Measuring martian organics and methane: The Curiosity rover has been sampling on Mars for the past 5 years (see the Perspective by ten Kate). Eigenbrode et al. used two instruments in the SAM (Sample Analysis at Mars) suite to catch traces of complex organics preserved in 3-billion-year-old sediments. Heating the sediments released an array of organics and volatiles reminiscent of organic-rich sedimentary rock found on Earth. Most methane on Earth is produced by biological sources, but numerous abiotic processes have been proposed to explain martian methane. Webster et al. report atmospheric measurements of methane covering 3 martian years and found that the background level varies with the local seasons. The seasonal variation provides an important clue for determining the origin of martian methane. Science , this issue p. 1096 , p. 1093 ; see also p. 1068

The Mars Science Laboratory Curiosity rover has been traversing strata at the base of Aeolis Mons (informally known as Mount Sharp) since September 2014. The Murray formation makes up the lowest exposed strata of the Mount Sharp group and is composed primarily of finely laminated lacustrine mudstone intercalated with rare crossbedded sandstone that is prodeltaic or fluvial in origin. We report on the first three drilled samples from the Murray formation, measured in the Pahrump Hills section. Rietveld refinements and FULLPAT full pattern fitting analyses of X-ray diffraction patterns measured by the MSL CheMin instrument provide mineral abundances, refined unit-cell parameters for major phases giving crystal chemistry, and abundances of X-ray amorphous materials. Our results from the samples measured at the Pahrump Hills and previously published results on the Buckskin sample measured from the Marias Pass section stratigraphically above Pahrump Hills show stratigraphic variations in the mineralogy; phyllosilicates, hematite, jarosite, and pyroxene are most abundant at the base of the Pahrump Hills, and crystalline and amorphous silica and magnetite become prevalent higher in the succession. Some trace element abundances measured by APXS also show stratigraphic trends; Zn and Ni are highly enriched with respect to average Mars crust at the base of the Pahrump Hills (by 7.7 and 3.7 times, respectively), and gradually decrease in abundance in stratigraphically higher regions near Marias Pass, where they are depleted with respect to average Mars crust (by more than an order of magnitude in some targets). The Mn stratigraphic trend is analogous to Zn and Ni, however, Mn abundances are close to those of average Mars crust at the base of Pahrump Hills, rather than being enriched, and Mn becomes increasingly depleted moving upsection. Minerals at the base of the Pahrump Hills, in particular jarosite and hematite, as well as enrichments in Zn, Ni, and Mn, are products of acid-sulfate alteration on Earth. We hypothesize that multiple influxes of mildly to moderately acidic pore fluids resulted in diagenesis of the Murray formation and the observed mineralogical and geochemical variations. The preservation of some minerals that are highly susceptible to dissolution at low pH (e.g., mafic minerals and fluorapatite) suggests that acidic events were not long-lived and that fluids may not have been extremely acidic (pH>2). Alternatively, the observed mineralogical variations within the succession may be explained by deposition in lake waters with variable Eh and/or pH, where the lowermost sediments were deposited in an oxidizing, perhaps acidic lake setting, and sediments deposited in the upper Pahrump Hills and Marias Pass were deposited lake waters with lower Eh and higher pH

Measuring martian organics and methane: The Curiosity rover has been sampling on Mars for the past 5 years (see the Perspective by ten Kate). Eigenbrode et al. used two instruments in the SAM (Sample Analysis at Mars) suite to catch traces of complex organics preserved in 3-billion-year-old sediments. Heating the sediments released an array of organics and volatiles reminiscent of organic-rich sedimentary rock found on Earth. Most methane on Earth is produced by biological sources, but numerous abiotic processes have been proposed to explain martian methane. Webster et al. report atmospheric measurements of methane covering 3 martian years and found that the background level varies with the local seasons. The seasonal variation provides an important clue for determining the origin of martian methane. Science , this issue p. 1096 , p. 1093 ; see also p. 1068

The depths of an ancient lake on Mars: Gale crater on Mars was once a lake fed by rivers and groundwater. Hurowitz et al . analyzed 3.5 years of data from the Curiosity rover’s exploration of Gale crater to determine the chemical conditions in the ancient lake. Close to the surface, there were plenty of oxidizing agents and rocks formed from large, dense grains, whereas the deeper layers had more reducing agents and were formed from finer material. This redox stratification led to very different environments in different layers, which provides evidence for Martian climate change. The results will aid our understanding of where and when Mars was once habitable. Science , this issue p. eaah6849

The sample analysis at Mars instrument evolved gas analyzer (SAM‐EGA) has detected evolved water, H₂, SO₂, H₂S, NO, CO₂, CO, O₂, and HCl from two eolian sediments and nine sedimentary rocks from Gale Crater, Mars. These evolved gas detections indicate nitrates, organics, oxychlorine phase, and sulfates are widespread with phyllosilicates and carbonates occurring in select Gale Crater materials. Coevolved CO₂ (160 ± 248–2373 ± 820 μgC_(CO2)/g) and CO (11 ± 3–320 ± 130 μgC_(CO)/g) suggest that organic C is present in Gale Crater materials. Five samples evolved CO₂ at temperatures consistent with carbonate (0.32 ± 0.05–0.70 ± 0.1 wt % CO₃). Evolved NO amounts to 0.002 ± 0.007–0.06 ± 0.03 wt % NO₃. Evolution of O₂ suggests that oxychlorine phases (chlorate/perchlorate) (0.05 ± 0.025–1.05 ± 0.44 wt % ClO₄) are present, while SO₂ evolution indicates the presence of crystalline and/or poorly crystalline Fe and Mg sulfate and possibly sulfide. Evolved H₂O (0.9 ± 0.3–2.5 ± 1.6 wt % H₂O) is consistent with the presence of adsorbed water, hydrated salts, interlayer/structural water from phyllosilicates, and possible inclusion water in mineral/amorphous phases. Evolved H₂ and H₂S suggest that reduced phases occur despite the presence of oxidized phases (nitrate, oxychlorine, sulfate, and carbonate). SAM results coupled with CheMin mineralogical and Alpha‐Particle X‐ray Spectrometer elemental analyses indicate that Gale Crater sedimentary rocks have experienced a complex authigenetic/diagenetic history involving fluids with varying pH, redox, and salt composition. The inferred geochemical conditions were favorable for microbial habitability and if life ever existed, there was likely sufficient organic C to support a small microbial population.

Organic synthesis on Mars occurs by the electrochemical reduction of CO ₂ , a reaction that is highly relevant for abiotic organic synthesis on early Earth.