Volumetric Changes of Mud on Mars: Evidence From Laboratory Simulations

Subtle mounds have been discovered in the source areas of Martian kilometer-sized flows and on top of summit areas of domes. These features have been suggested to be related to subsurface sediment mobilization, opening questions regarding their formation mechanisms. Previous studies hypothesized that they mark the position of feeder vents through which mud was brought to the surface. Two theories have been proposed: (a) ascent of more viscous mud during the late stage of eruption and (b) expansion of mud within the conduit due to the instability of water under Martian conditions. Here, we present experiments performed inside a low-pressure chamber designed to investigate whether the volume of mud changes when exposed to a Martian atmospheric pressure. Depending on the mud viscosity, we observe a volumetric increase of up to 30% at the Martian average pressure of similar to 6 mbar. The reason is that the low pressure causes instability of the water within the mud, leading to the bubble formation that increases the volume of the mixture. This mechanism bears resemblance to the volumetric changes associated with the degassing of terrestrial lava or mud volcano eruptions caused by a rapid pressure drop. We conclude that the mounds associated with putative Martian sedimentary volcanoes might indeed be explained by volumetric changes in the mud. We also show that mud flows on Mars and elsewhere in the Solar System could behave differently to those found on Earth because mud dynamics are affected by the formation of bubbles in response to the different atmospheric pressures. Mars is a planet whose surface atmospheric pressure is similar to 160 times weaker than that on Earth. This means that water cannot be present on the Mars surface since in these conditions, lit should boil and evaporate. At the same time, many edifices previously observed on the planet's surface are believed to be the result of mud movement over the Martian surface. Many authors proposed that these structures are the result of a process of sedimentary volcanism during which the sediment was mobilized by liquid water. However, until now, it remained unclear how muds of various viscosities would behave under current Martian conditions. Here, we show that depending on the mud viscosity, a volumetric increase in up to 30% might occur. The reason is that the low pressure causes instability of the water within the mud, leading to the formation of bubbles that increase the volume of the mud mixture. This shows that mud flows on Mars and elsewhere in the Solar System could behave differently to those found on Earth and therefore we might encounter different shapes of edifices formed on Mars by sedimentary volcanism than on Earth. High viscosity muds are able to prevent the easy escape of water vapor bubbles leading to the increase in the volume of such mixturesDepending on the thickness of the mudflow, the volumetric increase can reach up to 30% and thus change the resulting shape of the mudflowPresence of small mounds associated with putative Martian sedimentary volcanoes might be explained by volumetric changes in the mud