EVIDENCE OF EXTENSIONAL AND COMPRESSIONAL TECTONISM ON SATURN'S MOON TITAN

Saturn's largest satellite, Titan, is at 2500 km radius larger than the planet Mercury. Its crust is dominantly water ice, which at Saturn's position has surface temperatures of 90 K and behaves as rock. Titan boasts an Earth-like geomorphology on its icy crust, with mountains, dunes, volcanoes, lakes, and river valleys carved by an active hydrologic cycle involving liquid methane/ethane. These features have all been observed by the currently operating Cassini spacecraft, mostly by the RADAR Synthetic Aperture Radar (SAR) instrument at resolutions of ~300 m (e.g. Elachi et al. 2005).

Titan's water ice mountains are relatively low-relief features at heights of up to 3000 m, but they can attain slopes of 40 degrees (Radebaugh et al. 2007). They appear as elevated blocks, eroded mesas, and rugged mountain chains eroded by methane rainfall. Many of these ridges are oriented W-E near Titan's equatorial region, suggesting regional or global tectonism was involved in their formation (Radebaugh et al. 2007; Mitri et al. 2009). Mountains are sometimes organized into arcuate trends of bright peaks we call belts. These arcuate belts are elevated, are concave southward, and give evidence of having been formed by compression.

Many of the margins of widened valleys and mountain blocks on Titan have straight morphologies. Some of these lineations stretch for up to 100 km; many are subparallel to one another. These patterns indicate there are regional tectonic controls, likely extensional, on the evolution of the mountain blocks and associated river valleys. In some locations, drainages have an angular or trellis pattern, indicating surface fracturing from extension.

In most regions on Titan, it appears that first compression, followed by extension, occurred. This progression is typical of other icy satellites (although evidence of compression is rare elsewhere) and is related to the release of internal heat, due to the decay of radioactive elements in its rocky core plus tidal heating from interactions with Saturn. Finally, the heavily eroded morphologies of Titan's mountainous regions indicate these may be old terrains existing under an active atmosphere.