1. Los Alamos National Laboratory (LANL), MS D462, Los Alamos, New Mexico 87545, USA
2. Indiana University, 1001 E 10th St, Bloomington, Indiana 47405, USA
3. Los Alamos National Laboratory (LANL), MS D466, Los Alamos, New Mexico 87545, USA

Correspondence to: David T. Vaniman¹ Email: vaniman@lanl.gov

Abstract

Recent reports of 30 wt% of sulphate within saline sediments on Mars^{1, 2}—probably occurring in hydrated form³—suggest a role for sulphates in accounting for equatorial H₂O observed in a global survey by the Odyssey spacecraft⁴. Among salt hydrates likely to be present³, those of the MgSO₄nH₂O series have many hydration states. Here we report the exposure of several of these phases to varied temperature, pressure and humidity to constrain their possible H₂O contents under martian surface conditions. We found that crystalline structure and H₂O content are dependent on temperature–pressure history, that an amorphous hydrated phase with slow dehydration kinetics forms at <1% relative humidity, and that equilibrium calculations may not reflect the true H₂O-bearing potential of martian soils. Mg sulphate salts can retain sufficient H₂O to explain a portion of the Odyssey observations⁵. Because phases in the MgSO₄nH₂O system are sensitive to temperature and humidity, they can reveal much about the history of water on Mars. However, their ease of transformation implies that salt hydrates collected on Mars will not be returned to Earth unmodified, and that accurate in situ analysis is imperative.

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