Hydrogen and chlorine abundances in the Kimberley formation of Gale crater measured by the DAN instrument on board the Mars Science Laboratory Curiosity rover

Abstract

The Dynamic Albedo of Neutron (DAN) instrument on board the Mars Science Laboratory Curiosity rover acquired a series of measurements as part of an observational campaign of the Kimberley area in Gale crater. These observations were planned to assess the variability of bulk hydrogen and neutron‐absorbing elements, characterized as chlorine‐equivalent concentration, in the geologic members of the Kimberley formation and in surface materials exposed throughout the area. During the traverse of the Kimberley area, Curiosity drove primarily over the “Smooth Hummocky” unit, a unit composed primarily of sand and loose rocks, with occasional stops at bedrock of the Kimberley formation. During the Kimberley campaign, DAN detected ranges of water equivalent hydrogen (WEH) and chlorine‐equivalent concentrations of 1.5–2.5 wt % and 0.6–2 wt %, respectively. Results show that as the traverse progressed, DAN observed an overall decrease in both WEH and chlorine‐equivalent concentration measured over the sand and loose rocks of the Smooth Hummocky unit. DAN measurements of WEH and chlorine‐equivalent concentrations in the well‐exposed sedimentary bedrock of the Kimberley formation show fluctuations with stratigraphic position. The Kimberley campaign also provided an opportunity to compare measurements from DAN with those from the Sample Analysis at Mars (SAM) and the Alpha‐Particle X‐ray Spectrometer (APXS) instruments. DAN measurements obtained near the Windjana drill location show a WEH concentration of ~1.5 wt %, consistent with the concentration of low‐temperature absorbed water measured by SAM for the Windjana drill sample. A comparison between DAN chlorine‐equivalent concentrations measured throughout the Kimberley area and APXS observations of corresponding local surface targets and drill fines shows general agreement between the two instruments.

Geologic overview of the Mars Science Laboratory rover mission at the Kimberley, Gale crater, Mars

“…Litvak et al . [] describe how DAN observations at the Kimberley are best described by a two‐layer model with ~2.3% water in the top 15 cm and 1.4% water in the bottom layer. This inferred hydration in the upper layer is consistent with estimates from SAM EGA analyses of the Windjana sample [ Litvak et al ., ].…”

“…Curiosity approached the Kimberley waypoint and performed initial reconnaissance during sols 535-570, surveyed and characterized the units at the Kimberley during sols 571-609, completed a drilling and sampling campaign at the target Windjana during sols 610-629, and departed the Kimberley during sols 630-634. Key results of the rover's geologic investigation during these periods are presented in this special section of Journal of Geophysical Research-Planets [Le Deit et al, 2016;Lasue et al, 2016;Litvak et al, 2016;Mangold et al, 2016;Thompson et al, 2016;Treiman et al, 2016;Vasconcelos et al, 2016] and elsewhere [e.g., Grotzinger et al, 2015;Lanza et al, 2016]. This introduction provides the context for specific findings at the Kimberley and integrates them together into a broader understanding of the geologic history and past habitability of Gale crater.…”

Chemistry, mineralogy, and grain properties at Namib and High dunes, Bagnold dune field, Gale crater, Mars: A synthesis of Curiosity rover observations

“…The units inFigure 10aare reported waterequivalent hydrogen; no discrimination between H 2 O and OH is implied. The units inFigure 10bare indicative of the Cl wt % present, but very large variations in other neutron absorbers like Fe, Ti, and Mn could modify chlorine-equivalent concentrations from the true concentration of chlorine by 5-10% (relative; formally, the axis is ξCl as in Mitrafanov et al[2014] andLitvak et al, 2016]).…”

A rover's geologic field campaign: Exploration of the Kimberley by Curiosity

“…The lack of evidence of hydrous alteration is consistent with results from the Dynamic Albedo of Neutrons investigation showing the Kimberley to be drier than any terrains traversed by the rover before arriving at the Kimberley [ Litvak et al ., ]. Indeed, the Dillinger member is the most volatile poor of the Kimberley units [ Litvak et al ., ]. Together, these observations suggest sediment transportation with low water‐rock ratio and/or under cold conditions to limit alteration [ Thompson et al ., ; Treiman et al ., ].…”

“…The preservation and retention of fine‐grained minerals in the Windjana sample such as sanidine, plagioclase, feldspar, and olivine that are easily altered by water suggest little alteration occurred during sediment transport or after deposition [ Treiman et al ., ; Thompson et al ., ]. The lack of evidence of hydrous alteration is consistent with results from the Dynamic Albedo of Neutrons investigation showing the Kimberley to be drier than any terrains traversed by the rover before arriving at the Kimberley [ Litvak et al ., ]. Indeed, the Dillinger member is the most volatile poor of the Kimberley units [ Litvak et al ., ].…”