November 9, 2013
Should those on Earth control and restrict the use of off-Earth real estate or should people use and profit from what they find in space? We have conducted reconnaissance and mapping of celestial bodies for centuries using telescopes, orbital and landing spacecraft, and (forty years ago) explored it with people. Earth’s scientists have studied the returned data and we’ve dreamed of returning to the Moon and to new places where humanity has never set foot. Entrepreneurs and social engineers see a time in the near future when we will make that next step and they each hold somewhat different views — some want to develop and capitalize on their investment, some want to preserve and permit only limited access.
In a recent Popular Science article, Veronique Greenwood argues for having the Moon declared an “International Park – an off-World Heritage site.” And not just the Apollo sites but all 14 million square miles of the lunar surface. Greenwood likes the legal model of Antarctica, an entire continent that the nations of the world agreed to not develop but use solely for scientific study. Understanding that profit motives will be behind the drive to the Moon, she allows there may be carve-outs for mining (after environmental impact studies) but legally, the Moon will be protected as a preserve for history and science, serving as the template for human expansion beyond the Moon. She doesn’t want it “damaged.”
Greenwood’s concerns stem from her belief that humans (even when they’re careful) “tromp all over things” and that without government preservation and oversight, cultural artifacts on the Moon (such as the Apollo 11 crew’s “One Small Step” footprints and various “important craters”) are in danger of destruction. She argues that “because the Moon was part of Earth until 4.5 billion years ago” (a proposition not yet established), the United Nations should have legal sovereignty over its use and disposition. She notes that the 1979 Moon Treaty was never ratified (“flopped spectacularly”), a presumed “victim of the Cold War era.” In fact, the treaty’s “flop” had nothing to do with the Cold War – a concerted lobbying effort by various space advocacy groups (such as the L-5 Society) was largely responsible for the Senate’s refusal to ratify it. No nation that had space faring capability at that time ratified the Moon Treaty.
Her article illustrates that the “green” anti-development worldview has expanded to include opposition to unfettered space utilization. Because we’re not dealing with anything green, I suggest that we dub the lunar environmentalists “Grays.” Stemming from their belief that humans are harming the Earth, the Grays fear that it is not right to allow unrestricted access and development of the Moon. Fifty years after those interloping Apollo astronauts tromped on, drove over and kicked up a lot of dust on the Moon, a more enlightened humanity will return to peacefully – and carefully – explore its surface and, in the words of the National Park Service, “Taking only photographs, leaving only footprints.” If environmental impact studies allow it, some limited mining activity might be permitted, presumably to pay for these Luna Park overseers.
The analogy to Antarctica, beloved of academics, is of limited value in this instance. The reason nations of the world do not bother to mine or drill for oil in Antarctica is that there are alternative and cheaper sources of oil and minerals that do not require the costly build up of infrastructure in that challenging environment. Such is not true for the Moon; the alternative to using the resources of the Moon is to bring everything you need with you from the deep gravity well of the Earth. With launch costs of thousands of dollars per pound (and unlikely to come down significantly for the foreseeable future), it makes good sense to look for and obtain as much of the required “dumb mass” (i.e., air, water, shielding and propellant) needed for extended presence from “local” sources – the extraterrestrial bodies themselves. Launch from Earth should be reserved only for high information density items – high-technology equipment, instruments and people. The raw materials of space will provision us – and we need to learn how to do it out there, starting with the Moon. You cannot lock up new territory and then expect entrepreneurs to invest their capital in getting you there.
While Greenwood uses Antarctica as a model for the Moon, in my mind, a better analogy is Alaska, a vast area (656,424 square miles) of great natural beauty and abundant resources. Alaska serves a multitude of purposes, including mining, fishing, oil and gas production, tourism, recreation and settlement, as well as maintaining and protecting vast reserves of national and state wilderness. No one could call Alaska a decimated paradise or an industrial wasteland – it is an immense landscape with room for every imagined activity, commercial and non-commercial. It is a harsh place, yet one where self-reliant humans migrated for profit, play and its wide-open spaces. It also has the virtue of being part of a self-governing republic, not an “administrative area” controlled by international bureaucrats. And yet, even though the land has been developed and used, the people have conserved, protected and managed the landscape and resources of the state. But Greenwood points to the Antarctica “peaceful and scientific use of” model, whereby the U.N. would own and control the Moon, thereby setting a precedent for the rest of the Solar System. Talk about throwing cold water on pioneering outer space! Greenwood’s suggestions certainly do that.
Setting aside the obvious objection that the United Nations has not shown any particular management capability (nor does it possess the ability to oversee natural resources 250,000 miles from Earth), a more important objection to this proposal is the negative impact it will have on investment toward the development and support of commercial space activity. If advocates of commercial spaceflight think dealing with the federal government is difficult, they haven’t seen anything until they start dealing with a U.N. authority. Greenwood wants “important craters” protected from defacement by ATVs, but that begs the question as to who decides which craters are “important,” what needs to be protected, and who gets those limited mining rights? Would she leave these environmental assessments and commercial allocation judgments in the hands of U.N. decision makers and arbitrators?
The basic problem with the attitude of the Grays is that it is misdirected. There is no “ecology” to preserve on the Moon because there is no life there. The only thing that can be preserved is the Moon’s pristine state – an ancient surface unsullied by the tread of endless footprints. It would take tens of thousands of years, if then, (since few would live on the Moon) to put a footprint on every square meter of the lunar surface, an area greater than the continent of Africa. Even the most rare and valuable terrains on the Moon – the water-containing areas near the poles – are enormous regions, hundreds of square kilometers in extent, containing tens of billions of tons of water ice and other valuable deposits. As these materials are the most accessible and useful products in near Earth space, they are crucial to the creation of new space faring capability.
If the entire territory of the Moon is designated the property of Earth with U.N. oversight, we will handicap ourselves from becoming a space faring species. We must learn how to use what we find in space to create new capabilities. Even the most ardent developers would not object to preserving the historical sites of the first impacts of spacecraft on the Moon (Luna 2), the first soft-landers (Luna 9 and Surveyor 1), and of course, the site of the first human landing on another world (Apollo 11). But the rest of the Moon should be open to exploration, development and use. It is wrong to restrict the use and development of whole new worlds in order to assuage the overly emotional and misguided aesthetic sensibilities of the Grays, as opposed to opening up of a frontier that can be profitably used and enjoyed for the benefit of all humanity.
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.
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.
April 8, 2013
Though unremarkable in appearance compared to the roughly 4,000 craters on the Moon in its size range, the 20 km diameter crater Shackleton has been the source of relentless scientific controversy for the past 20 years. Shackleton is located at the south pole of the Moon; indeed, its near side rim is the precise location of the geographic pole itself. Its location makes observation by Earth-based telescopes difficult and it was not well photographed by the Lunar Orbiter series (our principal source of lunar images) of the 1960s. That all changed in 1994 with the flight of the joint DoD-NASA mission to the Moon, Clementine.
Clementine carried cameras that globally imaged the Moon in eleven visible and near-infrared wavelengths. In addition, it mapped the surface and lighting of the poles of the Moon at uniform resolution over the course of almost three lunar days (74 Earth days). When the Science Team first saw the south polar mosaic, the extent of darkness in the map was striking. Because the Moon’s spin axis is close to perpendicular to the ecliptic plane, the Sun is always at the horizon at the lunar poles. Instead of rising and setting, the Sun circles around the poles at or near the horizon. Because of this grazing incidence, an area in a topographic depression may be in permanent shadow. And so it appeared for Shackleton crater in the Clementine data, setting off bells in the heads of the Science Team.
A key controversy of the post-Apollo era was whether the lunar poles might contain water or not. Although the Apollo samples had been studied and found to be “bone-dry,” we had not been to the poles on any Apollo mission. We knew that any shadowed areas had to be extremely cold as well as permanently dark. As water-bearing debris in the form of asteroids and comets constantly strike the Moon, it was thought that some of that water might get into a polar “cold trap” and would be kept there (essentially) forever – billions of years of impacting cosmic “debris” can add up.
Clementine was not configured to measure the presence of water, but a cleverly improvised experiment used the spacecraft’s data transmitter to beam radio waves into the dark regions near the poles and listen to their reflected echoes on the enormous (70 m) dish antenna of NASA’s Deep Space Network. Interestingly, the reflections indicated an enhancement of “same sense” polarization within the (very large) resolution cell that contained Shackleton crater. A collect of data from a nearby sunlit area (taken as an experimental control) did not show this peak. The Clementine team interpreted the RF peak as evidence for the presence of a few percent water ice within the dark, cold interior of Shackleton crater. The media quickly spread the startling news about water on our “bone-dry” Moon.
Such a controversial conclusion did not go unchallenged. Some in the radar community argued that abundant wavelength-sized rocks on the surface were the source of the enhanced same sense reflection. Since the lunar surface is indeed rocky, this interpretation could not be ruled out. Then a few years later, the Lunar Prospector (LP) mission found an enhancement of hydrogen concentration at both poles of the Moon; as hydrogen is a major constituent of water, the idea ice exists in the dark areas gained credence and has lead to a decade-long scientific search (using a variety of techniques) for lunar polar ice. Though many areas near the poles were studied in detail, attention continued to be drawn back to Shackleton and the area near the south pole.
From studying Clementine images, we discovered that part of the rim crest of Shackleton is one of the most sunlit areas on the Moon. Now we had a double-attraction: constant sunlight with water ice nearby. At a press briefing in 1996, I called this area of water and sunlight “the most valuable piece of real estate in the Solar System.” Nothing found subsequently has changed my mind on that judgment.
So what have we learned about Shackleton lately? Many different, new sensors have flown to the Moon in the last few years, including radar, ultraviolet (UV) imaging, laser reflections, and low-light level imaging. And yet again, Shackleton crater continues to confound us with contradictory evidence, both for and against the presence of water ice in its interior.
In 2009, the question regarding the presence of water ice somewhere near the lunar south pole was answered when the LCROSS impactor threw up a cloud of water vapor and ice particles during its collision with the floor of the nearby crater Cabaeus. Spectral mapping instruments on three different spacecraft (Chandrayaan-1, Cassini, and EPOXI) documented the presence of adsorbed water on the lunar surface, increasing in concentration with latitude toward both poles. A small impact probe flown by India (MIP) passed through a water vapor zone in the exosphere just above the lunar south pole. And radar images from Mini-RF, our radar imaging experiment on both Chandrayaan-1 and Lunar Reconnaissance Orbiter (LRO), found evidence of high same sense reflections (just as Clementine had suggested in 1994) within the interior of Shackleton crater. These new lines of supporting evidence were countered by Japanese researchers, whose Kaguya spacecraft imaged the interior of the crater and found morphology similar to other lunar craters in the same size-class. But no one had ever claimed that the interior of Shackleton was a skating rink of pure ice – the lunar polar ice is partly covered by waterless dust and mixed with an unknown amount of dry regolith.
Interpretation of the new data continues to vex us. The LOLA (laser altimeter) team on LRO recently published a paper that documents the high reflectivity (at 1 micron wavelength) of the walls of Shackleton. Although the team’s favored interpretation is that this is caused by a constant exposure of fresh material on a steep slope, they also note that it is consistent with the presence of water ice on the walls of the crater. In addition, a team analyzing neutron spectrometer data from both LP and LRO found evidence in the fast neutron data (never before analyzed) that water in the interior of Shackleton is a possible explanation for its signal. Detailed analysis of the Mini-RF data for Shackleton corrected for its steep wall slopes and found that the presence of 5-10 wt.% water there provides the best model fit to the observed data. Newly obtained UV images from LRO show the existence of water frost in the interiors of the craters Haworth and Shackleton, and the neutron detector on LRO shows enhanced hydrogen within both Shoemaker and Shackleton craters. The Japanese team from Kaguya continue to insist that the no-ice interpretation is the correct one.
So we are left with a mystery. Some evidence is pro-ice and some is contra-ice. I find it interesting that for most of the investigators, new data does not necessarily change any minds, but tends to be interpreted in a way most favorable to their previously published ideas. This should not be terribly surprising; the people who have argued for some specific interpretation presumably did so for good reasons and desire hard and clear-cut evidence to the contrary before abandoning a previously held position, one no doubt reached after much thought and soul-searching.
The way to unravel the water-ice mystery is to go to the surface of the lunar south pole (or both poles) and measure the composition of the surfaces in question. Getting a definitive answer about the nature of lunar water would be game changing. Some say the bigger mystery is: Why hasn’t the United States sent a rover to the south pole of the Moon to take a closer look?
December 2, 2012
Mercury – the planet, not the element – was in the news this past week. For some time, we had suspected that the poles of Mercury might harbor deposits of water ice. This – on a planet so close to the Sun that the surface temperature at the equator is hot enough to melt lead!
Yet like the Moon, Mercury’s spin axis is perpendicular to the plane in which it orbits the Sun. This means that large craters near Mercury’s poles lie in permanent shadow (“shivering” around -170° C), unaffected by the Sun’s searing heat (equivalent to more than eleven times the solar flux we get on Earth). As on the Moon, these permanently shadowed areas get heat from only two sources – the 3 K background heat of space, created during the Big Bang some 15 billion years ago, and whatever heat is being generated now from the deep interior (a quantity that geophysicists call the heat flow of a planet).
Large planets (like Earth) generate heat mostly from the decay of radioactive elements deep inside them. This heat is lost largely through the phenomenon of volcanism, in which melted rock from the interior is erupted onto a planet’s surface as lava and ash. Smaller planets and moons likewise experience this heating and volcanism, but because they are have lower overall contents of heat-producing elements, their volcanic episodes occurred in the distant past. Much of the heat of these smaller planets has been largely dissipated. Thus, on Mercury, we suspect that the overall heat flow is very low, resulting in extremely cold temperatures on the floors of its permanently shaded polar craters.
For many years, astronomers have studied Mercury with radio telescopes from Earth (using radar to make images of its surface). Because the orbital inclination of Mercury is relatively high (about 7°), we can get a fairly good look into the interiors of the polar craters. Interestingly, even though Mercury is much farther away than the Moon, we can see more of the mercurian polar areas because of this relatively high orbital inclination (the Moon’s orbital plane is inclined only 5°). These radar pictures showed an amazing and unexpected feature – the dark areas are filled with material that is highly reflective at radio frequencies, properties similar to the surfaces of the icy moons of Jupiter (Europa, Ganymede and Callisto).
These results were so unexpected and startling that debate raged for many years whether these deposits really were what they appeared to be: water ice. Facts are stubborn things and few materials have radio properties similar to ice. Some suggested that sulfur might be an alternative explanation, but provided little evidence for such behavior. Moreover, another moon of Jupiter, Io, which has a surface largely composed of sulfur, does not show the radar brightness or “glint” seen on the other, ice-rich Jovian moons.
The debate on the nature of the Mercury polar deposits has now been settled with the release of new data from the MESSENGER mission. Launched on August 3, 2004, with insertion into obit around the planet on March 18, 2011, the spacecraft has been taking pictures and making measurements of Mercury for the last two years. We have mapped the extent of darkness near the poles, measured the temperatures of the surface inside these regions, and detected the presence of significant amounts of hydrogen there. All of these results are strongly supportive of the water ice interpretation.
The existence of ice near the poles of Mercury supports the case for water ice on our own Moon, although there are some significant differences between the two occurrences. Like Mercury, the Moon’s spin axis is nearly perpendicular to the plane of its orbit around the Sun. The similarity of the terrain of both bodies results in deep holes that hide large expanses of terrain from the glare and heat of the Sun. Both objects have been volcanically active in the past, but not today, meaning that the average rates of heat flow on both are low. These properties result in the creation of polar “cold traps” in which any entering volatile substance (such as water molecules) cannot escape.
The solid bodies of the inner Solar System are constantly hit by debris from comets and asteroids. This material contains water, both in free form and bound within hydrous minerals. On smaller objects (like the Moon and Mercury), most of this water is lost to space, but we suspected that some of it might be retained within these dark cold traps near the poles. Now we know that such a process does occur.
Differences between the Moon and Mercury result in differing amounts and settings for their polar deposits. Being much closer to the Sun, one might expect Mercury to contain less water ice, but a variety of evidence suggests that the opposite is the case. The polar ice of Mercury appears to be greater in extent and thickness than comparable deposits on the Moon. This probably results from two factors. First, Mercury is a bigger object, with a surface gravity about twice that of the Moon. Thus, it is more difficult for water to “escape” from Mercury. Second, the closeness of Mercury to the Sun (the edge of biggest gravity well of the Solar System) results in a higher flux of cometary impacts there than experienced in the Earth-Moon system. So more water is being added to Mercury, where it is more easily retained.
Nonetheless, both Moon and Mercury have similar polar environments and processes. The long debate – a scientific controversy for over 50 years – about water at the poles of these objects has been resolved. The next steps will be to characterize these deposits in situ using a soft lander and selected instruments to measure the amounts, states and distributions of water in the polar areas. Because of the great difficulty in even getting into orbit around Mercury (let alone landing there), doing this first on the Moon will mostly likely happen first. So, here again is another rationale for sending a robotic surveying lander and rover mission to the poles of the Moon – in addition to characterizing these areas for our future presence there, by inference, we will also learn about the polar processes on and environment of Mercury.
A planetary “two-fer.” Let’s get on with it.
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