A Mushy Source For The Geysers Of Enceladus

By Keith Cowing
Press Release
September 4, 2022
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A Mushy Source For The Geysers Of Enceladus
Model for a mushy region around the tiger stripes of Enceladus: (left) Schematic for the composition of Enceladus’ shell demonstrating strike-slip friction along a tiger stripe fracture. Shear heating on the fracture generates a mushy zone of salty water surrounding ice crystals, providing a liquid source for the geyser material. Darker colors qualitatively correspond to higher expected salt content (Buffo et al., 2021a). (right) Equilibrium phase diagram for a eutectic binary alloy (e.g. NaCl and H2O) as a function of temperature and concentration, after Worster (2000). At temperatures above the liquidus, the mixture is entirely liquid. In the region between the liquidus and solidus, there is two-phase coexistence, i.e. a mushy zone, where saline liquid occupies the interstices of a solid ice matrix. Below the eutectic point (Ce, Te), a solid forms that is a composite of salt and ice.

Enceladus is a primary target for astrobiology due to the salty plume ejecta measured by the Cassini spacecraft and the inferred subsurface ocean sustained by tidal heating.

Sourcing the plumes via a direct connection from the ocean to the surface requires a fracture through the entire ice shell (∼10 km).

Here we explore an alternative mechanism in which shear heating within the shallower tiger stripe fractures produces partial melting in the ice shell, allowing the interstitial fluid to be ejected as geysers. We use a two-dimensional multiphase reactive transport model to simulate the thermomechanics of a mushy region generated by localized shear heating in a salty ice shell.

From our model, we predict the temperature, porosity, melting rate, and liquid volume of an intrashell mushy zone surrounding a fracture. We find that there is sufficient brine volume within the mushy zone to sustain the geysers for ∼250 kyr, without additional melting, and that the rate of internal melting can match the observed ice ejection rate.

The composition of the liquid brine within the mushy zone is, however, distinct from the ocean, due to partial melting. This shear heating mechanism for geyser formation applies to Enceladus and other moons and has implications for our understanding of the geophysical processes and astrobiological potential of icy satellites.

Colin R. Meyer, Jacob J. Buffo, Francis Nimmo, Andrew J. Wells, Samuel Boury, Tara C. Tomlinson, Jamie R. G. Parkinson, Geoffrey M. Vasil

Comments: 10 pages, 3 figures, 1 supplement (11 pages, 3 figures)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Fluid Dynamics (physics.flu-dyn)
Cite as: arXiv:2208.06714 [astro-ph.EP] (or arXiv:2208.06714v1 [astro-ph.EP] for this version)
Submission history
From: Colin Meyer
[v1] Sat, 13 Aug 2022 19:15:36 UTC (2,754 KB)
Full paper: https://arxiv.org/abs/2208.06714

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