EMBARGOED for release 2 p.m. MDT Thursday, Sept. 3, 1998
NEW ANALYSES FROM LUNAR PROSPECTOR PUBLISHED
LOS ALAMOS, N.M., Sept. 4, 1998 -- Refined calculations of lunar water amounts and unique lunar compositional maps appeared today in the journal Science as part of the first publications of detailed analyses of data returned from NASA's Lunar Prospector mission.
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:
o the first application of neutron spectroscopy to planetary exploration, used on Lunar Prospector principally to look for the
presence of water, but showing unexpected value for studying lunar composition as well;
o the first mapping of the entire lunar surface in gamma rays, which reveals compositional variations across the surface;
o and a comparison between Lunar Prospector neutron measurements and spectroscopic data from the Clementine spacecraft, which orbited the moon in 1994.
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.
-30-
RSS