Stability of methane clathrate hydrates under pressure: Influence on outgassing processes of methane on Titan

Research areas:
Year:
2010
Authors:
Journal:
ICARUS
Volume:
205
Number:
2
Pages:
581-593
Month:
February
ISSN:
0019-1035
Abstract:
We have conducted high-pressure experiments in the H2O-CH4 and
H2O-CH4-NH3 systems in order to investigate the stability of methane
clathrate hydrates, with an optical sapphire-anvil cell coupled to a
Raman spectrometer for sample characterization. The results obtained
confirm that three factors determine the stability of methane clathrate
hydrates: (1) the bulk methane content of the samples: (2) the presence
of additional gas compounds such as nitrogen; (3) the concentration of
ammonia in the aqueous solution. We show that ammonia has a strong
effect on the stability of methane clathrates. For example, a 10 wt.\%
NH3 solution decreases the dissociation temperature of methane
clathrates by 14-25 K at pressures above 5 MPa. Then, we apply these new
results to Titan's conditions. Dissociation of methane clathrate
hydrates and subsequent outgassing can only occur in Titan's icy crust,
in presence of locally large amounts of ammonia and in a warm context.
We propose a model of cryomagma chamber within the crust that provides
the required conditions for methane outgassing: emplacement of an ice
plume triggers the melting (if solid) or heating (if liquid) of large
ammonia-water pockets trapped at shallow depth, and the generated
cryomagmas dissociate surrounding methane clathrate hydrates. We show
that this model may allow for the outgassing of significant amounts of
methane, which would be sufficient to maintain the presence of methane
in Titan's atmosphere for several tens of thousands of years after a
large cryovolcanic event. Published by Elsevier Inc.