Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p13d1735b&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P13D-1735
Other
[5462] Planetary Sciences: Solid Surface Planets / Polar Regions, [5494] Planetary Sciences: Solid Surface Planets / Instruments And Techniques
Scientific paper
Introduction: Recently the Lunar Exploration Neutron Detector (LEND) instrument onboard the NASA Lunar Reconnaissance Orbiter (LRO) provided data on the flux of epithermal neutrons in the Polar Regions showed that areas of low epithermal neutron flux, presumably due to high hydrogen content, were not closely related to the Permanently Shadowed Regions (PSRs) [1]. The areas which had suppressed epithermal neutron flux were referred to as Neutron Suppressed Regions (NSRs) to show a clear distinction from the PSRs. This work will first discuss the nature of the NSRs, and then discuss in more general terms the distribution of hydrogen and what that can tell us about processes acting in the Polar Regions. Mapping the NSRs: The raw LEND data from the collimated epithermal neutron sensors are processed to generate a records corrected for changes in sensor efficiency during warm-up and for times when one or more detectors may have been shut off. The neutrons are generated by interaction of cosmic rays with the surface, so the data have also been corrected for both short- and long-term cosmic ray variations in the cosmic ray flux. Maps of the epithermal neutron counting rates in the Polar Regions were made by binning the counts using HEALPix [2] bins of 1.7 km. The maps are smoothed by a box filter and the uncertainties associated with each bin are also calculated. Discussion: In our work, we found that there are two populations of hydrogen distribution in the polar areas. A plot of the histogram of number of bins vs. neutron flux shows a bimodal distribution with the low flux regions clearly distinguished from the non-suppressed flux in the other areas. We found that when the areas associated with the NSRs are removed, the remaining areas show a clear decrease in flux with decreasing distance to the pole showing a significant increase in hydrogen content. The two different populations of hydrogen distribution argue for at least two different processes being responsible. If the NSRs were associated with the PSRs, it would be clear that the cold surfaces in the PSRs were acting as cold traps for hydrogen, probably as water in some form. That not being the case, however, it is hard to imagine what could allow some PSRs to have high hydrogen content and others to have no significant enrichment at all. If the hydrogen rich areas were buried under a meter or more of hydrogen poor regolith, it would avoid detection via orbiting neutron sensors, but it is difficult to imagine how one PSR, like the Shoemaker Crater can have high hydrogen and a nearby one of comparable size show little or no enrichment of hydrogen. The enrichment of H in the non NSR areas is directly related to latitude and thus suggests surface temperature as a driving parameter for the enrichment. and argues for a process of migration of H. Water molecules will be released from the surfaces of grains at a rate controlled by temperature and this rate of migration will slow as the temperatures drop getting closer to the poles.
Boynton Willam V.
Droege G.
McClanahan Timothy P.
Mitrofanov Igor
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