Scientists investigate the mystery of the missing delta of Titan

For scientists looking to learn about the planet’s geological history, the Delta River is a great place to start. Deltas gather sediment from a large area into one location. This can be studied to reveal the history of climate and structures and past life signs. That’s why NASA sent a recent Mars Rover to Jezero Crater.

And that’s why planetary scientists are also interested in finding the delta in Saturn’s moon Titan. Titan is the only planetary body in the solar system, besides Earth, and its delta could become a scientific treasure trove as liquids flow across its surfaces.

One problem: Titan discovers what new research has discovered despite its large rivers of liquid methane and ethane.

“Because Deltas should preserve much of Titan’s history,” said Sam Burch, an assistant professor at the Department of Environmental Planetary Sciences at Brown University’s Earth Bureau.

But the absence of a delta raises many new questions.

“If you have rivers or sediment, we take it for granted that you get a delta,” Burch said. “But Titan is odd. It’s a playground for studying the processes we thought we understood.”

Titan is the largest of Saturn’s 274 confirmed moons. Its thick nitrogen and methane atmosphere creates many earth-like climate and weather features. Titan has clouds, wind, rain, rivers, lakes and oceans. However, instead of water, Titan’s liquids contain methane and ethane. Methane is a liquid at Titan’s chilly surface temperatures.

Scientists learned about Titan’s liquid body when Cassini’s spaceship flew in 2006. Peering into the thick atmosphere of Titan with Cassini’s synthetic aperture radar (SAR), the spacecraft revealed a large body of liquid and a large flat area with a large flat area.

But what is largely missing from Cassini’s SAR images is the delta, even the mouth of the large river. However, it was not clear whether Delta was really absent or not shown in Cassini’s SAR data. That’s a question that Burch and his colleagues tried to answer in this new study. This is featured in the Journal of Geophysical Research: Planets.

The problem with Cassini’s SAR data is that shallow liquid methane is largely transparent in every image. Therefore, while images of the SAR allowed us to see wide ocean and river channels, it is difficult to create coastal features with confidence, as it is difficult to see where the coast ends and where the seabed begins.

For this study, Birch develops a numerical model and simulates what Cassini’s SAR checks whether landscape scientists understand well: Earth. In the model, Earth’s rivers and ocean waters were replaced by Titan methane liquids with different radar absorption properties compared to water.

“We basically created a synthetic SAR image of the Earth that assumes the properties of Titan’s liquids instead of the Earth,” Burch said. “If you look at the SAR images of the landscape, you know so well that you can go back to Titan and get a little better at what you’re looking at.”

This study found that synthetic SAR images of the Earth clearly resolved large deltas and many other large coastal landscapes.

“If there’s a size of a delta at the mouth of the Mississippi River, we should be able to see it,” Burch said. “If we have a big barrier island or similar coastal landscape that we see along the US Gulf Coast, we should be able to see them.”

However, when Burch and his colleagues looked at images of Titan in light of their new analysis, they were almost empty. The rest of the moon’s river was completely delta free, except for two possible deltas near Titan’s Antarctic. Researchers found that only about 1.3% of the large rivers of Titan that end on the coastline have a delta. In contrast, on Earth, almost every river of similar size has a delta.

It’s not entirely clear why Titans generally lack delta, Burch says. The liquid properties of Titan rivers must be able to carry and deposit sediment. Researchers say that the sea level on Titans rises very rapidly, and is smeared faster than the delta can accumulate in a single location. Winds and tidal currents along Titan’s coast can play a similarly large role in preventing delta formation.

And the only delta of mystery posed by new research is not missing. A new analysis of Cassini SAR data on the Titan coast reveals holes of unknown origin deep within the lake and ocean. This study found deep waterways on the ocean floor, which appears to have been carved by river flows, but it is not clear how they reached it.

All of these surprises, Burch says, will require more research to fully understand.

“This is not really what we expected,” Burch said. “But Titan does this a lot for us, and I think that’s what makes it an attractive place to study.”