Universe Today
Titan, the largest moon of Saturn, looks more Earth-like on its surface than any other place in the Solar System. With its thick atmosphere and liquid methane rain, it has lakes, rivers, sand dunes and seas. But appearances can be deceiving and in other ways, Titan is in fact a very alien world. One baffling difference, recently discovered, is that Titan's rivers do not seem to form deltas when they reach the sea.
Titan is the largest moon of Saturn. It was discovered in 1655 by Christiaan Huygens, and was the 6th moon to be discovered after our own, and the 4 Galilean moons around Jupiter. It is the second largest moon in the Solar System, and orbits Saturn at an average distance of roughly 1.2 million kilometers. Although it is larger than Mercury, it has less than half as much mass. It also has a thick, cloudy atmosphere, and until a few decades ago, that was almost all we knew.
Similarities with Earth
When the Cassini mission arrived, we learned that Titan was surprisingly similar to Earth. It has an atmosphere thicker than our own and is the only place in the entire universe, outside of Earth, where we have observed the presence of free-running liquids on the surface. Despite the extreme cold, it has weather systems complete with rain falling from the clouds.
This rain, when it lands, rolls downhill to form streams and rivers, which eventually empty out into lakes and seas. Like on Earth, these rivers carve channels unto the ground, forming river beds, and they carry sediment.
But despite these similarities, they are still very different worlds. Titan is a place of extreme cold. Being so far from the Sun, it doesn't receive a lot of warming sunlight, and it doesn't have a massive molten iron core. At these temperatures, water is frozen so hard that it is just another kind of rock. The liquids raining from the sky and flowing on the surface? Super-chilled ethane and methane.
Hydrology
On Earth, water circulates around the planet in a cycle. Liquid water gives up some of its molecules to the atmosphere, driven out by their internal heat energy, in a process we call evaporation. The water vapour in the atmosphere circulates around the globe until it finds a region where the pressure is high enough, the temperature low enough, that it condenses into tiny droplets around nucleation sites: specks of dust or airborne bacteria. Sometimes these droplets stay liquid, and combine to form larger and larger droplets, sometimes they freeze into ice crystals, but either way we can see them from the ground as clouds. If the droplets get big enough, they start to fall, and we get precipitation (rain, snow, hail, depending on conditions).
If the rain falls from low enough that it doesn't simply evaporate again, it reaches land and wets the ground. Some soaks into the soil, the rest trickles down to form small streams, which in turn combine to form rivers, and eventually flow into the sea (or not! Some rivers in arid areas simply fade away, either soaking into the parched earth or evaporating away entirely). As rivers flow, they erode the ground beneath them, carving river beds, and transporting silt. This silt can be deposited wherever the flow is slow, and eventually builds up enough to change the course of the river. When this happens at a river mouth, the mouth begins to block up with silt and the river eventually breaks a new path around the blockage. Over enough time, this happens often enough that you are left with the classic triangular river delta formation.
But for some reason, this doesn't seem to happen on Titan!
Cassini
Titan's thick soupy atmosphere makes it hard to observe any of these features. None of this would be known without Cassini's synthetic aperture radar (SAR). Unfortunately liquid methane and ethane are completely transparent to the SAR instrument, so many of the details are inferred. We don't observe rivers or seas directly, but instead we see what they've done to the ground beneath: river beds cutting across the landscape, emptying to large basins that make up lake and sea beds.
Given that Deltas are formed from silt accumulated over very long times, blocking up river mouths and forcing rivers to find new paths, you might expect these formations to be easy to spot. But researchers studying Cassini mission data have not found them.
It's kind of disappointing as a geomorphologist because deltas should preserve so much of Titan's history," said Sam Birch, an assistant professor in Brown University's Department of Earth, Environmental and Planetary Sciences. "We take it for granted that if you have rivers and sediments, you get deltas. But Titan is weird. It's a playground for studying processes we thought we understood.
The hunt continues
To test his assumptions, Birch developed a numerical model to process similar data from a more familiar world: Earth. The model simulated what Earth's underwater features might look like to the same SAR instruments, if they were under liquid methane and ethane instead of water, and confirmed that river deltas should have been easily visible.
If there are deltas the size of the one at the mouth of the Mississippi River, we should be able to see it," Birch said. "If there are large barrier islands and similar coastal landscapes like those we see all along the U.S. Gulf Coast, we should be able to see those.
But when Birch and his colleagues returned to the Cassini data they did not find the missing features: Only two rivers, near the South Pole of Titan, showed possible delta formations. By their count based on the Cassini data, only 1.3% of large rivers on Titan terminate in deltas, compared to almost all comparable rivers on Earth.
We're unlikely to know for certain what's going on until another mission can be sent to Saturn to study its moons more closely. But Birch and his team do have some ideas: Perhaps the sea level rises and falls fast enough that the sediments are regularly submerged, washing the silt away before it has time to form a proper delta. Or possibly strong winds and coastal currents are doing the same thing. After all, radar imaging has also revealed deep river channels cut into the sea beds themselves, another mystery that hasn't yet been solved.
As usual, it will take more data, and a lot more hard work from planetary scientists to find answers.
This is really not what we expected," Birch said. "But Titan does this to us a lot. I think that's what makes it such an engaging place to study.
