Hydrogen Variability in the Murray Formation, Gale Crater, Mars

Abstract

The Mars Science Laboratory (MSL) Curiosity rover is exploring the Murray formation, a sequence of heterolithic mudstones and sandstones recording fluvial deltaic and lake deposits that comprise over 350 m of sedimentary strata within Gale crater. We examine >4,500 Murray formation bedrock points, employing recent laboratory calibrations for ChemCam laser-induced breakdown spectroscopy H measurements at millimeter scale. Bedrock in the Murray formation has an interquartile range of 2.3-3.1 wt.% H₂O, similar to measurements using the Dynamic Albedo of Neutrons and Sample Analysis at Mars instruments. However, specific stratigraphic intervals include high H targets (6-18 wt.% H₂O) correlated with Si, Mg, Ca, Mn, or Fe, indicating units with opal, hydrated Mg sulfates, hydrated Ca sulfates, Mn-enriched units, and akageneite or other iron oxyhydroxides, respectively. One stratigraphic interval with higher hydrogen is the Sutton Island unit and Blunts Point unit contact, where higher hydrogen is associated with Fe-rich, Ca-rich, and Mg-rich points. A second interval with higher hydrogen occurs in the Vera Rubin ridge portion of the Murray formation, where higher hydrogen is associated with Fe-rich, Ca-rich, and Si-rich points. We also observe trends in the H signal with grain size, separate from chemical variation, whereby coarser-grained rocks have higher hydrogen. Variability in the hydrogen content of rocks points to a history of water-rock interaction at Gale crater that included changes in lake water chemistry during Murray formation deposition and multiple subsequent groundwater episodes.

Keywords: Mars Science Laboratory Curiosity rover; aqueous geochemistry; hydrogen.

Figure 1

(a) The Gale crater stratigraphic column (Fedo et al., 2018) with drill sites marked as solid black points and (b and c) the normalized H peak area versus elevation for all ChemCam measured Murray formation bedrock targets. The smaller points are individual point observations, and the larger points are target averaged. Colors in (b) indicate Gini index mean score approximated grain size classes, when available (from Rivera‐Hernández et al., 2020). Colors in (c) indicate compositions of high Si, Fe, Mg, or Ca.

Figure 2

Boxplots showing the distribution of normalized H peak areas for (a) all Murray formation bedrock measurements, (b) each member of the Murray formation, and (c) each Gini index mean score categorized grain size class. The members are Pahrump Hills (PH), Hartmann's Valley (HV), Karasburg (K), Sutton Island (SI), Blunts Point (BP), Pettegrove Point (PP), and Jura (J). The grain size classes are coarse silt and smaller (GS1), coarse silt to very fine sand (GS2), very fine to medium sand (GS3), and medium to very coarse sand (GS4). The box extends from the lower to upper quartile values of the data, with a line at the median. The whiskers extend from the box to show the range of the data. Outlier points are defined as above or below 1.5 times the interquartile range from the median.

Figure 3

Normalized H peak area, or wt.% H₂O, versus ChemCam major oxides for all Murray formation bedrock observation points. Colors indicate grain size as estimated by the Gini index mean score when available. Vertical dashed line signifies 3‐sigma high H outliers. Shaded regions are typical Murray bedrock compositions (Frydenvang, 2020).

Figure 4

Principal component analysis of Murray formation bedrock observation points containing high H (>1 sigma) chemical composition. Colors indicate grain size as measured by the Gini index mean score. The arrows are the original major oxide and H data axes, projected into the eigenvector space. The arrow lengths represent amount of variance explained by each dimension.

Figure 5

Mastcam mosaics and Remote Micro‐Imager images of high normalized H peak area targets: (a) opal measured at Marias Pass, (b) hydrated Mg‐sulfates at Norwood Cove, (c) H‐rich Mn‐oxides at Newport Ledge, (d) hydrated Ca‐sulfate cement at Heron Island, and (e) akaganeite at Rock Hall.

Figure 6

Principal components analysis of Murray formation bedrock observation points containing high H (>1 sigma) chemical composition in the Rock Hall area (elevation greater than −4,150 m and less than −4,140 m). Colors indicate grain size as measured by the Gini index mean score. The arrows are the original major oxide and H data axes, projected into the eigenvector space. The arrow lengths represent amount of variance explained by each dimension.