Thermal and radiation stability of the hydrated salt minerals epsomite, mirabilite, and natron under Europa environmental conditions

We report studies on the thermal and radiolytic stability of the hydrated s alt minerals epsomite (MgSO4. 7H(2)O), mirabilite (Na2SO4. 1OH(2)O), and na tron (Na2CO3. 10H(2)O) under the low-temperature and ultrahigh vacuum condi tions characteristic of the surface of the Galilean satellite Europa. We pr epared samples, ran temperature-programmed dehydration (TPD) profiles and i rradiated the samples with electrons. The TPD profiles are fit using Arrhen ius-type first-order desorption kinetics, This analysis yields activation e nergies of 0.90 +/- 0.10, 0.70 +/- 0.07, and 0.45 +/- 0.05 eV for removal o f the hydration water for epsomite, natron, and mirabilite, respectively. A simple extrapolation indicates that at Europa surface temperatures (less t han or equal to 130 K), epsomite should remain hydrated over geologic times cales (similar to 10(11)-10(14) years), whereas natron and mirabilite may d ehydrate appreciably in approximately 10(8) and 10(3) years, respectively. A small amount of SO2 was detected during and after 100 eV electron-beam ir radiation of dehydrated epsomite and mirabilite samples, whereas products s uch as O-2 remained below detection limits. The upper limit for the 100 eV electron-induced damage cross section of mirabilite and epsomite is similar to 10(-19) cm(2). The overall radiolytic stability of these minerals is pa rtially due to (1) the multiply charged nature of the sulfate anion, (2) th e low probability of reversing the attractive Madelung (mostly the attracti ve electrostatic) potential via Auger decay, and (3) solid-state caging eff ects, Our laboratory results on the thermal and radiolytic stabilities of t hese salt minerals indicate that hydrated magnesium sulfate and perhaps oth er salts could exist for geologic timescales on the surface of Europa.