions were not only bone-dry, but crystallized in a very reducing environment, suggesting that any indigenous lunar water, if present, must have been a very minor component.  Apollo scientist Jim Arnold resurrected the Watson et al. hypothesis forty years ago, concluding that their original proposal of water ice at the poles was still feasible and that a polar lunar orbiter was needed to search for such deposits.

We know that over geologic time, the Moon was bombarded by water-bearing objects.  Meteorites contain water, and just as they’ve landed on Earth, they’ve also hit the Moon.  Moreover, we’ve detected water vapor in the tails of comets with Earth-based telescopes.  But it was widely speculated that all this water must be lost from the Moon, which left the issue of polar ice unresolved.

Fifteen years ago, the 1994 Clementine orbiter mission revived our interest in the Moon’s polar regions.  When Clementine’s images of the Moon’s poles revealed large areas of shadowed terrain, it reminded Gene Shoemaker and the science team of the Watson and Arnold papers.  Large shadowed areas suggested that polar cold traps might really exist, so an experiment was improvised using the spacecraft transmitter to beam RF energy into the shadowed areas.  Analysis of the radio echoes suggested the presence of ice in shadowed areas near the south pole.  This result was questioned, largely because our team couldn’t repeat the passes using the improvised experiment.

In 1998, Lunar Prospector found evidence for excess hydrogen in the surface soils of both lunar poles.  These data could not show what form the hydrogen was in and had very low spatial resolution.  The issue, as to whether the observed polar hydrogen represented water ice in the dark cold traps or elemental hydrogen implanted by solar wind protons, was vigorously debated.  The preponderance of evidence in the years since Lunar Prospector, suggests that water ice is present in the polar areas, but its form, distribution and physical state are completely unknown.

The current flotilla of lunar orbiting spacecraft carry several advanced sensors, all designed to better characterize the environment and deposits of the polar regions of the Moon.  We have seen extremely low temperatures in the polar dark regions using the DIVINER instrument on the American Lunar Reconnaissance Orbiter (LRO) spacecraft.  The Japanese Kaguya mission mapped the topography and terrain of the polar areas and showed us the extent of the shadowed areas.  The Indian Chandryaan mission sent a probe into the south pole, mapped the extent of sunlight and carried two NASA instruments – the Moon Mineralogy Mapper (M3) and Mini-SAR radar.  In September, the M3 instrument found significant amounts of water bound into mineral structures at high latitudes.  The Mini-SAR instrument has made maps showing the interior of dark polar craters.  These maps are being analyzed for scattering characteristics to determine whether water ice might be present there; our initial results will be announced soon.

Now, the LCROSS impactor – sent to kick up the dust of the polar dark regions – has shown us that water ice does exist there.  We still don’t know how much water ice in total may be present; from Clementine,  we estimated there are billions of metric tones of water ice present in the south polar area.  Complete analysis of all of the remote sensing information in the next couple of years will ultimately give us a good estimate of the total amount of water available.  Clementine also revealed peaks of near-permanent sunlight in proximity to regions of permanent darkness at the poles (where the sun’s circular rotation keeps temperatures benign).

If you don’t know where you’re going, any path will get you there.

The Moon has the resources needed to bootstrap a sustained, permanent human presence.  It is the place where we can learn how to live and work productively in space.  The Moon has put out a welcome mat.  What are we waiting for?