Clathrate blankets as (in)surmountable barriers for hydrothermal systems
A key question pertaining to Europa’s habitability is whether
hydrothermal activity could be sustained for long periods of time,
enabling redox and nutrient exchange between the ocean and rocky
interior [e.g. 1, 2]. Europa’s early ocean, if formed during
differentiation, could have been infused with gases . A
consequence of this initial infusion is that clathrate hydrates may have
been stable within the ocean. These clathrates could then rise to the
bottom of the ice shell, or blanket the seafloor, depending on their
density relative to the ocean. Accumulations of floating and sinking
clathrates would affect the geological and thermal evolution of Europa
because of their high heat capacity and low thermal conductivity
compared to ice Ih, but sinking clathrates could also inhibit chemical
exchange between the ocean and the rocky interior. We calculate the
stability and density of CH4 and CO2 clathrates, and predict the volumes
precipitated at the seafloor or accumulated at the base of the ice
shell, for ocean compositions evolved from the interior of Europa during
metamorphism on the path towards formation of a metallic core .
For a chemically reduced ocean derived from heating a mix of chondritic
material near Jupiter , plus cometary volatiles,
~2 x 10^7 km^3 of methane clathrates form. These
are less dense than the ocean (Fig. 1), and float to the base of the ice
shell. However, for a CO2-rich ocean derived from CI or CM chondrites,
~3 x 10^8 – 2 x 10^9 km^3 of CO2 clathrates
could form, i.e., sufficient feedstock to form a 13–77 km global layer
on the seafloor. A salty ocean (e.g. 10 % MgSO4) or a warm seafloor
(316 K) may be needed to prevent the accumulation of a CO2 clathrate
blanket (Fig. 1), although the blanketing effect would thin the
equilibrium thickness of the clathrate layer to ~500 m
for allowable heat fluxes (~50 mW/m^2). 
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https://doi.org/10.1089/ast.2007.0075  Klimczak, C. et al.
(2019). 50th Lunar. Planet Sci. Conf., Abstract #2132, p. 2912.
Melwani Daswani, M. et al. (2021). A metamorphic origin for Europa’s
ocean (preprint). https://doi.org/10.1002/essoar.10507048.1 
Desch, S. J. et al. (2018). Astrophys. J., Suppl. Ser., 238(1), 11.