Abstract
Water on the present day Martian surface is thought to exist in two thermally distinct sub-surface reservoirs: as ice in the cryosphere and as groundwater located deeper in the crust. These sub-surface environments are thought to contain saline, rather than pure, water and laboratory studies on whether or not clathrate hydrates can form in such environments are lacking. We fill this gap by performing synchrotron radiation X-ray powder diffraction to investigate the formation and evolution of clathrate hydrates in weak chloride solutions at CO2 pressures, and over temperature ranges, that are similar to those found in the Martian regolith. We have found that clathrate hydrates can form under conditions relevant to the Martian cryosphere, despite the presence of chloride salts. We find that the dissociation temperatures for CO2 clathrate hydrates formed in saline solutions are depressed by 10–20 K relative to those formed in pure water, depending on the nature of the salt and the CO2 pressure. We suggest that the inhibiting effect that salts such as MgCl2, CaCl2 and NaCl have on clathrate hydrate formation could also be related to the salts’ effect on the formation of the low temperature phase of ice. However, despite the inhibiting effect of the salts, we conclude that the presence of clathrate hydrates should still be possible under conditions likely to exist within the Martian cryosphere.
Highlights
Clathrate hydrates are cage-like structures formed at high pressures ( J 0:6 MPa) and low temperatures ( K 300 K) (Sloan and Koh, 2007) in which water molecules, bonded via hydrogen bonds, can encase guest gas phase molecules (Sloan and Koh, 2007)
The dissociation temperatures for clathrates formed in the presence of CaCl2 at 10 bar do not differ significantly from those of clathrates formed in the presence of MgCl2, despite the higher concentration of CaCl2 in our experiments
We have found that the dissociation temperature for clathrates formed in saline solutions is consistently $ 20 K lower at 10 bar CO2 pressure than it is for clathrates formed in pure water
Summary
Clathrate hydrates (hereafter ‘‘clathrates”) are cage-like structures formed at high pressures ( J 0:6 MPa) and low temperatures ( K 300 K) (Sloan and Koh, 2007) in which water molecules, bonded via hydrogen bonds, can encase guest gas phase molecules (Sloan and Koh, 2007). SI clathrates are constructed of two small cages for every six larger ones, and host relatively large molecules such as CO2 and CH4. Safi et al / Geochimica et Cosmochimica Acta 245 (2019) 304–315
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