Geology of Titan

Lore & Background

The possibility of hydrocarbon seas on Titan was first suggested based on Voyager 1 and 2 data showing a thick atmosphere of suitable temperature and composition. Direct evidence emerged in 1995 from Hubble and other observations suggesting liquid methane, either in disconnected pockets or satellite-wide oceans. The Cassini mission confirmed the former hypothesis. Upon arrival in 2004, no specular reflections were initially observed, but near Titan's south pole a dark feature named Ontario Lacus was identified and later confirmed as a lake. Following a flyby on 22 July 2006, Cassini's radar imaged the northern latitudes and found several large, smooth patches near the pole. In January 2007, scientists announced definitive evidence of lakes filled with methane. The Cassini–Huygens team concluded these are the first stable bodies of surface liquid found outside Earth. Some appear to have channels associated with liquid and lie in topographical depressions. Liquid erosion features appear very recent; channels in some regions have created surprisingly little erosion, suggesting erosion on Titan is extremely slow or recent phenomena may have wiped out older riverbeds. Overall, lakes cover only a small percentage of the surface, making Titan much drier than Earth. Most lakes are concentrated near the poles, but several long-standing hydrocarbon lakes in equatorial desert regions have also been discovered, including one near the Huygens landing site in Shangri-La, about half the size of the Great Salt Lake. These equatorial lakes are probably 'oases' supplied by underground aquifers.

Reader's Guide

The geology of Titan is significant for revealing a world with active surface liquids, a feature unique in the solar system beyond Earth. The Cassini mission provided definitive evidence of hydrocarbon lakes and seas, primarily methane and ethane, concentrated near the poles. The discovery of specular reflections from Ontario Lacus and Jingpo Lacus confirmed smooth, mirror-like liquid surfaces. Depth measurements showed Ontario Lacus is extremely shallow (0.4–3 m average), while northern seas like Ligeia Mare reach depths over 200 m, containing enough liquid methane to fill three Lake Michigans. The Vid Flumina channels, a drainage network connected to Ligeia Mare, were found to be deep canyons (up to ~570 m) with steep sides, currently filled with liquid at the same elevation as the sea, indicating drowned river valleys. This is the first direct evidence of liquid channels on Titan and hundred-meter deep canyons. The rarity of river deltas (only 1.3% of large rivers ending at coastlines have a delta) contrasts sharply with Earth. Titan's interior is inferred to have a deformable icy shell dozens of kilometers thick floating atop a global ocean, which may lie no more than 100 km below the surface. The ocean may be as salty as the Dead Sea. Studies suggest methane rainfall may interact with an underground layer of icy materials called an 'alkanofer' to produce ethane and propane that feed rivers and lakes. The few impact craters discovered show evidence of extensive erosion, indicating active surface modification.

Did You Know?

• The Cassini mission confirmed the presence of hydrocarbon lakes on Titan in January 2007.
• Only 1.3% of large rivers on Titan that end at coastlines have a river delta.