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

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.