P43B-2113
Observation of Very High Passive Mode Thermal Neutron Counts by the MSL DAN Instrument at Marias Pass in Gale Crater

Thursday, 17 December 2015
Poster Hall (Moscone South)
Insoo Jun1, Igor G. Mitrofanov2, Maxim L Litvak2, Anton B. Sanin2, Luz Maria Martinez Sierra1, Jens Frydenvang3, Fedor Fedosov2, Dmitry Golovin2, Craig J Hardgrove4, Karl Harshman5, Alexander Kozyrev2, Alexey V Malakhov2, Michael A Mischna1, Jeffrey Moersch6, Maxim Mokrousov2, Sergey Nikiforov2, Christopher G Tate6 and The MSL Science Team, (1)NASA Jet Propulsion Laboratory, Pasadena, CA, United States, (2)Space Research Institute RAS, Moscow, Russia, (3)Los Alamos National Laboratory, Los Alamos, NM, United States, (4)Arizona State University, Tempe, AZ, United States, (5)Univ Arizona, Tucson, AZ, United States, (6)University of Tennessee, Knoxville, TN, United States
Abstract:
Since landing in August 2012, DAN has provided a wealth of scientific data from the successful surface operation in both Active mode and Passive mode. The main scientific objectives of DAN are twofold. The primary objective is to measure the bulk hydrogen abundance (in forms of water or hydrated minerals) of the subsurface. The secondary objective is to measure the background neutron environment at the surface. DAN achieves the first objective by using the pulsed neutron generator (PNG) as a pulsed 14 MeV neutron source (active mode). When no neutron pulse is used (passive mode), DAN measures the neutron background environment at the Martian surface. The DAN active data have extensively and successfully used in estimating the near-surface contents of hydrogen (in terms of “water equivalent hydrogen” or WEH) and chlorine (in terms of “absorption equivalent chlorine” or AEC) at multiple locations in Gale crater. In the mean time, DAN has been accumulating the passive mode data in almost all sols since landing, and it is the main topic of this presentation. Particularly, we want to show the DAN data on Sol 991 and 992 where we observed very high passive thermal neutron count – a factor of ~2 higher than usual count. As a reminder, note that many different factors would influence the amplitude of DAN passive thermal neutron counts – GCR condition, soil compositions, water/chlorine contents, etc.

The Sol 991/992 location, which is at a region near a rock target called Elk in Marias Pass, was also analyzed by another MSL instrument ChemCAM. And interestingly, its data show that the Elk target is estimated to contain as high as 80 wt% of SiO2, which is radically different from what we have used as a reference soil composition for the interpretation of the DAN passive data. This means that the DAN passive data simulations have to be revisited to account for different soil composition data at the Elk target region. In the final presentation, we will present new simulation results based on the ChemCAM measured soil composition at Elk and the impact of water/chlorine content variation to the DAN passive thermal neutron counts. The DAN active data at Elk will be discussed in a companion paper by M. Litvak.

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