Cornell University
June 3, 1999
These first three-dimensional topographic images of the lunar polar regions will provide essential data for the proposed crash of the orbiting Lunar Prospector spacecraft into the lunar south pole in late July. If NASA approves, the controlled, high-speed dive into a massive crater, 50 kilometers (30 miles) across and 2.5 kilometers (1.5 miles) deep, will attempt to provide absolute proof of the existence of water on the moon.
The new images, obtained through a technique called radar interferometry, are published in the latest edition (June 4) of Science magazine. They are a leap forward in settling the "significant argument" about the existence of water ice on the moon, says Donald Campbell, professor of astronomy at Cornell, and one of the paper's authors.
In the solar system, ice has a unique, and not totally understood, "signature" when probed by radar beams. This was first discovered by Campbell and others when they used the radar system of the Cornell-operated Arecibo Observatory in Puerto Rico to get radar echoes from the icy Galilean satellites of Jupiter.
In 1996, researchers working with radar data from the orbiting lunar spacecraft Clementine reported indications of ice at the south pole of the moon. However, in 1997 researchers, Campbell among them, published a paper in Science reporting on Arecibo's radar imaging of the lunar poles that showed no evidence of ice. Both the Clementine and Arecibo radars would only have detected ice if it had been in the form of large chunks or slabs. The absence of an Arecibo radar detection did not preclude ice being present in small chunks or crystals mixed in with the lunar "soil."
Last year, the neutron spectrometer aboard the Lunar Prospector orbiter, launched in January 1998, detected significant deposits of hydrogen at the moon's north and south poles. This was interpreted as indicating the presence of water ice, since hydrogen in water molecules is thought to be the most likely source of the element at the poles. However, without detailed topographic maps of the poles, it was not possible to identify potential ice-containing regions -- so-called cold traps, or areas where the sun never shines and the temperature hovers around 100 degrees Kelvin (minus 280 degrees Fahrenheit).
With these first detailed altimetry data for the polar regions, the Cornell researchers have been able to map out the positions of most of these cold traps. Because the sun's limb rises less than two degrees above the horizon at the south pole, the floors of impact craters and other low areas can be in permanent shadow. In contrast, the radar beam from Goldstone reaches up 6 to 7 degrees above the horizon, allowing many polar features hidden from the sun to be imaged by radar.
The floors of five large craters in the south polar region are hidden from the sun, the researchers say. These five crater floors constitute the largest potential deposits of water ice at the south pole and would be expected to display an excess of hydrogen if they contained ice, says Jean-Luc Margot, who carried out the observations for his doctoral research at Cornell. He is now a research associate at Arecibo Observatory.
Using these new topographical maps, NASA is considering trying to settle the debate about the existence of water on the moon with a controlled crash of the Lunar Prospector spacecraft, which is nearing the end of its useful life. The orbiter has detected significant amounts of hydrogen in the chosen south pole crash site, the informally named Mawson crater. The hope is that the kinetic energy from the plunge into the crater will evaporate the water ice into a plume detectable from terrestrial and space telescopes. "In order to impact the spacecraft at the desired location, very accurate knowledge of the topography is needed," says Campbell, who is also the associate director of the National Astronomy and Ionosphere Center, headquartered at Cornell, which operates Arecibo for the National Science Foundation.
Says Margot: "The argument for targeting that particular crater is that it is both in permanent shadow, as shown by our radar data, and also has a high hydrogen abundance, as shown by new Lunar Prospector data. This makes it a prime candidate for water ice deposits."
To obtain the topographic features of the hidden lunar polar regions, Margot, Campbell and Martin Slade and Raymond Jurgens of JPL used the Goldstone 70-meter antenna to transmit the radar signals. Two separate 34-meter antennas, 20 kilometers (12 miles) apart at the Goldstone site, received the echoes. By comparing the images from the two antennas, Margot derived a three-dimensional digital elevation model of the lunar poles, with measurements every 150 meters (500 feet) over the imaged area and a height accuracy of 50 meters (165 feet).
To calculate which areas were in permanent shadow, Margot wrote a computer program that calculated whether each point in the three-dimensional image would be in shadow for any allowed position of the sun. "The program simulated light rays from the sun to each point on the map and tested to see if the ray was intercepted by the surrounding topography," Margot says. "If a single light ray was received, that point was in sunlight. If not, it was in shadow."
This detailed topography, says Margot, also has applications in cratering and other, studies. "The data is of such fine resolution that we can find out much about crater shape and impact mechanics," he says.
IMAGE CAPTIONS:
[Image 1]
Locations of lunar south pole cold traps on a radar image obtained with the
3.5 centimeter wavelength Goldstone Solar System. Areas visible to the
radar but in the sun's permanent shadow are marked in white. Several regions
not visible to the radar are shown in gray, indicating that they are predicted
to be hidden from the sun. Crater Mawson (51 kilometers, or 32 miles, in
diameter) is shown in white and gray at about two o'clock on the inner circle.
Crater Shackleton (20 kilometers, or 12 miles, in diameter) is at the south
pole. The size of the area is about 325 kilometers (200 miles) by 300
kilometers (185 miles). J.L. Margot/Cornell University.
[Image 2]
Radar image of lunar south pole obtained with the 3.5 centimeter wavelength
Goldstone Solar System Radar. Crater Mawson is crater at about two o'clock
on the inner circle. Crater Shackleton is at the south pole. The total size of
the imaged area is approximately 325 kilometers (200 miles) by 300
kilometers (185 miles). J.L. Margot/Cornell University.
[Image 3]
Locations of cold traps at the lunar north pole shown on a radar image
obtained with the 3.5 centimeter wavelength Goldstone Solar System Radar.
Areas visible to the radar but in the sun's permanent shadow are marked in
white. Several regions not visible to the radar are shown in grey, indicating
that they are predicted on the basis of the topography of the surrounding
terrain also to be hidden from the sun. The total size of the imaged area is
about 375 kilometers (230 miles) by 300 kilometers (185 miles). J.L. Margot/
Cornell University.
[Image 4]
Radar image of the lunar north pole obtained with the 3.5 centimeter
wavelength Goldstone Solar System Radar. The total size of the imaged
area is about 375 kilometers (230 miles) by 300 kilometers (185 miles).
J.L. Margot/Cornell University.
Los Alamos National Laboratory
Sept. 4, 1998
Scientists from the U.S. Department of Energy's Los Alamos National Laboratory are lead authors on four of the papers in Science, with significant contributions from the Observatoire Midi-Pyrenees in Toulouse, France. Los Alamos built three of Lunar Prospector's five onboard instruments.
Refined calculations of lunar water amounts are tenfold higher than the lower limit -- based on preliminary, conservative estimates -- released in March. The additional analysis also shows the water is likely confined to localized areas near the poles, rather than spread out evenly across the polar regions, as was assumed in making the earlier estimates.
Water amounts, inferred from measurements of hydrogen in the lunar soil, are of great interest because of their potential impact on plans for colonization.
Compositional measurements show that the well-known impact basin Mare Imbrium -- one of the large, dark areas visible in the full moon -- is unlike any other spot on the moon, which theories of lunar evolution will have to account for. "This mission has been an overwhelming success," said Los Alamos' Bill Feldman "We've gotten beautiful science from two of our three instruments. The third, we just haven't had time to analyze the data yet."
"These data will generate ripples that will spread throughout the planetary science community," said Rick Elphic. "We're barely scratching the surface of the analysis; we haven't begun to touch on the many ramifications for the origin and evolution of the moon."
The Los Alamos papers describe:
Los Alamos scientists built Lunar Prospector's neutron spectrometer, gamma ray spectrometer and alpha particle spectrometer. Spectrometers measure the numbers and energies of particles or photons encountered. Data from the neutron and gamma ray spectrometers figure into the Science papers; the alpha particle data are yet to be analyzed.
Neutrons and gamma rays emanate from the moon's surface as a result of cosmic rays -- high-energy particles traveling through space in all directions -- striking nuclei in the lunar soil. When a cosmic ray hits a nucleus it can eject neutron particles or high-energy gamma ray photons in response. Some of the neutrons and gamma rays travel upward where instruments aboard Lunar Prospector intercept them.
"The gamma ray measurements are ideal for spotting elements incorporated into materials that formed below the moon's crust," said Los Alamos' David Lawrence. The moon once was hot and molten and as it cooled minerals crystallized and sank to form the core, if they were heavy, or floated upward to form the crust, if they were light. The last material to solidify contained thorium, potassium, gadolinium and samarium, which do not readily incorporate into minerals. These elements are signatures of the moon's subsurface mantle region, and their presence on the surface indicates some process -- volcanic events or impacts strong enough to punch through the crust -- must have dredged them up from the interior. "Studies of these materials provides us a window into the moon's interior," Elphic said.
Thorium and potassium create standout gamma-ray signals, and their emissions neatly trace out Mare Imbrium's outer rim. Lawrence said this signal "provides a telltale sign of deposition by ejecta. This indicates that around Mare Imbrium the dredge-up process, at least in part, was related to an impact."
A different compositional story appears at the South-Pole Aitkin basin, the largest impact crater in the solar system and, therefore, presumably from an event strong enough to poke through the lunar crust.
Although the Aitken basin region shows enhanced gamma ray emissions from thorium, it is not nearly as bright as Mare Imbrium. The impact event apparently dredged up much less potassium- and thorium-rich materials than at Mare Imbrium.
For an independent look at the distribution of dredged-up lunar mantle, the Los Alamos scientists compared their neutron spectrometer data with Clementine data.
"You can see compositional variations with neutrons in ways people had not realized previously," Lawrence said. "We've obtained far more composition information from the neutron data than we expected we would."
The elemental makeup of the lunar soil affects the energies of neutrons emanating from it. Over regions rich in iron and titanium, for example, Lunar Prospector will encounter an abundance of fast-moving neutrons and a deficit of slow ones. Other elements don't produce as many energetic neutrons yet don't absorb slow ones efficiently, leading to enhanced numbers of these. By looking at the relative numbers of neutrons of different energies scientists can determine what elements are in the lunar soil.
Gadolinium and samarium, key indicators of material from the moon's interior, interact very efficiently with slow neutrons. They can appear in small concentrations in the soil yet have a large impact on the low-energy neutron emissions.
By comparing their neutron measurements against Clementine's data for iron and titanium, the Los Alamos scientists found a large residual signal around Mare Imbrium they attribute to the presence of gadolinium and samarium. This signal did not appear in other locations where scientists would expect to see subsurface material dredged up.
"Something special happened around Imbrium; you don't see this sort of chemistry anywhere else on the moon," Elphic said. "It also confirms that the moon is very inhomogeneous -- at least for these elements. These data are going to be fairly restricting to theorists: whatever happened did not happen all over the moon, just in this one spot." Another element that provides a unique signature in the neutron measurements is hydrogen. Scientists think hydrogen is most likely bound up in water molecules in the lunar soil, trapped frozen in regions of craters near the poles that never see direct sunlight. "The data show clearly where the hydrogen is," Feldman said. "It's localized in spots near the poles, and it has to be buried, about half a meter or so.
"In making our initial estimates, we assumed the water was spread over the 'footprint' of the instrument," Feldman said, which is how much surface area the instrument can detect at any moment, a square approximately 120 miles on a side at Lunar Prospector's current altitude. "As we've gotten more data we've found that it's not spread out as we first assumed, but concentrated."
When they presented their initial results in March, the scientists said the water was likely in the form of a fine frost spread through the lunar soil. Further data analysis now allows the possibility of deposits of solid ice, Feldman said.
Feldman currently estimates there may be as much as three billion metric tons of water ice at each of the poles, with 15 percent more at the north pole than at the south pole.
Scientists assume comets carry the water ice to the moon. The comets basically vaporize on impact, and the water molecules migrate to the permanently shaded regions at the poles. These regions are so cold that once a water molecule enters them it gets stuck.
Lunar Prospector, part of NASA's Discovery Program of low-cost, fast-track space missions, was launched in January and its first scientific results were announced in March. Alan Binder of the Lunar Research Institute is the principal investigator for the mission.
Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.
Full text of the technical papers in SCIENCE are available for free access at http://www.sciencemag.org/content/current
NASA Headquarters, Washington, DC
Ames Research Center, Moffett Field, CA
September 3, 1998
Growing evidence now suggests that water ice deposits of relatively high concentration are trapped beneath the soil in the permanently shadowed craters of both lunar polar regions. The researchers believe that alternative explanations, such as concentrations of hydrogen from the solar wind, are unlikely.
Mission scientists also report the detection of strong, localized magnetic fields; delineation of new mass concentrations on the surface; and the mapping of the global distribution of major rock types, key resources and trace elements. In addition, there are strong suggestions that the Moon has a small, iron-rich core. The new findings are published in the Sept. 4 issue of Science magazine.
"The Apollo program gave us an excellent picture of the Moon's basic structure and its regional composition, along with some hints about its origin and evolution," said Dr. Carl Pilcher, science director for Solar System exploration in NASA's Office of Space Science, Washington, DC. "Lunar Prospector is now expanding that knowledge into a global perspective. The indications of water ice at the poles are tantalizing and likely to spark spirited debate among lunar scientists."
In March, mission scientists reported a water signal with a minimum abundance of one percent by weight of water ice in rocky lunar soil (regolith) corresponding to an estimated total of 300 million metric tons of ice at the Moon's poles. "We based those earlier, conscientiously conservative estimates on graphs of neutron spectrometer data, which showed distinctive dips over the lunar polar regions," said Dr. Alan Binder of the Lunar Research Institute, Gilroy, CA, the Lunar Prospector principal investigator. "This indicated significant hydrogen enrichment, a telltale signature of the presence of water ice.
"Subsequent analysis, combined with improved lunar models, shows conclusively that there is hydrogen at the Moon's poles," Binder said. "Though other explanations are possible, we interpret the data to mean that significant quantities of water ice are located in permanently shadowed craters in both lunar polar regions.
"The data do not tell us definitively the form of the water ice," Binder added. "However, if the main source is cometary impacts, as most scientists believe, our expectation is that we have areas at both poles with layers of near-pure water ice." In fact, the new analysis "indicates the 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 15 percent stronger at the Moon's north pole than at the south."
How much water do scientists believe they have found? "It is difficult to develop a numerical estimate," said Dr. William Feldman, co-investigator and spectrometer specialist at the Department of Energy's Los Alamos National Laboratory, NM. "However, we calculate that each polar region may contain as much as three billion metric tons of water ice."
Feldman noted he had cautioned that earlier estimates "could be off by a factor of ten," due to the inadequacy of existing lunar models. The new estimate is well within reason, he added, since it is still "one to two orders of magnitude less than the amount of water predicted as possibly delivered to, and retained on, the Moon by comets," according to earlier projections by Dr. Jim Arnold of the University of California at San Diego.
In other results, data from Lunar Prospector's gamma ray spectrometer have been used to develop the first global maps of the Moon's elemental composition. The maps delineate large compositional variations of thorium, potassium and iron over the lunar surface, providing insights into the Moon's crust as it was formed. The distribution of thorium and potassium on the Moon's near side supports the idea that some portion of materials rich in these trace elements was scattered over a large area as a result of ejection by asteroid and comet impacts.
While its magnetic field is relatively weak and not global in nature like those of most planets, the Moon does contain magnetized rocks on its upper surface, according to data from Lunar Prospector's magnetometer and electron reflectometer. The resultant strong, local magnetic fields create the two smallest known magnetospheres in the Solar System.
"The Moon was previously interpreted as just an unmagnetized body without a major effect on what is going on in the solar wind," explained Dr. Mario Acuna, a member of the team located at NASA's Goddard Space Flight Center, Greenbelt, MD. "We are discovering that there is nothing simple about the Moon as an obstacle to this continuous flow of electrically charged gas from the Sun."
These mini-magnetospheres are located diametrically opposite to large impact basins on the lunar surface, leading scientists to conclude that the magnetic regions formed as the result of these titanic impacts. One theory is that these impacts produced a cloud of electrically charged gas that expanded around the Moon in about five minutes, compressing and amplifying the pre-existing, primitive ambient magnetic field on the opposite side. This field was then "frozen" into the surface crust and retained as the Moon's then-molten core solidified and the global field vanished.
Using data from Prospector's doppler gravity experiment, scientists have developed the first precise gravity map of the entire lunar surface. In the process, they have discovered seven previously unknown mass concentrations, lava-filled craters on the lunar surface known to cause gravitational anomalies. Three are located on the Moon's near side and four on its far side. This new, high-quality information will help engineers determine the long-term, altitude-related behavior of lunar-orbiting spacecraft, and more accurately assess fuel needs for possible future Moon missions.
Finally, Lunar Prospector data suggests that the Moon has a small, iron-rich core approximately 186 miles (300 kilometers) in radius, which is toward the smaller end of the range predicted by most current theories. "This theory seems to best fit the available data and models, but it is not a unique fit," cautioned Binder. "We will be able to say much more about this when we get magnetic data related to core size later in the mission." Ultimately, a precise figure for the core size will help constrain models of how the Moon originally formed.
Lunar Prospector was launched on Jan. 6, 1998, aboard a Lockheed Martin Athena 2 solid-fuel rocket and entered lunar orbit on Jan. 11. After a one-year primary mission orbiting the Moon at a height of approximately 63 miles (100 kilometers), mission controllers plan to the lower the spacecraft's orbit substantially to obtain detailed measurements. The $63 million mission is managed by NASA's Ames Research Center, Moffett Field, CA.
Further information about Lunar Prospector, its science data return, and relevant charts and graphics:
Moon Handbook foretold discovery of ice on the Moon by Lunar Prospector
Clementine data already pointed to water ice on the south pole of the Moon
PERMANENT: Projects to Employ Resources of the Moon and Asteroids Near Earth in the Near Term
NASA Headquarters, Washington, DC
Ames Research Center, Moffett Field, CA
Los Alamos National Laboratory, Los Alamos, NM
Department of Energy
Jet Propulsion Laboratory, Pasadena, CA
March 5, 1998
The Discovery Program mission also has produced the first operational gravity map of the entire lunar surface, which should serve as a fundamental reference for all future lunar exploration missions, project scientists announced today at NASA's Ames Research Center, Moffett Field, CA.
Just two months after the launch of the cylindrical spacecraft, mission scientists have solid evidence of the existence of lunar water ice, including estimates of its volume, location and distribution. "We are elated at the performance of the spacecraft and its scientific payload, as well as the resulting quality and magnitude of information about the Moon that we already have been able to extract," said Dr. Alan Binder, Lunar Prospector Principal Investigator from the Lunar Research Institute, Gilroy, CA.
The presence of water ice at both lunar poles is strongly indicated by data from the spacecraft's neutron spectrometer instrument, according to mission scientists. Graphs of data ratios from the neutron spectrometer "reveal distinctive 3.4 percent and 2.2 percent dips in the relevant curves over the northern and southern polar regions, respectively," Binder said. "This is the kind of data 'signature' one would expect to find if water ice is present."
However, the Moon's water ice is not concentrated in polar ice sheets, mission scientists cautioned. "While the evidence of water ice is quite strong, the water 'signal' itself is relatively weak," said Dr. William Feldman, co-investigator and spectrometer specialist at the Department of Energy's Los Alamos National Laboratory, NM. "Our data are consistent with the presence of water ice in very low concentrations across a significant number of craters." Using models based on other Lunar Prospector data, Binder and Feldman predict that water ice is confined to the polar regions and exists at only a 0.3 percent to 1 percent mixing ratio in combination with the Moon's rocky soil, or regolith.
How much lunar water ice has been detected? Assuming a water ice depth of about a foot and a half (.5 meters) -- the depth to which the neutron spectrometer's signal can penetrate -- Binder and Feldman estimate that the data are equivalent to an overall range of 11 million to 330 million tons (10-300 million metric tons) of lunar water ice, depending upon the assumptions of the model used. This quantity is dispersed over 3,600 to 18,000 square miles (10,000-50,000 square kilometers) of water ice- bearing deposits across the northern pole, and an additional 1,800 to 7,200 square miles (5,000-20,000 square kilometers) across the southern polar region. Furthermore, twice as much of the water ice mixture was detected by Lunar Prospector at the Moon's north pole as at the south.
Dr. Jim Arnold of the University of California at San Diego previously has estimated that the most water ice that could conceivably be present on the Moon as a result of meteoritic and cometary impacts and other processes is 11 billion to 110 billion tons. The amount of lunar regolith that could have been "gardened" by all impacts in the past 2 billion years extends to a depth of about 6.5 feet (2 meters), he found. On that basis, Lunar Prospector's estimate of water ice would have to be increased by a factor of up to four, to the range of 44 million to 1.3 billion tons (40 million to 1.2 billion metric tons). In actuality, Binder and Feldman caution that, due to the inadequacy of existing lunar models, their current estimates "could be off by a factor of ten in either direction."
The earlier joint Defense Department-NASA Clementine mission to the Moon used a radar-based technique that detected ice deposits in permanently shadowed regions of the lunar south pole. It is not possible to directly compare the results from Lunar Prospector to Clementine because of their fundamentally different sensors, measurement "footprints," and analysis techniques. However, members of the Clementine science team concluded that its radar signal detected from 110 million to 1.1 billion tons (100 million to 1 billion metric tons) of water ice, over an upper area limit of 5,500 square miles (15,500 square kilometers) of south pole terrain.
There are various ways to estimate the economic potential of the detected lunar water ice as a supporting resource for future human exploration of the Moon. One way is to estimate the cost of transporting that same volume of water ice from Earth to orbit. Currently, it costs about $10,000 to put one pound of material into orbit. NASA is conducting technology research with the goal of reducing that figure by a factor of 10, to only $1,000 per pound. Using an estimate of 33 million tons from the lower range detected by Lunar Prospector, it would cost $60 trillion to transport this volume of water to space at that rate, with unknown additional cost of transport to the Moon's surface.
From another perspective, a typical person consumes an estimated 100 gallons of water per day for drinking, food preparation, bathing and washing. At that rate, the same estimate of 33 million tons of water (7.2 billion gallons) could support a community of 1,000 two-person households for well over a century on the lunar surface, without recycling.
"This finding by Lunar Prospector is primarily of scientific interest at this time, with implications for the rate and importance of cometary impacts in the history and evolution of the Solar System," said Dr. Wesley Huntress, NASA Associate Administrator for Space Science. "A cost-effective method to mine the water crystals from within this large volume of soil would have to be developed if it were to become a real resource for drinking water or as the basic components of rocket fuel to support any future human explorers."
Before the Lunar Prospector mission, historical tracking data from various NASA Lunar Orbiter and Apollo missions had provided evidence that the lunar gravity field is not uniform. Mass concentrations caused by lava which filled the Moon's huge craters are known to be the cause of the anomalies. However, precise maps of lunar mass concentrations covering the moon's equatorial nearside region were the only ones available.
Lunar Prospector has dramatically improved this situation, according to co-investigator Dr. Alex Konopliv of NASA's Jet Propulsion Laboratory, Pasadena, CA. Telemetry data from Lunar Prospector has been analyzed to produce a full gravity map of both the near and far side of the moon. Konopliv also has identified two new mass concentrations on the Moon's nearside that will be used to enhance geophysical modeling of the lunar interior. This work has produced the first-ever complete engineering-quality gravity map of the moon, a key to the operational safety and fuel-efficiency of future lunar missions.
"This spacecraft has performed beyond all reasonable expectations," said NASA's Lunar Prospector mission manager Scott Hubbard of Ames. "The findings announced today are just the tip of the iceberg compared to the wealth of information forthcoming in the months and years ahead."
Lunar Prospector is scheduled to continue its current primary data gathering mission at an altitude of 62 miles (100 kilometers) for a period of ten more months. At that time, the spacecraft will be put into an orbit as low as six miles (10 kilometers) so that its suite of science instruments can collect data at much finer resolution in support of more detailed scientific studies. In addition, surface composition and structure information developed from data returned by the spacecraft's Gamma Ray Spectrometer instrument will be a crucial aspect of additional analysis of the polar water ice finding over the coming months.
The third launch in NASA's Discovery Program of lower cost, highly focused planetary science missions, Lunar Prospector is being implemented for NASA by Lockheed Martin, Sunnyvale, CA, with mission management by NASA Ames. The total cost to NASA of the mission is $63 million.
Additional informaiton about the Lunar Prospector mission can be found on the Internet at URL:
NASA Headquarters, Washington, DC
Ames Research Center, Moffett Field, CA
March 2, 1998
Participants in the press briefing are:
NASA Television is available on GE-2, transponder 9C at 85 degrees West longitude, with vertical polarization. Frequency is on 3880.0 megahertz, with audio on 6.8 megahertz.
Information on Lunar Prospector is available on the Internet.
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