Scott J. V. VanBommel

Elemental Geochemistry of Sedimentary Rocks at Yellowknife Bay, Gale Crater, Mars.

Sedimentary rocks examined by the Curiosity rover at Yellowknife Bay, Mars, were derived from sources that evolved from an approximately average martian crustal composition to one influenced by alkaline basalts. No evidence of chemical weathering is preserved, indicating arid, possibly cold, paleoclimates and rapid erosion and deposition. The absence of predicted geochemical variations indicates that magnetite and phyllosilicates formed by diagenesis under low-temperature, circumneutral pH, rock-dominated aqueous conditions. Analyses of diagenetic features (including concretions, raised ridges, and fractures) at high spatial resolution indicate that they are composed of iron- and halogen-rich components, magnesium-iron-chlorine–rich components, and hydrated calcium sulfates, respectively. Composition of a cross-cutting dike-like feature is consistent with sedimentary intrusion. The geochemistry of these sedimentary rocks provides further evidence for diverse depositional and diagenetic sedimentary environments during the early history of Mars.

Geochemical diversity in first rocks examined by the Curiosity Rover in Gale Crater: Evidence for and significance of an alkali and volatile‐rich igneous source

The first four rocks examined by the Mars Science Laboratory Alpha Particle X-ray Spectrometer indicate that Curiosity landed in a lithologically diverse region of Mars. These rocks, collectively dubbed the Bradbury assemblage, were studied along an eastward traverse (sols 46–102). Compositions range from Na- and Al-rich mugearite Jake_Matijevic to Fe-, Mg-, and Zn-rich alkali-rich basalt/hawaiite Bathurst_Inlet and span nearly the entire range in FeO* and MnO of the data sets from previous Martian missions and Martian meteorites. The Bradbury assemblage is also enriched in K and moderately volatile metals (Zn and Ge). These elements do not correlate with Cl or S, suggesting that they are associated with the rocks themselves and not with salt-rich coatings. Three out of the four Bradbury rocks plot along a line in elemental variation diagrams, suggesting mixing between Al-rich and Fe-rich components. ChemCam analyses give insight to their degree of chemical heterogeneity and grain size. Variations in trace elements detected by ChemCam suggest chemical weathering (Li) and concentration in mineral phases (e.g., Rb and Sr in feldspars). We interpret the Bradbury assemblage to be broadly volcanic and/or volcaniclastic, derived either from near the Gale crater rim and transported by the Peace Vallis fan network, or from a local volcanic source within Gale Crater. High Fe and Fe/Mn in Et_Then likely reflect secondary precipitation of Fe^(3+) oxides as a cement or rind. The K-rich signature of the Bradbury assemblage, if igneous in origin, may have formed by small degrees of partial melting of metasomatized mantle.

A global Mars dust composition refined by the Alpha‐Particle X‐ray Spectrometer in Gale Crater

Modern Martian dust is similar in composition to the global soil unit and bulk basaltic Mars crust, but it is enriched in S and Cl. The Alpha Particle X-ray Spectrometer (APXS) on the Mars Science Laboratory Curiosity rover analyzed air fall dust on the science observation tray (o-tray) in Gale Crater to determine dust oxide compositions. The o-tray dust has the highest concentrations of SO3 and Cl measured in Mars dust (SO3 8.3%; Cl 1.1 wt %). The molar S/Cl in the dust (3.35 ± 0.34) is consistent with previous studies of Martian dust and soils (S/Cl = 3.7 ± 0.7). Fe is also elevated ~25% over average Mars soils and the bulk crust. These enrichments link air fall dust with the S-, Cl-, and Fe-rich X-ray amorphous component of Gale Crater soil. Dust and soil have the same S/Cl, constraining the surface concentrations of S and Cl on a global scale.

APXS-derived chemistry of the Bagnold dune sands: Comparisons with Gale crater soils and the global martian average

We present APXS data for the active Bagnold dune field within the Gale impact crater (MSL mission). We derive an APXS-based Average Basaltic Soil (ABS) composition for Mars based on past and recent data from the MSL and MER missions. This represents an update to the Taylor and McLennan (2009) average martian soil, and facilitates comparison across martian datasets. The active Bagnold dune field is compositionally distinct from the ABS, with elevated Mg, Ni and Fe, suggesting mafic mineral enrichment, and uniformly low levels of S, Cl and Zn, indicating only a minimal dust component. A relationship between decreasing grain size and increasing felsic content is revealed. The Bagnold Sands possess the lowest S/Cl of all martian unconsolidated materials.. Gale soils exhibit relatively uniform major element compositions, similar to Meridiani Planum and Gusev Crater basaltic soils (MER missions). However, they show minor enrichments in K, Cr, Mn and Fe, which may signify a local contribution. The lithified eolian Stimson Formation within the Gale impact crater is compositionally similar to the ABS and Bagnold sands, which provide a modern analogue for these ancient eolian deposits. Compilation of APXS-derived soil data reveals a generally homogenous global composition for martian soils, but one that can be locally modified due to past or extant geologic processes that are limited in both space and time.

Oxidation of manganese in an ancient aquifer, Kimberley formation, Gale crater, Mars

The Curiosity rover observed high Mn abundances (>25 wt % MnO) in fracture-filling materials that crosscut sandstones in the Kimberley region of Gale crater, Mars. The correlation between Mn and trace metal abundances plus the lack of correlation between Mn and elements such as S, Cl, and C, reveals that these deposits are Mn oxides rather than evaporites or other salts. On Earth, environments that concentrate Mn and deposit Mn minerals require water and highly oxidizing conditions; hence, these findings suggest that similar processes occurred on Mars. Based on the strong association between Mn-oxide deposition and evolving atmospheric dioxygen levels on Earth, the presence of these Mn phases on Mars suggests that there was more abundant molecular oxygen within the atmosphere and some groundwaters of ancient Mars than in the present day.

Potassium‐rich sandstones within the Gale impact crater, Mars: The APXS perspective

The Alpha Particle X-ray spectrometer (APXS) onboard the Curiosity rover at the Kimberley location within Gale crater, Mars, analyzed basaltic sandstones that are characterized by potassium enrichments of two to eight times estimates for average martian crust. They are the most potassic rocks sampled on Mars to date. They exhibit elevated Fe, Mg, Mn and Zn, and depleted Na, Al and Si. These compositional characteristics are common to other potassic sedimentary rocks analyzed by APXS at Gale, but distinct from other landing sites and martian meteorites. CheMin and APXS analysis of a drilled sample indicate mineralogy dominated by sanidine, Ca-rich and Ca-poor clinopyroxene, magnetite, olivine and andesine. The anhydrous mineralogy of the Kimberley sample, and the normative mineralogy derived from APXS of other Bathurst class rocks, together indicate provenance from one or more potassium-rich magmatic or impact-generated source rocks on the rim of Gale crater or beyond. Elevated Zn, Ge and Cu suggest that a localized area of the source region(s) experienced hydrothermal alteration, which was subsequently eroded, dispersed and diluted throughout the unaltered sediment during transport and deposition. The identification of the basaltic, high potassium Bathurst class and other distinct rock compositional classes by the APXS, attests to the diverse chemistry of crustal rocks within and in the vicinity of Gale crater. We conclude that weathering, transport and diagenesis of the sediment did not occur in a warm and wet environment, but instead under relatively cold and wet conditions, perhaps more fitting with processes typical of glacial/periglacial environments.

High concentrations of manganese and sulfur in deposits on Murray Ridge, Endeavour Crater, Mars

Abstract Mars Reconnaissance Orbiter HiRISE images and Opportunity rover observations of the ~22 km wide Noachian age Endeavour Crater on Mars show that the rim and surrounding terrains were densely fractured during the impact crater-forming event. Fractures have also propagated upward into the overlying Burns formation sandstones. Opportunity’s observations show that the western crater rim segment, called Murray Ridge, is composed of impact breccias with basaltic compositions, as well as occasional fracture-filling calcium sulfate veins. Cook Haven, a gentle depression on Murray Ridge, and the site where Opportunity spent its sixth winter, exposes highly fractured, recessive outcrops that have relatively high concentrations of S and Cl, consistent with modest aqueous alteration. Opportunity’s rover wheels serendipitously excavated and overturned several small rocks from a Cook Haven fracture zone. Extensive measurement campaigns were conducted on two of them: Pinnacle Island and Stuart Island. These rocks have the highest concentrations of Mn and S measured to date by Opportunity and occur as a relatively bright sulfate-rich coating on basaltic rock, capped by a thin deposit of one or more dark Mn oxide phases intermixed with sulfate minerals. We infer from these unique Pinnacle Island and Stuart Island rock measurements that subsurface precipitation of sulfate-dominated coatings was followed by an interval of partial dissolution and reaction with one or more strong oxidants (e.g., O2) to produce the Mn oxide mineral(s) intermixed with sulfate-rich salt coatings. In contrast to arid regions on Earth, where Mn oxides are widely incorporated into coatings on surface rocks, our results demonstrate that on Mars the most likely place to deposit and preserve Mn oxides was in fracture zones where migrating fluids intersected surface oxidants, forming precipitates shielded from subsequent physical erosion.

MSL‐APXS titanium observation tray measurements: Laboratory experiments and results for the Rocknest fines at the Curiosity field site in Gale Crater, Mars

The Mars Science Laboratory (MSL) rover, Curiosity, has a titanium science observation tray (o-tray), upon which portions from drilled and scooped Martian samples can be delivered for analyses by the Alpha-Particle X-ray Spectrometer (APXS). The standard APXS calibration approach to derive elemental concentrations cannot be applied to samples on the o-tray because they (1) have a nonuniform three-dimensional distribution within the APXS field of view and (2) are thin ( 90 µm). To develop techniques for interpreting MSL-APXS o-tray measurements, we conducted laboratory measurements of thin particulate basalt samples on Ti metal with the Flight Equivalent APXS Unit. The experiments demonstrate that, relative to an “infinitely thick” sample, increasing areal coverage of particulates on a Ti metal substrate results in a proportional decrease in the Ti signal and increase in the sample signal. Count rates for heavier elements (Mn and Fe) drop with decreasing sample thickness because the mean thickness is smaller than the APXS information depth. Similar effects were seen in the MSL-APXS o-tray measurement of Rocknest fines on Martian solar day 95, an aliquot of material delivered to Sample Analysis at Mars and Chemistry and Mineralogy. The thin layer effect caused a drop in Mn and Fe signals, which cannot be quantitatively compared to the in situ Rocknest target “Portage” because sample thickness was unknown. Otherwise, Rocknest fines on the o-tray had no significant compositional differences from Portage, except for slight increases in S and Cl.

Diverse Lithologies and Alteration Events on the Rim of Noachian‐Aged Endeavour Crater, Meridiani Planum, Mars: In Situ Compositional Evidence

We report the results of geological studies by the Opportunity Mars rover on the Endeavour Crater rim. Four major units occur in the region (oldest to youngest): the Matijevic, Shoemaker, Grasberg, and Burns formations. The Matijevic formation, consisting of fine‐grained clastic sediments, is the only pre‐Endeavour‐impact unit and might be part of the Noachian etched units of Meridiani Planum. The Shoemaker formation is a heterogeneous polymict impact breccia; its lowermost member incorporates material eroded from the underlying Matijevic formation. The Shoemaker formation is a close analog to the Bunte Breccia of the Ries Crater, although the average clast sizes are substantially larger in the latter. The Grasberg formation is a thin, fine‐grained, homogeneous sediment unconformably overlying the Shoemaker formation and likely formed as an airfall deposit of unknown areal extent. The Burns formation sandstone overlies the Grasberg, but compositions of the two units are distinct; there is no evidence that the Grasberg formation is a fine‐grained subfacies of the Burns formation. The rocks along the Endeavour Crater rim were affected by at least four episodes of alteration in the Noachian and Early Hesperian: (i) vein formation and alteration of preimpact Matijevic formation rocks, (ii) low‐water/rock alteration along the disconformity between the Matijevic and Shoemaker formations, (iii) alteration of the Shoemaker formation along fracture zones, and (iv) differential mobilization of Fe and Mn, and CaSO_4‐vein formation in the Grasberg and Shoemaker formations. Episodes (ii) and (iii) possibly occurred together, but (i) and (iv) are distinct from either of these.

Retrieval of Compositional End‐Members From Mars Exploration Rover Opportunity Observations in a Soil‐Filled Fracture in Marathon Valley, Endeavour Crater Rim

The Opportunity rover investigated a gentle swale on the rim of Endeavour crater called Marathon Valley where a series of bright planar outcrops are cut into polygons by fractures. A wheel scuff performed on one of the soil-filled fracture zones revealed the presence of three end-members identified on the basis of Pancam multispectral imaging observations covering ~0.4 to 1 μm: red and dark pebbles, and a bright soil clod. Multiple overlapping Alpha Particle X-ray Spectrometer (APXS) measurements were collected on three targets within the scuff zone. The field of view of each APXS measurement contained various proportions of the Pancam-based end-members. Application of a log maximum likelihood method for retrieving the composition of the end-members using the 10 APXS measurements shows that the dark pebble end-member is compositionally similar to average Mars soil, with slightly elevated S and Fe. In contrast, the red pebble end-member exhibits enrichments in Al and Si and is depleted in Fe and Mg relative to average Mars soil. The soil clod end-member is enriched in Mg, S, and Ni. Thermodynamic modeling of the soil clod end-member composition indicates a dominance of sulfate minerals. We hypothesize that acidic fluids in fractures leached and oxidized the basaltic host rock, forming the red pebbles, and then evaporated to leave behind sulfate-cemented soil.