Understanding hypervelocity sampling of biosignatures in space missions

The atomic scale fragmentation processes involved in molecules undergoing hyper- velocity impacts (HVI; defined as > 3km=s) are challenging to investigate and still not well understood. This is especially relevant for solar system robotic missions sampling atmospheres and plumes at hypervelocities. Light gas guns, magnetic particle accelerators, or laser field shock experiments are used on earth in an attempt to replicate space conditions during HVI events. Experimental measurements on the HVI effects on small molecular weight neutral organics are extremely difficult, both in terms of accelerating uncharged species and in isolating the multiple transition states on extremely rapid time scales (< 1ps). First-principles based simulations are of high value in modeling the non-adiabatic, non-equilibrium phenomena involved, so as to extend the range of physical parameters and species of interest beyond what is possible with experiments. We report on high fidelity simulations of the fragmentation of small organic biosignature molecules over the range v = 1–12km=s, using a first-principles based approach. We demonstrate that the fragmentation fraction is a sensitive function of velocity, impact angle, molecular structure, impact surface material, and the presence or absence of surrounding ice-shells, and generate interpretable fragmentation pathways and spectra for velocity values above the fragmentation thresholds. We demonstrate how organic molecules encased in ice grains, as would likely be the case for those in 'ocean worlds', preserve these molecules at even higher velocities. Our results place ideal spacecraft encounter velocities between 3-5km/s for bare amino and fatty acids and within 4-6 km/s for the same species encased in ice grains, which is consistent with recent experiments exploring HVI effects using impact-induced ionization and analysis via mass spectrometry.1 The predicted results match the major amino acid fragments observed in lab spectra, and the onset of organic fragmentation in ice grains at > 5km/s from the analysis of Enceladus organics in Cassini Data.2 We show that HVI energy is dissipated by ice casings via thermal velocity resistance from the impact shock wave, and that there is a limit defined by the tensile forces caused by water molecules during the expansion phase at which ultimately all organic contents will fragment, independent of the number of ice shells. These results contribute to our understanding of molecular fragmentation under HVI, and allow us to bracket instrument design and future mission parameters.