Jeff A. Berger

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.

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.

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.

Effect of halite coatings on thermal infrared spectra

Characterizing the occurrence and distribution of soluble salts on planetary surfaces allows us to model or monitor aqueous and geochemical conditions. Thermal infrared (TIR) remote sensing is useful for identifying some types of salt deposits; however, mineral abundance determinations are based on the interaction of infrared with only the top few hundred micrometers of the surface. Thus, distinguishing massive deposits from coatings presents a challenge to TIR remote sensing. To better understand the TIR properties of spectrally transmissive halite coatings, we investigated the effects of coating thickness and texture on the TIR reflectance spectra of halite-coated glasses. We analyzed two coating textures with variable thickness: (1) continuous and (2) discontinuous particulate coatings. As halite coating thickness increases, the intensity of substrate absorption bands decreases nonlinearly. The substrate is not detected with halite coatings >150 µm thick. Therefore, when coatings are present, it is not possible to apply TIR models of mineral abundances using linear deconvolution algorithms. We show that continuous and coarse particulate halite coatings increase the reflectance minimum (emissivity maximum), making them potentially detectable on planetary surfaces by the methods of Osterloo et al. (2008). However, the reflectance minimum does not change if coatings have low areal coverage (<50%) or are composed of fine particles (<5 µm). Thus, halite that forms as efflorescent coatings or has been redistributed as fine dust is not detected using TIR spectroscopy. Our results explain why Cl salts are not detected with TIR spectroscopy at many locations on Mars despite high Cl contents.

Desiccation cracks provide evidence of lake drying on Mars, Sutton Island member, Murray formation, Gale Crater

Mars Science Laboratory (MSL) Curiosity rover data are used to describe the morphology of desiccation cracks observed in ancient lacustrine strata at Gale crater, Mars, and to interpret their paleoenvironmental setting. The desiccation cracks indicate subaerial exposure of lacustrine facies in the Sutton Island member of the Murray formation. In association with ripple cross-stratification and possible eolian cross-bedding, these facies indicate a transition from longer-lived perennial lakes recorded by older strata to younger lakes characterized by intermittent exposure. The transition from perennial to episodically exposed lacustrine environments provides evidence for local to regional climate change that can help constrain Mars climate models.

Observation of > 5 wt % zinc at the Kimberley outcrop, Gale crater, Mars: ZN DETECTION AT KIMBERLEY WITH CHEMCAM

Zinc-enriched targets have been detected at the Kimberley formation, Gale crater, Mars, using the Chemistry Camera (ChemCam) instrument. The Zn content is analyzed with a univariate calibration based on the 481.2 nm emission line. The limit of quantification for ZnO is 3 wt % (at 95% confidence level) and 1 wt % (at 68% confidence level). The limit of detection is shown to be around 0.5 wt %. As of sol 950, 12 targets on Mars present high ZnO content ranging from 1.0 wt % to 8.4 wt % (Yarrada, sol 628). Those Zn-enriched targets are almost entirely located at the Dillinger member of the Kimberley formation, where high Mn and alkali contents were also detected, probably in different phases. Zn enrichment does not depend on the textures of the rocks (coarse-grained sandstones, pebbly conglomerates, and resistant fins). The lack of sulfur enhancement suggests that Zn is not present in the sphalerite phase. Zn appears somewhat correlated with Na2O and the ChemCam hydration index, suggesting that it could be in an amorphous clay phase (such as sauconite). On Earth, such an enrichment would be consistent with a supergene alteration of a sphalerite gossan cap in a primary siliciclastic bedrock or a possible hypogene nonsulfide zinc deposition where Zn, Fe, Mn would have been transported in a reduced sulfur-poor fluid and precipitated rapidly in the form of oxides.