Could CH4 clathrates have formed in the subsurface of Gale Crater?

In situ measurements of atmospheric CH₄ taken by the Curiosity rover in Gale Crater revealed strong seasonal variations in background gas levels (Webster et al. 2018, 2021). Endogenous CH₄ could have been stored in the past in subsurface reservoirs in the form of clathrate hydrates (Prieto-Ballesteros et al. 2006; Chastain and Chevrier 2007; Oehler and Etiope 2017), and remain partially stable in the present-day cryosphere (Gloesener et al. 2021).

The stability of clathrates—including their thermodynamic properties and their formation-dissociation kinetics—depends not only on pressure and temperature conditions, but also on the presence of other compounds with which they coexist to form a mineral association, and with which they could interact physically and chemically. Thus, in this research we have reviewed the mineralogy of the Gale Crater subsurface—i.e., the region where clathrates could coexist with other mineral phases—in order to study the stability of CH₄ clathrates in such associations.

Data from missions to that region suggest that Gale Crater may once have been home to an ancient lake. The crater formed between the late Noachian and early Hesperian periods, while the subsequent formation of the sedimentary strata (composed of clay minerals and sulfate layers) occurred during the Hesperian period (Thomson et al. 2011; Palucis et al. 2014; Grant et al. 2014; Grotzinger et al. 2015). Furthermore, a wide variety of sedimentary layers have been revealed on Mount Sharp that record multiple episodes of ancient aqueous alteration processes (e.g., Achilles et al. 2020), including groundwater activity (Thorpe et al. 2022). In particular, a coexistence of clay minerals and sulfates was found in large quantities in several drill samples collected in the Murray Formation (Bristow et al. 2018) and in the Glen Torridon area (Thorpe et al. 2022).

Based on these premises, we experimentally studied the kinetics of CH₄ clathrate formation and dissociation, at pressures up to 100 bar, in the presence of MgSO₄ brines and two clay minerals—a montmorillonite (2:1 type) and a kaolinite (1:1 type)—using high-pressure differential scanning calorimetry (DSC). The results showed that the presence of both sulfates and clay minerals can favor the induction time of clathrate formation, but considerably reduces the final stabilized amount. The salting-out effect caused by the salts and the water strongly bound to the clay surfaces are the main factors that inhibit clathrate growth. These results have important implications for the assessment of potential CH₄ sources in Gale, as the experiments demonstrate the inhibition of clathrate formation within the mineral assemblages found in the crater.

References:  Achilles et al. 2020, J. Geophys. Res. Planets, 125, e2019JE006295; Bristow et al. 2018, Sci. Adv., 4, eaar3330; Chastain and Chevrier 2007, Planet. Space Sci., 55, 1246 ; Gloesener et al. 2021, Icarus, 353, 114099 ; Grant et al. 2014, Geophys. Res. Lett., 41, 1142 ; Grotzinger et al. 2015, Science, 350, aac7575 ; Oehler and Etiope 2017, Astrobiology, 17, 1233 ; Palucis et al. 2014, J. Geophys. Res. Planets, 119, 705 ; Prieto-Ballesteros et al. 2006, Geology, 34, 149 ; Thomson et al. 2011, Icarus, 214, 413 ; Thorpe et al. 2022, J. Geophys. Res. Planets, 127, e2021JE007099 ; Webster et al. 2018, Science, 360, 1093 ; Webster et al. 2021, A&A, 650, A166

Acknowledgements: The authors acknowledge support from the MCIN/AEI/ http://doi.org/10.13039/501100011033 projects PID2022-142490OB-C31, and PCI2023-145992-2, the SOS-Mars project (PID2020-119412RJ-I00) from MICINN Spain, the Marie Curie Postdoctoral Fellowship program (HORIZON-MSCA-2022-PF-01), grant nº 101105979 – SECRECY; and the RYC2024-050522-I grant, funded by MICIU/AEI/10.13039/501100011033 and the ESF+. Part of this work has been conducted at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.