Abstract
Success of a future Mars Sample Return mission will depend on the correct choice of samples. Pyrolysis-FTIR can be employed as a triage instrument for Mars Sample Return. The technique can thermally dissociate minerals and organic matter for detection. Identification of certain mineral types can determine the habitability of the depositional environment, past or present, while detection of organic matter may suggest past or present habitation. In Mars' history, the Theiikian era represents an attractive target for life search missions and the acquisition of samples. The acidic and increasingly dry Theiikian may have been habitable and followed a lengthy neutral and wet period in Mars' history during which life could have originated and proliferated to achieve relatively abundant levels of biomass with a wide distribution. Moreover, the sulfate minerals produced in the Theiikian are also known to be good preservers of organic matter. We have used pyrolysis-FTIR and samples from a Mars analog ferrous acid stream with a thriving ecosystem to test the triage concept. Pyrolysis-FTIR identified those samples with the greatest probability of habitability and habitation. A three-tier scoring system was developed based on the detection of (i) organic signals, (ii) carbon dioxide and water, and (iii) sulfur dioxide. The presence of each component was given a score of A, B, or C depending on whether the substance had been detected, tentatively detected, or not detected, respectively. Single-step (for greatest possible sensitivity) or multistep (for more diagnostic data) pyrolysis-FTIR methods informed the assignments. The system allowed the highest-priority samples to be categorized as AAA (or A*AA if the organic signal was complex), while the lowest-priority samples could be categorized as CCC. Our methods provide a mechanism with which to rank samples and identify those that should take the highest priority for return to Earth during a Mars Sample Return mission. Key Words: Mars—Astrobiology—Search for Mars' organics—Infrared spectroscopy—Planetary habitability and biosignatures. Astrobiology 18, 556–570.
Highlights
Attempts to discover whether life exists or has previously existed on Mars have not yet been conclusive
The system allowed the highest-priority samples to be categorized as AAA, while the lowest-priority samples could be categorized as CCC
Hydroxyl peaks were observed in strong form for wood-containing samples and bank sediments from the flowing stream and a microbial mat–containing sample from the dry stream (FlowWJ1b, FlowBS1, FlowBS2, FlowBS3, DryMJ1a), and they were observed in weak form for wood, microbial mat, and quartz sand–containing samples from the flowing stream (FlowWG1a, FlowWG1b, FlowMG1c, FlowMJ1a, FlowMJ1b, FlowQ1)
Summary
Attempts to discover whether life exists or has previously existed on Mars have not yet been conclusive. To seek greater certainty when reading the rock record of habitability and habitation on Mars, mission concepts are being proposed that involve the return of samples to Earth (McLennan et al, 2012). Once on Earth, samples of Mars can be distributed to multiple laboratories that host the most powerful analytical techniques available. The probability of successfully obtaining conclusive evidence of past or present life on Mars is inevitably influenced by the choice of sample to be returned to Earth (Sephton and Carter, 2015). Hydrated minerals represent evidence of water on early Mars (Bibring et al, 2006; Poulet et al, 2005). Isotopic evidence suggests the presence of a thicker atmosphere in the past (Owen and Bar-Nun, 1995)
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