Role of Fluids and Atmosphere in the Tectonic Evolution of Titan

Abstract:

The Cassini Synthetic Aperture Radar (SAR) has revealed elevated mountains and hummocky terrains, which have been interpreted to be related to tectonic processes. The geomorphic evidence strongly suggests contractional strain has formed most of the mountains on Titan. Analysis of SAR images and SARTopo profiles reveals mountain belts with linear-to-arcuate morphology and relatively gentle slopes (< 2^o). These observations suggest that Titan’s mountains are likely folds and thrusts related to small amounts of deformation.

If the mountains on Titan are contractional in origin, another problem arises: relatively large stresses are required to cause contractional deformation, and sources of such stresses are difficult to obtain for most icy satellites. We estimate Titan’s lithospheric strength as a function of depth in the brittle and ductile regime based on Byerlee’s law and flow law. Our calculation shows that a stress of less than 8-10 MPa is required to cause contractional strain on Titan. Since liquid hydrocarbons have been identified on Titan’s surface and may flow in the subsurface, by modifying the Mohr-Coulomb law of frictional shear strength, we show that fluid pressures associated with liquid hydrocarbons in the subsurface significantly reduce frictional resistance to sliding and enables contractional structures to form without requiring large stresses. On Titan, fluid pressures weaken the upper crust and make contraction easier.

Titan’s volatile endowment (methane and ammonia) and consequently thick atmosphere has sustained the stability of surface and subsurface hydrocarbon liquids, weakening the upper crust. Other icy satellites, such as Europa, Callisto and Ganymede, lack significant proportions of these volatiles, and hence lack atmospheres and hydrospheres with ground-liquids. Consequently, they have very different tectonic systems than Titan and they lack extensive contractional tectonic features. Thus, the volatile component obtained during accretion directly affects the style of tectonics on icy satellites, just as it does on Earth.