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.
May 29, 2013
For every problem there is a solution that is simple, elegant and wrong. – H. L. Mencken
Accuracy in scientific reporting (and thus the education of the public) is wholly dependent on a reporter’s understanding of the material they’re covering. Making a reporter’s job even more challenging is the fact that some research results themselves can be misleading. A variant of my post title above appeared recently over a story reporting the results of a paper published in the journal Nature Geoscience. That study used computer modeling to simulate the effects of a low velocity impact on the Moon. Computer models of natural phenomena are made in an attempt to understand complex processes that we could otherwise not be able to address.
To briefly set the stage on this new work, we believe that the vast majority of craters on the Moon and planets are formed by the collision of solid objects with these bodies. These impacts occur at very high speeds; on the Moon, the average velocity of impact is about 20,000 meters per second. At such speeds, geological materials will vaporize and the mechanics of the formation of a crater are complex. These results have been painstakingly described through laboratory and field studies of both natural and artificial impact craters of a wide range of sizes.
Because we needed to fully understand the mechanics of impact cratering to understand the record in the Apollo lunar samples, much work was conducted toward characterizing the physical and chemical effects of impact on typical rocks. Because impact velocities are typically high, there is little preservation of the projectile in impact craters. Most of the impactor is vaporized and this super-hot silicate vapor is partly lost to space and partly incorporated into the shock melted rocks of the crater interior.
The soils returned from the Apollo missions contained a recognizable fraction of material that must have been added by the impacting objects that created its craters. In most soils, this fraction is on the order of a few weight percent. Interestingly, this “meteoritic component” tends to be defined chemically and actual fragments of meteorite in the lunar soil are extremely rare. This observation would seem to support the notion that most of the impacting debris is vaporized at impact and does not occur as fragments on the surface.
However, the speed of impacting projectiles cited above is an average speed, meaning that while some impacts occur at higher velocities, others must occur at lower speeds. As the encounter velocity decreases, there is an increasing likelihood that some portions of the impacting fragments might be preserved on the surface. It is this last possibility that the new paper considers. The authors modeled the effects of the impact of a relatively slow-moving body with the Moon and found that more fragments of the object are preserved than in high velocity impacts. Moreover, by tracing the paths of impactor particles during cratering flow, they find that much of this preserved material ends up on or near the central peak of the resulting crater.
That last finding is interesting because in remote sensing studies of the lunar surface, it is in the central peaks where we find “unusual” compositions, in the sense that those compositions are different from the average upper lunar surface. The traditional explanation for this relation is that because central peaks are derived from well below the impact target, they are exposing deep-seated compositions (lower levels of the crust of the Moon contain different rock types than occur on the surface). The study’s new interpretation suggests instead that the central peaks are covered in debris from the impacting projectile.
One problem with this interpretation is that the “debris covering” of central peaks should occur in a distinct minority of craters (i.e., those created by low velocity impacts). But the exposure of unusual compositions within central peaks of lunar craters is quite common and occurs globally. Moreover, there are as many impacts at higher velocity as at lower velocity. Yet slow impacts would produce less total volume of impact melt and most of the central peak craters on the Moon have abundant melt deposits.
The most serious flaw in the new study is the assumption that the “unusual minerals” olivine and spinel (found in many central peaks) are rare on the Moon. They are not rare; although spinel is somewhat sparse on the lunar surface (requiring high pressure for its formation), it has been described as present in lunar rocks from the first sample return and more recently has been found in remote sensing data of impact basin deposits. Olivine is a very abundant mineral on the Moon and typically makes up a significant fraction of the dark mare basalts (including some lavas that consist only of olivine and glass.) Olivine is also not uncommon in highland rocks, usually occurring within the rock type troctolite, a 50-50 mixture of olivine and plagioclase. The presence of olivine does not indicate either “deep” origins or “lunar mantle” provenance; virtually all olivine in lunar samples has high calcium content, indicating a relatively shallow origin (probably in magmas that crystallized within a few kilometers of the surface). In short, there is no compelling reason to believe that the central peaks of many lunar craters are dusted with exotic minerals from asteroids, although such a possibility is certainly not excluded. The minerals that we see in central peaks are all indigenous to the Moon and in some cases, abundant in the lunar crust.
Computer modeling in science has both value and pitfalls. An impact event is extremely messy and complicated. Simultaneously, gigantic shock pressures and temperatures occur, putting billions of particles in motion. Computers are good at keeping track of these particles and the codes developed to model complex, multi-variable phenomena have been shown to at least partly describe the behavior of crater formation on Earth. However, the results of computer models must be interpreted cautiously; small changes in input variables or the conditions of the simulation sometimes result in drastic changes in the output of the model. In addition, there is a tendency in science to believe in numbers, regardless of their provenance. Because a model holds together does not mean that it describes reality.
In science, it is dangerous to embrace a model because it “works” (i.e., comes to closure). Much of the current fracas over human-induced climate change comes from those who contend that the results of computer models constitute “settled science” (whatever that is). Because the computer models say that it may happen, people assume (and some journalists report) that it is happening. In actual fact, we have no direct observational evidence that human-caused emissions of carbon dioxide are causing the climate to change. That conclusion comes from computer models that “show” (project) that humanity’s introduction of “excess” carbon dioxide into the atmosphere by industrialization will increase the magnitude of the greenhouse effect and raise the mean global temperature. But climate (like impact) is a complex, chaotic phenomenon and we still do not fully understand how the Earth’s atmosphere interacts with itself and the cosmos.
In questions of complex natural processes, beware of accepting the results of computer modeling too easily. Computer models are useful tools, but the old software adage of “garbage in, garbage out” still applies. Be familiar with whom and from where the information comes, understand how it is processed and then carefully consider the likelihood of reported accounts.
May 14, 2013
Science Magazine recently published a paper that reports that minute quantities of water contained in lunar volcanic glass appear to be identical in isotopic composition to terrestrial water. According to subsequent press reports, this finding revolutionizes our understanding of the origin of Earth and Moon. But does it?
Water is a simple molecule, made up of two hydrogen atoms and one oxygen atom. However, these atoms are not all made the same – they always contain the same number of protons and electrons but the number of neutrons they contain varies. In particular, some naturally occurring hydrogen contains an extra neutron and hence has twice the mass of normal hydrogen. This “heavy hydrogen” (called deuterium, for its atomic weight of two) is much less abundant than its lighter version. Planetary scientists use the amounts of deuterium, relative to normal hydrogen, as a measure of the provenance of the material, i.e., where it formed relative to the Sun.
Ultimately, substances that have identical deuterium/hydrogen ratios are presumed to have come from the same source. We have reason to believe this ratio increases systematically outward from the Sun, depending upon where in the early “solar nebula” the material condensed and its subsequent geological processing. Oxygen (the other element in water) also has an isotopic variation; normal oxygen has 16 protons in its nucleus, but the other isotopes of oxygen can have an additional neutron or two. As with hydrogen, the variation in the ratios of normal to “heavy” oxygen is thought to be indicative of where the material comes from.
Of course nothing is ever quite so simple and straightforward. Subsequent processing, such as interaction with cosmic rays, can sometimes alter the composition of samples but if these effects can be accounted for and eliminated, isotopic composition can be used as a tool to map the ultimate sources of Solar System debris. This has been done with many different elements and compounds, but oxygen and hydrogen are very volatile and thus, sensitive indicators of the thermal environment in which they formed.
When the isotopic composition of an element like oxygen is plotted for the various groups of Solar System materials – meteorites, lunar, martian and terrestrial samples – they all form distinct groups, indicating that the source reservoirs of these materials formed in different locations of the nebula. The most primitive type of meteorite – carbonaceous chondrite – appears to have formed at the farthest distance from the Sun. These rocks are thought to have originated within once icy bodies, the cores of objects known as comets. Comets form in the outer Solar System where low temperature substances are abundant and are occasionally perturbed by gravity to enter the inner Solar System, i.e., inside the orbit of Jupiter. Once there, they are heated by the Sun and their most volatile components are sublimed away; after multiple passes through the inner planet zone, only a small fraction of this primitive material remains.
The new findings indicate that the isotopic composition of the hydrogen in water in the mantle (deep interior) of the Moon is nearly identical to that in the water of Earth’s mantle, and both appear to have come from carbonaceous chondrite (most primitive) meteorites. When compared to a variety of data from other Solar System objects (including the giant planets, icy outer planet satellites and meteorite groups) the Earth-Moon system is compositionally distinct and identical, indicating that, whatever our origins, the description of Earth and Moon as a double-planet is even more appropriate than we had thought.
What does this mean for lunar origin and what does it say about the water at the Moon’s poles? The bulk composition of the Moon has long been recognized as a key constraint on models of lunar origin. A basic question is whether the Moon is made of the same material as the Earth or not. The new results indicate that it is and as such, is another contributory piece of evidence that the materials of the Earth and Moon were brewed in the same pot. Interestingly, this pot of material is distinct from virtually every other Solar System object (as near as we can tell based on limited information from the other planets). Whatever process formed the Moon, it involved objects that were created more or less in this neighborhood of the Solar System. The new results also suggest that both Earth and Moon had a significant component of water early in its history. Earlier studies had suggested that the terrestrial hydrosphere was a late addition, a veneer of cometary debris from deep space that was added to the Earth late in its history. We now know that this water was incorporated into the Earth very early, possibly from the beginning of accretion. The Moon shares this trait – and the same source of water.
So is the giant impact model of lunar origin still viable? The existence of water in the lunar interior is not a prediction of the giant impact model but as has happened previously, the model will probably be modified to accommodate the new findings. We have a tendency to imagine (and desire) simple systems in chemical and thermal equilibrium, in which materials and energy behave in a straightforward, predictable manner. But this event (if it occurred) was a singular one, possibly involving complex, chaotic behavior. Thus, some of the difficulties created by the new data will probably be explained away. A hypothesis elastic enough to be stretched to fit any new discordant observation isn’t particularly useful and certainly isn’t scientific.
How does this affect our thinking about the water ice trapped at the Moon’s poles? As we continue to find that the interior of the early Moon was more water rich than previously thought, we must add lunar water to the long list of possible sources for polar-trapped water. (As a reminder, the previous idea was that polar water was derived from external sources – the Sun via the solar wind hydrogen, water-bearing meteorites and comets). Could at least some of the water at the poles be of lunar origin? One problem that we still don’t understand is the geological age of the polar cold traps – they exist because the spin axis of the Moon is normal to the ecliptic plane. How long has the Moon been in this orientation? We suspect that the Moon has been stable for at least the last 2 billion years but water is being found in volcanic glass over 3 billion years old and thus, released before the current polar cold traps existed. So at least for now, it seems that the Moon’s own water is an unlikely contributor to the ice at its poles. But that story could change too.
The Moon’s surprisingly complex and interesting history continues to confound the experts. We may have already “been there” but we still don’t fully understand the Moon’s story and true potential.
April 24, 2013
Imagine a system of molten silicate material, where low-density minerals float and higher density minerals sink. Minerals rich in iron and magnesium (such as olivine and pyroxene) will settle toward the bottom of the magma body while those rich in the elements aluminum and calcium (such as plagioclase feldspar) will float. Just such a scenario – on a global basis – is thought to have created the crust of the Moon.
Before Apollo, many believed that the Moon was a primitive, undifferentiated lump of cosmic debris. By studying the samples returned by Apollo 11, scientists identified small fragments of white, plagioclase-rich rocks (anorthosite). There are no known magma compositions corresponding to this rock type – anorthosite is created by removing low-density plagioclase from a crystallizing system and concentrating it by floatation. From the evidence of fragments in the lunar soil, large amounts of anorthosite were inferred to be present in the nearby highlands of the Moon. As the highlands make up more than 85% of the surface of the Moon, it was postulated that the crust of the Moon formed early in its history by global melting, an episode termed the “magma ocean.”
Expecting only minor volcanic activity and perhaps a local igneous intrusion, the concept of a global ocean of magma was surprising to most scientists. Given its small size and consequent paucity of radioactive heat-producing elements, the idea that most of the Moon might have melted and differentiated was astounding. The existence of an early magma ocean, which implied high-energy processes, provided us with clues to lunar origin. Once it was recognized that the Moon had a crust, it was important to gain an understanding of its composition and physical nature.
On subsequent missions, Apollo astronauts were tasked with laying out a series of seismic stations across the near side. These stations allowed us to measure “moonquakes” – both natural events as well as those created artificially by slamming spent rocket stages and satellites into the Moon. Seismic recording allowed us to infer the speed at which seismic waves traveled through the lunar interior. These estimated speeds indicated densities that implied composition, allowing us to deduce the probable chemical and mineral composition of the lunar interior.
The Apollo seismic network indicated that the crust of the Moon was about 50-60 km thick in the central near side, a surprisingly large value, especially compared to the thickness of the crust of the Earth (which varies from as thin as 5-10 km under the ocean basins to over 30 km in continental areas). Such a thick crust for the Moon led to the postulation of a global magma ocean, as so much anorthosite could only be produced under the conditions of near global melting. Subsequent studies incorporating gravity data from Lunar Orbiter and other missions suggested that the lunar crust is variable in thickness, with values exceeding 100 km in some regions of the far side highlands.
Re-analysis of the Apollo seismic data gave the first indication that those values might be overestimated. Using modern techniques on these old data, new analysis revealed that the crust might be thinner than we had originally thought, on the order of 40-50 km thick. This lower value of crustal thickness had some implications for estimating the bulk chemical composition of the Moon, but because it was considered to be a relatively minor adjustment, it caused no major difficulties for the rest of lunar science.
However, the recent GRAIL mission to the Moon (using high precision gravity mapping) ascertained the thickness of its crust to be 34-43 km. Why should this new value worry some scientists? Because we are now entering realms in which the new estimates of crustal thickness create consistency problems for other aspects of lunar science. A crust as thin as 35 km on the near side of the Moon implies that the largest impacts – the multi-ring basins – should have excavated considerable amounts of material from the layer below the crust, the mantle of the Moon. One might object that, as this region of the interior is inaccessible, we don’t know what the mantle would look like. But in fact, the density constraints imposed by the seismic and gravity data dictate that it must be a rock type rich in iron and magnesium, made up mostly of the minerals olivine and pyroxene. Such rocks are not unknown in the lunar collections, but they possess chemical and mineralogical characteristics indicating their origins at much shallower (crustal) depths. In other words, there does not appear to be any material from the lunar mantle in the Apollo collections. Given our obviously incomplete sampling of the Moon, should this be a problem?
Several Apollo landing sites (e.g., Apollo 14 and 15) were specifically chosen to maximize the chances of sampling ejecta from the enormous 1100 km diameter Imbrium basin (one of the biggest impact features on the Moon). Virtually any reconstruction of the dimensions of the excavation cavity of this basin indicates that it should have dug up material from tens of kilometers depth, much deeper than the new value of crustal thickness implied by the GRAIL data. So where is this debris from the mantle of the Moon? True enough, it is possible that it may have been missed during the limited exploration time available to the Apollo crews, but the astronauts were trained to recognize such rocks and none were found. Additionally, because we can map rock types by remote sensing (both from spacecraft and from Earth), we have an understanding of the regional distribution of rocks around these large impact features. Despite a 30-year, exhaustively detailed search of the Imbrium impact basin (an area larger than Texas), we have found no convincing evidence for mantle material on the surface of the Moon.
So where does this leave us? In science, new data can solve some problems but at the same time, it may also create new ones. Modern analyses of the old seismic data and new information on the Moon’s gravity field both suggest a relatively thin crust, with mantle material being very close to the surface (a few km) in some areas. On the other hand, none of the ubiquitous impact basins and large craters of the Moon show evidence for mantle material in their ejecta, either in the Apollo collections or in remote sensing data. Could our understanding of impact mechanics be completely wrong? Or are we misunderstanding the new gravity data? How could an event that formed an impact crater thousands of kilometers across excavate only a few kilometers deep?
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