Complex flow of mud over cold and frozen surfaces in low pressure: Insights from physical experiments

Extensive fields of sub-kilometre- to kilometer-scale edifices have been discovered on Mars and the process of subsurface sediment mobilization has been proposed as their formation mechanism. However, as igneous volcanism might form similarly looking features, it is currently unknown how they formed. Previously it was shown that when low viscosity muds would be exposed over cold sandy surfaces under the reduced martian atmospheric pressure, such muds would behave in similar fashion as Pahoehoe lavas on Earth. This shows how difficult it can be to distinguish mud volcanoes from igneous volcanoes based on morphology alone. However, the composition of the propagating mud as well as of the substrate might be crucial to the overall dynamics and the finite pattern of developed flow features on Mars. On the Red planet, a wide range of substrates is expected to be present globally; covering a transition from dry and warm unconsolidated regolith to permafrost with a higher content of ice.  Therefore, to get a better understanding of the behavior of muds exposed to reduced atmospheric pressure and the resulting shapes of putative martian mud volcanoes, we performed a set of experiments, in which we studied the effect of warm, pre-cooled or continuously frozen substrates on general flow properties. We also considered different granular materials, transitional compositions or their spatial sequencing, using mainly silica sand, flour or pure water ice.  All tested scenarios showed a significantly contrasting style in mud spreading over the various surfaces. The streaming style and finite morphology of the flows differed from fast, flat spreadings, with the levitation component of transport, to slow and narrow flows with a characteristic ropy pattern. The most important observed feature was an alternation of melting and recrystallization of the ice substrate, caused by interplay between the latent heat release and consumption in between the mud and substrate. Importance of ice in the substrate was also shown through rapidly extended boiling potential and prolonged flow ability of mud, probably due to combination of phase transitions in mud-permafrost and mechanical properties of the substrate itself.  These findings are interesting for an evaluation of mud behavior in various environments occurring on Mars or other bodies within the Solar system where the sedimentary volcanism or cryovolcanism might be expected.