BULK HYDRATION OF A CLAY-RICH UNIT IN GALE CRATER, MARS

The Mars Science Laboratory rover, Curiosity, is currently investigating the clay-bearing ‘Glen Torridon’ (GT) region in Gale crater, where abundant phyllosilicates have been observed in x-ray diffraction experiments by the CheMin instrument. The Dynamic Albedo of Neutrons (DAN) instrument is a neutron spectrometer that, in active mode, emits neutrons which interact with subsurface materials and some of which scatter back to the DAN detectors. DAN is sensitive to bulk hydrogen (reported as water-equivalent hydrogen or WEH), and neutron absorbing elements in the top ~50 cm of the subsurface. Hydrogen efficiently thermalizes neutrons, increasing observed neutron flux, whereas neutrons absorbers reduce neutron flux. Iron and boron are neutron absorbers of particular interest. Iron is abundant in phyllosilicates observed in Gale crater (Fe-smectites), and boron is abundant in terrestrial phyllosilicates, can promote prebiotic chemistry, and can help to preserve organic signatures. By comparing DAN observations to simulations with variable subsurface compositions, we can discriminate between the effects of variable hydration and neutron absorber content.

Curiosity began exploring GT on sol (martian day) 2297. DAN observed increasing thermal neutron flux as Curiosity transitioned into GT (sols 2297 – 2309). Simulation results show an increase in WEH in this sol range from 2.7 wt.% up to 4.4 wt.%, before leveling off between ~3.5 – 4.5 wt.% for about 100 sols (330 m distance). This is consistent with orbital identification of phyllosilicates, which can contain up to tens of wt.% WEH. The GT area explored thus far is rubble covered with scattered exposures of bedrock. Dedicated DAN active measurements were taken on sol 2407 over bedrock from which phyllosilicate-rich samples had been retrieved by Curiosity. However, DAN measured the lowest WEH in GT at this location (2.4 wt.%). This suggests that the phyllosilicates are collapsed (partially dehydrated) and that the rubble, which dominates the GT surface, contains even more abundant phyllosilicates and/or a distinct hydrogen reservoir. Further hydration mapping of GT using the DAN instrument with refined simulations using in situ geochemistry, including boron and iron from ChemCam, will help constrain the WEH-bearing phases and abundances in GT.