Jennifer C. Stern

Volatile, Isotope, and Organic Analysis of Martian Fines with the Mars Curiosity Rover

Samples from the Rocknest aeolian deposit were heated to ~835°C under helium flow and evolved gases analyzed by Curiosity’s Sample Analysis at Mars instrument suite. H2O, SO2, CO2, and O2 were the major gases released. Water abundance (1.5 to 3 weight percent) and release temperature suggest that H2O is bound within an amorphous component of the sample. Decomposition of fine-grained Fe or Mg carbonate is the likely source of much of the evolved CO2. Evolved O2 is coincident with the release of Cl, suggesting that oxygen is produced from thermal decomposition of an oxychloride compound. Elevated δD values are consistent with recent atmospheric exchange. Carbon isotopes indicate multiple carbon sources in the fines. Several simple organic compounds were detected, but they are not definitively martian in origin.

Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases

All terrestrial organisms depend on nucleic acids (RNA and DNA), which use pyrimidine and purine nucleobases to encode genetic information. Carbon-rich meteorites may have been important sources of organic compounds required for the emergence of life on the early Earth; however, the origin and formation of nucleobases in meteorites has been debated for over 50 y. So far, the few nucleobases reported in meteorites are biologically common and lacked the structural diversity typical of other indigenous meteoritic organics. Here, we investigated the abundance and distribution of nucleobases and nucleobase analogs in formic acid extracts of 12 different meteorites by liquid chromatography–mass spectrometry. The Murchison and Lonewolf Nunataks 94102 meteorites contained a diverse suite of nucleobases, which included three unusual and terrestrially rare nucleobase analogs: purine, 2,6-diaminopurine, and 6,8-diaminopurine. In a parallel experiment, we found an identical suite of nucleobases and nucleobase analogs generated in reactions of ammonium cyanide. Additionally, these nucleobase analogs were not detected above our parts-per-billion detection limits in any of the procedural blanks, control samples, a terrestrial soil sample, and an Antarctic ice sample. Our results demonstrate that the purines detected in meteorites are consistent with products of ammonium cyanide chemistry, which provides a plausible mechanism for their synthesis in the asteroid parent bodies, and strongly supports an extraterrestrial origin. The discovery of new nucleobase analogs in meteorites also expands the prebiotic molecular inventory available for constructing the first genetic molecules.

The imprint of atmospheric evolution in the D/H of hesperian clay minerals on Mars

Of water and methane on Mars The Curiosity rover has been collecting data for the past 2 years, since its delivery to Mars (see the Perspective by Zahnle). Many studies now suggest that many millions of years ago, Mars was warmer and wetter than it is today. But those conditions required an atmosphere that seems to have vanished. Using the Curiosity rover, Mahaffy et al. measured the ratio of deuterium to hydrogen in clays that were formed 3.0 to 3.7 billion years ago. Hydrogen escapes more readily than deuterium, so this ratio offers a snapshot measure of the ancient atmosphere that can help constrain when and how it disappeared. Most methane on Earth has a biological origin, so planetary scientists have keenly pursued its detection in the martian atmosphere as well. Now, Webster et al. have precisely confirmed the presence of methane in the martian atmosphere with the instruments aboard the Curiosity rover at Gale crater. Science, this issue p. 412, p. 415; see also p. 370 A measurement with the Curiosity rover probes the Hesperian era and constrains the timing of hydrogen loss. [Also see Perspective by Zahnle] The deuterium-to-hydrogen (D/H) ratio in strongly bound water or hydroxyl groups in ancient martian clays retains the imprint of the water of formation of these minerals. Curiosity’s Sample Analysis at Mars (SAM) experiment measured thermally evolved water and hydrogen gas released between 550° and 950°C from samples of Hesperian-era Gale crater smectite to determine this isotope ratio. The D/H value is 3.0 (±0.2) times the ratio in standard mean ocean water. The D/H ratio in this ~3-billion-year-old mudstone, which is half that of the present martian atmosphere but substantially higher than that expected in very early Mars, indicates an extended history of hydrogen escape and desiccation of the planet.

Abundances and implications of volatile‐bearing species from evolved gas analysis of the Rocknest aeolian deposit, Gale Crater, Mars

The Sample Analysis at Mars (SAM) instrument on the Mars Science Laboratory (MSL) rover Curiosity detected evolved gases during thermal analysis of soil samples from the Rocknest aeolian deposit in Gale Crater. Major species detected (in order of decreasing molar abundance) were H2O, SO2, CO2, and O2, all at the µmol level, with HCl, H2S, NH3, NO, and HCN present at the tens to hundreds of nmol level. We compute weight % numbers for the major gases evolved by assuming a likely source and calculate abundances between 0.5 and 3 wt.%. The evolution of these gases implies the presence of both oxidized (perchlorates) and reduced (sulfides or H-bearing) species as well as minerals formed under alkaline (carbonates) and possibly acidic (sulfates) conditions. Possible source phases in the Rocknest material are hydrated amorphous material, minor clay minerals, and hydrated perchlorate salts (all potential H2O sources), carbonates (CO2), perchlorates (O2 and HCl), and potential N-bearing materials (e.g., Martian nitrates, terrestrial or Martian nitrogenated organics, ammonium salts) that evolve NH3, NO, and/or HCN. We conclude that Rocknest materials are a physical mixture in chemical disequilibrium, consistent with aeolian mixing, and that although weathering is not extensive, it may be ongoing even under current Martian surface conditions.

Evidence for indigenous nitrogen in sedimentary and aeolian deposits from the Curiosity rover investigations at Gale crater, Mars

Significance We present data supporting the presence of an indigenous source of fixed nitrogen on the surface of Mars in the form of nitrate. This fixed nitrogen may indicate the first stage in development of a primitive nitrogen cycle on the surface of ancient Mars and would have provided a biochemically accessible source of nitrogen. The Sample Analysis at Mars (SAM) investigation on the Mars Science Laboratory (MSL) Curiosity rover has detected oxidized nitrogen-bearing compounds during pyrolysis of scooped aeolian sediments and drilled sedimentary deposits within Gale crater. Total N concentrations ranged from 20 to 250 nmol N per sample. After subtraction of known N sources in SAM, our results support the equivalent of 110–300 ppm of nitrate in the Rocknest (RN) aeolian samples, and 70–260 and 330–1,100 ppm nitrate in John Klein (JK) and Cumberland (CB) mudstone deposits, respectively. Discovery of indigenous martian nitrogen in Mars surface materials has important implications for habitability and, specifically, for the potential evolution of a nitrogen cycle at some point in martian history. The detection of nitrate in both wind-drifted fines (RN) and in mudstone (JK, CB) is likely a result of N2 fixation to nitrate generated by thermal shock from impact or volcanic plume lightning on ancient Mars. Fixed nitrogen could have facilitated the development of a primitive nitrogen cycle on the surface of ancient Mars, potentially providing a biochemically accessible source of nitrogen.

Sulfur‐bearing phases detected by evolved gas analysis of the Rocknest aeolian deposit, Gale Crater, Mars

The Sample Analysis at Mars (SAM) instrument suite detected SO2, H2S, OCS, and CS2 from ~450 to 800°C during evolved gas analysis (EGA) of materials from the Rocknest aeolian deposit in Gale Crater, Mars. This was the first detection of evolved sulfur species from a Martian surface sample during in situ EGA. SO2 (~3–22 µmol) is consistent with the thermal decomposition of Fe sulfates or Ca sulfites, or evolution/desorption from sulfur-bearing amorphous phases. Reactions between reduced sulfur phases such as sulfides and evolved O2 or H2O in the SAM oven are another candidate SO2 source. H2S (~41–109 nmol) is consistent with interactions of H2O, H2 and/or HCl with reduced sulfur phases and/or SO2 in the SAM oven. OCS (~1–5 nmol) and CS2 (~0.2–1 nmol) are likely derived from reactions between carbon-bearing compounds and reduced sulfur. Sulfates and sulfites indicate some aqueous interactions, although not necessarily at the Rocknest site; Fe sulfates imply interaction with acid solutions whereas Ca sulfites can form from acidic to near-neutral solutions. Sulfides in the Rocknest materials suggest input from materials originally deposited in a reducing environment or from detrital sulfides from an igneous source. The presence of sulfides also suggests that the materials have not been extensively altered by oxidative aqueous weathering. The possibility of both reduced and oxidized sulfur compounds in the deposit indicates a nonequilibrium assemblage. Understanding the sulfur mineralogy in Rocknest materials, which exhibit chemical similarities to basaltic fines analyzed elsewhere on Mars, can provide insight in to the origin and alteration history of Martian surface materials.

The Nitrate/(Per)chlorate Relationship on Mars

Nitrate was recently detected in Gale Crater sediments on Mars at abundances up to ~600 mg/kg, confirming predictions of its presence at abundances consistent with models based on impact-generated nitrate and other sources of fixed nitrogen. Terrestrial Mars analogs, Mars meteorites, and other solar system materials help establish a context for interpreting in situ nitrate measurements on Mars, particularly in relation to other cooccuring salts. We compare the relative abundance of nitrates to oxychlorine (chlorate and/or perchlorate, hereafter (per)chlorate) salts on Mars and Earth. The nitrate/(per)chlorate ratio on Mars is  103), suggesting not only the absence of biological activity but also different (per)chlorate formation mechanisms on Mars than on Earth.