166 Los Alamos Science Number 28 2003

Eyes in Space

An outgrowth of the space-basedTreaty Verification andProliferation Detection

Programs at Los Alamos was the field-ing of experiments to determine the ele-mental composition of the moon andMars. Our instruments, which includedneutron, gamma-ray, and alpha-particlespectrometers, flew aboard LunarProspector and Mars Odyssey missionssponsored by NASA and were tailoredinto a small package to meet the scien-tific objectives of these missions. Allfour Los Alamos experiments aboardthe Lunar Prospector mission wereextremely successful. Measurementsfrom the neutron and gamma-ray spec-trometers enabled the first global map-ping of the seven most abundantrock-forming elements (oxygen, magne-sium, aluminum, silicon, calcium, tita-nium, and iron); the trace radioactive

elements thorium (refer to Figure A forthorium’s spatial distribution on themoon), uranium, and potassium; and theminor elements hydrogen, gadolinium,and samarium. This combined packageof instrumentation marked the firstapplication of neutron spectroscopy toplanetary exploration. Several highlightsemerged from our data collecting. Welearned that a unique, thorium-rich geo-chemical province exists on the earth-facing side of the moon. Heat from thedecay of radioactive elements is nodoubt responsible for the iron-richbasalt flows that give the moon itsblack-splotched appearance. In anotherlandmark success of these experiments,we discovered water-ice deposits thatreside within the permanently shadowedcraters near both lunar poles. Asteroids,comets, and interplanetary dust grainsstriking the surface were most likely

responsible for delivering the water tothe moon. In addition, using the alpha-particle spectrometer, we detected radonand its daughters that escape from ventsin the lunar crust. Our experimentalresults help unravel the origin and evo-lution of the moon and show the exis-tence and locations of lunar resourcesthat could be used to support themanned exploration of the moon.Firstreturns from the Los Alamos neutronspectrometer aboard Mars Odyssey haveproved the power of neutron spec-troscopy to map the volatile inventoryof nearly airless planetary bodies. Wehave discovered that a vast region ofMars south of –60° latitude is rich inburied water ice. Figure B shows lower-bound estimates of water on Mars.Although water ice has been predictedto be stable at these cold Martian lati-tudes, we were surprised by the extentand richness of this deposit (up to50 percent water ice by mass). Initialdata were taken during the southern latesummer and early fall, when the south-ern cap is smallest; our analysis clearlyshows that the residual south polar capof Mars is permanently covered by dryice—or frozen carbon dioxide (CO2). Amuch more extensive dry-ice cap coversthe northern region, poleward of about55°, which was in the grip of late winterand early spring. As time went on andthe sun returned to the north, the north-ern cap shrank and the southern capgrew, as CO2 evaporated and precipi-tated, respectively. This exchange ofCO2 between the Martian poles is amajor driving force for Mars’ atmos-pheric circulation, and a factor in thelong-term climate variability on Mars(which is also driven by the changingobliquity, eccentricity, and perihelion ofthe Martian orbit). The shrinking northpolar dry-ice cap reveals a basement as

Figure A. Global Map of Thorium Abundance on the MoonThe spatial distribution of thorium looks asymmetric, being strongly concentrated in asingle province on the earth-facing side of the moon. This province witnessed much ofthe volcanism that has distributed large quantities of basalt filling many of the large-impact basins, which are also concentrated on the earth-facing side of the Moon.

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Geochemical Studies of the Moon and PlanetsWilliam C. Feldman

rich in water ice as that in the southerncap.Future work for our group includesthe development of a neutron spectrom-eter, to be launched to Mercury in 2004,and a combined neutron and gamma-rayspectrometer, which is scheduled forlaunch in 2006 for a rendezvous withthe asteroids Vesta and Ceres. �

Number 28 2003 Los Alamos Science 167

Eyes in Space

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Figure B. The Distribution of Water on MarsData taken between February 2002 and April 2003 by the neutron spectrometer aboard the Mars Odyssey were used to deter-mine the minimum water distribution on Mars. Measurements of the epithermal neutron flux streaming from the planet’s surfaceallow us to determine the amount of hydrogen trapped in the upper one meter of soil. We then assume that the hydrogen ismost likely in the form of water ice and uniformly distributed throughout the soil layer, and convert the data to water massequivalent. (a) This cylindrical projection map of the midlatitudes shows the water equivalent hydrogen abundances by massoverlayed on a shaded relief map of the planet’s surface. The relief map was derived from tomography data taken by the MarsOrbiter laser altimeter (MOLA), an instrument designed at the Goddard Space Flight Center. The data are presented in stereo-graphic projection in (b) and (c), in which the latitudes poleward of +60° and –60° are shown, respectively.

William Feldman earned his bachelor’s in physics from the Massachusetts Institute of Technologyin 1961 and his Ph.D. in physics from Stanford University in 1968. A technical staff member at Los

Alamos National Laboratory since 1971, he has extensive experience in theanalysis and interpretation of solar wind and magnetospheric plasma data. Hewas responsible for a fast neutron spectrometer aboard the Air Force LACEmission that was launched into low-Earth orbit in 1990, the neutron subsys-tem of the Mars Observer gamma-ray spectrometer that was launched toMars in 1992, the neutron, gamma-ray, and alpha particle spectrometers thatwere launched to the Moon aboard Lunar Prospector in 1998, and the neutronspectrometer component of the gamma-ray spectrometer that is presentlyreturning data from Mars. He provided the design for the neutron spectrome-ter aboard Messenger, which will be launched to Mercury in 2004 and isresponsible for the gamma-ray and neutron spectrometer aboard DAWN,

which will be launched to Vesta and Ceres in August 2006.

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