EXTREME KARST: DISSOLUTION GEOLOGY ON OTHER PLANETS

We propose that dissolution geology can occur wherever there is a circulating fluid in contact with a soluble geological material for a long enough time. Here on Earth, we are used to materials such as halite, gypsum, calcite, dolomite, and even silica dissolving in water to create karstic landscapes. The amount of material that can be dissolved in a given volume coupled with the amount of circulating fluid determines how quickly a karstic system can develop. Highly soluble materials such as halite will be eroded quickly in high precipitation areas, while karstic systems in quartzite can take hundreds of millions of years to fully develop.

In a hyperarid landscape such as Mars, episodic circulating water has been proposed to have created polygonal “eggbox terrain” in kieserite (MgSO₄) observed in Western Tithonium Chasma. On Earth, this material would be hydrated and dissolved quickly. On Mars, the overall volume of circulating surface fluids was not sufficient to allow complete dissolution and removal of this material. We can thus use the partial removal of soluble materials to constrain environmental conditions.

Saturn’s moon Titan extends geological processes to include cryogenic hydrocarbon fluids such as methane and ethane. The Cassini mission has provided evidence of a hydrocarbon-based cycle on Titan similar to the terrestrial water cycle. On Titan, recirculating hydrocarbon fluids are capable of dissolving some of the surface organic molecules derived from complex atmospheric photochemistry. Several terrain features resembling terrestrial karst have been observed on Titan, including polygonal karst-like terrain, closed valleys, possible poljes, and steep-sided lakes with morphologies similar to terrestrial karstic lakes. These observations, particularly the closed valleys, suggest that there is a material on Titan capable of being dissolved and transported. Recent theoretical calculations and laboratory experimentation has provided further support that dissolution can occur on a reasonable timescales and amounts even in cryogenic hydrocarbon fluids such as those found on Titan.

We suggest that karst is a fundamental planetary surface process that occurs wherever circulating fluids are capable of dissolving materials and developing subsurface drainage.