Abstract
In 2020, the ESA ExoMars and NASA Mars 2020 missions will be launched to Mars to search for evidence of past and present life. In preparation for these missions, terrestrial analog samples of rock formations on Mars are studied in detail in order to optimize the scientific information that the analytical instrumentation will return. Desert varnishes are thin mineral coatings found on rocks in arid and semi-arid environments on Earth that are recognized as analog samples. During the formation of desert varnishes (which takes many hundreds of years), organic matter is incorporated, and microorganisms may also play an active role in the formation process. During this study, four complementary analytical techniques proposed for Mars missions (X-ray diffraction [XRD], Raman spectroscopy, elemental analysis, and pyrolysis-gas chromatography-mass spectrometry [Py-GC-MS]) were used to interrogate samples of desert varnish and describe their capacity to sustain life under extreme scenarios. For the first time, both the geochemistry and the organic compounds associated with desert varnish are described with the use of identical sets of samples. XRD and Raman spectroscopy measurements were used to nondestructively interrogate the mineralogy of the samples. In addition, the use of Raman spectroscopy instruments enabled the detection of β-carotene, a highly Raman-active biomarker. The content and the nature of the organic material in the samples were further investigated with elemental analysis and methylated Py-GC-MS, and a bacterial origin was determined to be likely. In the context of planetary exploration, we describe the habitable nature of desert varnish based on the biogeochemical composition of the samples. Possible interference of the geological substrate on the detectability of pyrolysis products is also suggested. Key Words: Desert varnish-Habitability-Raman spectroscopy-Py-GC-MS-XRD-ExoMars-Planetary science. Astrobiology 17, 1123-1137.
Highlights
In 2018, the ExoMars Rover mission (ESA/Roscosmos) will be launched to explore the surface of Mars (ExoMars mission, 2015)
Suitable samples for powder diffraction should contain tens of thousands of randomly oriented crystallites to obtain a pattern representative of all minerals present in the sample (Pecharsky and Zavalij, 2009). This was not the case for the samples studied here, which were generally composed of a few big grains
This sampling statistics problem can lead to strongly biased relative intensity ratios or even to missing reflections, making it very difficult for the search algorithms to identify the minerals in the powder
Summary
In 2018, the ExoMars Rover mission (ESA/Roscosmos) will be launched to explore the surface of Mars (ExoMars mission, 2015). That question was a key aspect of the science goals of the NASA’s MSL Curiosity rover, and is a key objective of the NASA Mars 2020 mission (planned for launch in 2020) (NASA Mars 2020, 2014) In preparation for these missions, developing an understandings of: the appearance and evolution of life on Earth (the only location in the Universe where life as we comprehend it was confirmed), as well as the preservation of evidence of life, are of utmost importance. Some of these analogue samples comprise microorganisms that survive on or in rocks
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