October 30, 2013
The Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft is currently circling the Moon. With the spacecraft safely settled into its observation orbit, the mission science team is busy testing and calibrating its instruments. This U.S. mission was designed to characterize the lunar “atmosphere” – the extremely tenuous zone of gases that vary in time in the space above the Moon. Technically called an exosphere, this region contains extremely low concentrations of a variety of elements and compounds, of varied origins and a largely unknown life cycle. LADEE is designed to monitor and characterize these species, with the goal of identifying the process and sources of the gases and how they vary with time.
Initially a precursor to human lunar return, LADEE was selected for development early in 2008, as we wanted to understand the lunar exosphere before the lunar environment was contaminated by humans. The LADEE spacecraft is designed to observe the Moon in its natural, pristine state. However, the very act of going to the Moon inadvertently (though briefly) modifies the lunar atmosphere. When a spacecraft arrives at the Moon, it uses its on-board rocket engines to brake into lunar orbit or to descend to the surface. These rockets spew large quantities of exhaust gas into space and as the vehicles get captured into the Moon’s gravity field, so too does this exhaust product.
From estimates drawn on the Apollo landings, the rocket exhaust expended from each Lunar Module temporarily doubled the total mass of the natural lunar atmosphere. This artificial addition of gases eventually dissipates, driven off by solar interactions and other complex effects. In time, the Moon resumes its normal state of near-vacuum. The creation of a temporary artificial atmosphere created by rocket effluent and its subsequent dissipation is imperfectly understood, except to the extent that we know that it happens. The one-month “commissioning phase” that the LADEE mission is currently experiencing was largely designed to ensure that the exhaust from the orbital braking burn of the spacecraft (and subsequent low-rate out-gassing from the spacecraft) is largely complete. We want to measure the Moon’s environment, not the products of the craft that brought us there.
But the U.S. will not be the only one conducting a mission at the Moon for the next few months. The long-planned Chinese robotic mission Chang’E 3 is scheduled for launch to the Moon in early December. Their lander mission will place a fairly large (1200 kg) spacecraft on Sinus Iridum in the northwestern quadrant of the near side, deliver a small roving vehicle and examine and measure the properties of the lunar surface over the course of several months. But before it begins its surface mission, the Chang’E 3 spacecraft will burn roughly 2600 kg of rocket fuel in the vicinity of the Moon’s exosphere. I have not seen any documentation on the fuel this spacecraft will use, but it is highly likely that it will be the chemicals unsymmetrical dimethylhydrazine (UDMH; H2NN(CH3)2) and nitrogen tetroxide (N2O4). These propellants are widely used in spacecraft because they are liquid at room temperature and can be easily stored in tanks for long periods of time (a requirement for long-duration spaceflight to destinations beyond low Earth orbit).
When UDMH and nitrogen tetroxide are burned in a rocket engine, they produce a variety of exhaust gases; the dominant combustion products are water (H2O), nitrogen (N2), carbon monoxide (CO), carbon dioxide (CO2), and a few trace species, including hydrogen (H2) and hydroxyl (OH). Expelled by a rocket nozzle, these gases rapidly expand in all directions in the vacuum of space. Because most of the burn occurs after the spacecraft has been “captured” by the gravity of the Moon, this rocket exhaust is also captured by the Moon. Thus, exhaust from an orbital or a landing vehicle becomes (temporarily) part of the lunar atmosphere.
If you’re thinking that this “rude” addition of alien gases will mess up the very delicate phenomena that LADEE was designed to map and measure, you’re correct – it does. You might even expect the scientists of the LADEE team would be very upset at this disruption of their carefully planned measurement strategy. But you would be wrong. This problem is actually an opportunity.
The coincidence of Chang’E 3 arriving at the Moon after LADEE has begun observations has developed into a serendipitous occurrence for lunar science. Because we don’t understand very well how exospheric gases are added to and removed from the Moon, what has landed in our laps is an unplanned (but controlled) experiment. A known quantity of gases – of known composition – will be added to the lunar atmosphere at a precisely known time, in a precisely known place. One could have not designed a better experiment to measure how this addition of material is distributed, how its distribution evolves over time, and how these expelled gases dissipate into cislunar space. Even better, LADEE will have almost a full month to monitor and characterize the lunar atmosphere before Chang’E arrives, thus allowing us to first observe the “natural” Moon and then the “contaminated” Moon and how the lunar atmosphere recovers from its defilement.
None of this was prearranged – the Chinese schedule their missions on the basis of their own time-table and programmatic needs (just as NASA’s lunar goals have changed over the last 5 years). But because of a fortuitous alignment of schedules, we have a unique opportunity to observe in real time how the Moon works. Hopefully, the Chinese will provide us with detailed mass numbers of their spacecraft and exactly what variety of fuel it carries, but even if they don’t, physics dictates a certain mass and volume of the exhaust gas and its composition will be measured by LADEE (allowing us to know the type of fuel used). China’s December lander mission to the Moon will provide our U.S. mission with a welcome bit of “traffic exhaust,” giving scientists the opportunity to learn more from LADEE than we’d originally envisioned.
October 4, 2013
If all goes according to plan in the next few days, the latest NASA robotic mission to the Moon will enter lunar orbit. Launched last month from the Wallops Island site, LADEE (for Lunar Atmosphere and Dust Environment Explorer) will spend the next few months orbiting the Moon. This small spacecraft will attempt to characterize and measure the lunar “atmosphere,” while also looking for dust that might be electrostatically levitated above the surface or thrown into ballistic flight by impacts.
Wait a minute. Did I say “atmosphere?” Isn’t the Moon renowned for its lack of an atmosphere? Indeed it is. In fact, the 10-12 torr surface pressure of the Moon is a better vacuum than we can achieve with even the most advanced equipment in Earth laboratories. (For comparison, sea level pressure on the Earth is about 760 torr, making the lunar surface pressure over one hundred trillion times less dense.) A better term for the tenuous gas near the Moon is “exosphere,” meaning free flying gas molecules that may or may not be gravitationally bound to the Moon. In such an “atmosphere,” there may be only a few thousand molecules in a cubic centimeter of space. This is very tenuous indeed.
LADEE is designed to investigate from where these atoms and molecules come. Presently, we think the lunar exosphere consists mostly of helium, sodium and perhaps argon atoms, each coming from a completely different source. Helium likely comes from the Sun, as the solar wind continually “breathes” onto the surface of the Moon. Some atoms stick to surface dust grains but many simply bounce off, randomly moving in the space above the lunar surface. Easy to detect, lunar sodium has been observed from Earth-based telescopes. It most likely comes from rocks vaporized by the continual rain of micrometeorites. At least some fraction of this vaporous sodium must hang around the surface, unable to escape the Moon. Argon might have a solar wind origin, but at least some of it comes from the natural decay of radioactive potassium in the lunar interior (potassium-40 (40K) decays to argon-40 (40Ar) with a half-life of a bit more than one billion years). Gases like argon, venting from the interior of the Moon, were observed by subsatellites left in lunar orbit by the departing Apollo spacecraft over 40 years ago (these small spacecraft have long since crashed into the Moon).
Although helium, sodium and argon are the principal expected components of the lunar exosphere, the LADEE team will search for other species. An interesting possibility is water (H2O) or its related species, hydroxyl (OH). One of the most surprising results of recent lunar exploration was the discovery of adsorbed (surface) water and hydroxyl on the dust grains of the lunar surface (observed by the Moon Mineralogy Mapper (M3) aboard the Indian Chandrayaan-1 lunar orbiter in 2009). Occurring in the form of a monolayer of molecules on dust grains in the cooler portions of the Moon, a clear water signal is best seen above latitudes of 65°, increasing in strength (i.e., increasing water abundance) toward each pole.
The surprise from M3 was not only the presence of water but observing that its abundance increases with decreasing surface temperatures. This means that water being made or deposited on the surface is in motion, with a net movement toward the poles. The same Chandrayaan-1 spacecraft also carried an impact probe with a mass spectrometer. During the probe’s half-hour descent to the South Pole, it passed through a cloud of water in space, just above the lunar surface. The water cloud at this high latitude had a density a hundred times higher than at the equator, providing additional evidence that exospheric water is in motion, moving from lower, hotter latitudes towards higher, cooler ones.
LADEE cannot directly measure this water in a neutral state, but if some process ionizes it (e.g., if a water molecule breaks apart into a proton and a hydroxyl by UV radiation from the Sun), it will be visible to the ultraviolet spectrometer aboard the spacecraft. If the process of water migration on the lunar surface is correct, we should be able to observe exospheric water and by measuring its density with time, track the water migration to higher latitudes.
LADEE will also tackle another controversial issue – the amounts and mechanisms of dust movement on and around the Moon. During the unmanned Surveyor lander missions over 50 years ago, a strange illumination or glow was observed by television for several hours after local sunset, just above the horizon. This phenomenon was termed “horizon glow” by surprised Surveyor investigators. At a loss to explain it, the team postulated that some mechanism was lofting dust up above the surface and this dust was scattering sunlight. Exactly how the dust was lofted was uncertain; some thought it must be fragments in ballistic flight from distant impacts, while others thought that it might be levitated by electrostatic force, thus “hovering” above the surface.
A few years later, just before his orbiting spacecraft emerged into the daylight side of the Moon, Apollo 17 Commander Gene Cernan observed and sketched an illuminated limb and “streamers” that could be seen extending into space above where the lunar horizon would be. At the time, this phenomenon was thought to be the same as that seen in the Surveyor pictures, although they have totally different scales (the Surveyor horizon glow must occur within a few meters of the surface, while Cernan’s horizon glow extended many kilometers above the Moon). Dust (probably of lunar provenance) is certainly involved in whatever causes this horizon glow.
As the Moon slowly rotates once every 708 hours, the line between the sunlit and dark hemispheres (the terminator) slowly moves across the lunar surface. The day and night hemispheres have different fluxes of electrons from the solar wind and thus, the presence of the terminator can induce an electrical charge in surface materials. It is postulated that this charge might levitate smaller dust particles such that they would hover above the surface. LADEE will attempt to detect and map this dust, both by searching for scattered sunlight with its ultraviolet spectrometer and via the direct detection of dust particles in flight with an instrument on the top of the orbiting spacecraft.
The issue of levitated dust is thought to be relevant to the future habitation of the Moon. If dust is lofted above the surface by the passage of the terminator, the particles could degrade clean surfaces and create a hazard for inhabitants of the Moon. Such a process could have major effects near the poles of the Moon, areas that are in the near-constant presence of a day-night terminator. Although it is unlikely that levitated dust on the Moon is an environmental hazard, we currently are working in near total absence of hard data. Thus, it makes sense to at least try to make some direct measurements of the dust environment around the Moon to assess the importance of this proposed surface process.
LADEE arrives in lunar orbit this Sunday. We wish it well on its mission to give us fresh (and welcome) data on a poorly understood aspect of lunar processes and history.
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
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