- © 2015 by the Mineralogical Society of America
The science investigations enabled by Curiosity rover's instruments focus on identifying and exploring the habitability of the Martian environment. Measurements of noble gases, organic and inorganic compounds, and the isotopes of light elements permit the study of the physical and chemical processes that have transformed Mars throughout its history. Samples of the atmosphere, volatiles released from soils, and rocks from the floor of Gale Crater have provided a wealth of new data and a window into conditions on ancient Mars.
The investigations by Curiosity rover as part of the Mars Science Laboratory (MSL) mission were designed to explore the geology and geochemistry in Gale Crater (Grotzinger 2009) and to determine and evaluate the habitability potential at present and in the past. The fundamental parameters of a habitable environment are the presence of liquid water and sources of energy to feed metabolism of even the simplest of prokaryotic organisms. Chemical attributes, such as the availability of elements necessary for life (e.g. C, H, O, N, etc.) and redox state, and environmental physical characteristics, such as temperature excursions and ionizing radiation, also play critical roles in determining the habitability of Martian environments. In the case of Mars, orbital characteristics (such as changes in the axial tilt, or obliquity; Touma and Wisdom 1993), the size of the planet relative to Venus and Earth, its distance from the Sun, gravitational perturbations by migrating giant planets, and impacting asteroids have profoundly influenced its evolution and have certainly had a significant impact on surface environmental conditions and habitability. Moreover, the early loss of a magnetic dynamo, as indicated by remnant crustal magnetization data (Acuna et al. 1999), and evidence for a lack of recent extensive crustal recycling through Earth-like plate tectonics (Yin 2012) are factors that caused environments on Mars to diverge dramatically from those on Earth. The prevalence or rarity of liquid water also carries consequences for habitability. There is much evidence that liquid water was abundant on the surface of early Mars, but an outstanding question is how long conditions favorable for the emergence and sustenance of early microbial life could have persisted (Fig. 1). The ambitious investigations by Curiosity will advance our understanding of environmental evolution and habitability by making a wide range of measurements.
Here we provide an overview of the volatile and isotopic measurements made by Curiosity that not only may help us to understand current surface environments but also may document milestones in the evolution of Mars. These data provide information on atmospheric loss; on the chemical composition, isotopic composition, and redox states of soils and rocks; on the clays that preserve an isotopic signature from more than 3 billion years ago; and on the signatures in surface rocks of nuclear transformations caused by cosmic radiation. The search for methane in the atmosphere and organic compounds in the rocks are also core mission objectives. Although the central mound of Gale Crater (Mt. Sharp), with its layers of clays and hydrated minerals—evident from orbital spectroscopy (e.g. Milliken et al. 2010)—is a primary exploration target, the landing site fortuitously provided the mission with an opportunity to meet mission goals more immediately in Yellowknife Bay (Fig. 2).
The Sample Analysis at Mars (SAM) instrument suite is a chemical laboratory (Mahaffy et al. 2012) miniaturized to the size of a microwave oven to fit in the belly of Curiosity (Fig. 3) and was selected to complement the capabilities of nine other science instruments. With 54 valves, dozens of heaters, 2 turbomolecular pumps, gas getters and scrubbers, thermoelectric cooler–driven hydrocarbon traps, pressurized helium tanks, multiple calibration samples, a sample manipulation system, and a sophisticated experiment sequencer implemented on a powerful central processing unit (CPU), it is the most complex instrument to have operated on another planetary surface. SAM's three instruments are a tunable laser spectrometer (TLS) designed to measure carbon dioxide and water and their isotopes and to search for trace levels of methane; a quadrupole mass spectrometer (QMS) with a mass range of 2–535 daltons; and a gas chromatograph designed to work with the mass spectrometer (GCMS) mode in the search for organic compounds in surface materials. Primary measurement modes of SAM are listed in Table 1. In order to use the power resources efficiently, SAM typically operates at night when the rover is stationary and its CPU is asleep.
ATMOSPHERIC TRACERS OF LOSS TO SPACE
The isotopic composition of the noble gases and the light elements C, O, N, and H in the atmospheric gases CO2, N2, and H2O reflects both the sources of these gases and the processes that isotopically fractionate them over billions of years. Gas sources include initial outgassing of the mantle shortly after the formation of Mars (Elkins-Tanton et al. 2005), volcanic venting over most of the history of Mars, and exogenous input from meteorites and larger bodies such as asteroids and comets (Owen et al. 1992; Lunine et al. 2003). Atmospheric loss can be to surface reservoirs through weathering or precipitation and/or to space through a variety of mechanisms (e.g. Jakosky and Jones 1997). Such mechanisms include early hydrodynamic escape, thermal escape, sputtering erosion by ions picked up and accelerated into the top of the atmosphere by the fields of the solar wind, and dissociative recombination. In this latter mechanism, an electron and an ion recombine in the upper atmosphere, resulting in an atom with sufficient energy to escape the gravity of Mars. Atmospheric loss to space preferentially removes the lighter isotopes, leaving the atmosphere enriched in the heavier isotope (e.g. Bogard et al. 2001; Pepin 2006).
Previous in situ measurements of isotope ratios in a variety of gases provided by instruments on the Viking missions (Biemann et al. 1976), ground-based spectroscopy (e.g. Krasnopolsky et al. 2007), and analysis of Martian meteorites (e.g. Bogard et al. 2001) were refined during the early stages of the MSL mission using experiments carried out by SAM's TLS (Webster et al. 2013) and QMS (Atreya et al. 2013; Mahaffy et al. 2013; Wong et al. 2013). Each of the elements studied showed the imprint of atmospheric escape (Fig. 4; Table 2). It is evident that this mechanism dominates in the case of the relevant isotopes in argon, water, nitrogen, and carbon dioxide. One ratio of particular importance is the primordial 36Ar/38Ar. Argon, once released to the atmosphere, does not return to surface reservoirs; thus, it provides one of the cleanest measurements of long-term atmospheric escape. Models of the extent of atmospheric loss established by this isotope pair alone (Hutchins and Jakosky 1996) lead to an estimate of loss that is at least several times the volume of gas in the present atmosphere.
GASES EVOLVED FROM ANCIENT LAKE BED SEDIMENTS
The first rocks drilled by Curiosity at Yellowknife Bay were found to be mudstones containing ~20% smectite clay in addition to the basaltic primary silicate minerals feldspar, pyroxene, and olivine. The presence of phyllosilicates was confirmed independently (Vaniman et al. 2014) by Curiosity's X-ray diffractometer (CheMin), and it is consistent with the observation of a high-temperature water release in evolved gas analysis (EGA) mode by SAM (Ming et al. 2014). Nodules and lighter-tone veins observed in the rock suggest subsequent diagenesis (Grotzinger et al. 2014). Gases thermally released by heating tens of milligrams of drilled material to ~900 °C utilizing a continuous temperature increase (Fig. 5) included, in decreasing order of abundance, H2O, SO2, CO2, O2, H2, NO, H2S, and HCl, as well as trace hydrocarbons. The volatile sources constituted ~5% of the mass of the sample and revealed the following features: smectite clay signatures from characteristic low and high temperatures of water release; degradation of an oxychlorine compound; possible self-combustion of organic compounds by the oxygen released from the oxychloride compound; the presence of both reduced and oxidized sulfur compounds; and a small number of trace organic compounds that survived the self-combustion. These results are summarized in Table 3. When these results were combined with the elemental and mineralogical data obtained by other instruments on MSL and the stratigraphic context inferred from images acquired by Curiosity's cameras, it was concluded that Curiosity had characterized an ancient lake bed with a high potential for being habitable.
RADIOGENIC AND COSMOGENIC NOBLE GAS ISOTOPES
By removing the chemically active gases with SAM's scrubber and getter through the heating of ~150 mg of a powdered Yellowknife Bay sample to ~900 °C, evolved volumes of 3He, 21Ne, 36Ar, and 40Ar were measured (Farley et al. 2014). With the thin Martian atmosphere, high-energy cosmic radiation penetrates only the first 2–3 meters of the surface, producing spallation products (including 3He, 21Ne, and 36Ar) and neutron-capture products (including 36Ar). A near-surface exposure age of 78 ± 30 million years was independently determined using these three noble gas measurements. 40Ar, on the other hand, is produced primarily from the radiogenic decay of potassium, and the SAM measurement of 40Ar combined with potassium measurements from the alpha particle X-ray spectrometer enabled a rock formation age of 4.21 ± 0.35 billion years to be determined. Even with uncertainties regarding the partitioning of potassium between the basaltic and amorphous portions of the sample, the measurement allowed us to conclude with 95% confidence that the materials in the Gale Crater wall that washed down to form these mudstones are older than 3.6 billion years. This is consistent with the cratering record in the source region for these materials.
The exposure-age results have important implications for the MSL mission objective to find organic compounds produced on ancient Mars because long, near-surface exposure to cosmic radiation can destroy or transform these organic compounds (e.g. Pavlov et al. 2012). The combined 3He, 21Ne, and 36Ar cosmic radiation exposure-age results were interpreted in the context of a mechanical-weathering model in which wind-driven dust and sand erosion produced a scarp that started the noble gas production clock on the sample analyzed by SAM at about 78 million years ago, when the ancient rock's scarp face was exposed. This result motivated the MSL science and operations teams to search for the most recently exposed materials near the base of an outcrop (Fig. 6); this would optimize the possibility of finding organic compounds only minimally transformed after deposition of the host sediments.
D/H RATIO LOCKED INTO ANCIENT CLAYS
Water released from the mudstone samples at low temperature included adsorbed water, water from hydrated compounds in the amorphous fraction of the samples, and interlayer water in the smectite clays derived from basalt alteration (Fig. 5). Water released at high temperature, on the other hand, resulted from the dehydroxylation of the OH groups in these clays. Because OH is structurally bound once the clays are formed, its thermal decomposition should reflect the deuterium/hydrogen (D/H) ratio of the water in which the clay precipitated. Multiple thermal evolved-gas analyses were carried out on samples of Yellowknife Bay mudstones, and the D/H ratio of the evolved water was measured using both the QMS and the TLS. A clear trend was observed: D/H decreased as the release temperature increased, and low-temperature released water showed a D/H value close to Mars' atmospheric value of ~6 times that of Earth's standard mean ocean water (SMOW). However, mixing of the low- and high-temperature components in the nominal SAM EGA experiment prevented a clean measurement of the D/H ratio in the high-temperature water release, so a special stepped-heating experiment was designed and implemented to remove the water that initially evolves over an extended time period below 550 °C. The subsequent release of water gave a consistent D/H value halfway between SMOW and the current greatly enriched atmospheric value (Mahaffy et al. 2015). This experiment showed that at the time of clay formation in the Yellowknife Bay aqueous environment, substantial atmospheric loss must have already taken place, if we make the reasonable assumption that the D/H ratios of early Earth and Mars were similar.
THE ONGOING SEARCH FOR MARTIAN ORGANICS
If early microbial life existed on Mars, the most direct evidence for not only an ancient habitable environment but also an occupied habitat could come from molecular fossils of microbes. On Earth, examples of these biosignatures at the chemical level include the utilization by life of a limited set of amino acids, which are the building blocks of proteins, and the ordered odd/even patterns in long-chain hydrocarbons, which make up cell walls. In addition to this molecular order imposed by life, biological activity also leaves its imprint on isotope ratios such as 13C/12C and 15N/14N because metabolism uses the most energetically efficient paths. The MSL mission goal is to identify and characterize a habitable environment, not to find extant or extinct life. However, if near-surface organic compounds are found in a potentially habitable environment, then the search for biosignatures can become much more focused in the future, using robotic in situ explorers or by returning samples to Earth for more detailed analysis. The organic compounds that are the target of the MSL mission could originate from indigenous geochemistry (Steele et al. 2012a, b), from life, or simply represent exogenous organic compounds that rain down on Mars at the rate of 240,000 kg per year or more (Flynn and McKay 1990) as a result of meteorite and interstellar dust infall. This deposition of micrograms of organic compounds per square meter on Mars every year from space could be expected to produce tens of parts per million of organics if there were no organic destruction and if mixing in the regolith over billions of years were only to a depth of tens of meters.
Many environmental factors on present-day Mars conspire to oxidize and degrade organics. Ultraviolet radiation passes readily through the thin Martian atmosphere; hydrogen peroxide is present in the atmosphere and periodically increases in the soil as a result of chemical reactions induced by dust (Atreya et al. 2006); organics are oxidized by compounds such as superoxides (e.g. Yen et al. 2000) or peroxides (e.g. Leshin et al. 2013) in the soil; and organics are transformed by high-energy solar and cosmic particles that are minimally impeded by the thin Martian atmosphere.
In the distant past, a thicker atmosphere and a less oxidizing surface environment may have limited these effects. Thus the strategy to be used in a search for ancient organic compounds must focus on locating sites where organic compounds were present in ancient times, where plausible sedimentary processes concentrated organic materials during deposition, and where rapid burial preserved and protected them from ionizing radiation until relatively recent exposure allowed their detection. The recent SAM analysis of Yellowknife Bay rocks allows some optimism that this strategy may be successful, with the determination of a cosmic radiation exposure age, the discovery of indications of possible self-combustion of organic compounds to produce CO2 as the oxychloride compounds decompose, and the detection of traces of simple chlorinated organic compounds.
The latest generation of tools to locate and demonstrate the existence of a habitable environment has now been successfully used by Curiosity in the Yellowknife Bay formation on the floor of Gale Crater. As a result of the volatiles investigation by Curiosity, we have provided information on carbon, chlorine, oxygen, sulfur, and nitrogen compounds in rocks, made the first in situ rock age determination on another planet; characterized the cosmic radiation that can transform organic compounds; and measured the deuterium to hydrogen ratio in clay minerals that are our tie point to an ancient environment. The clays, sulfates, and hydrated-mineral layers of Mt. Sharp have been identified as prime exploration targets for Mars Science Laboratory's extended mission. The detailed compositional and isotopic studies of the Martian volatiles that SAM has made provide important information about the environmental conditions on early Mars. Could Mars have supported microbial life at or before the time it emerged on Earth? This intriguing question continues to motivate our current and future exploration of our neighboring terrestrial planet.