Abstract
We report studies on the thermal and radiolytic stability of the hydrated
salt minerals epsomite (MgSO4. 7H(2)O), mirabilite (Na2SO4. 1OH(2)O),
and natron (Na2CO3. 10H(2)O) 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 (less than or equal
to 130 K), epsomite should remain hydrated over geologic timescales
(similar to 10(11)-10(14) years), whereas natron and mirabilite may
dehydrate appreciably in approximately 10(8) and 10(3) 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 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 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.
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