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 salt minerals epsomite (MgSO₄*7H₂O), mirabilite (Na₂SO₄*10H₂O), and natron (Na₂CO₃*10H₂O) under the low-temperature and ultrahigh vacuum conditions characteristic of the surface of the Galilean satellite Europa. We prepared samples, ran temperature-programmed dehydration (TPD) profiles and irradiated the samples with electrons. The TPD profiles are fit using Arrhenius-type first-order desorption kinetics. This analysis yields activation energies of 0.90±0.10,0.70±0.07, and 0.45±0.05 eV for removal of the hydration water for epsomite, natron, and mirabilite, respectively. A simple extrapolation indicates that at Europa surface temperatures (<130 K), epsomite should remain hydrated over geologic timescales (∼10¹¹-10¹⁴ years), whereas natron and mirabilite may dehydrate appreciably in approximately 10⁸ and 10³ years, respectively. A small amount of SO₂ was detected during and after 100 eV electron-beam irradiation of dehydrated epsomite and mirabilite samples, whereas products such as O₂ remained below detection limits. The upper limit for the 100 eV electron-induced damage cross section of mirabilite and epsomite is ∼10^-19 cm². The overall radiolytic stability of these minerals is partially due to (1) the multiply charged nature of the sulfate anion, (2) the low probability of reversing the attractive Madelung (mostly the attractive electrostatic) potential via Auger decay, and (3) solid-state caging effects. Our laboratory results on the thermal and radiolytic stabilities of these salt minerals indicate that hydrated magnesium sulfate and perhaps other salts could exist for geologic timescales on the surface of Europa.