Detecting Low-Contrast Features in the Cosmic Ray Albedo Proton Yield Map of the Moon
Abstract
High energy cosmic rays constantly bombard the lunar regolith, producing (via nuclear evaporation[1]) secondary 'albedo' or 'splash' particles like protons and neutrons, some of which escape back to space. Lunar Prospector and the Lunar Reconnaissance Orbiter (LRO), have shown that the energy distribution of albedo neutrons is modulated by the elemental composition of the lunar regolith[2-5], and by ice deposits[6] in permanently shadowed polar craters. Here we investigate an analogous phenomenon with high energy lunar albedo protons. Using the CRaTER instrument (Cosmic Ray Telescope for the Effects of Radiation) on LRO, we measure albedo protons (60 to 150 MeV) to construct a cosmic ray albedo proton map of the Moon. Our current map is a significant improvement over the proof-of-concept map of Wilson et al.[7]. In addition to using more numerous minimum ionizing GCR protons for normalization, we filter out all solar particle enhancement periods and make use of all six of CRaTER's detectors to reduce contamination from spurious non-proton events in the data stream. The average yield of albedo protons from the maria is 0.8% × 0.4% higher than the yield from the highlands. In addition there appear to be localized peaks in the albedo proton yield that are co-located with peaks in trace elemental abundances as measured by the Lunar Prospector Gamma Ray Spectrometer. More data may reveal subtler proton yield variations correlated with latitude, time of day, or the locations of permanently shadowed craters, due to the presence of water frost. Given that the most obvious features in the map have a proton yield only 2σ above average, the search for more subtle regions of enhancement or reduction in proton yield will require precise corrections for small but systematic effects of time and spacecraft altitude on the apparent proton yield. We will show the effects of these trends as well as the latest version of the albedo proton map. References: [1] Bethe (1937) Rev. Mod. Phys., 9, 69. [2] Feldman W. C. et al. (1998) Science, 281, 1496-1500. [3] Gasnault, O. et al. (2001) GRL, 28, 3797-3800. [4] Maurice, S. et al. (2004) JGR, 109, E07S04. [5] Mitrofanov I. G. et al. (2010) Science, 330, 483-486. [6] Feldman W. C. et al. (1997) JGR, 102, 25565-25574. [7] Wilson, J. K. et al. (2012) JGR, 117, E00H23. Figure 1. Top: Color-coded lunar albedo proton map, with two high-yielding mare regions labeled 'A' and 'B'. Bottom: Clementine white-light mosaic of lunar surface.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.P51E1778W
- Keywords:
-
- 6250 PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS Moon;
- 5421 PLANETARY SCIENCES: SOLID SURFACE PLANETS Interactions with particles and fields;
- 5464 PLANETARY SCIENCES: SOLID SURFACE PLANETS Remote sensing;
- 5494 PLANETARY SCIENCES: SOLID SURFACE PLANETS Instruments and techniques