The presence of salts changes the architecture of potential mudflows on Mars - insights from laboratory simulations

The behavior and the rheology of mud during the emplacement of terrestrial sedimentary volcanism has been extensively investigated (e.g., [1,2]). In contrast, this is not the case for Mars and other planetary bodies within the Solar System for which sedimentary volcanism has been proposed [e.g., 3]. The propagation behavior of low viscosity mud in a low-pressure chamber, that partly simulated the environment of Mars, was firstly experimentally studied by [4,5]. Their work revealed that bentonite-based mud could flow in a completely different manner in such conditions. On Mars, mud flowing over cold surfaces would rapidly freeze due to evaporative cooling [6] forming an icy-crust leading to the behavior of some of the mud flows in a similar manner to pahoehoe lava on Earth [4]. However, we lack the knowledge how variations of salt types and their content would affect the flow style and finite pattern of such mudflows as a presence of various salts can be natural on Mars as well (e.g., [7,8]). Therefore increased content of salts can strongly affect the P-T-t dependent cooling and at the same time the rheology of mud which can lead to significantly different propagation potential and finite geometry. 

In a set of experiments, performed in the Mars Simulation Chamber (Open University, UK), we tested several selected salts relevant for the Mars environment (namely NaCl, MgSO4, Na2SO4 and CaSO4) and various salinities of these salts (0.5-15 wt%). These experiments were performed in metallic trays infilled with dry and precooled sand to -25 °C (to simulate the martian surface) and which were inclined to 5°. A container filled with 500 ml mud was positioned above the tray. Then we decreased the pressure to 4.5-6 mbar and released mud. Experiments were documented by a system of video cameras situated around the model box. At the same time, referential cooling experiments of binary solutions (water-salt) were performed. 

Results revealed contrasting scenarios of mud propagation which result in a wide range of shapes. We also found several transitional regimes in behavior between current concentrations and various salts. It was confirmed that the high content of salt in a mud or mud composed by different salts can undergo slightly to significantly different cooling according to thermodynamic equilibria which shifts both freezing and boiling point. Thus, the resultant style of flow process and finite morphology of such mudflows can be highly variable. For example, high content of MgSO4 (typically 5-10 wt%) leads to development of long and narrow streams and with increasing content also develops a “ropy pattern” structure, whereas the same behavior occurs for 2.5 wt% of the NaCl.  

References: [1] O’Brien and Julien (1988), Journal of Hydraulic Engineering 114 [2] Laigle and Coussot (1997), J. Hydraul. Eng., 123 [3] Ruesch et al. (2019) Nature Geoscience 12 [4] Brož et al. (2020), Nature Geoscience [5] Brož et al. (2020), EPSL 545 [6] Bargery et al. (2010), Icarus 210(1), Chevrier et al. (2020), The planetary science journal, 1(3) [8] Nuding, et al. (2014), Icarus, 243.