Martian Caves as Special Region Candidates : A simulation in ANSYS Fluent on how caves on Mars are, and what their conditions would be for being considered as special regions.

Abstract: One of the most interesting questions about Mars is if life ever existed on it. One of the main requirements for life to exist as we know it is the presence of liquid water. It has been suggested by Martín-Torres et al. (2015a) that a daily-transient liquid water cycle takes place on the surface of Mars through deliquescence and efforescence (binding and releasing of water vapour) of perchloratic salts in the Martian soil. Given the right conditions regarding water activity and temperature, certain planetary areas have been defined as Special Regions where there is a chance of life-form reproduction to occur (Kminek et al. 2017). Sub-surface cavities and caves are defined as such and are still a relatively unexplored and not yet studied feature of the Martian surface. This report is an assessment of the environmental conditions in Martian subsurface cavities such as caves and how it can be considered as a Special Region. Based on observations of lava tubes made by Cushing and Titus (2010) with atmospheric and thermal data from REMS on board the Curiosity rover by Martín-Torres et al. (2015b), simulation models were set up in ANSYS Fluent to examine the behaviour of the temperature and relative humidity within these caves. Different properties of the studied models included size, shape, inclination, materials of the ground composition and air flow behaviour. The results showed that a cave roof with a thickness greater than 1-2 m prevents the ground temperature variation during the day to have any considerable impact on the air temperature in the cave which implies that the thermal waves are the main driving factor of the thermal environment in larger models. The average temperature and relative humidities throughout the entire models resulted in unfavourable conditions (relative humidity under 20% RH) to allow for any perchloratic salts to hydrate or form brines. The most interesting results were found in smaller models where different phenomena with higher relative humidity near the floor and in corners occurred for several hours during the same day. This happened at certain times during the day (LMST 7 and 17) when the inlet temperature surpassed the average temperature in the cave and resulted in relative humidities of up to 90% RH which potentially could allow perchloratic salts to stay in brine form, or at least in a hydrated state throughout the day. While the low temperatures in today's Martian caves may be too harsh for life forms to exist, a previous warmer climate might have allowed for extremophiles to thrive in highly saline solutions. This could be an implication that Martian caves should be defined as Special Regions and that further studies should be done on the subject.