The 6 August 2012 arrival of the curiosity rover on the surface of Mars delivered the most technically advanced geochemistry laboratory ever sent to the surface of another planet. Its 10 instruments (1)* were commissioned for operations and were tested on a diverse set of materials, including rocks, soils, and the atmosphere, during the first 100 martian days (sols) of the mission. The five articles presented in full in the online edition of Science (www.sciencemag.org/extra/curiosity), with abstracts in print (pp. 1476–1477), describe the mission's initial results, in which Curiosity's full laboratory capability was used.
Curiosity was sent to explore a site located in Gale crater, where a broad diversity of materials was observed from orbit. Materials representing interactions with aqueous environments were targeted for study because of the emphasis on understanding habitable environments. In addition, the mission's science objectives also include characterizing the geologic diversity of the landing site at all scales, including loose surface materials such as impact ejecta, soils, and windblown accumulations of fine sediments. In certain cases, such characterization may even provide constraints on the evolution of the planet as a whole. Two notable points along Curiosity's initial 500-m traverse included Jake_M, a loose rock sitting on the plains, and Rocknest, an accumulation of windblown sand, silt, and dust that formed in the lee of some rocky outcrops. Sparse outcrops of lithified fluvial conglomerate were also encountered (2).
As described by Stolper et al., Jake_M was encountered ∼282 m away from the landing site and is a dark, macroscopically homogeneous igneous rock representing a previously unknown martian magma type. In contrast to the relatively unfractionated Fe-rich and Al-poor tholeiitic basalts typical of martian igneous rocks, it is highly alkaline and fractionated. No other known martian rock is as compositionally similar to terrestrial igneous rocks; Jake_M compares very closely with an uncommon terrestrial rock type known as a mugearite, typically found on ocean islands and in rift zones. It probably originates from magmas generated by low degrees of partial melting at high pressure of possibly water-rich, chemically altered martian mantle that is different from the sources of other known martian basalts.
Over the first 100 sols of the mission, the ChemCam instrument returned >10,000 laser-induced breakdown spectra, helping to characterize surface material diversity. ChemCam's laser acts effectively as a microprobe, distinguishing between fine soil grains and coarser ∼1-mm grains. Based on these data, Meslin et al. report that the coarse soil fraction contains felsic (Si- and Al-rich) grains, mimicking the composition of larger felsic rock fragments found during the traverse and showing that these larger components probably break apart to form part of the soil. In contrast, the fine-grained soil component is mafic, similar to soils observed by the Pathfinder and Mars Exploration Rover missions.
Curiosity scooped, processed, and analyzed a small deposit of windblown sand/silt/dust at Rocknest that has similar morphology and bulk elemental composition to other aeolian deposits studied at other Mars landing sites. Based solely on analysis of CheMin x-ray diffraction (XRD) data from Mars, calibrated with terrestrial standards, Bish et al. estimate the Rocknest deposit to be composed of ∼71% crystalline material of basaltic origin, in addition to ∼29% x-ray–amorphous materials. In an independent approach, Blake et al. used Alpha Particle X-ray Spectrometer data to constrain the bulk composition of the deposit and XRD data and phase stoichiometry to constrain the chemistry of the crystalline component, with the difference being attributed to the amorphous component, resulting in estimates of ∼55% crystalline material of basaltic origin and ∼45% x-ray–amorphous materials. The amorphous component may contain nanophase iron oxide similar to what was observed by earlier rovers. The similarity between basaltic soils observed at Rocknest and other Mars sites implies either global-scale mixing of basaltic material or similar regional-scale basaltic source material or some combination of both. No hydrated phases were detected. However, as shown by Leshin et al., pyrolysis of Rocknest fines using the Sample Analysis at Mars (SAM) instrument suite revealed volatile species, probably in the amorphous component, including H2O, SO2, CO2, and O2, in order of decreasing abundance. ChemCam measurements of these materials also revealed the presence of H. It is likely that H2O is contained in the amorphous component and CO2 was liberated via the decomposition of Fe/Mg carbonates present below the XRD detection limit of 1 to 2%. Isotopic data from SAM indicate that this H2O, and possibly the CO2, were derived from the atmosphere. SAM analysis also revealed oxychloride compounds similar to those found by earlier missions, suggesting that their accumulation reflects global planetary processes. The evolution of CO2 during pyrolysis and the observation of simple chlorohydrocarbons during SAM gas chromatograph mass spectrometer analyses could be consistent with organic carbon derived from a terrestrial instrument background source, or a martian source, either exogenous or indigenous.
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↵* References may be found on page 1477 after the abstracts.