November 17, 2012
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 19th 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.
October 17, 2012
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.
October 10, 2012
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.
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.
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…..”
September 8, 2012
Rick Tumlinson of the Space Frontier Foundation published a “free-enterprise” critique of the Republican platform in regard to the American civil space program. Indeed, the text of the space plank is vague (no doubt intentionally, so as to give the candidate maximum flexibility to structure the space program to align with his vision and goals for the country). But what I found most interesting was the underlying premise and assumptions in Tumlinson’s article, a worldview that I find striking.
In brief, Tumlinson approves of the current administration’s direction for our civil space program. The U.S. has stepped back from pushing toward the Moon, Mars and beyond and redirected NASA on a quest for “game-changing” technologies (to make spaceflight easier and less costly), while simultaneously transitioning launch to low Earth orbit (LEO) operations to private “commercial space” companies selected by our government to compete for research and development funding and contracts. Many see this as gutting NASA and the U.S. national space program. To be clear, the term “commercial space” in this context does not refer to the long-established commercial aerospace industry (e.g., Lockheed-Martin, Boeing) but to a collection of startup companies dubbed “New Space” (typically, companies founded by internet billionaires who have spoken much and often about lofty space plans, but have actually flown in space very little).
Tumlinson criticizes the Republican space plank because it does not explicitly declare that a new administration would continue the current policy. In his view, the very idea of a federal government space program, including a NASA-developed and operated launch and flight system, is a throwback to 1960’s Cold War thinking. Instead, he envisions space as a field for new, flexible and innovative companies, untainted by stodgy engineering traditions or bloated bureaucracy. Many space advocates on the web hold this viewpoint – “If only government would get out of the way and give New Space a chance, there will be a renaissance in space travel!” But travel to where? And why?
The idea that LEO flight operations should be transitioned to the commercial sector is not new. It was a recommendation of the 2004 Aldridge Commission report on implementing the Vision for Space Exploration (VSE). NASA itself started the Commercial Orbital Transportation Services program (COTS) in 2006, designed to nurture a nascent spaceflight industry by offering subsidies to companies to develop and fly vehicles that could provision and exchange crew aboard the International Space Station. That effort was envisioned as an adjunct to – not a replacement of – federal government spaceflight capability.
The termination of the VSE and the announcement of the “new direction” in space received high cover from the 2009 Augustine committee report, which concluded that the current “program of record” (e.g., Constellation) was unaffordable. The Augustine Committee received presentations with options to reconfigure Constellation whereby America could have returned to the Moon (to learn how to use resources found in space) under the existing budgetary cap, but they elected to start from first principles. Hence, we have something called Flexible Path, which doesn’t set a destination or a mission but calls on us “to develop technology” to go anywhere (unspecified) sometime in the future (also unspecified). With target dates of 2025 for a “possible” human mission to a near-Earth asteroid and a trip to Mars “sometime in the 2030’s,” timelines and milestones for the Flexible Path offer no clarity or purpose. Try getting a loan or finding investors using a “flexible” business plan.
Tumlinson argues that both political parties should embrace this new direction because New Space will create greater capability for lower cost sooner. He also makes much about the philosophical inclinations of the Republican Party (the “conservative” major party in American politics) – Why don’t the Republicans support free enterprise in space? Why are they putting obstacles in the way of all these new trailblazing entrepreneurs? As to those obstacles, it is unclear exactly what they are. True enough, there are regulatory and liability issues with private launch services, but not of such magnitude that they cannot be handled through the traditional means of indemnification (e.g., launch insurance).
The COTS program record of the past decade largely has not been a contract let for services, but a government grant for the technical development of launch vehicles and spacecraft. Close reading reveals the real issue: Tumlinson wants more of NASA’s shrinking budget to finance New Space companies. He is concerned that a new administration might cut off this flow of funding. However, what will cut off the flow of funding is having no market, no direction, and no architectural commitment – regardless of who occupies the White House.
The belief of many New Space advocates is that once they are established to supply and crew the ISS, abundant and robust private commercial markets will emerge for their transportation services. Although many possible services are envisioned, space tourism is the activity most often mentioned. Whether such a market emerges is problematic. Although Richard Branson’s Virgin Galactic has a back-listed manifest of dozens of people desiring a suborbital thrill ride (at a cost of a few hundred thousand dollars), those journeys are infinitely more affordable than a possible orbital trek (which will cost several tens of millions of dollars, at least initially). Nevertheless, there will no doubt be takers for a ticket. But what will happen to a commercial space tourism market after the first fatal accident? New Space advocates often tout their indifference to danger, but such bravado is neither a common nor wise attitude in today’s lawsuit-happy society (not to mention, the inevitable loss of confidence from a limited customer base). My opinion is that after the first major accident with loss of life, a nascent space tourism industry will become immersed in an avalanche of litigation and will probably fully or partly collapse under the ensuing financial burden. We are no longer the barnstorming America of the 1920’s and spaceflight is much more difficult than aviation.
Despite labeling themselves “free marketers,” New Space (in its current configuration) looks no different than any other contractor furiously lobbying for government sponsorship through continuation of its subsidies. True free-market capitalists do not seek government funding to develop a product. Rather, they devise an answer to an unmet need, identify a market, seek investors and invest their own capital, provide a product or service and only remain viable by making a profit through the sale of their goods and services.
Tumlinson bemoans the attitude of some politicians, ascribing venal and petty motives as to why they do not fully embrace the administration’s new direction, e.g., the oft-thrown label “space pork” to describe support for NASA’s Space Launch System. In regard to New Space companies, Tumlinson asserts that, “[We] have to both give them a chance and get out of the way.” But in fact, he does not want government to “get out of the way” – at least not while they’re still shoveling millions into New Space company coffers – nor when they need (and they will) a ruling on, or protection of, their property rights in space. Any entity that accepts government money is making a “deal with the devil,” whereby it is understood that such money comes with oversight requirements (as well it should, consisting of taxpayer dollars).
Successful commercialization of space has occurred in the past (e.g., COMSAT) and will occur in the future. But the creation of a select, subsidized, quasi-governmental industry is not by any stretch of the imagination what we commonly understand free market capitalism to mean. It is more akin to oligarchical corporatism, a common feature of the post-Soviet, Russian economy. True private sector space will be created and welcomed, but not through this mechanism, whose most worrisome accomplishment to date has been to effectively distract Americans from noticing the dismantling of their civil space program and preeminence in space.
July 19, 2012
The Moon, unlike Earth, has no global magnetic field but many surface locales of limited extent (tens of kilometers across) are magnetized. In many instances, these small areas of high magnetic intensity are associated with unusual patterns of surface brightness (albedo, or degree of reflectance) that occur in curved, blotchy or other strange “swirl-like” shapes. First observed by telescope, lunar scientists have been puzzled by the possible origin of what they imaginatively named “swirls.”
An example of a lunar swirl is a feature named Reiner γ (pronounced “Reiner gamma”), a bright splotch in southern Oceanus Procellarum, the dark mare region of the western near side. The name indicates that initially this feature was thought to be an isolated peak of highland material that juts up through the mare (lowercase Greek letters were assigned to such prominences in the old nomenclature.) However, even at very low sun elevations, close examination shows that this bright patch does not cast a shadow. It is simply a bright patch on the surface, one with diffuse and nebulous edges, yet clearly more reflective than the surrounding dark mare material. It does not appear to be associated with any crater or other surface feature. It’s as though someone smudged a finished painting of the lunar surface.
During later Apollo missions, orbiting vehicles released “subsatellites” (small spacecraft that continued to orbit the Moon long after the crews had left for home) carrying instruments to measure the Moon’s magnetic field. Interestingly, they found a very strong magnetic field enhancement around the Reiner γ feature. Moreover, numerous other swirls were found elsewhere on the Moon, especially on the floor of the huge South Pole-Aitken (SPA) basin in Mare Ingenii on the far side, and on the eastern limb of the Moon near Mare Marginis. Each newly seen swirl was found to be associated with a magnetic anomaly. However, the converse statement is not true – not all magnetic anomalies have associated swirls.
Two principal models emerged to explain these relations. One model held that the swirls and the magnetism were contemporaneous – the swirls were surficial deposits caused by the scouring of the surface during the impact of a comet. In this model, the cometary coma (i.e., the dense gaseous “atmosphere” surrounding the icy nucleus) struck the Moon at high velocity, scouring the surface and increasing its brightness while at the same time embedding the soil with a strong magnetic field caused by the creation of an impact-generated plasma (high temperature, low density matter).
The other model suggested that the magnetic anomalies pre-dated and were the cause of the swirls. The lunar surface darkens and becomes redder with time owing to exposure to the solar wind (the stream of energetic particles – mostly protons – from the Sun). Strong, localized magnetic fields serve as protective “bubbles” that caused the incident solar wind to flow around these tiny areas, darkening the edges of the field bubbles with enhanced flow but preserving the inner zones (which were shielded from the solar wind) as bright patches. Thus, the bright parts of the swirls are areas that have not undergone “weathering” by the solar wind while the dark parts are zones that have experienced excessive space weathering.
It remained uncertain whether this postulated “magnetic bubble” effect would actually work but recent experiments suggest that these bubbles might well operate on the Moon. Scientists from the UK’s Rutherford Appleton Laboratory, creating a “solar wind tunnel” to observe the interactions of streaming plasma and confined magnetic bubbles, successfully produced a magnetic bubble under simulated space conditions. They have compared the flow field around the laboratory magnetic bubble with the observations from orbiting spacecraft of lunar surface magnetism and find that the solar wind would be diverted around these magnetic anomalies on the Moon. If solar wind darkening is the primary process that darkens the surface, we may have an explanation for the creation of the bright swirls.
The astute reader will note that while this bubble model might account for the origin of the swirls, it begs the question about what caused the magnetic field anomalies in the first place. That remains a mystery. It was noted many years ago by my colleague Lon Hood of the University of Arizona that many of the magnetic anomalies on the Moon are at the antipodes (i.e., 180° away from the center) of some of the youngest, large impact basins on the Moon. The largest concentration of both surface magnetic anomalies and swirls are on the floor of the large SPA basin, near Mare Ingenii on the lunar far side. This area is directly antipodal to the large, young Imbrium basin on the near side. Likewise, the Mare Marginis swirls and magnetic fields are antipodal to the Orientale basin on the western limb (the last of the large lunar multi-ring impact basins). Furthermore, as basins tend to cover the entire Moon, one can find a basin near the antipode of almost any given feature (note well: the swirl that started all this hubbub, Reiner γ, isn’t antipodal to anything in particular). But an even more significant issue is that while the basin antipodal association of many swirls is intriguing, it does not explain why we should see a zone of enhanced magnetization at such locations. Igneous intrusion, concentration of impact-generated plasmas and converging ballistic ejecta have all been proposed but no specific mechanism seems to emerge as the magnetic field creating event.
We are left with a continuing and highly unsatisfactory situation – a possible explanation for the development of surface swirls on the Moon and of their association with magnetic field bubbles, but we still don’t understand the origins of these fields, the cause of their shapes and intensities and how they fit into the continually vexing problem of lunar magnetism in general. Two steps forward and one step back. Lunar science marches on.
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