p-Cresyl sulfate Moreover despite the generally severe surface conditions that destroy

Moreover, despite the generally severe surface conditions that destroy organic molecules, p-Cresyl sulfate can be preserved for billions of years in the martian subsurface (Kanavarioti and Mancinelli, 1990; Bada, 1991 ; Septhon, 2010). Therefore, it is necessary to sample below the influence of surface oxidants and cosmic ionizing radiation, when searching for present or past evidence of life. This is the reason why, for instance, the ExoMars 2018 rover has been designed to drill down to 2 m depth, i.e. where amino acids are not degraded substantially after 3 byr of exposure (Kminek and Bada, 2006). Hence, below the first dusty layer, organic matter could still be preserved in the proposed landing region. Hence, wherever life emerged, it would have reached this location transported by water flows.
Ancient life hosted in the Simud–Tiu vallis floor could have been preserved as biosignature. In the first meters of the subsurface the sediments deposited inside the recognized paleochannels could retain organisms coated by minerals and entombed in a fine-grained matrix. In particular, inverted channels constitute a convenient place for drilling, being already exposed by the erosion. Such channels are located less than 4 km from the proposed landing ExoMars ellipse center (i.e., well inside the traverse requirements) and would also provide a unique opportunity to sample contextualized cemented sedimentary outcrops that have preserved the late watery stages occurring in this area.
The NASA Mars 2020 rover, thanks to its greater mobility compared to ExoMars 2018, could be able to analyze more samples from different geological units can be analyzed. Thus, the late stages fluvial deposits, that postdate the Simud Vallis floor, could be reached attained and would provide ample opportunity to sample fresher outcrops on their incisions, both on the sides and on the floor of an incision. Nonetheless, one of the main limitations of the Mars 2020 rover is that incomplete dominance will only be able to obtain cores a few centimeters deep, much less than the 2 m generally recognised as necessary for the preservation of organic moleucles. Hence, if that coring depth is within the sterile layer, there will be no chance to study these potential traces of past martian life. For this reason, the two Mars 2020 landing sites called XX and ZZ, have been located in close proximity to two high sections that can be easily reached and sampled by the rover. In these cases, the sediments experienced erosion after deposition, but not transport, thus preserving the original sedimentary context. Unfortunately, this is not the case of debris flows fronts that, on one hand can provide information from a broader area, but on the other hand they cannot guarantee the determination of the original sedimentary context.
7. Engineering constraints
The Simud Vallis landing site described above was selected and presented as one of the eight finalists for the ESA ExoMars 2018–2020 landing site selection context: http://exploration.esa.int/mars/53944-proposed-landing-sites-for-exomars-2018-mission/ (Flahaut et al., 2014). In addition, the Simud Vallis area as a possible joint landing site for both the ExoMars and the Mars 2020 rovers has been presented for the Eight International Conference on Mars (2014): http://www.hou.usra.edu/meetings/8thmars2014/pdf/1209.pdf (Pajola et al., 2014b). Nowadays, this site is no longer considered the final choice for both ExoMars 2018 and Mars 2020 missions; nonetheless, the presented data can be of great interest for future in situ missions.

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