Mars Science Laboratory
KeywordsAtmosphere Biomarkers Chlorohydrocarbons Curiosity Derivatization Gale GC-MS In situ Mars Mars Science Laboratory Methane MSL Organics Pyrolysis Regolith Rover
Mars Science Laboratory is a large instrumented vehicle (rover) built by NASA that landed on Mars in August 2012 (launched November 2011) and will operate for more than 1 Martian year (i.e., 2 Earth years). This rover, also called Curiosity, characterizes and interprets the geology of the landing region at all scales, assesses the biological potential of encountered targets, investigates planetary processes of relevance to past habitability, and characterizes radiation arriving at Mars’ surface. Aboard MSL, an analytical laboratory is especially devoted to atmospheric analysis and search for organics in the ground.
Ancient Martian history, from its formation up to about three billion years ago, looks close to Earth’s (Hadean-Archaean on Earth,Noachian-EarlyHesperian on Mars). We may then ask, was it possible for life to appear on Mars? One faces three possibilities: (a) life appeared and is extinct, but organic remnants or biominerals can be found; (b) only complex organic molecules, “bricks of life,” were formed, and some of them still exist; and (c) Mars has been absolutely sterile from the beginning.
In 1976, NASA sent the Viking orbiters and landers to Mars. On landers Viking 1 and 2, instruments were devoted to the search for biosignatures and molecular analysis of the ground. They were (1) a gas exchange experiment: nutrients were brought to the soil, variations in concentrations of O2, CO2, CH4, etc. were searched for (Oyama and Berdahl 1977); (2) two experiments using radioactive 14-C: CO2 exchanges between atmosphere, soil, and nutrient were monitored (Horowitz et al. 1977) (Levin and Straat 1977); (3) a pyrolysis-gas chromatograph-mass spectrometer (GC-MS) experiment: Mars soil was heated and analyzed (Bieman et al. 1977).
Ambiguous responses to the first three experiments were understood as due to the presence of nonbiological reactants rather than metabolic activity. One knows, now, that Mars’ atmosphere contains gaseous oxidants, such as H2O2, due to atmospheric photochemistry (Atreya et al. 2006; Zanhle et al. 2008) and that oxidizing compounds may also be present in the soil: Phoenix lander, when characterizing Mars soil near the North pole, observed the presence of perchlorates (ClO4 −) (Hecht et al. 2009).
Consequently, (i) the analytical laboratory of Curiosity, SAM, has been built (2005–2009) taking into account the fact that detection of organic matter by pyro-GC-MS needs higher temperatures or some preparation before pyrolysis; (ii) the potential production of chlorohydrocarbons has to be kept in mind during the analysis processes that occur at Mars surface, during MSL/Curiosity’s activities.
Water on Mars
Mars Science Laboratory
From 1999, MEPAG (Mars Exploration Program Analysis Group, NASA) defined four goals linked by the motto “follow the water”: (1) Life, (2) Climate, (3) Geology, (4) Human exploration. In this frame, MSL (Mars Science Laboratory) was developed and launched in November 2012 (the vehicle to be landed on Mars surface was named “Curiosity”). MSL project has, in particular, for objectives to assess the past and present habitability, to characterize carbon cycling in its geochemical context, and to assess whether life is or was present (Grotzinger et al. 2012).
MSL for 2012
Operations began as soon as the rover was deposited on Mars surface (see Fig. 7). It can be noticed that the way Curiosity is operated leads to quasi-daily downlink (data from Mars to Earth) or uplink (commands from Earth to Mars), seven days on seven at the beginning and, later, limited to working days; note that operations are linked to the Martian diurnal cycle (the “sol”) that slips of about 40 min each “Earth” day. During the first three months (i.e., commissioning), teams were operating exclusively from JPL, then they came back to their laboratories. Results from Curiosity’s science payload are embargoed until MSL project decides/authorizes to release them (Press conferences from NASA, international conferences, papers in peer-reviewed journals, for example).
Exobiology Aboard MSL: SAM
Six different gas chromatographic modules can be used, which cover a range from low-mass permanent gases or low carbon number (C1-C5) hydrocarbons to high masses (> C15), with the possible use of a “chiral” column to search for enantiomers. Specific detectors (thermal conductivity detectors, TCDs) are used that can measure down to some 10−10 mol of a given species in a He flow. The Quadrupolar Mass Spectrometer precisely identifies molecular compositions: the mass measurement ranges from 2 to 535 Da, with a detection limit ∼ 1 ppm (∼1 pmole in a He flow) and isotope ratios down to 1 per mil for noble gases. In the case of GC analysis, the MS can be used as detector to lower the detection level (some pmoles for GC-MS compared to some 10−10 mol for GC-TCD). The cell of Tunable Laser Spectrometer instrument can detect down to 2 ppb of CH4 and 2 ppm of water without enrichment and measure down to <10 per mil isotope ratios for C and O. Isotope measurements from TLS and MS will, in particular, help to solve the methane concern (Lefèvre and Forget 2009): what are the origins and sinks of the Martian CH4, some tens of ppbs observed using teledetection (cf Atreya et al. 2007), that could originate from well-defined places?
Gases sampled in the atmosphere are pumped through GC-MS-TLS; on their path, the Chemical Separation and Processing Laboratory (CSPL: getters, scrubbers, cold traps) may help to eliminate/separate some species (CO2, H2O) or to concentrate the sample.
Soil or rock powder need to be sieved (diameter <150 μm) before introduction into SAM’s inlets. They can be heated up to 1,000 °C (74 cells potentially reusable, in the Sample Manipulation System, SMS), and exhausted gases are analyzed; in the case of GC analysis, an He flow (pressurized tanks onboard) goes through the oven and pushes the mixture toward GC columns. In case of refractory species, some cups are equipped for wet chemistry (derivatization): a chemical reagent transforms the molecule into a vaporizable one, while preserving the structure (Buch et al. 2009). Combustion of samples in O2 atmosphere (pressurized tank onboard) may be also used to determine the isotopic ratios.
Gases from heating of the sample can be directly analyzed by the MS, using the EGA (Evolved Gas Analysis) mode: the release of typical molecules (H2O, CO2, O2, SO2) is monitored by the MS when the temperature of the cell that contains the sample is varied. Then, in a function of temperature, molecules show up and disappear, corresponding to their release from the sample at given temperatures. They can be adsorbed molecules, easily released at low temperatures, or weakly bonded molecules (e.g., H2O from Gypsum CaSO4 (H2O)2), or they can be molecules that come, at higher temperatures, from a breakup (e.g., CO2 from CaCO3) or a dehydroxylation (e.g., H2O from kaolinite Al2Si2O5(OH)4). Monitoring organic molecules when heating the sample is very useful, as far as it allows a better understanding of the processes and reactions that can release and/or modify the structure of organic molecules. Moreover, a precise investigation on the temperature of release could help to decide on the (a)biotic origin of some minerals (Stalport, 2005; Stalport, 2007). During the heating process, for a given temperature range, one can decide to send gases to TLS or GC-MS for further analyses.
In order to have a complete understanding of the results that are obtained on Mars, using GC and/or MS and/or TLS, devoted experiments intended to check the possible pathways are performed on Earth. For this purpose, teams can use, in their laboratories, separate instruments, most of them presenting the same characteristics (e.g., spare models) as the ones that are used on SAM; a complete spare model of SAM is also used, as a full testbed, at NASA/GSFC, in Martian conditions.
Key Research Findings
Curiosity landed safely on Mars surface (landing site was named “Ray Bradbury”) on August 5, 2012 and, from then, performed measurements, using all the instruments of the Science Payload and moving on Mars surface. The embargo policy does not allow quoting nonvalidated results; Nasa’s websites (e.g., http://mars.jpl.nasa.gov/msl/) allow to follow Curiosity in its progress and results.
The main idea, before landing, was to drive out the landing ellipse, quite rapidly, in direction of sulfate- and phyllosilicate-bearing strata (lower part of the mound). Curiosity was deposited near the center of the landing ellipse (see Fig. 5), and it appeared that, at short distances (less than 300 m), there were targets that were worth the visit, in full agreement with the expectations that the team had, from Gale’s choice. It took from August 2012 to June 2013 to carry out this task. Here are described the main results, obtained during the first 6 months, that are linked to Astrobiology; other results (Meteorology, Mineralogy, Geology, etc.) may be found on devoted websites and science papers.
Obviously, this conclusion is compatible with the position of Curiosity at this time, at the end of an alluvial fan from Peace Vallis (see Fig. 5), and it strengthens the idea of water flowing on Mars, some billion years ago, in a mild and dense atmosphere.
SAM used its TLS instrument to search for CH4 in the atmosphere and compare with the previous results of 10 ppb or more (Kerr 2012) that were obtained from remote observations. Measurements of TLS show a very low level of CH4: 0,55 +/− 1.46 ppb, at the 1-sigma level (Webster et al. 2013), which allows to think that, if CH4 is present, it has a concentration that is well below the ones that one could expect from teledetection. Concerning atmospheric escape that helped to lead to the present 7 mbars, SAM observed a decrease of 36Ar/38Ar, compared to Solar or Earth values, that is fully compatible with the values observed for Martian meteorites and corroborates the theories of a dense atmosphere for the first billion years of Mars history. The same trend was obtained by measuring D/H ratio in the atmosphere or from H2O released when heating soil samples (at Rocknest, see below).
Chemin observed (Vaniman et al. 2013) that all of the identified crystalline phases it identifies are volatile free; H2O, SO2, and CO2 volatile releases are then associated with the amorphous or poorly ordered materials. Water vapor is released at rather low temperatures, corresponding to a mixture of adsorbed or weakly bonded water for the lowest part of the spectrum and more or less tightly bonded water for the highest part. Oxygen release can be attributed to the decomposition of perchlorates. CO2 comes, at low temperatures, from adsorbed molecules, and at high temperatures from carbonates decomposition, a source for CO2 may be the action of O2 on organic matter. Using GC-MS, low amounts of CH3Cl and CH2Cl2 are detected, which might correspond to action of perchlorates on organics either present in the samples or brought from Earth. The complexity of the results, the interaction of various species between themselves, leads to cautious interpretations; laboratory experiments are in progress, in order to reproduce and explain the results observed on Mars.
Concerning sulfur, it was concluded to the presence of hydrogen sulfide (H2S, reduced S) and sulfate (SO4, oxidized S). Similarly to Rocknest, chlorohydrocarbons are observed by GC-MS, and laboratory experiments are in progress for a better understanding.
NASA’s Mars Atmosphere and Volatile EvolutioN (MAVEN) mission is scheduled for launch in late 2013. One of the goals of this orbiter is to determine the role that losses of atmospheric gases (e.g., CO2, N2, H2O) to space played in changing the Martian climate through time, giving insights into the history of Mars atmosphere and climate, liquid water, and planetary habitability.
In the frame of its Exomars program, and in cooperation with RosCosmos, ESA will launch, in 2016, the trace gas orbiter, TGO, devoted to spatial and temporal localization, and possible sources of methane and other light hydrocarbons or nitriles but also of nitrogen and sulfur compounds, O3, HCl, HO2, etc., with required detection sensitivities down to 1 ppt. An Entry Descent and Landing demonstrator module will be associated, which could perform some science measurements. In the same frame, ESA will land, in 2018, the Exomars Rover, Pasteur, equipped with a drill, able to collect samples down to 2 m below the surface, and a science payload mainly devoted to astrobiology. Pasteur will explore Mars for one or more seasons; a surface platform should be associated to Pasteur to investigate Martian environment.
To continue its Mars Exploration program, NASA has in mind to send to Mars, in 2020, a rover with almost the technical capabilities as Curiosity and an enhanced drilling system. The mission would provide an isotopic datation of Mars terrains (instead of ages obtained from craterization rates) and address key questions about the potential for life on Mars. The mission would also provide opportunities to gather knowledge and demonstrate technologies that address the challenges of future human expeditions to Mars. The rover could acquire rock cores with a core drill and place the cores in a cache. The cache would be left on the ground, while the rover would continue exploring the planet. This could be the first step of an exploration leading ultimately to the arrival, at Earth, of Mars Sample Return mission.
References and Further Reading
- Bieman K et al (1977) The search for organic substances and inorganic compounds in the surface of Mars. Science 82:4641–464Google Scholar
- Grotzinger et al (2013) editors: Mars Science Laboratory (Springer) 860 pp; this book brings together the 21 papers that were published in Space Science Reviews 170(1–4) (2012)Google Scholar
- Horowitz NH et al (1977) Viking on Mars: the Carbon assimilation experiment. Science 82:4659–4662Google Scholar
- Levin GW, Straat PA (1977) Recent results from the labeled release experiment. Science 82:4663–4669Google Scholar
- Mahaffy PR, Cabane M, Webster CR et al (2013) Curiosity’s sample analysis at Mars (SAM) investigation: overview of results from the first 120 Sols on Mars. In: 44th Lunar and Planetary Science Conference, 18–22 Mar 2013, The Woodlands. LPI Contribution No. 1719, p 1395Google Scholar
- Murchie S et al (2009) Evidence for the origin of layered deposits in Candor Chasma, Mars, from mineral composition and hydrologic modeling. JGR 114 doi:10.1029/2009JE003343Google Scholar
- Newsom HE et al (2013) Regional and global context of soil and rock chemistry from ChemCam and APXS at Gale crater. In: 44th Lunar and Planetary Science Conference, 18–22 Mar 2013, The Woodlands. LPI Contribution No. 1719, p 1832Google Scholar
- Oyama VI, Berdahl BJ (1977) The Viking gas exchange experiment results from Chryse and Utopia. Science 82:4669–4676Google Scholar
- Vaniman D et al (2013) Data from the Mars Science Laboratory CheMin XRD/XRF instrument. EGU General Assembly EGU2013-6272Google Scholar
- Webster CR, PR Mahaffy et al (2013) Measurements of Mars Methane at Gale Crater by the SAM Tunable Laser Spectrometer on the Curiosity Rover. In: 44th Lunar and Planetary Science Conference, 18–22 Mar 2013, The Woodlands. LPI Contribution No. 1719, p 1366Google Scholar