Martian Glacial Morphology, Geomorphology, and Atmospheric Methane

Introduction: We have earlier proposed [1] that methane clathrate hydrate in martian glacial ice could be responsible for both current martian atmospheric methane and for signs of past martian glacial melt. We further explore this idea, and compare it to the similar hypothesis of Prieto-Ballesteros, et al. [2]. Background: The presence of methane in the martian atmosphere [3-5] is intriguing, because methane’s photochemical stability on Mars is short [3], and nearly all terrestrial methane is biogenic. The mixing ratio is highest in water-rich areas, but varies greatly, suggesting that it is released from highly localized sources [4]. The existence of geomorphology resembling terrestrial glacial meltwater features is also puzzling, as martian surface temperatures, atmospheric pressure, gravity, and geothermal heat flux suggest that martian glaciers since the Noachian should be coldbased [6-7], even during periods of warm climate [1]. Methane release loci: Data from the Planetary Fourier Spectrometer (PFS) show highly variable mixing ratios, implying localized sources [4]. Allen et al. [8] seek correlations between methane and geologic structures. Work by McMenamin and McGill [1] and Prieto-Ballesteros, et al. [2] suggests that the correlation will be with remnant glacial features, such as those demonstrated by Head, et al. [9]. PrietoBallesteros et al. [2] propose that methane is released from cryospheric hydrates destabilized by the retreat of sublimating glaciers. As seen in Figure 1, this is minor compared to the amount destabilized within the remnant glacier itself. A glacial or cryospheric reservoir of methane does not constrain the ultimate source of the methane, which could be produced by microbe or abiotic geological processes. Meltwater sources: The most likely sources of subglacial meltwater are local geothermal activity (limited to certain times and places), increased basal temperature due to the insulating properties of dust or gas hydrates, or lowered melting point due to inclusion of salts, gases, or gas hydrates [1]. Catastrophic dissociation of cryospheric gas hydrates may have caused chaos terrain and outflow channels [10,11]. Why methane hydrate: The fact that methane hydrate can explain both geomorphological features and atmospheric chemistry makes it an elegant solution to both problems. Carbon dioxide hydrate, which may also exist on Mars, would form where the CO2-rich atmosphere interacts with ice, i.e., at the surface, and probably only near the poles [11,12]; concentrations of CO2 hydrate are unlikely [10]. Methane hydrate formation, whether due to microbes or geologic processes, can occur within the hydrate stability zone (HSZ). In the HSZ, hydrates form spontaneously if the appropriate molecules are present, though the presence of salts inhibits them. Max and Clifford [10, 13] showed that a HSZ should begin at ~15 m below the surface of bare, ice-saturated regolith. Extrapolating from their results and assuming a glacial ice density of ~910 kg m [14], methane hydrate should be stable beneath a mere ~38.5 m of glacial ice [1]. Methane hydrate dissociates below the melting point of water ice, and increases greatly in volume

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