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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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