Evidence for perchlorates and the origin of chlorinated hydrocarbons detected by SAM at the Rocknest aeolian deposit in Gale Crater

A single scoop of the Rocknest aeolian deposit was sieved (< 150 µm), and four separate sample portions, each with a mass of ~50 mg, were delivered to individual cups inside the Sample Analysis at Mars (SAM) instrument by the Mars Science Laboratory rover's sample acquisition system. The samples were analyzed separately by the SAM pyrolysis evolved gas and gas chromatograph mass spectrometer analysis modes. Several chlorinated hydrocarbons including chloromethane, dichloromethane, trichloromethane, a chloromethylpropene, and chlorobenzene were identified by SAM above background levels with abundances of ~0.01 to 2.3 nmol. The evolution of the chloromethanes observed during pyrolysis is coincident with the increase in O_2 released from the Rocknest sample and the decomposition of a product of N‐methyl‐N‐(tert‐butyldimethylsilyl)‐trifluoroacetamide (MTBSTFA), a chemical whose vapors were released from a derivatization cup inside SAM. The best candidate for the oxychlorine compounds in Rocknest is a hydrated calcium perchlorate (Ca(ClO_4)_2·nH_2O), based on the temperature release of O_2 that correlates with the release of the chlorinated hydrocarbons measured by SAM, although other chlorine‐bearing phases are being considered. Laboratory analog experiments suggest that the reaction of Martian chlorine from perchlorate decomposition with terrestrial organic carbon from MTBSTFA during pyrolysis can explain the presence of three chloromethanes and a chloromethylpropene detected by SAM. Chlorobenzene may be attributed to reactions of Martian chlorine released during pyrolysis with terrestrial benzene or toluene derived from 2,6‐diphenylphenylene oxide (Tenax) on the SAM hydrocarbon trap. At this time we do not have definitive evidence to support a nonterrestrial carbon source for these chlorinated hydrocarbons, nor do we exclude the possibility that future SAM analyses will reveal the presence of organic compounds native to the Martian regolith

QnAs with John P. Grotzinger. Interview by Prashant Nair.

In late November 2011, the National Aeronautics and Space Administration (NASA) plans to launch its robotic explorer to scour Mars for signs of the planet’s ability to support life. The Mars Science Laboratory (MSL) spacecraft is scheduled to lift off from Cape Canaveral Air Force Station in Florida, shuttling Curiosity, an SUV-sized rover with a hefty scientific payload, to the red planet’s surface. John Grotzinger, a member of the National Academy of Sciences and professor of geology at the California Institute of Technology, helps oversee the mission. He became involved in the quest after studying how changes in the Earth’s environment helped influence animal diversity in some parts of our planet. Here, Grotzinger discusses the MSL with PNAS

Physicochemical properties of concentrated Martian surface waters

Understanding the processes controlling chemical sedimentation is an important step in deciphering paleoclimatic conditions from the rock records preserved on both Earth and Mars. Clear evidence for subaqueous sedimentation at Meridiani Planum, widespread saline mineral deposits in the Valles Marineris region, and the possible role of saline waters in forming recent geomorphologic features all underscore the need to understand the physical properties of highly concentrated solutions on Mars in addition to, and as a function of, their distinct chemistry. Using thermodynamic models predicting saline mineral solubility, we generate likely brine compositions ranging from bicarbonate-dominated to sulfate-dominated and predict their saline mineralogy. For each brine composition, we then estimate a number of thermal, transport, and colligative properties using established models that have been developed for highly concentrated multicomponent electrolyte solutions. The available experimental data and theoretical models that allow estimation of these physicochemical properties encompass, for the most part, much of the anticipated variation in chemistry for likely Martian brines. These estimates allow significant progress in building a detailed analysis of physical sedimentation at the ancient Martian surface and allow more accurate predictions of thermal behavior and the diffusive transport of matter through chemically distinct solutions under comparatively nonstandard conditions

Numerical Modeling of Ooid Size and the Problem of Neoproterozoic Giant Ooids

Temporal variation in ooid size reflects important changes in physical and chemical characteristics of depositional environments. Two numerical models are used to evaluate the effects of several processes influencing ooid size. The first demonstrates that low supply of new ooid nuclei and high cortex growth rate each promote growth of large ooids. The second model demonstrates that high average water velocity and velocity gradient also enhance ooid growth. Several Neoproterozoic oolites contain unusually large ooids, some reaching diameters of up to 16 mm. While lower nuclei supply and higher ooid growth rate may have prevailed prior to the evolution of carbonate-secreting organisms, neither difference can explain the presence of giant ooids in Neoproterozoic deposits because Archean through Mesoproterozoic ooids rarely exceed 5 mm in diameter. In the presence of lower nuclei supply and higher growth rate, high average water velocity may have allowed growth of such large ooids. Higher average water velocity could have been due to a prevalence of carbonate ramps over rimmed shelves during Neoproterozoic time

Anomalous Carbonate Precipitates: Is the Precambrian the Key to the Permian?

Late Permian reefs of the Capitan complex, west Texas; the Magnesian Limestone, England; Chuenmuping reef, south China; and elsewhere contain anomalously large volumes of aragonite and calcite marine cements and seafloor crusts, as well as abundant microbial precipitates. These components strongly influenced reef growth and may have been responsible for the construction of rigid, open reefal frames in which bryozoans and sponges became encrusted and structurally reinforced. In some cases, such as the upper biostrome of the Magnesian Limestone, precipitated microbialites and inorganic crusts were the primary constituents of the reef core. These microbial and inorganic reefs do not have modern marine counterparts; on the contrary, their textures and genesis are best understood through comparison with the older rock record, particularly that of the early Precambrian. Early Precambrian reefal facies are interpreted to have formed in a stratified ocean with anoxic deep waters enriched in carbonate alkalinity. Upwelling mixed deep and surface waters, resulting in massive seafloor precipitation of aragonite and calcite. During Mesoproterozoic and early Neoproterozoic time, the ocean became more fully oxidized, and seafloor carbonate precipitation was significantly reduced. However, during the late Neoproterozoic, sizeable volumes of deep ocean water once again became anoxic for protracted intervals; the distinctive "cap carbonates" found above Neoproterozoic tillites attest to renewed upwelling of anoxic bottom water enriched in carbonate alkalinity and ^(12)C. Anomalous late Permian seafloor precipitates are interpreted as the product, at least in part, of similar processes. Massive carbonate precipitation was favored by: 1) reduced shelf space for carbonate precipitation, 2) increased flux of Ca to the oceans during increased continental erosion, 3) deep basinal anoxia that generated upwelling waters with elevated alkalinities, and 4) further evolution of ocean water in the restricted Delaware, Zechstein, and other basins. Temporal coincidence of these processes resulted in surface seawater that was greatly supersaturated by Phanerozoic standards and whose only precedents occurred in Precambrian oceans

New constraints on Precambrian ocean composition

The Precambrian record of carbonate and evaporite sedimentation is equivocal. In contrast to most previous interpretations, it is possible that Archean, Paleoproterozoic, and to a lesser extent, Meso to Neoproterozoic seawater favored surplus abiotic carbonate precipitation, as aragonite and (hi-Mg?) calcite, in comparison to younger times. Furthermore, gypsum/anhydrite may have been only rarely precipitated prior to halite precipitation during evaporation prior to about 1.8 Ga. Two effects may have contributed to these relationships. First, sulfate concentration of seawater may have been critically low prior to about 1.9 Ga so the product m_(Ca)^(++) • m_(SO_4)^(--) would not have produced gypsum before halite, as in the Mesoproterozoic to modern ocean. Second, the bicarbonate to calcium ratio was sufficiently high so that during progressive evaporation of seawater, calcium would have been exhausted before the gypsum field was reached. The pH of the Archean and Paleoproterozoic ocean need not have been significantly different from the modern value of 8.1, even at CO_2 partial pressures of a tenth of an atmosphere. Higher CO_2 partial pressures require somewhat lower pH values

The origin of life as a planetary phenomenon

We advocate an integrative approach between laboratory experiments in prebiotic chemistry and geologic, geochemical, and astrophysical observations to help assemble a robust chemical pathway to life that can be reproduced in the laboratory. The cyanosulfidic chemistry scenario described here was developed by such an integrative iterative process. We discuss how it maps onto evolving planetary surface environments on early Earth and Mars and the value of comparative planetary evolution. The results indicate that Mars can offer direct evidence for geochemical conditions similar to prebiotic Earth, whose early record has been erased. The Jezero crater is now the chosen landing site for NASA’s Mars 2020 rover, making this an extraordinary opportunity for a breakthrough in understanding life’s origins

Singularity analysis: a tool for extracting lithologic and stratigraphic content from seismic data

In this work, we test an amplitude-independent method of seisimic data analysis designed to extract lithologic information about stratigraphic horizons. We apply the method of singularity characterization in an attempt to determine the sharpness of lithologic boundaries. We infer the sharpness of the boundary based upon a fractional integration of the seismic trace. The order of fractional integration is taken to represent the abruptness of the lithologic transition responsible for a given reflector. We find that the method output behaves in a geologically reasonable manner which suggests that our method is responding to lithologic variations along boundaries responsible for prominent reflectors in the data