Rivers run through Titan, and they take a different type of path than those on Earth or ancient Mars. Comparing riverbeds on the three worlds in our solar system that have them show that Earth is the odd world out.
Titan, Saturn’s largest moon, is one of the more mysterious worlds in our solar system. It has methane rivers, clouds and haze, mountains and valleys – all of which superficially remind observers of Earth. But how all of those features came to be is not well understood.
“Rivers and topography are right there at the intersection of what’s happening on the inside of the planet and the surface,” says Benjamin Black at City University of New York. “If you can figure out what’s happening at that interface, then you can access this whole rich history.”
But Titan is much more difficult to observe. We have only a few images of the surface from the 2005 Huygens probe, and the rest are fairly low-resolution or blocked by the moon’s dense haze.
Looks can be deceiving
In order to compare the topography of these three worlds, Black and his colleagues had to dial down the resolution of sharp images of Mars and Earth to match the images of Titan from the Cassini spacecraft.
“When you look at these images of Titan or Mars, you see so much – valleys, mountains, lake beds – that strike this familiar chord that associates with Earth,” says Black. “But appearances can be deceiving.”
Despite the familiarity of the landscapes, the researchers found that the origins of topography on Titan and Mars are very different from Earth. Here, the terrain is constantly reshaped by plate tectonics, which push up the mountain ranges that often sharply deflect rivers.
Neither Titan nor Mars has plate tectonics, but they both have much longer-range topography – features that span at least one-fortieth (or 2.5 per cent) of the world’s circumference.
But the two worlds came by their mountains and valleys in different ways. “The fact that Earth is so different from the other two could be misconstrued as demonstrating that Mars and Titan are extremely similar,” says Jonathan Lunine at Cornell University. “They’re not. It’s really a tale of three bodies that are all different from each other.”
On Mars, Black and his team found, ancient rivers flowed along the large-scale topographical features, indicating that the terrain came first and directed the flow of rivers.
On Titan, though, they found evidence for recent, maybe ongoing, geological activity which creates the largest features on the hazy moon. Its large-scale topography may be generated by changes in thickness of its global ice shell due to tidal forces from Saturn.
Such shifts in the global shell could potentially help explain how Titan gets its methane. The atmosphere is full of it, so it must be constantly replenished somehow – the method for this replenishment remains one of Titan’s biggest mysteries.
“There’s so much that we don’t know about Titan’s geologic history,” says Black. “Getting this glimpse of how different it is from Earth is pretty exciting.”
Journal reference: Science, DOI: 10.1126/science.aag0171
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