  Halite (NaCl mineral) has exhibited the potential to preserve microorganisms
for millions of years on Earth. This mineral was also identified on Mars and in
meteorites. In this study, we investigated the potential of halite crystals to
protect microbial life forms on the surface of an airless body (e.g.,
meteorite), for instance, during a lithopanspermia process (interplanetary
travel step) in the early Solar System. To investigate the effect of the
radiation of the young Sun on microorganisms, we performed extensive simulation
experiments by employing a synchrotron facility. We focused on two exposure
conditions: vacuum (low Earth orbit, 10^{-4}Pa) and vacuum-ultraviolet (VUV)
radiation (range 57.6 - 124 nm, flux 7.14 W m^{-2}), with the latter
representing an extreme scenario with high VUV fluxes comparable to the amount
of radiation of a stellar superflare from the young Sun. The stellar VUV
parameters were estimated by using the very well-studied solar analog of the
young Sun, k^{1}Cet. To evaluate the protective effects of halite, we entrapped
a halophilic archaeon (Haloferax volcanii) and a non-halophilic bacterium
(Deinococcus radiodurans) in laboratory-grown halite. Control groups were cells
entrapped in salt crystals (mixtures of different salts and NaCl) and
non-trapped (naked) cells, respectively. All groups were exposed either to
vacuum alone or to vacuum plus VUV. Our results demonstrate that halite can
serve as protection against vacuum and VUV radiation, regardless of the type of
microorganism. In addition, we found that the protection is higher than
provided by crystals obtained from mixtures of salts. This extends the
protective effects of halite documented in previous studies and reinforces the
possibility to consider the crystals of this mineral as potential preservation
structures in airless bodies or as vehicles for the interplanetary transfer of
microorganisms.
