Leeb hardness of salty Europa ice analogs exposed to high-energy electrons

Abstract

Constraint of Europa's surface material properties is critical for future lander missions to Europa, particularly for drilling and sample retrieval. Surface ice on Europa, which is thought to contain water, hydrated acids and salts, and other trace materials at temperatures of ~80 K at the polar regions to ~120 K at the equator, is drenched in radiation from Jupiter's magnetosphere. Because 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 these extreme environments, we have initiated a set of laboratory studies to examine the effect of temperature, composition, and radiation on ice hardness. By utilizing a commercially-available Leeb rebound hardness tester, we have determined that ice-containing salts such as NaCl and MgSO4 exhibit increased hardness relative to pure water ices at Europa-relevant temperatures. 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 ‘worst-case scenarios’ (upper limits) for hardness of ice on Europa 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 the Leeb Hardness (HL) ranges from 477 to 527.