Effect of Salts on the Formation and Hypervelocity-Induced Fragmentation of Icy Clusters with Embedded Amino Acids

Abstract

The search for biomolecules via flyby or orbiting missions is prime for hypervelocity sampling where there is a water-rich plume or exosphere that can be sampled without landing (e.g., Europa, Enceladus, and possibly Triton). Under hypervelocity conditions, meaning relative speeds of km/s, these molecules may fragment upon impact with spacecraft surfaces or instrument inlets in ways that are not fully understood, potentially leading to incorrect identification and/or quantitation. Experiments on earth have attempted to reproduce the fragmentation process; however, accelerating single neutral molecules above several km/s over short distances (in a lab) is extremely challenging, and even if successful, such experiments are hard-pressed to yield insights into molecular reaction pathways. In this work, we use first-principles-based simulations to describe the effect of salts in the hypervelocity fragmentation processes of the amino acids arginine (Arg), alanine (Ala), and histidine (His) when encased in ice grains at different concentrations of sodium chloride (NaCl), between 0.25 and 2.0 M, under normal impacts at velocities between 3 and 10 km/s. We find that salt ions affect the fragmentation pathways and velocity thresholds of encased amino acids. Although most fragmentation starts by 3 km/s, the salinity effect can be considered second order, compared to differences resulting from velocity. These changes are attributed to weak interactions between Na⁺ and Cl⁻ with particular amino acid groups, during the flash-freezing process of ejected particles from Enceladus (and possibly Europa). Such interactions may weaken amino acid bonds (e.g., N–H), electrostatically shield them from surrounding waters undergoing high-strain rates, change the amino acid placement and conformation within the ice clusters (due to salting-in and salting-out effects), or disrupt the mechanical response of the ice clusters (interfere with hydrogen-bond network). These effects become more pronounced at higher velocities and provide valuable information for the interpretation of data from the Cassini spacecraft, and motivate future missions to characterize ocean worlds via hypervelocity sampling of atmospheres and plumes.