Leeb Hardness of Salty Europa Ice Analogs Exposed to MeV Electrons

Europa is a key target for exploration of habitability in the Solar System. Due to the intense radiation impinging upon the surface, it is likely that future lander missions to Europa will need to gain access to subsurface material to obtain pristine samples that are unaffected by radiation. Since Europa's ice composition and surface structure are not yet fully characterized, technological requirements for mobility, drilling, and caching mechanisms are also not well constrained. To begin to understand the physical properties of ice under Europa's extreme environments, we have initiated a set of laboratory studies to examine the effect of temperature, composition, and MeV electron radiation on the hardness of ices that contain hydrated salts. By utilizing a commercially-available Leeb rebound hardness tester, we have determined that ices that contain salts such as NaCl and MgSO₄ exhibit increased hardness relative to pure water ices under temperatures that are relevant to Europa (which experiences average temperatures of 80 K at the polar regions to 120 K at the equator). However, exposure of these ices to 10-25 MeV electron radiation (such as is received by the trailing hemisphere of Europa) led to measurable softening, which indicates that the increases in hardness due to the presence of salt may be somewhat tempered by surface radiation effects. We found that hydrated MgSO₄ (close to 1:1 by mass with H₂O) is the hardest of these materials at Europa-relevant temperatures and ranges from 477 to 527 HL.