Destabilization of methane clathrate hydrate by meteorite impacts on present-day Mars

<p>The series of methane detection and non-detection in the atmosphere of Mars over the last two decades has raised numerous questions about methane generation and destruction mechanisms, which still remain unexplained.  It has been suggested that Martian methane could have a biological origin and be generated by organisms living in the subsurface where conditions are more hospitable [1]. Methane could also be produced through several abiologic processes, including Fischer-Tropsch Type (FTT) reactions where H<sub>2</sub> reacts with CO<sub>2</sub> in the presence of a metal catalyst [2]. The H<sub>2</sub> necessary for the FTT reactions can be produced by several processes and notably by serpentinization [3]. Many of the proposed generation mechanisms for methane would take place hundreds of meters to several kilometers deep in the crust of Mars. Once produced, methane can migrate upwards and be either directly released at the surface or trapped in subsurface reservoirs, such as clathrate hydrates, where it could accumulate over long time before being episodically liberated during destabilizing events. These phenomena leading to surface degassing imply a change in temperature/pressure conditions of the methane reservoirs and are multiple: faulting and landslide generated by seismicity, impact, climatic changes...</p> <p>In this study, we investigate the capacity of small-sized (a few tens to a few hundred meters diameter) impact craters to thermally penetrate the Martian ground and release methane through the dissociation of subsurface clathrate reservoirs. The impacts of small meteorite are more frequent on present-day Mars and could represent a likely process that would sporadically destabilize shallow gas reservoirs, inducing the degassing of methane in the atmosphere of the planet. We use a one-dimensional finite difference thermal model of the subsurface to calculate the depth of stable methane clathrate hydrates. The impact-induced heat, calculated using the Murnaghan equation of state, is then added to geothermal temperatures to obtain the post-impact temperature distributions similarly to [4]. We apply our model to different case studies in order to constrain the impactor radius, velocity and impact angle required to destabilize a subsurface clathrate layer that would discharge methane amounts corresponding to the observations.</p> <p><strong>Acknowledgements</strong></p> <p>This work was supported by the Fonds de la Recherche Scientifique - FNRS and by the Research Foundation Flanders (FWO) under Grant n° EOS-30442502.</p> <p><strong>References</strong></p> <p>[1] Atreya, S. K. et al., <em>Planet. Space Sci.</em> 55, 358-369, 2007.</p> <p>[2] Oehler, D. Z. and Etiope, G., <em>Astrobiology</em> 17, 1233-1264, 2017.</p> <p>[3] Oze, C. and Sharma, M., <em>Geophys. Res. Lett.</em> 32, 2005.</p> <p>[4] Schwenzer, S. P. et al., <em>Earth Planet. Sci. Lett.</em> 335-336, 9-17, 2012.</p>

Unraveling nucleation pathway in methane clathrate formation

Methane clathrates are widespread on the ocean floor of the Earth. A better understanding of methane clathrate formation has important implications for natural-gas exploitation, storage, and transportation. A key step toward understanding clathrate formation is hydrate nucleation, which has been suggested to involve multiple evolution pathways. Herein, a unique nucleation/growth pathway for methane clathrate formation has been identified by analyzing the trajectories of large-scale molecular dynamics (MD) simulations. In particular, ternary water-ring aggregations (TWRAs) have been identified as fundamental structures for characterizing the nucleation pathway. Based on this nucleation pathway, the critical nucleus size and nucleation timescale can be quantitatively determined. Specifically, a methane hydration layer compression/shedding process is observed to be the critical step in (and driving) the nucleation/growth pathway, which is manifested through overlapping/compression of the surrounding hydration layers of the methane molecules, followed by detachment (shedding) of the hydration layer. As such, an effective way to control methane hydrate nucleation is to alter the hydration layer compression/shedding process during the course of nucleation.

Numerical modelling of permafrost spring discharge and open-system pingo formation induced by basal permafrost aggradation

In the high Arctic valley of Adventdalen, Svalbard, sub-permafrost groundwater feeds several pingo springs distributed along the valley axis. The driving mechanism for groundwater discharge and associated pingo formation is enigmatic because wet-based glaciers in the adjacent highlands and the presence of continuous permafrost seem to preclude recharge of the sub-permafrost groundwater system by either a sub-glacial source or a precipitation surplus. Since the pingo springs enable methane that has accumulated underneath the permafrost to escape directly to the atmosphere, our limited understanding of the groundwater system brings significant uncertainty to the understanding of how methane emissions will respond to changing climate. We address this problem with a new conceptual model for open-system pingo formation wherein pingo growth is sustained by sub-permafrost pressure effects during millennial scale basal permafrost aggradation. We test the viability of this mechanism for generating groundwater flow with decoupled heat (1D-transient) and groundwater (2D-steady-state) transport modelling experiments. Our results show that the pingos in lower Adventdalen easily conform to this conceptual model. Simulations suggest that the generally low-permeability hydrogeological units cause groundwater residence times that exceed the duration of the Holocene. The likelihood of such pre-Holocene groundwater ages is also supported by the hydrogeochemistry of the pingo springs, which demonstrate a sea-wards freshening of groundwater, potentially supplied by paleo-subglacial melting during the Weichselian. Such waters form a sub-permafrost fresh water wedge that progressively thins inland, where the duration of permafrost aggradation is longest. The mixing ratio of the underlying marine waters therefore increases in this direction because less unfrozen freshwater is available for mixing. Although this unusual hydraulic system is most likely governed by permafrost aggradation, the potential for additional pressurization is also explored. We conclude that methane production and methane clathrate formation may also affect hydraulic the pressure in sub-permafrost aquifers, but additional research is needed to fully establish their influence.

Cage occupancy of methane clathrate hydrates in the ternary H2O–NH3–CH4 system

Suggested effect of NH3 on methane cage occupancy.