Mathematics – Logic
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999nvm..conf...29j&link_type=abstract
Workshop on New Views of the Moon 2: Understanding the Moon Through the Integration of Diverse Datasets, p. 29
Mathematics
Logic
Chemical Composition, Lunar Maria, Lunar Resources, Lunar Rocks, Lunar Soil, Lunar Surface, Moon, Neutrons, Solar Wind, Clementine Spacecraft, Lunar Prospector, Magnetic Effects, Neutron Spectrometers, Remote Sensing, Water
Scientific paper
The epithermal neutron distribution from Lunar Prospector (LP) and the H abundance and maturity of returned lunar soils are being used synergistically to estimate global H distribution. The ability to determine lunar H abundance using a combination of remote sensing and laboratory analyses broadens our understanding of the retentivity of volatiles in the lunar regolith and the nature of the sources from which the volatiles arrived (i.e., solar wind, cometary impacts). This is relevant to the search for water ice at the lunar poles as well as other potential resources such as H-3. Knowledge of these possible resource-rich regions will be important for future mission planning, instrument selection, and landing-site analyses. The Level I calibrated epithermal neutron distribution maps (initially calibrated to remove major instrumental effects by the LP team) have been used for first-order calibration to H abundance. The equation of Feldman et al. i.e., (epithermal* = epithermal - (0.068 * thermal) was used to minimize the effects of compositional variations between mare and highlands (likely due to major-element concentrations such as Ti, Fe) on the epithermal neutron counting rates. Comparison of the average H abundances for the Apollo landing site soils with the epithermal* neutron counts for the landing sites from results in the rather poor correlation shown. Neglecting the Apollo 11 sample, the sense of the correlation is correct (fewer epithermal* neutrons implies greater H abundance), but the correlation is still mediocre (r=-0.574). This may result from using the average values for the landing sites as opposed to the individual soil sample values. It can be seen in the epithermal* map that the highest counting rates (likely the lowest H abundances) correspond well to regions of fresh, Copernican-age impacts (e.g., Tycho, Hayn, Jackson, possibly Stevinus). This implies a dearth of solar-wind-implanted gases, which could be explained by the immaturity of the regolith in these regions. But there are also mare regions near the limbs (Orientale, Crisium) with high values that require other explanations. Further, while the darker regions in the polar areas have been interpreted as water-ice-bearing regions, the low-valued highland terrain west of Sinus Iridium requires further study. Overall, the solar-wind fluence model's predictions of highest volatile abundances on the central farside and lowest values on the central nearside are not immediately apparent in the epithermal* map. However, if the lower counting rates (higher H abundances) at longitudes 75-180E are real, it would be consistent with the solar wind model (at least for that portion of the eastern limb and farside). Hydrogen contents of 54 Apollo bulk lunar soils will be compared to the epithermal neutron counts for the landing sites to determine mare precisely an average correlation. This will be applied to the LP data to estimate H content on the lunar surface. Because exposure age affects retained solar-wind-implanted volatile abundances, incorporation of soil maturity using the Is/FeO parameter (reduced:total Fe ratio) and/or spectrally-derived maturity data from lunar soils will also be used in conjunction with H abundances to refine the correlation. Once a H map is constructed, it will be used to test a solar-wind fluence model for the Moon developed previously that represents relative solar-wind-implanted elemental abundances (assuming minimal variations in impact history and chemical composition of the soils, as well as minimal saturation of soils with volatiles). Deviations of the H distribution from the idealized solar fluence model will be investigated by incorporating the effects of local magnetic fields, composition (e.g., Ti, Fe), and soil maturity. further, previous work by the authors has shown that abundances of H-3 (a potential lunar resource relevant as a fuel for future nuclear fusion power) in the lunar regolith can be estimated using the fluence model in combination with surface maturity and Ti maps constructed using Clementine multispectral data. The modeled solar-wind fluence distribution map will be updated based on observations of the epithermal neutron-derived H maps, and the Clementine and LP Ti-abundance maps will be incorporated with Clementine surface maturity maps to construct revised estimates of H-3 abundance. The calibration of the LP neutron spectrometer data to provide maps of H abundance will be important in understanding the nature of the solar-wind-implanted volatiles in the lunar regolith, particularly the distribution, migration, and retentivity of H relevant to understanding the possible presence of water ice at the lunar poles. Hydrogen maps will be useful in separating the near surface geologic history (e.g., solar-wind implantation and micrometeorite bombardment history related to surface maturity) from the underlying crustal evolution, the knowledge of which is a required component of our total understanding of the planetary history of the Moon. The preliminary observations made here regarding correlations of epithermal* neutron counts with geologic features are intriguing and will require further study. Potential improvements to models of solar-wind fluence will assist in mapping other volatiles in the regolith, particularly H-3, which along with H and water ice are some of the most promising potential lunar resources available on the Moon. Additional information contained in the original
Johnson Jay Robert
Lucey Paul G.
Swindle Timothy D.
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