Lunar Prospector Provides a World of Data
12 Mar 2001
(Source: Los Alamos National Laboratory)
Los Alamos National Laboratory
Shelley Thompson, firstname.lastname@example.org, (505) 665-7778
LOS ALAMOS, N.M., March 12, 2001 - This week scientists from The Department of Energy's Los Alamos National Laboratory present their latest findings from NASA's Lunar Prospector mission at the Lunar and Planetary Science Conference in Houston, Texas.
"Lunar Prospector has revolutionized our view of the Moon -- we just didn't realize how much it revolutionized it. And there is a whole community of people out there in the planetary science world that is excited about and interested in the Lunar Prospector data," said Rick Elphic, a Los Alamos scientist.
The Los Alamos studies include data on Moonquake activity, further confirmation of the presence of water-ice on the moon, and mapping of iron and titanium using gamma-rays emitted when cosmic rays slam into the lunar surface.
Los Alamos scientists built three of the five instruments that were aboard the Lunar Prospector spacecraft that orbited the Moon for nearly 19 months gathering data, and then was intentionally crashed in the Moon's south pole in a final attempt to extract additional information about water on the Moon.
The data gathered by the Lunar Prospector and analyzed by Los Alamos scientists also provided the first global elemental lunar study to date.
"You can't take samples of just a few locations on the Moon, like the Apollo missions did, and say you know the composition of the whole Moon. It would be like taking a rock from Paris and Los Angeles and a snap shot of Tokyo and saying you know everything about Earth's composition. It's like a detective story -- you have to put all of the pieces of information together to see the whole picture," David Lawrence, a Los Alamos researcher, said.
Los Alamos scientists present a few pieces from this puzzle this week at the conference. The scientific goals for the Lunar Prospector were to answer long-standing questions about the Moon, its resources -- including water -- its structure, and its origins.
Los Alamos scientists at the conference include Lawrence and Tom Prettyman, who present the first Lunar Prospector Gamma-Ray Spectrometer measurements of lunar iron abundance. Elphic and Prettyman, who present titanium data that suggest previous measurements were twice as high as the actual abundance. Olivier Gasnault, who presents neutron data that correlates the atomic mass of the soil to neutron flux. William Feldman, who provides further evidence that the enhanced hydrogen abundance at the lunar poles is in the form of water ice. Stefanie Lawson, who presents the first Alpha-Particle Spectrometer measurements on tectonic activity and Elphic also presents computer simulation data that confirm actual measurement techniques.
Lawrence's gamma-ray spectrometry data give the first overall iron composition measurements of the entire lunar surface. Iron abundance is a crucial piece in understanding lunar composition because iron is a major constituent of all lunar rocks, and iron abundance reveals information about how the Moon formed and changed over time.
Both iron and titanium are fundamental elements in the lava -- or maria flood basalt -- plains on the Moon's surface that are seen from Earth as the "dark spots" of the Moon.
The Department of Defense's earlier Clementine mission measured iron and titanium by using ultraviolet, visible, and near-infrared spectroscopy and concluded there is more iron and titanium in the dark spots and less in the light-colored spots. "Their overall measurements of the entire Moon's elemental composition is based on empirical relationships. They empirically identified that a certain reflectance correlates with a given abundance. But this correlation was based upon the limited soil samples gathered on the Apollo missions," Lawrence said.
Lawrence's results show that the Clementine abundance measurements for returned sample locations agree quite well with the Lunar Prospector's gamma-ray spectrometer measurements; however the abundance derived by Clementine in locations not sampled by Apollo will need to be revised based on the Lunar Prospector gamma-ray spectrometer data.
Elphic's Neutron Spectrometer data on the concentration of titanium in the maria flood basalt regions contradicts the concentrations inferred by the correlation drawn between the Clementine and Apollo missions. Elphic's findings indicate there is half as much titanium in the basalt regions as the previous data suggested.
"These results have potential ramifications for two areas. First, for understanding the source regions and history of lunar basaltic volcanism and secondly for the future colonization of the Moon. If we intend to use indigenous resources when we colonize it's important to know what we have available and where we can find it," Elphic said.
"No one has ever looked at the Moon -- or any other planetary body -- with 'neutron eyes.' It is a whole different way of seeing things and it is very exciting," said Elphic. Lunar Prospector carried neutron detectors that measured neutrons in three energy ranges emitted by the lunar surface.
In another study, Gasnault determined the relationship between the fast neutron fluxes emitted from the lunar surface and the average weight -- atomic mass -- of the soil. "The calculations show that the number of neutrons produced is proportional to the atomic mass of the soil," Gasnault said.
The first neutrons that escape after the lunar soil is hit by cosmic rays are indicative of the composition of the soil. Different elements, when hit by cosmic rays, will produce a different number of neutrons, which will eventually reach different energies called fast, thermal or epithermal. Neutrons termed fast neutrons are indicative of iron and titanium. This information helps determine the elemental composition of the soil.
"The remarkable difference in composition between the two faces of the Moon is surprising for a small planet. This is one of the great mysteries of the Moon," said Gasnault.
Gasnault also worked out a combined analysis of fast neutron data and thorium abundance on the lunar surface. Thorium is a subsurface material brought to the surface by meteor impacts and volcanic activity. Using this analysis Gasnault believes they have found another impact basin near the large Imbrium basin.
Feldman's Neutron Spectrometer data coupled with calculations of sublimation processes of hydrogen compounds confirmed previous indications that the hydrogen in the permanently shaded regions of the lunar poles is in the form of water ice.
Sublimation is the process by which solids are transformed directly to the vapor state without passing through the liquid phase.
"Sublimation is the only mechanism that can account for observed differences between the hydrogen content of sunlit and permanently shaded craters near lunar poles," Feldman said.
The partially shaded -- or partially sunny -- regions of the poles reach minus 234 degrees Fahrenheit but the permanently shaded regions remain at minus 315 F. Going above or below minus 279 F determines whether the water-ice will sublimate or be trapped forever. So in the partially sunny regions of the poles the water-ice will sublimate, whereas in the permanently shaded regions it will be trapped indefinitely. Hydrogen by itself is not stable at these temperatures and will only remain if it forms bonds -- becomes water-ice for example -- so the hydrogen detected in the permanently shaded regions of the poles must be in the form of water-ice.
It is estimated that each pole may contain up to one billion tons of frozen water ice spread throughout the soil.
Feldman said, "These data suggest an exciting scenario for lunar colonization. The polar regions that border the permanently shaded craters are also in the sunlight 80-85 percent of the time and would make optimal space station sites. The stations would have access to the water-ice and the sunlight would provide solar power. And by being near the poles you see Earth most of the time, which means you can communicate."
Lawson presents measurements of near-surface uranium gained from the Alpha-Particle Spectrometer data. The Alpha-Particle Spectrometer measured radon and polonium gasses that escaped from below the surface through conduits created by tectonic, or Moonquake, activity. Polonium is a decay product of radon, which in turn is a decay product of uranium. When detected, they both indicate the presence of uranium.
Elphic presents a poster on computer simulations used to model neutron emissions from the surface, their flight into space and their detection by Lunar Prospector spectrometers. "The good news about these simulations," Elphic said, "is that they agree with measurements from the Lunar Prospector. If they didn't it would have raised doubts about our data."
The three instruments built by Los Alamos scientists were the Neutron Spectrometer designed to measure the surface abundance of lunar materials with special emphasis placed on the search for polar water-ice deposits as implied by hydrogen abundances; the Gamma-Ray Spectrometer designed to provide maps of the major and trace elements in the lunar surface; and the Alpha-Particle Spectrometer designed to measure the history of gas release events on the Moon.
Los Alamos' Lunar Prospector team of the Space and Atmospheric Sciences Group submitted 15 abstracts to the conference and is represented by nine scientists presenting both posters and talks and attending meetings.
Prospector is a NASA Discovery Mission that places emphasis on science and "faster, better, cheaper" mission design and development.