Phenomena

Where are Titan’s Deltas?

Titan's Ligeia Mare.

Saturn’s moon Titan is the only other planetary body in our solar system known to have bodies of liquid on its surface. But where Earth has lakes and seas of water, Titan’s are hydrocarbon-based, primarily ethane and methane. As on Earth, these liquids rain from skies and run down rivers and streams into larger bodies. What they do not do, as far as scientists can tell, is form deltas.

On Earth (and ancient Mars), rivers tend to slow and branch out as they run into larger, still bodies. Many of these river deltas — like the Nile, Ganges, and Mississippi — are visible from space. But so far we’ve seen no equivalent formations on Titan, even though the radar resolution of Cassini should have allowed for it.

There are currently two hypotheses to explain this absence. One posits that density differences between hydrocarbon rivers and lakes mean that deltas do not form. On Titan, the larger bodies are warmer and do not absorb as much atmospheric nitrogen, making them lighter overall. That means a cold, dense river might just sink immediately beneath the lake without slowing to deposit sediment.

Another hypothesis is that deltas do form but that the shifting shorelines of Titan’s seas wash them out and make them unrecognizable. There’s evidence that Titan’s northern and southern hemispheres can swap their liquid hydrocarbons back and forth on a 100,000 year timescale. If that’s true, those shifts could obscure any evidence of deltas.

Experiments are underway to test the first hypothesis, but the final answers may have to wait until NASA’s Dragonfly mission reaches Titan in 2034. (Image credit: Titan – NASA/JPL-Caltech/ASI/Cornell, Alaska – NOAA; via AGU Eos; submitted by Kam-Yung Soh)

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