Earth’s coastlines are fascinating places where liquids mix and materials are shaped into distinct landforms like river deltas. Similar active coastlines exist on Saturn’s moon Titan, where liquid hydrocarbons (methane and ethane) take the place of water. However, studying Titan’s coastlines, especially river deltas, is challenging due to limited imaging data and the unknown nature of its materials. To overcome these challenges, planetary scientists from Brown University, MIT, MIT-WHOI Joint Program in Oceanography, Woods Hole Oceanographic Institution and Cornell University developed a new model that simulates Earth’s coastlines as if seen by NASA’s Cassini spacecraft. They discovered that large landforms can be detected on Titan with the right contrasts. They then returned to Titan and re-mapped its coastlines. Surprisingly, they found that many of Titan’s rivers do not end in deltas, unlike on Earth where many large rivers host river deltas. They also found submerged features on Titan’s seafloors that suggest changing sea level and/or active flows under the sea surface.
This composite image shows an infrared view of Titan. In this image blue represents wavelengths centered at 1.3 microns, green represents 2.0 microns, and red represents 5.0 microns. A view at visible wavelengths would show only Titan’s hazy atmosphere; the near-infrared wavelengths in this image allow Cassini’s vision to penetrate the haze and reveal the moon’s surface. The view looks toward terrain that is mostly on the Saturn-facing hemisphere of Titan. Image credit: NASA / JPL-Caltech / Space Science Institute.
“It’s kind of disappointing as a geomorphologist because deltas should preserve so much of Titan’s history,” said Dr. Sam Birch, a researcher at Brown University.
“But the absence of deltas raises a host of new questions.”
“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.”
Titan is the largest of Saturn’s 274 confirmed moons. Its thick nitrogen and methane atmosphere gives rise to a host of Earth-like climate and weather features.
Titan has clouds, wind and rain as well as rivers, lakes and seas. But instead of water, Titan’s fluid bodies contain methane and ethane, which are liquid at Titan’s chilly surface temperatures.
Scientists learned of Titan’s liquid bodies when the Cassini spacecraft flew by in 2006. Peering through Titan’s thick atmosphere with Cassini’s synthetic aperture radar (SAR), the spacecraft revealed spidering channels and large flat areas consistent with large bodies of liquid.
Largely missing from Cassini’s SAR images, however, were deltas — even at the mouths of large rivers.
It wasn’t clear, however, whether the deltas were truly absent, or whether they just didn’t show up in Cassini’s SAR data.
That’s the question Dr. Birch and his colleagues tried to answer with their new study.
The problem with Cassini’s SAR data is that shallow liquid methane is largely transparent in any images.
So while the SAR images could see the broad seas and river channels, it’s harder to confidently make out coastal features because it’s difficult to see where the coast ends and where the sea floor begins.
For the study, the authors developed a numerical model to simulate what Cassini’s SAR would see if it looked at a landscape scientists understand well: Earth.
In the model, the water in Earth’s rivers and oceans was replaced by Titan’s methane liquid, which has different radar absorption properties compared to water.
“We basically made synthetic SAR images of Earth that assume properties of Titan’s liquid instead of Earth’s,” Dr. Birch said.
“Once we see SAR images of a landscape we know very well, we can go back to Titan and understand a bit better what we’re looking at.”
The researchers found that the synthetic SAR images of Earth clearly resolved large deltas and many other large coastal landscapes.
“If there are deltas the size of the one at the mouth of the Mississippi River, we should be able to see it,” Dr. 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 the scientists combed over the Titan images in light of their new analysis, they came up mostly empty.
Aside from two probable deltas near Titan’s south pole, the rest of the moon’s rivers were entirely delta-free.
They found that only about 1.3% of Titan’s large rivers that terminate at coastlines have deltas. On Earth, in contrast, nearly every river of similar size has a delta.
“It’s not entirely clear why Titan generally lacks deltas,” Dr. Birch said.
“The fluid properties of Titan’s rivers should make them perfectly capable of carrying and depositing sediment.”
“It could be that sea levels on Titan rise and fall so rapidly that deltas are smeared across the landscape more quickly than they can be built up in a single spot.”
“Winds and tidal currents along Titan’s coasts may also play an equally large role in preventing delta formation.”
And missing deltas aren’t the only mystery raised by the new research.
The new analysis of Cassini SAR data of Titan’s coasts revealed pits of unknown origin deep within lakes and seas.
The researchers also found deep channels on the floors of the seas that seem to have been carved by river flows, but it’s not clear how they got there.
“All of these surprises will require more research to fully understand,” Dr. Birch said.
“This is really not what we expected. But Titan does this to us a lot. I think that’s what makes it such an engaging place to study.”
The study was published in the Journal of Geophysical Research: Planets.
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S.P.D. Birch et al. 2025. Detectability of Coastal Landforms on Titan with the Cassini RADAR. JGR Planets 130 (3): e2024JE008737; doi: 10.1029/2024JE008737
