Is there ice on the Moon?
On 5 March 1998 it was announced that data returned by the Lunar Prospector spacecraft indicated that water ice might be present at both the north and south lunar poles, in agreement with Clementine results for the south pole reported in November 1996. Later work has called this interpretation into question, so the issue is still unresolved. The ice originally appeared to be mixed in with the lunar regolith (surface rocks, soil, and dust) at low concentrations conservatively estimated at 0.3 to 1 percent. Subsequent data from Lunar Prospector taken over a longer period has indicated the possible presence of discrete, confined, near-pure water ice deposits buried beneath as much as 18 inches (40 centimeters) of dry regolith, with the water signature being stronger at the Moon's north pole than at the south (1). The ice was thought to be spread over 10,000 to 50,000 square km (3,600 to 18,000 square miles) of area near the north pole and 5,000 to 20,000 square km (1,800 to 7,200 square miles) around the south pole, but the latest results show the water may be more concentrated in localized areas (roughly 1850 square km, or 650 square miles, at each pole) rather than being spread out over these large regions. The estimated total mass of ice is 6 trillion kg (6.6 billion tons). Uncertainties in the models mean this estimate could be off considerably.
How was the ice detected?
The Lunar Prospector, a NASA Discovery mission, was launched into lunar orbit in January 1998. Included on Lunar Prospector is an experiment called the Neutron Spectrometer. This experiment is designed to detect minute amounts of water ice at a level of less than 0.01%. The instrument concentrated on areas near the lunar poles where it was thought these water ice deposits might be found. The Neutron Spectrometer looks for so-called "slow" (or thermal) and "intermediate" (or epithermal) neutrons which result from collisions of normal "fast" neutrons with hydrogen atoms. A significant amount of hydrogen would indicate the existence of water. The data show a distinctive 4.6 percent signature over the north polar region and a 3.0 percent signature over the south, a strong indication that water is present in both these areas. The instrument can detect water to a depth of about half a meter.
How can ice survive on the Moon?
The Moon has no atmosphere, any substance on the lunar surface is exposed directly to vacuum. For water ice, this means it will rapidly sublime directly into water vapor and escape into space, as the Moon's low gravity cannot hold gas for any appreciable time. Over the course of a lunar day (~29 Earth days), all regions of the Moon are exposed to sunlight, and the temperature on the Moon in direct sunlight reaches about 395 K (395 Kelvin, which is equal to about 250 degrees above zero F). So any ice exposed to sunlight for even a short time would be lost. The only possible way for ice to exist on the Moon would be in a permanently shadowed area.
The Clementine imaging experiment showed that such permanently shadowed areas do exist in the bottom of deep craters near the Moon's south pole. In fact, it appears that approximately 6000 to 15,000 square kilometers (2300 to 5800 square miles) of area around the south pole is permanently shadowed. The permanently shadowed area near the north pole appears on Clementine images to be considerably less, but the Lunar Prospector results show a much larger water-bearing area at the north pole. Much of the area around the south pole is within the South Pole-Aitken Basin (shown at left in blue on a lunar topography image), a giant impact crater 2500 km (1550 miles) in diameter and 12 km deep at its lowest point. Many smaller craters exist on the floor of this basin. Since they are down in this basin, the floors of many of these craters are never exposed to sunlight. Within these craters the temperatures would never rise above about 100 K (280 degrees below zero F) . Any water ice at the bottom of the crater could probably exist for billions of years at these temperatures.
Where did the ice come from?
The Moon's surface is continuously bombarded by meteorites and micrometeorites. Many, if not most, of these impactors contain water ice, and the lunar craters show that many of these were very large objects. Any ice which survived impact would be scattered over the lunar surface. Most would be quickly vaporized by sunlight and lost to space, but some would end up inside the permanently shadowed craters, either by directly entering the crater or migrating over the surface as randomly moving individual molecules which would reach the craters and freeze there. Once inside the crater, the ice would be relatively stable, so over time the ice would collect in these "cold traps", and be buried to some extent by meteoritic gardening. Such a possibility was suggested as early as 1961 . However, loss of ice due to photodissociation, solar wind sputtering, and micrometeoroid gardening is not well quantified .
Is there any other evidence for ice?
In a Science magazine article on 29 November 1996, it was announced that interpretation of data from a Clementine spacecraft experiment suggested the possibility of ice on the surface of the Moon. The ice was believed to be in the bottom of a permanently shadowed crater near the Moon's south pole (at the center of the Clementine mosaic shown at the top of the page). It was also thought likely that other frozen volatiles, such as methane, were in the deposit. The deposit was estimated to be approximately 60,000 to 120,000 cubic meters in volume. This would be comparable to a small lake in size, four football fields in surface area and 16 feet deep. This estimate was very uncertain, however, due to the nature of the data.
One of the problems in studying a permanently shadowed area is that no pictures can be obtained. The Clementine spacecraft searched for the ice using an investigation known as the Bistatic Radar Experiment. Basically, this experiment consisted of having the Clementine spacecraft transmit an S-band radio signal through its high gain antenna towards a lunar target. The signals reflected off the Moon and were received by a 70 meter Deep Space Network (DSN) antenna on the Earth. Frozen volatiles such as water ice are much more reflective to S-band radio waves than lunar rocks. Radio waves also have different characteristics when reflected off ice than off silicate rock. An analysis of the signals returned from orbit 234 showed reflection characteristics suggestive of water ice for the permanently shadowed areas near the south pole. Reflections from regions which are not permanently shadowed do not show these characteristics. It is possible that other scattering mechanisms could be responsible for this result, but the interpretation of the radio returns and the fact that they are associated only with the permanently shadowed regions seem to indicate that water ice is the most likely possibility. However, Arecibo radio telescope studies using the same radio frequency as Clementine showed similar reflection patterns from areas which are not permanently shadowed. These reflections have been interpreted as being due to rough surfaces, and it was suggested that the Clementine results may have been due to roughness, rather than water ice, as well.
Bistatic Radar Experiment Parameters
9-10 April 1994
Transmission: S-Band 2.273 GHz (13.19 cm wavelength)
Polarization: Right Circular (RCP)
Signal Power: 6 Watts
Axial Tilt: 4.5 to 5.5 degrees (Moon to Earth)
Orbits Used: 234 and 235
Why is ice on the Moon important?
The ice could represent relatively pristine cometary or asteroid material which has existed on the Moon for millions or billions of years. A robotic sample return mission could bring ice back to Earth for study, perhaps followed by a human mission for more detailed sampling. The simple fact that the ice is there will help scientists constrain models of impacts on the lunar surface and the effects of meteorite gardening, photodissociation, and solar wind sputtering on the Moon. Beyond the scientifically intriguing aspects, deposits of ice on the Moon would have many practical aspects for future manned lunar exploration. There is no other source of water on the Moon, and shipping water to the Moon for use by humans would be extremely expensive ($2,000 to $20,000 per kg). The lunar water could also serve as a source of oxygen, another vital material not readily found on the Moon, and hydrogen, which could be used as rocket fuel. Paul Spudis, one of the scientists who took part in the Clementine study, referred to the lunar ice deposit as possibly "the most valuable piece of real estate in the solar system". It appears that in addition to the permanently shadowed areas there are some higher areas such as crater rims which are permanently exposed to sunlight and could serve as a source of power for future missions.
In contrast to some recent claims, this debate is still open and nothing has occurred in the last few years to cause participants in the debate to abandon their positions. In a nutshell, poor or incomplete coverage by a variety of marginal data has led to much heat, while casting little light on the issue of lunar polar water. Here, I present the evidence to the reader, noting the strengths and weaknesses of each data set, and attempt to identify the remaining unanswered questions.
Clementine bistatic radar. As the Clementine spacecraft orbited the Moon, it transmitted radio waves toward the poles and we listened to the reflected radio waves bounced back to Earth. This experiment was bistatic, i.e., the transmitter and receiver were in different places. Bistatic radar has the advantage of observing reflections through the phase angle, the angle between transmitted and received radio rays. This phase dependence is important. It’s similar to the effect one gets from looking at a bicycle reflector at just the right angle: at certain angles, the internal planes in the transparent plastic align and a very bright reflection is seen. Similarly, in both radio and visible wavelengths on the Moon, we see an “opposition surge”, an apparent increase in brightness looking directly down from the sun (zero phase). Clementine orbited the Moon such that we could observe its phase dependence and we specifically looked for this “opposition surge”, called the Coherent Backscatter Opposition Effect (CBOE). CBOE is particularly valuable to identify ice on planetary surfaces.
Clementine transmitted right circular polarized (RCP) radio and we listened on Earth in both right- and left-circular polarized (LCP) channels. The ratio of power received in these two channels is called the circular polarization ratio (CPR). The dry, equatorial Moon has CPR less than one, but the icy satellites of Jupiter all have CPR greater than one. We know these objects have surfaces of water ice; in this case, the ice acts as a radio-transparent media in which waves penetrate the ice, are scattered and reflected multiple times, and returned such that some of the waves are received in the same polarization sense as they are sent—they have CPR greater than unity.
The problem with CPR alone is that we can also get high values from very rough surfaces, such as a rough, blocky lava flow, which has angles that form many small corner reflectors. In this case, a radio wave could hit a rock face (changing RCP into LCP) and then bounce over to another rock face (changing the LCP back into RCP) and hence to the receiver . This “double-bounce” effect also creates high CPR in that “same sense” reflections could mimic the enhanced CPR one gets from ice targets.
Bistatic geometry can help in the interpretation of radar scattering. Both monostatic and bistatic radar measure CPR but bistatic radar also measures the angular dependence on reflection, which is distinctly narrower for volume (ice) scattering. In the case of the Clementine experiment, we measured two orbits of the lunar south pole, one over an area of polar darkness and the other over a nominally sunlit zone near the pole. The results are intriguing; we see evidence of a CPR enhancement (symmetric about the zero phase angle; see peak in orbit 234 curve) over the dark region, where ice would be stable, but not over the control (orbit 235) sunlit area. The Clementine team interpreted this response as CBOE, caused by ice in dark areas near the south pole. From the strength of the enhancement and its angular width, they reasoned that ice was mixed with regolith dirt and present in a deposit about 2 meters thick with an average concentration of about 1.5 wt. %. It should be noted that this doesn’t require an intimate mixture of ice and dirt, but is the average over hundreds of square kilometers. Thus, areas could exist of nearly pure ice in some places, and virtually none elsewhere.
This conclusion was tempered by the recognition that Clementine found enhanced CPR only during one observation; the limited time of the mission at the Moon (71 days) precluded repeating that measurement. In addition, the Clementine spacecraft was not optimized for this experiment, so the data have very low resolution—basically a spot about 300 kilometers across. Nevertheless, the results of this experiment have not been refuted. The most recent Earth-based radar studies confirm that high CPR does indeed exist within the dark area near the south pole. Given the size of the Clementine resolution cell, the observed CPR enhancement could be explained by the same area of high CPR observed in groundbased radar images of the crater Shackleton. The controversy is not whether an area of high CPR exists in the permanently shadowed interior of Shackleton crater, but over what is causing the high CPR signature.
Lunar Prospector Neutron Spectrometer. NASA’s Lunar Prospector spacecraft carried an instrument that measured neutrons emitted from the Moon as a function of their energy. Medium-energy neutrons are strongly absorbed by hydrogen. Thus, by measuring the flux of neutrons in this energy range, we can estimate how much hydrogen is present in the lunar soil. The LP neutron experiment sampled only the upper 40 centimeters or so of the Moon. As the spacecraft was a spinner, its instruments looked simultaneously in all directions and the effect of such a view is to limit surface resolution to roughly the altitude of the spacecraft. The best resolution of the LP neutron data is 30–40 kilometers. Unlike both the Clementine radar experiment and Earth-based radar, the LP instrument looked directly into the entire polar dark area of the Moon.
The Lunar Prospector observed strong absorption of medium-energy neutrons at both poles. Initially it was thought that there was more hydrogen at the north pole, but later analysis showed roughly equal amounts at both poles. The actual enrichment (up to 200 parts per million) is only about a factor of two greater than the highest concentrations of solar wind hydrogen seen in the Apollo soil samples. But the LP team suggested that if this hydrogen was present as water ice (which is stable only in polar dark areas), the average concentration of ice was around 1.5 wt. %, a significant value. Moreover, with the low resolution of the LP neutron data, significantly higher concentrations within the shadow cannot be ruled out; a uniform, low average ice concentration of about 1–2 wt.% or a very heterogeneous distribution with very high concentrations (in some places up to over 40 wt.%) are equally consistent with the data.
Lunar Prospector neutron spectrometer maps of the lunar poles. These low resolution data indicate elevated concentrations of hydrogen at both poles; it does not tell us the form of the hydrogen. Map courtesy of D. Lawrence, Los Alamos National Laboratory.
Curiously, data from fast neutrons detected by Lunar Prospector suggest that the uppermost surface is depleted in hydrogen, down to about 10 centimeters below the surface. Such a depletion suggests a non-solar wind origin for the polar hydrogen, as hydrogen implanted by solar wind would be expected to be high in the uppermost lunar surface.
As you might expect, the LP neutron results have been questioned. Some have suggested that the reduction in neutrons is caused by the presence of another light element, such as sulfur. However, cometary ice is very abundant and known to constantly hit the Moon. Lunar sulfur is not rare, but is relatively low in cosmic abundance and any process that would concentrate sulfur in the polar dark areas would also concentrate the more abundant extra-lunar hydrogen. Recent claims that the LP neutron data indicate a low, uniform concentration are not correct; we know nothing about the distribution of the hydrogen below the resolution of the neutron spectrometer (i.e., scales smaller than 30 kilometers.)
Earth-based radar data. Radar has been used to study the Moon for decades with many observations made in preparation for the Apollo missions. This work largely concentrated on the equatorial regions (target sites for Apollo), but later work has focused on the lunar poles. Although some of their early work supported the concept, the most strident objections to the presence of lunar polar ice has come from planetary radar astronomers.
Nick Stacy mapped the south pole of the Moon using the Arecibo telescope in 1992 for his Ph.D. dissertation. The Arecibo group found several zones of high CPR, although its distribution is patchy and discontinuous. They noted that some areas of high CPR occur within craters that might be permanently shadowed (at that time, lighting maps of the poles did not exist). Although couched in appropriately cautious terms, Stacy noted that one high CPR zone occurs within the crater Shackleton and that it appears to continue down into the portion of the crater floor in Earth shadow, out of view of the Arecibo dish. Attributing most of the high CPR to blocky, rough surfaces associated with craters, Stacy reserved the possibility that some high CPR spots could be ice if they occurred deep within permanently dark crater floors.
Subsequent work by the Arecibo group has moved away from this cautiously positive interpretation to a definitive assertion that none of the high CPR zones seen around the pole are caused by the presence of ice. In at least four papers published between 1997 and 2006, they have presented increasingly more detailed image data, each showing the same relations: patchy, high CPR found in both sunlight areas and in permanent darkness. The latest paper from the Arecibo group, published in October 2006 to a barrage of publicity (including an overwrought press release in which one investigator called the “door on the debate” on lunar polar ice detected by radar “closed”) shows the south pole of the Moon in unprecedented surface resolution, about 20 meters per pixel. Yet again, we see the high CPR patch in Shackleton , but this time, it is accompanied by an image and analysis of another crater, Schomberger G, which is alleged to have the same distribution of high CPR within it. As Schomberger G is in sunlight (and has high CPR in portions of its interior), the authors conclude that the high CPR in Shackleton is similarly caused by surface roughness and not by the presence of ice within the permanently dark area of the crater.
As all parties agree that high CPR is found in the polar regions of the Moon, the debate is over what this relation means. The Arecibo group claims that the distribution patterns in Schomberger G and Shackleton are the same; hence, the high CPR patches represent rocky outcrops on and within these craters, not ice. However, high CPR can be caused by either roughness or ice; in itself, high CPR is not uniquely diagnostic of either . I contend that because of its non-unique nature, high CPR within Shackleton could be ice; as near as can be determined, the high CPR patch occurs within a zone of permanent darkness.
Why even entertain this notion? After all, if ice is unstable on any part of the Moon that sees sunlight, doesn’t that mean that high CPR here indicates roughness, not ice? In fact, similar relations are seen on the planet Mercury . The polar features of Mercury were initially discovered by Dewey Muhleman and colleagues at Caltech using very low resolution, global disk images. Although these images show a prominent high CPR zone near the north pole of Mercury , they also show high CPR zones in mid-latitudes and equatorial regions. The interpretation of the authors of this work was that two mechanisms produce high CPR on Mercury; near the equator, surface roughness must be the cause of high CPR, but at the poles, water ice in permanent shadow could not ruled out (like the Moon, Mercury’s pole is normal to the plane of its orbit around the sun). Thus, two scattering mechanisms were invoked. In principle, there is no reason why such a relation would not also occur on our Moon. In such a case, high CPR can be caused by both roughness and ice. If a spot is in sunlight, it must be surface roughness, but if it’s in the permanent darkness, ice cannot be ruled out.
It is claimed by the Arecibo group that the distribution of high CPR within the two craters Shackleton and Schomberger G are identical. As Schomberger G is in partial sunlight, high CPR seen within it cannot be caused by ice. As a planetary geologist, I see significant differences in the distribution of CPR in the two craters. In Schomberger G, high CPR is found as a quasi-continuous upper “layer,” with CPR values decreasing deeper into the crater. At Shackleton, the upper crater wall is complex and high CPR is discontinuous; the large zone of high CPR within the crater at about 8 o’clock starts below the rim, but continues down into the crater, disappearing into the shadow caused by the Earth-Moon geometry. I leave it to readers to decide for themselves whether the distribution of high CPR is identical in these two craters. All of the interior of Shackleton is in permanent darkness, shielded from sunlight and has been continuously for at least the last two billion years. So in theory, ice may have accumulated within it. Thus, three data sets exist, each unique, on the possibility of lunar polar ice. But what are they telling us?
Synthesis: Best guess on polar volatiles
No single piece of evidence for lunar ice is decisive, but I think the preponderance of evidence indicates that water ice exists in permanently dark areas near the poles. However, its origin and the processes associated with its deposition are unclear. The ice could be of cometary, meteoritic, or solar wind origin; each mode would have interesting implications for its composition. If largely of cometary origin, other volatile species of great utility may also be present, such as ammonia (NH3), methane (CH4), and various organic substances. Nitrogen is particularly useful in supporting human life, both for breathing air and for agriculture. Whatever the source, polar ice is a useful resource for future lunar inhabitants.
Much remains unclear about the nature of ice is on the Moon. Rates of deposition of polar ice and implications for its physical nature are unknown. We can, however, make some inferences from the data in hand. Ice deposits cover a minority of the polar terrain and concentrations of it could vary widely over a small area, leading to a very heterogeneous deposit. This supposition is suggested by the patchy distribution of high CPR spots in the Earth-based radar data (not all of which are caused by ice). The concentration and distribution of the ice is unknown, but if very heterogeneous as suggested, deposits could locally cover between 10–50 percent of a given patch of dark area. Individual bodies of trapped ice could be on the order of meters to tens of meters in size, as suggested by the patchy extent of high CPR areas seen in the polar darkness.
From the fast neutron data of Lunar Prospector, the uppermost 10 centimeters or so of the polar dark regions are depleted in hydrogen. Radar data suggest volume scattering at depths on the order of several tens of wavelengths of the S-band radar (~13 centimeters). Thus, ice occurs between depths of 10 centimeters and 2–3 meters. From our current understanding of the creation, turnover, and evolution of the lunar soil, the ice is probably not “pure” but contains contaminants and solid inclusions of varying concentrations. Although water ice is expected to dominate the deposit, other minor species of cometary origin could be present in useful quantities. The terrain of a lunar highlands region (found at both lunar poles) can be very rugged, with local slopes exceeding 30 degrees. However, as shown by the Apollo 16 highland landing site, such areas can be negotiated reasonably well, if the correct paths are chosen.
So what?
Water ice on the Moon makes living there easier, cheaper, and thus, more likely. Solar wind hydrogen is found everywhere on the Moon, but in vanishingly small quantities. Ice at the poles is a concentrated source of both hydrogen and oxygen—two substances vital to supporting human life and creating a space transportation infrastructure. We can extract what we need out of the equatorial regolith, but it’s much harder and more energy intensive than at the poles. Extracting solar wind hydrogen requires heating soil to about 700° C, at which point 90 percent of the adsorbed gas is driven off. In contrast, icy regolith heated to about 100° C gives off water as an easily collected and stored gas. Per unit mass, it takes roughly two orders of magnitude less energy to extract hydrogen from icy polar regolith than it does by roasting soil at the equator.
Although polar ice is important, it is not a requirement to successfully live and work on the Moon. The poles of Moon are primarily attractive due to the near-permanent sunlight found in several areas. Such lighting is significant from two perspectives. First, it provides a constant source of clean power and allows humans to live on the Moon without having to survive the two-week-long lunar night experienced on the equator and at mid-latitudes. Second, because these areas are illuminated by the Sun at grazing angles of incidence, the surface never gets very hot or very cold. Sunlit areas near the poles are a benign thermal environment, with an estimated temperature of about –50° ± 10°C. Having water near these locales would be a huge bonus. The most compelling reason to go to the poles is to solve the problem of surviving the extended lunar night—a task that, at most other places on the Moon, would probably require spending billions of dollars for a nuclear reactor.
Science is an imperfect process. At any given point in time, we have limited data of less than optimum quality and nearly always imperfectly or incompletely understood. Our information on lunar polar ice is limited in both quality and quantity. No question in modern science is “solved” and the presence of “consensus,” while a useful concept in marketing and politics, has no real value to the truth or falsity of scientific questions. The way the universe is put together and works is quite independent of the collective opinions of the experts.
To answer the question of lunar polar ice, we need more and better data. We must first thoroughly map the polar deposits from lunar orbit. India is preparing to launch their first mission to the Moon, Chandrayaan-1, in early 2008. I am on a team that will build and fly the radar mapping instrument on that mission. This radar will map both poles using a revolutionary new processing architecture that allows us to distinguish areas of high CPR caused by roughness and those caused by the presence of ice. An even more advanced radar instrument will be on the US Lunar Reconnaissance Orbiter (LRO) mission in late 2008, mapping in two frequency bands (potentially distinguishing roughness from ice) and in high resolution, showing patches of ice as small as 20 meters across. Chandrayaan will systematically map the polar regions at moderate resolution (75 meters/pixel.) On the subsequent LRO mission, we will get high-resolution coverage (10 meters/pixel) at multiple wavelengths of promising targets seen in that data. Since these two missions overlap and will orbit the Moon at the same time, we can use both instruments on the two spacecraft to make bistatic images of the polar deposits; such a mode of operation can observe scattering through the phase angle (looking for the CBOE effect, a good discriminator between ice and roughness). Together, these missions will map the extent and distribution of anomalous material in the polar regions of the Moon.
A key advantage of orbital mapping is the ability to look into all of the areas of permanent darkness. In a recent article in Scientific American (“Radar Images Fail to Detect Ice at Lunar Poles”, October 2006), Don Campbell of Cornell University, part of the Arecibo team, notes that the lunar orbiters LRO and Chandrayaan “will get a better view of the polar terrain than we can from Earth.”
The LRO spacecraft will carry other instruments, including a thermal mapper to determine temperatures of the dark areas, a laser altimeter to measure the topography of the poles (needed to make definitive maps of sunlight and darkness) and other instruments designed to characterize the environment and deposits of the polar regions. In addition, other nations (including China and Japan) are flying lunar orbiters carrying a variety of mapping instruments. The Moon, once the most poorly mapped body in the Solar System, appears ready to become the most thoroughly charted and remotely studied object in the history of mankind.
The next step is critical. After polar deposits have been mapped from orbit, we must land at a promising target and measure volatile substances in the soil. As descri
Image via Wikipedia
After the first lander, we should survey potential mining prospects, map the distribution of ice on small, local scales (hundreds of meters), and experiment with different extraction methods, water separation technologies, and resource processing and storage techniques. The goals of lunar resource utilization are challenging, but significant experience can be gathered from small robotic landers prior to the arrival of people. A program of robotic missions can provide critical strategic information as well as gaining operational experience and providing milestones for a human lunar return.
The Moon is not a hostile, barren rock in space—it is humanity’s stepping-stone into the Solar System. The poles of the Moon are inviting “oases” for humans in the desert of near-Earth space. To live there and at destinations beyond, we must identify resources that will support human life and enable the creation of a new spacefaring infrastructure.
0 comments:
Post a Comment