Impact generated modification of the mineralogy at Oxia Planum

 Phyllosilicate minerals have been detected at Oxia Planum, the ExoMars landing site, which indicate this area has been subject to aqueous activity during the Noachian [1,2,3]. The aim of the ExoMars rover mission is to search for signs of past or present life at Oxia Planum and to characterise the geochemical environment [1]. Evidence of past life may include physical biosignatures such as fossilised cells or stromatolites, or chemical biosignatures, such as biominerals and organic molecules produced by biological activity (biomarkers) [1,4,5]. The rover will be equipped with a suite of instruments, such as Multispectral Panoramic cameras (PanCam), Close-up imager (CLUPI), Raman Laser Spectrometer (RLS), and Mars Organic Molecule Analyser (MOMA), the latter of which includes gas chromatography-mass spectrometry (GC-MS) and laser desorption/ionisation-mass spectrometry (LDI-MS) capable of detecting biomarkers [6,7,8,9].

Mineralogy plays a vital role in the preservation of organic molecules [10]. Phyllosilicate minerals have been shown to adsorb organic compounds on and between their layered structures [11,12], protecting them from degradation by radiation and oxidation [13]. Given the identification of phyllosilicate mineralogy at Oxia Planum, detecting biomarkers there is plausible. However, alteration of biosignatures may have occurred at Oxia Planum through a range of geological processes, such as impacts.

Geomorphological data of Oxia Planum indicates that extensive impact cratering has occurred, which would have altered the surface mineralogy, and may have modified any biomarkers present in the sediments [14]. However, organic molecules detected in impact craters on Earth have been shown to have survived the impact process [15] and impact experiments have shown impact energy, angle of ejection and mineralogy will influence the decomposition rate of organic molecules [16]. In addition, glasses formed by impact processing have been shown to trap organic components and aid their preservation [17, 18]. However, the elevated temperatures and pressures experienced during impact events can cause the modification of organic molecules, such as bond breaking and formation, thermal oxidation and racemization [19,20].  Depending on the reaction dynamics of these changes, these modifications may differ according to particular mineral hosts [20]. 

In this study, we perform laboratory impact experiments using the all-axis light gas gun (LGG), at the Open University, to understand the effects of impacts on the local mineralogy at Oxia Planum. These experiments use a mineralogical simulant for Oxia Planum, which is composed of a mixture of unaltered basaltic minerals, a phyllosilicate component, iron oxides and an amorphous component, representing the possible mineral assemblage at Oxia Planum[21].The LGG will expose the simulant to the high pressures and temperatures associated with planetary impacts at a range of velocities relevant to impacts at Oxia Planum. Samples taken of the simulant will be analysed using Near-IR, Raman and XRD analysis to assess the changes to its mineralogy. Here, we will present the preliminary results from these experiments, and will outline their application to understanding the survival of biomarkers at the Oxia Planum landing site.

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