Latitude Variation of the Subsurface Lunar Temperature: Lunar Prospector Thermal Neutrons

Details Latitude Variation of the Subsurface Lunar Temperature: Lunar Prospector Thermal Neutrons Latitude Variation of the Subsurface Lunar Temperature: Lunar Prospector Thermal Neutrons

May 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm...p22a01l&link_type=abstract

American Geophysical Union, Spring Meeting 2001, abstract #P22A-01

Physics

5418 Heat Flow, 5464 Remote Sensing, 6250 Moon (1221)

Scientific paper

Planetary thermal neutron fluxes provide a sensitive proxy for mafic and feldspathic terranes, and are also necessary for translating measured gamma-ray line strengths to elemental abundances. Both functions require a model for near surface temperatures and a knowledge of the dependence of thermal neutron flux on temperature. We have explored this dependence for a representative sample of lunar soil compositions and surface temperatures using MCNP. For all soil samples, the neutron density is found to be independent of temperature, in accord with neutron moderation theory. The thermal neutron flux, however, does vary with temperature in a way that depends on D, the ratio of macroscopic absorption to energy-loss cross sections of soil compositions. The weakest dependence is for the largest D (which corresponds to the Apollo 17 high Ti basalt in our soil selection), and the largest dependence is for the lowest D (which corresponds to ferroan anorthosite, [FAN] in our selection). For the lunar model simulated, the depth at which the thermal neutron population is most sensitive to temperature is ~30 g/cm**2. These simulations were compared with the flux of thermal neutrons measured using the Lunar Prospector neutron spectrometer over the lunar highlands using a sub-surface temperature profile that varies with latitude, L, as (Cos L)**0.25. The fit is excellent. The best fitting equatorial temperature is determined to be, Teq=224+/-40 K. This temperature range brackets the average temperature measured below the thermal wave at the equator, Tmeas = 252+/-3K [Langseth and Keihm, 1977]. The present result represents the first measurement of subsurface temperature from orbit using neutrons.

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