Experimental evidence for lava-like mud flows under Martian surface conditions

Large outflow channels on ancient terrains of Mars have been interpreted as the products of catastrophic flood events. The rapid burial of water-rich sediments after such flooding could have led to sedimentary volcanism, in which mixtures of sediment and water (mud) erupt to the surface. Tens of thousands of volcano-like landforms populate the northern lowlands and other local sedimentary depocentres on Mars. However, it is difficult to determine whether the edifices are related to igneous or mud extrusions, partly because the behaviour of extruded mud under Martian surface conditions is poorly constrained. Here we investigate the mechanisms of mud propagation on Mars using experiments performed inside a low-pressure chamber at cold temperatures. We found that low viscosity mud under Martian conditions propagates differently from that on Earth, because of a rapid freezing and the formation of an icy crust. Instead, the experimental mud flows propagate like terrestrial pahoehoe lava flows, with liquid mud spilling from ruptures in the frozen crust, and then refreezing to form a new flow lobe. We suggest that mud volcanism can explain the formation of some lava-like flow morphologies on Mars, and that similar processes may apply to cryovolcanic extrusions on icy bodies in the Solar System

Exposed water ice on the nucleus of comet 67P/Churyumov-Gerasimenko

Although water vapour is the main species observed in the coma of comet 67P/Churyumov–Gerasimenko1, 2 and water is the major constituent of cometary nuclei3, 4, limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far5, 6. The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far7, 8, 9. The nucleus of 67P/Churyumov–Gerasimenko appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material10. Here we report the identification at infrared wavelengths of water ice on two debris falls in the Imhotep region of the nucleus. The ice has been exposed on the walls of elevated structures and at the base of the walls. A quantitative derivation of the abundance of ice in these regions indicates the presence of millimetre-sized pure water-ice grains, considerably larger than in all previous observations6, 7, 8, 9. Although micrometre-sized water-ice grains are the usual result of vapour recondensation in ice-free layers6, the occurrence of millimetre-sized grains of pure ice as observed in the Imhotep debris falls is best explained by grain growth by vapour diffusion in ice-rich layers, or by sintering. As a consequence of these processes, the nucleus can develop an extended and complex coating in which the outer dehydrated crust10 is superimposed on layers enriched in water ice. The stratigraphy observed on 67P/Churyumov–Gerasimenko11, 12 is therefore the result of evolutionary processes affecting the uppermost metres of the nucleus and does not necessarily require a global layering to have occurred at the time of the comet’s formation