Properties of CO₂ clathrate hydrates formed in the presence of MgSO₄ solutions with implications for icy moons

¹ Astrophysics Group, Lennard-Jones Laboratories, Keele University, Keele, Staffordshire, ST5 5BG, UK
e-mail: e.safi@keele.ac.uk
² Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, UK

Received: 27 September 2016
Accepted: 25 January 2017

Abstract

Context. There is evidence to suggest that clathrate hydrates have a significant effect on the surface geology of icy bodies in the solar system. However the aqueous environments believed to be present on these bodies are likely to be saline rather than pure water. Laboratory work to underpin the properties of clathrate hydrates in such environments is generally lacking.

Aims. We aim to fill this gap by carrying out a laboratory investigation of the physical properties of CO₂ clathrate hydrates produced in weak aqueous solutions of MgSO₄.

Methods. We use in situ synchrotron X-ray powder diffraction to investigate clathrate hydrates formed at high CO₂ pressure in ice that has formed from aqueous solutions of MgSO₄ with varying concentrations. We measure the thermal expansion, density and dissociation properties of the clathrates under temperature conditions similar to those on icy solar system bodies.

Results. We find that the sulphate solution inhibits the formation of clathrates by lowering their dissociation temperatures. Hysteresis is found in the thermal expansion coefficients as the clathrates are cooled and heated; we attribute this to the presence of the salt in solution. We find the density derived from X-ray powder diffraction measurements is temperature and pressure dependent. When comparing the density of the CO₂ clathrates to that of the solution in which they were formed, we conclude that they should sink in the oceans in which they form. We also find that the polymorph of ice present at low temperatures is Ih rather than the expected Ic, which we tentatively attribute to the presence of the MgSO₄.

Conclusions. We (1) conclude that the density of the clathrates has implications for their behaviour in satellite oceans as their sinking and floating capabilities are temperature and pressure dependent; (2) conclude that the presence of MgSO₄ inhibits the formation of clathrates and in some cases may even affect their structure and (3) report the dominance of Ih throughout the experimental procedure despite Ic being the stable phase at low temperature.