An Experimental and Numerical Assessment of Ejected Martian Biosignatures Impacting Phobos

Emerland, Zoe Sara (2023). An Experimental and Numerical Assessment of Ejected Martian Biosignatures Impacting Phobos. PhD thesis The Open University.



Phobos, the larger and closer of Mars’ two moons, is not considered a location where extant or extinct life may exist. However, its close proximity to Mars means it could be a repository for impact-ejected material that may contain evidence of ancient martian life.

This thesis investigates the delivery of possible martian biosignatures to Phobos and their subsequent detection within the Phobos regolith. iSALE-2D hydrocode was used to investigate the conditions experienced by Mars-like projectiles impacting Phobos-like targets, that were then compared with a survival criteria of shock pressure and temperature for amino acids. The resultant amino acid survivability varied broadly. To validate the numerical simulations, hypervelocity impact experiments were conducted, whereby bespoke Mars-relevant basaltic projectiles doped with the potential organic biosignature glycine, were fired using the All-Axis Light Gas Gun (AALGG) into two Phobos regolith simulants at a range of velocities. The bespoke projectiles fragmented upon acceleration resulting in buckshot cluster impacts where the proportion of glycine that survived impacts at similar velocities varied broadly, regardless of Phobos simulant used. This presented a fundamentally different impact process than would be experienced on Phobos. Therefore, to validate the numerical simulations with an alternative approach, the impact experiment results were compared to numerical simulations on the spatial scale of the AALGG experiments, that focussed on cluster impacts. These simulations revealed that amino acid survival and detection could be significantly affected by how early within a cluster a projectile fragment impacts. Nevertheless, the small-scale numerical simulations aligned well with the impact experiments suggesting it is an adequate tool for estimating the temperatures and pressures in real impactors. Finally, comparing the results from the large-scale numerical simulations with the impact experiments it was revealed that the datasets could be fit by logistic function sigmoid curves with impact velocity.

This study highlights the stochastic nature of impact delivery, considers how some aspects of cluster impacts of martian ejecta on Phobos may result in resolvable biosignature survival, and provides a baseline from which more complex molecular biosignature survival and modification could be investigated. It supports the potential presence of martian material, including biosignatures, within Phobos’ regolith, which is significant for interpreting data from samples returned from Phobos by JAXA’s Martian Moons eXploration mission.

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