Self-preserving ice layers on CO₂ clathrate particles: Implications for Enceladus, Pluto, and similar ocean worlds

¹ Centre for Materials Science and Nanotechnology, Department of Physics, University of Oslo, PO Box 1048 Blindern, 0316 Oslo, Norway
e-mail: mathias.bostrom@smn.uio.no
² Multifunctional Optical Materials Group, Instituto de Ciencia de Materiales de Sevilla (Consejo Superior de Investigaciones Científicas - Universidad de Sevilla), Calle Américo Vespucio 49, 41092 Sevilla, Spain
e-mail: victoria.esteso@csic.es
³ Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Hermann-Herder-Str. 3, 79104 Freiburg, Germany
⁴ Department of Physics and Technology, University of Bergen, Allégaten 55, 5007 Bergen, Norway
⁵ Department of Energy and Process Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway
⁶ Institut für Physik, Universität Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany
⁷ Departamento de Física de Materiales, Universidad Autónoma de Madrid, 28049 Madrid, Spain
⁸ Department of Chemical and Geological Sciences, University of Cagliari, Cittadella Universitaria, 09042 Monserrato, CA, Italy
⁹ Discipline of Chemistry and Physics, C’SHEE, Murdoch University, 90 South St, Murdoch, WA 6150, Australia
¹⁰ Surface and Corrosion Science, Department of Chemistry, KTH Royal Institute of Technology, SE 100 44 Stockholm, Sweden
e-mail: corkery@kth.se
¹¹ Department of Applied Mathematics, Research School of Physics, ANU College of Science, Oliphant Building 60, Mills Road, The Australian National University, Canberra Acton ACT 2601, Australia

Received: 20 December 2020
Accepted: 8 March 2021

Abstract

Context. Gas hydrates can be stabilised outside their window of thermodynamic stability by the formation of an ice layer – a phenomenon termed self-preservation. This can lead to a positive buoyancy for clathrate particles containing CO₂ that would otherwise sink in the oceans of Enceladus, Pluto, and similar oceanic worlds.

Aims. Here we investigate the implications of Lifshitz forces and low occupancy surface regions on type I clathrate structures for their self-preservation through ice layer formation, presenting a plausible model based on multi-layer interactions through dispersion forces.

Methods. We used optical data and theoretical models for the dielectric response for water, ice, and gas hydrates with a different occupancy. Taking this together with the thermodynamic Lifshitz free energy, we modelled the energy minima essential for the formation of ice layers at the interface between gas hydrate and liquid water.

Results. We predict the growth of an ice layer between 0.01 and 0.2 μm thick on CO, CH₄, and CO₂ hydrate surfaces, depending on the presence of surface regions depleted in gas molecules. Effective hydrate particle density is estimated, delimiting a range of particle size and compositions that would be buoyant in different oceans. Over geological time, the deposition of floating hydrate particles could result in the accumulation of kilometre-thick gas hydrate layers above liquid water reservoirs and below the water ice crusts of their respective ocean worlds. On Enceladus, the destabilisation of near-surface hydrate deposits could lead to increased gas pressures that both drive plumes and entrain stabilised hydrate particles. Furthermore, on ocean worlds, such as Enceladus and particularly Pluto, the accumulation of thick CO₂ or mixed gas hydrate deposits could insulate its ocean against freezing. In preventing freezing of liquid water reservoirs in ocean worlds, the presence of CO₂-containing hydrate layers could enhance the habitability of ocean worlds in our Solar System and on the exoplanets and exomoons beyond.