September 5, 2013
Generally speaking, I hate “mop up” posts wherein stories, anecdotes, factoids and announcements are lumped together solely for the purpose of clearing the writer’s desk. But that’s what I have here, so let’s get on with it.
Despite being written off by many as a dead letter topic, the Moon (an object of scientific and commercial interest and utility) continues to confound experts and frustrate naysayers.
You may have recently learned about yet another discovery of lunar water. The “new” this time around is that we have apparently succeeded in identifying a form of hydration (i.e., the OH molecule) present in mineral structures in the central peak of the mid-latitude crater Bullialdus (20.7° S, 22.2° W; 60 km diameter). Past identifications of lunar water involve either the polar dark regions or high-latitude, solar wind implanted OH and H2O molecules. We’ve known about water-bearing minerals in the lunar samples for the past couple of years, but this is the first time we have identified them using remote sensing. This water is present in extremely minute amounts (tens of parts per million); it has nothing to do with the possibility of extracting water for human use, but rather, is a clue to the hydration state of the deep interior, and ultimately, the origin of the Moon.
We are finding that the early Moon had its own indigenous water, not an obvious consequence of the giant impact origin model, and that this water participated in early melting events. Water is an important compound in these processes by lowering the threshold temperatures of various significant reactions and creating an environment in which explosive, volatile-charged volcanic eruptions may occur. Work continues on understanding the meaning and significance of this interior water to the geological processes of the Moon.
The latest edition of the Global Space Exploration Roadmap has been released and to the astonishment of the press and many other observers, human lunar return is still prominently featured (minus NASA) in the strategic pathways considered by the world’s space agencies. This shouldn’t really surprise anyone – the international partners were taken aback (and angered) by the unilateral renunciation of lunar return by the U.S. in 2010. They have remained firm and consistent in their belief and knowledge that the Moon is a critical step toward developing genuine space faring capability, a path which they have no intention of abandoning. In this, our partners show more insight and sophistication than we do. There are simply too many advantages in developing technology and practicing operational skills on the Moon, all applicable to future human missions beyond low Earth orbit. In a sop to the reluctant Americans, human near-Earth asteroid missions are mentioned. But in the minds of the international partners, the benefits of human lunar return will not be subsumed by a domestic political agenda.
I am an occasional member and contributor to the Lunar Exploration and Analysis Group (LEAG), an informal working group of lunar scientists, engineers and developers who have devised a “roadmap” (i.e., a sequenced, strategic plan) for lunar exploration. This roadmap has been completed and we have developed a couple of ancillary products – an executive summary booklet (being readied for distribution), which will describe the major findings of the three-year road mapping exercise. It will be illustrated by wonderful Technicolor artwork of missions and surface activities (the creation of pretty pictures and graphics we have down pat), and a one-page “fact sheet” describing the value and rationale for human lunar return. The compact fact sheet is particularly good. It summarizes the main points about lunar return, its value to the nation and to science and society in general. This roadmap follows a lot of the concepts about which I write. If you visit Develop Cislunar Space Next, you will recognize many of the same themes and ideas. I am very happy with this product; it is concise and well crafted. I thank my LEAG colleagues for their scientific insight and technical acumen.
About 15 years ago, I wrote a reasonably well-received book published by the Smithsonian Institution Press titled The Once and Future Moon. In it I described the then-recent findings from the Clementine and Galileo missions about the Moon’s processes and history, and summarized what we had learned about the Moon from the Apollo missions. I also took the opportunity to make the case for a return to the Moon (some things never change) and how we might use it to create new capabilities in space. That book is now out of print, as well as rendered somewhat antiquated by the explosion this last decade of new information from data returning from lunar robotic missions and subsequent studies. Many have urged me to revise that book and I am considering writing an updated second edition. Unfortunately, the Smithsonian Press terminated their “Library of the Solar System” series and is not interested in publishing a new edition (but will give me copyright to the material). I am investigating the interest of other publishers and will keep you posted on what develops.
Next – an announcement. For some time I have watched the progress of many of the Google Lunar XPRIZE competitors. It’s a mixed bag, with some teams pretty much out of the running and some who have a decent chance to actually fly a mission. I have been very impressed with the team and the approach of one company, Moon Express (MoonEx), located at NASA Ames Research Center in California. Moon Express has plans for small and medium class lunar landers, using a soon-to-be-unveiled design that seems both robust and affordable. I have agreed to be associated with them on a part-time basis as their Chief Scientist. As such, I will evaluate possible mission scenarios and profiles, devise sample payloads, identify possible instruments and their investigators and vendors, and help define measurement requirements and operational scenarios.
I like working with small missions (my first mission experience was with Clementine, a small DoD-NASA mission in the 1990s and I was the Principal Investigator for the Mini-SAR radar experiment on India’s Chandrayaan-1 mission) and believe that these small missions deliver a lot of scientific and exploratory bang for a reasonably small amount of bucks. I have worked previously on projects with some of the Moon Express personnel, including Principal Systems Engineer Steve Bailey on the world’s first private lunar lander project (Blastoff.com in the late 1990s) and with CEO Bob Richards, when we were both affiliated with Odyssey Moon a few years ago. I am also happy that my longtime colleague and NASA Advisory Council member Jack Burns has joined the company on a similar part time basis as Chair of the Moon Express Science Advisory Board. I look forward to helping Moon Express achieve their goal of winning the Google Lunar XPRIZE and developing a truly commercial system to deliver payloads to the Moon.
Look for an article on the origin of the Moon written by yours truly, coming soon to a special web-based edition of Astronomy magazine. I’ll post the information when it appears. My recent post here at Air & Space describes the call for small lunar lander missions. The last of the (currently planned) NASA missions to the Moon is scheduled for launch this Friday, September 6, 2013. Here’s wishing LADEE a safe, successful and productive journey.
So I’m happy to report that there are signs of “life” about our future on the lunar frontier.
August 17, 2013
Wanted: lander spacecraft to deliver payloads to the Moon. Must be cheap and reliable.
NASA recently issued an “RFI” – a Request for Information – a method used by the agency to solicit concepts from various companies and gauge their ability to fulfill a future anticipated need. In this case, the need is for a small robotic lander, one capable of delivering two classes of payloads to the lunar surface: small (from 30 to 100 kg) and medium (from 250 to 450 kg).
Probably focused near-term with the RESOLVE (Regolith and Environment Science and Oxygen and Lunar Volatiles Extraction) payload, the intent of this RFI is to survey existing capabilities for the commercial delivery of a variety of payloads to the Moon. RESOLVE is a NASA experiment designed to test and demonstrate some techniques of in situ resource utilization (ISRU) on the Moon, specifically the generation of oxygen and the extraction of volatile elements (such as hydrogen) from lunar soil. The RESOLVE package consists of several highly integrated experiments designed to collect soil on the Moon, heat this feedstock to various temperatures and measure the amount and type of volatile elements released, and practice some techniques of processing the soil into useful products (such as water or oxygen).
Though we’ve been talking about using off-planet resources for years, this is the first time the agency would fly an experiment designed to evaluate the processes and difficulties involved. Some of us contend that until it is proven possible (by demonstrating it in space), space-based resource utilization (ISRU) will remain classified as “too risky” to incorporate into an architecture. Engineers don’t doubt the chemistry or physics behind ISRU, but to evaluate risk and return, they want demonstrations using real hardware versus theoretical concepts and paper studies.
Although it will not answer all ISRU questions, RESOVLE can provide useful data and would be an important milestone. Our ignorance is particularly vast in regard to the nature of the polar volatile deposits. Some near-polar sites are under consideration for RESOLVE, but because the lander must be able to communicate with Earth, sites near the poles must be in radio view of Earth. This eliminates the most promising polar volatile sites (permanently dark, out of radio sight) from consideration, at least for the first mission. However, we know that water ice occurs in some areas in view of Earth, so careful targeting will permit us to get ground truth for a critical area near the one of poles.
There are a wide variety of possible payloads (scientific and resource utilization) for lunar missions using small landers. A key priority for the lunar science community has been the deployment of a global network of geophysical instruments. Such a package would include a seismometer (to monitor and measure moonquakes), a heat flow probe (to take the Moon’s temperature) and other instruments, such as a magnetometer and a laser reflector. The five-station surface network laid out during the Apollo missions was operational for more than 7 years and gave us a first-order understanding of the nature of the deep lunar interior. A new global network – widely spaced and operating longer with more stations – would vastly improve on that knowledge.
The success of a network mission necessitates a long-lived power source to operate instruments during the very cold, 14-day lunar night (the Apollo network used nuclear power supplies), along with an inexpensive way to deploy the network stations. New technologies have developed small, reliable radioisotope generators that operate for many years. A small lander could deliver geophysical stations across the entire globe; each station is low mass, so the smaller (and presumably cheaper) the lander, the more likely that this mission will be realized. A global seismic network would decipher the crust and mantle structure of the Moon and could monitor its surface for large impacts. A precise measurement of lunar heat flow (measuring the abundance of radioactive elements in the Moon) will give us more information about the bulk composition of the Moon and advance our understanding of lunar origin. Laser ranging will also be useful in addressing some critical geophysical and astrophysical problems.
Single-point landers, making simple measurements, can investigate the surface composition and geology at select landing sites. If the landing sites and investigations are carefully chosen, they could significantly advance science by answering key questions. For example, a critical issue in the cratering history of the Moon is knowledge of the absolute age of some of the youngest craters on the Moon. The formation of the crater Copernicus marks a key time horizon in lunar history (the Copernican Period). We know its relative age very well but are uncertain about its absolute age. A small lander can be sent directly to the crater floor, where the impact melt is exposed and accessible, to analyze crater melt rocks for chemical composition and to learn the nature of the impact target (as well as determining the age of the rock by measuring the radiogenic potassium and argon in the rock). Although the potassium-argon technique is not the most precise method of radiometric dating, it can distinguish among the different proposed absolute ages, which vary over a billion years. By determining this age more precisely, we will better understand the impact flux in the Earth-Moon system, knowledge that will help us better interpret the surface ages of units on other terrestrial planets.
Small landers could deliver a variety of long-lived assets for future surface operations and resource utilization experiments. Techniques for making oxygen from lunar soil have been proposed but no comparative demonstration has been done on the Moon. A small laboratory could be send to the Moon to conduct simultaneous experiments on oxygen manufacture. The advantage of this experiment would be the use of identical feedstock under identical thermal and time constraints to compare their relative efficacy and identify any problems. This experiment would fit on a small lander (~ 50 kg capacity) and by using solar power, within the span of a single lunar day (2 weeks) could quickly complete its evaluation.
The larger version of the RFI lander opens up other possibilities. With a payload capacity on the order of 500 kg, this lander could deliver an advanced, automated surface rover (powered by an RTG – nuclear battery) able to undertake extensive and protracted exploration of the polar cold traps. Equipped with instruments utilizing well established technology, this rover would characterize the physical, chemical and isotopic make up of the polar volatiles – a task critical for mapping the extent and purity of deposits of water ice on the Moon, and evaluating their mining and extraction potential.
At this scale, it’s possible to deliver an ascent vehicle to the Moon to retrieve and return samples to Earth. Scientists have a long list of desired targets for sample return and the potential for low cost, commercial landers to deliver payloads simply and inexpensively to the Moon could revolutionize our understanding of the Moon’s (and Earth’s) history and processes. From remote sensing data, we know that many fascinating areas on the Moon display rocks either unrepresented or unrecognized in the existing collections from the American Apollo, Soviet Luna, and lunar meteorite samples. Samples from the oldest impact feature on the Moon – the floor of the South Pole-Aitken basin – are especially desired. Although a simple “grab” sample won’t answer all of our questions, rocks from this site could address major questions about the bombardment history of the Moon and the early Earth.
Small lander spacecraft will open up new horizons for science and exploration. Critical to their success is making them simple, robust and inexpensive. That’s been a tall order for NASA. Whether the commercial sector can provide this capability more effectively remains to be seen.
July 31, 2013
Prior to the Space Age, one of the longest running controversies in lunar science was over the origin of the Moon’s craters. Two camps emerged, one favoring an internal (volcanic) origin and the other an external (impact by solid bodies) origin. Although this debate was finally resolved in favor of impact, the argument was long and vehement, reigniting at one point during the flight of the last of the robotic precursor probes to the Moon, prior to the Apollo landings. Although the basic physics of impact were well understood by the mid-1960s, this newest argument centered around high-resolution pictures obtained by Lunar Orbiter 5 of the fresh (and therefore young) crater Tycho. These spectacular images showed a multitude of flows, smooth ponds, and fluid rock, seemingly draped over hills and hummocks (like a chocolate shell coating over a scoop of ice cream).
An asteroid possesses an enormous amount of kinetic energy when it strikes a planetary body at very high speeds. On contact, the asteroid vaporizes and the surface target rocks are intensely compressed. After the shock wave has passed, these rocks decompress and the release of this energy totally melts part of the crustal target. This material is said to be shock melted, with the resulting liquid called impact melt. Impact melt was first described from craters on the Earth, particularly some of the very large impact craters found on the ancient Canadian Shield. These rocks superficially resemble some volcanic rocks, having both fine-grained textures and partly melted inclusions. But unlike volcanic rocks, they have high concentrations of siderophile (“iron-loving”) elements, such as iridium. These elements are extremely rare in the Earth’s crust, but are more abundant in meteorites and asteroids. It is thought that they are added to the melt from the incoming projectile.
The newest chapter in the argument about the origin of craters came about because some landforms around Tycho look similar to small-scale volcanic features on Earth. The idea proposed was that the craters had been formed by impact, with those collisions triggering volcanic activity and producing multiple episodes of eruption at Tycho and other craters. At first glance, such a scenario seems plausible. After all, impact is a catastrophic event and one can imagine churning seas of subsurface liquid rock, released suddenly through the creation of fractures deep in the crust. But the Moon’s interior is relatively cool. If interior melt exists, it is at a level much too deep for any reasonably sized impact to tap. But these amazing landforms needed to be explained. What might they represent?
We found abundant physical and chemical evidence for impact (including shock-melted rocks) by studying the Apollo samples. They appear similar to volcanic lava, with inclusions, melt textures and even vesicles (holes), comparable to the ones produced by magmatic volatiles coming out of solution in basaltic lavas on Earth. Although it took a bit of study (and many more arguments) to establish their origin, shock melting became recognized as an important lunar (and Earth) impact process.
The images of the flows and ponds seen around Tycho and other fresh lunar craters led to a better understanding of how these rocks formed. Although we knew about impact melting from the study of Earth’s craters (and had found evidence of the same in lunar samples), some researchers still weren’t convinced that we were seeing flows of liquid impact melt on the Moon. The leading non-volcanic alternative was that these features were flows of dry, fine-grained granular debris. In part, this interpretation proceeded from the observation that the thermal signatures of some of these melt-like flows suggested the presence of fine debris rather than bare, jagged rock. Yet other data, such as radar backscatter, suggested that rough surfaces were common, while extremely high-resolution images showed abundant blocky craters on the surfaces of the flows, suggesting they were composed of solidified rock.
Images from the robotic Surveyor 7 spacecraft, which landed on the rim of Tycho in 1968, revealed the thinnest regolith (soil) covering of any site on the Moon. Visible in the surface panoramas were flow features covering the distant hills. It took a great deal of painstaking, detailed work to establish that these flows and ponds were composed of liquid rock, created simultaneously with their host crater and likely originated by impact melting and subsequent solidification.
For the last several years, NASA’s Lunar Reconnaissance Orbiter (LRO) has been sending us new and astonishing views of the Moon’s impact melt flows. Whereas fresh craters like Tycho, Aristarchus and Copernicus were well known from previous Lunar Orbiter frames, far side craters like the spectacular Giordano Bruno can now be seen with incredible clarity. G. Bruno is one of the very youngest craters on the Moon. A low density of craters overlying G. Bruno suggests an age of less than a couple million years (extremely young on a planet where most features count years in the billions). It is an astonishing spectacle of melt shapes and deposits (cracked floors, pools, flow festoons and lobes); the crater floor has an amazing whirlpool of solidified melt. All these features indicate that after the crater formed, the impact melt was mobile, flowing and collecting, and ponding in low areas.
Impact melts are of great interest to geologists. Unlike other crater ejecta, the radiometric clocks of impact melts are completely re-set by the melting. Thus, if a sample can be obtained first-hand, directly from an observed flow or pool of melt around a host crater, the age of that rock specifically and unambiguously dates the impact event. Unfortunately, we did not visit such deposits during the Apollo explorations. What we do have are loose samples of lunar impact melt but not their scientifically important corresponding geological context. It is for this reason that the age and sequence of early lunar history is so contentious – we must make educated guesses about where certain melt rocks come from. If we get the context wrong, then our conclusions about the history of the Moon are incorrect.
Increased understanding of the generation and deposition of impact melt comes from the new images obtained by the LRO camera of the geologic setting of impact melts. Future sample return missions to the Moon can be directed to landing sites that will provide us with samples of clear geological context (that they were from that area and not just flung there by an impact occurring elsewhere on the lunar surface). As features age on the Moon, subsequent geologic events (such as superposition of new units) bury or erase the original event making the context less clear. This problem is particularly acute for the oldest features on the Moon (multi-ring impact basins). By studying the geology of the freshest lunar features (such as Tycho and other fresh craters), we understand how the older impact features looked immediately after their formation. Thus, they serve as a guide to the interpretation of the older features. On the Moon, as on the Earth, as Charles Lyell, the 19th century author of the classic Principles of Geology aptly put it: The present is the key to the past.
A Collection of Spectacular Impact Melt Features Seen by LRO
June 26, 2013
As a naturally orbiting object, the Moon orbits Earth in an elliptical path, with the center of the Earth at one focus – more precisely, both Earth and Moon orbit each other around what it called the barycenter, the imaginary point about 1800 km below the surface of the Earth that constitutes their mutual center of gravity. Since the Moon is only about one percent the mass of Earth, the barycenter is much closer to the center of Earth than it is to the center of the Moon.
When the Moon comes closest to Earth in its elliptical orbit it is said to be at perigee. If the Sun, Earth and Moon come into alignment along a straight-line, a condition occurs that astronomers perversely have named syzygy (a great word to keep in your hip pocket the next time you play Scrabble, though you’ll need a blank to get there). Syzygy (alignment) is not the same as perigee (the closest approach of Moon to Earth) but on the occasion when syzygy and perigee coincide, we have what’s called a “Super Moon.”
During perigee, the Moon’s elliptical orbit causes it to be about 45,000 km closer to Earth than at farthest point (apogee). As the average distance between the two is about ten times that distance, the visual effects of this variation, though not large, is measurable. I’ve been skeptical about noticing this size difference by “eyeballing” the Moon at perigee (closest) and apogee (farthest). However, during an early morning walk with the dog this last weekend, I was somewhat startled to see the full Moon low in the sky, definitely appearing larger than usual.
In part, this appearance results because of the “Moon illusion,” whereby the Moon appears much larger on or near the horizon than when it is overhead, near zenith. The traditional explanation for this illusion is that when the Moon is near the horizon, we can compare the size of the Moon’s apparent disk to known objects on the Earth (such as a house, distant tree or hill). When the Moon is directly overhead, there is no nearby object with which to compare it. Many depictions in art show the Moon as an enormous lunar disc, glowing the night sky; it is to this optical illusion that such portrayals refer.
The Moon’s apparent diameter is about one-half of a degree of arc (same as the Sun), or roughly the dimensions of a small pea held at arm’s length. Although the biggest object in our sky, that size is much too small for the naked eye to resolve most surface features (except for the vague markings of light and dark that comprise the lunar maria, the “Man in the Moon”). In full phase, the Moon can be quite bright, illuminating the landscape at about -12 visual magnitude. While no one would mistake such conditions with daylight (the Sun is about -26 visual magnitude, about 400,000 times brighter than the full Moon), full moonlight is bright enough to cast strong shadows and to read by. This is one of the reasons astronomers “hate” the Moon – during full phase, the sky is typically too bright to reveal any but the very brightest stars and it interrupts their views of coinciding meteor showers. However, they’ll “love” the views that await them from the far side of the Moon, the only place in our Solar System where radio noise from Earth is silent and at times, when Earth blocks the Sun, the sky-viewing would be unsurpassed.
The most important effect of a “Super Moon” is on tides, which can be extraordinarily high during perigee. This effect can be especially significant in coastal areas that experience high tides, such as the famous Bay of Fundy in Canada. In this area, the combination of shore depth and geometry, prevailing winds and position create tidal height variations as high as 16 meters (over 52 feet) in the course of a day. At Super Moon, tidal variations are at their largest; during the passage of Hurricane Sandy up the East Coast last year, landfall occurred during full Moon (syzygy), resulting in both a storm surge (i.e., a large dome of water caused by low atmospheric pressure and wind) and high gravitational tides. As witnessed with Hurricane Sandy, the combination of both occurring together can be devastating.
Tidal effects are most notable in large bodies of water, but the solid Earth also deforms in response to the pull of the Moon’s gravity. A tidal bulge extends slightly above the mean radius of both Earth and Moon. This bulge is not perfectly aligned with the geometric line that connects the centers of the two objects because both Earth and Moon are rotating, and it takes time for the solid bodies to deform plastically. Thus, the tidal bulge of the rapidly spinning Earth slightly leads the Earth-Moon line, resulting in a constant increased tug at the Earth by the Moon, slightly slowing the rate of Earth’s rotation down. At the same time, this leading tidal bulge attracts the Moon more, making it speed up in its orbital path slightly and thus, move outward, away from the Earth. So over time, as the Earth spin rate slows, the Moon gradually recedes away from its grip; this rate of recession is about 4 cm per year. The Moon is currently about 60 Earth radii away; it was once much closer, possibly as close as a few Earth radii. It could not be closer than about 3 radii (the Roche limit) because at distances closer than the Roche limit, tidal forces would tear the Moon apart. In a few hundred million years, the Moon will be too far away to permit a total solar eclipse to be seen from Earth. A timely and good thing that we came along when we did!
Using information from a lunar seismic network deployed on the Moon during the Apollo missions, we know that “moonquakes” often correlate with the tidal flexing of the solid Moon induced by the Earth (which is much larger than the terrestrial bulge because Earth is much more massive). In fact, although there is a slight suggestion that the Moon might induce the initiation of an earthquake, in most cases there is no obvious connection. The Earth is an active, dynamic body and its great internal heat and complexity of configuration appear to be more important in determining when and where an earthquake occurs than by tidal effects caused by the Moon. But if the proper tidal conditions and the alignment of stress and magnitude of effect coincided, there is no reason that either syzygy or Super Moon could not induce an earthquake.
Our Moon is much more than the familiar, comforting nightlight orbiting Earth. Beyond touching us emotionally and affecting our planet physically, the Moon is also an orbiting treasure trove of, as yet unrealized (some imagined but mostly yet unimagined) scientific discoveries and technological breakthroughs. But before we make it our goal to settle the Moon, we must make it our goal to sail beyond it.
June 19, 2013
Although the Moon has no global magnetic field like the Earth, small areas on its surface are magnetized. These fields are not systematically distributed and in general are very weak. In trying to explain their mysterious presence and origin, several ideas have been advanced.
Rocks typically acquire magnetism (called remnant magnetism) by cooling in the presence of a magnetic field. At temperatures greater than about 570° C (the so-called Curie point), a rock cannot retain a magnetic signature. But if it cools below the Curie point, it assumes an induced magnetic field oriented in the same direction as the field in which it cooled. Unfortunately, on the Moon most rocks have been dislodged from their original orientations by impact processes, so we do not know whether a given rock cooled in the presence of a global (presumably uniform strength and direction) or local (randomized) magnetic field.
We knew the Moon had no global magnetic field before the Apollo crews landed, so it was a bit surprising to learn that some of the returned lunar rocks are strongly magnetized. Because these rocks are all very old (usually much older than 3 billion years), it was thought that they recorded an ancient epoch when the Moon might have had a global magnetic field, now vanished for some reason.
This finding from the lunar samples was complemented by measurements from orbit that show small areas (10s to 100s of kilometers across) of the surface to be magnetized. These areas occur all over the Moon and are not associated exclusively with either the dark volcanic maria or the bright highlands crust. However, they do tend to have two peculiar properties. First, we find strange “grooved” terrain associated with some of the strongest magnetic anomalies. This terrain is unlike any other lunar landform – it consists of ridges and valleys that cover the walls and sides of craters and mountains. Second, these magnetic anomalies tend to occur at the antipodes of (180° away on the opposite side of the globe from) the largest and youngest lunar multi-ring impact basins. These are curious properties indeed. What might it mean?
For years, many have pondered and worked on this dilemma. One idea was developed that perhaps these magnetic anomalies are formed during basin impact. It was proposed that seismic shaking from these enormous impacts created the grooved terrain and induced fractures in the crust at the antipode, into which hot volcanic magma was injected. After cooling these dikes assumed remnant magnetism from a global dipole field. Yet another idea contends that the concentration of magnetized material is a result of antipodal convergence of basin ejecta, which arrived hot from basin formation, collected at the antipode and cooled through the Curie point there. This last model has the advantage that it might also explain the presence of the grooved terrain, which might have formed by the arrival of basin ejecta on the surface from impacts coming from all directions simultaneously.
My colleague Lon Hood from the University of Arizona has been studying magnetic anomalies for many years and is an advocate of the last model described above. Hood was studying some previously ignored, smaller magnetic anomalies found around the Moon that had no explanation. He asked me about the geological setting of one particular magnetic anomaly on the Moon that had yet to be described in detail. This one occurs in highlands near the north pole of the Moon and had not been previously studied in detail.
I have been something of a skeptic for many years about the basin/antipode relation for magnetic anomalies. Part of the reason for my position is the problem of Reiner Gamma, which is a bright patch on the lunar surface that has one of the highest magnetic field strengths on the Moon. The problem is that Reiner Gamma is nowhere near the antipode of any basin and shows no evidence for any grooved terrain. So I thought that this was the exception that disproves the rule.
Nonetheless, I was intrigued by Hood’s finding and decided to examine the area. To my astonishment, I found wall textures very similar to the famous grooved terrain in the walls of the craters Lovelace and Froelich (not exactly coincident with the anomaly, but very close). I can see no obvious reason for such terrain development; it appears to be highly restricted in its distribution and is not a fresh feature. Judging from its degraded appearance, it is rather old.
So, is there a basin antipodal to Lovelace and Froelich? Indeed there is – the fabulous Schrödinger basin, one of the smaller lunar basins at 325 km diameter, located near the south pole of the Moon. Before our study, I probably would have thought that Schrödinger was too small to create any global-scale effects, but we don’t fully understand the effects of impact with increasing size and there is no good alternative explanation for the wall textures of these two craters. The presence of a significant magnetic anomaly nearby is unquestionable.
So have I changed my mind on the origin of lunar magnetic anomalies? Possibly. One of the most convincing ways to get a scientist to change his mind is to bludgeon him with an irrefutable fact that contradicts his worldview. I now realize the Reiner Gamma problem does not “disprove” the basin antipode model – it merely indicates that it may be incomplete. That distinction is subtle but significant. In science, we always look for “rules,” generalities that help us organize observations and suggest possible explanations. However, these rules sometimes have exceptions and we must carefully distinguish which actually have the force of a rule versus those that merely indicate some general tendencies.
To me, this discovery was surprising. The new finding still does not fully address exactly how these magnetic anomalies are formed at the antipodes, but the concept that magnetic anomalies and basin-forming impacts are intimately associated has been strengthened and extended. We will continue to work on this vexing problem.
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