Experimental Radar Technique Reveals the Composition of Titan’s Seas

The Cassini-Huygens mission to Saturn generated so much data that giving it a definitive value is impossible. It’s sufficient to say that the amount is vast and that multiple scientific instruments generated it. One of those instruments was a radar designed to see through Titan’s thick atmosphere and catch a scientific glimpse of the moon’s extraordinary surface.

Scientists are still making new discoveries with all this data.

Though Saturn has almost 150 known moons, Titan attracts almost all of the scientific attention. It’s Saturn’s largest moon and the Solar System’s second largest. But Titan’s surface is what makes it stand out. It’s the only object in the Solar System besides Earth with surface liquids.

Cassini’s radar instrument had two basic modes: active and passive. In active mode, it bounced radio waves off surfaces and measured what was reflected back. In passive mode, it measured waves emitted by Saturn and its moons. Both of these modes are called static modes.

But Cassini had a third mode called bistatic mode that saw more limited use. It was experimental and used its Radio Science Subsystem (RSS) to bounce signals off of Titan’s surface. Instead of travelling back to sensors on the spacecraft, the signals were reflected back to Earth, where they were received at one of NASA’s Deep Space Network (DNS) stations. Critically, after bouncing off of Titan’s surface, the signal was split into two, hence the name bistatic.

A team of researchers has used Cassini’s bistatic data to learn more about Titan’s hydrocarbon seas. Their work, “Surface properties of the seas of Titan as revealed by Cassini mission bistatic radar experiments,” has been published in Nature Communications. Valerio Poggiali, a research associate at the Cornell Center for Astrophysics and Planetary Science, is the lead author.

This schematic shows how Cassini’s bistatic radar experiment worked. The orbiter used its Radio Science Subsystem to send signals to Titan’s surface. The signals then reflected off Titan to Earth, where they were received by one of the DNS receivers at Canberra, Goldstone, or Madrid. The signals are either Right Circularly Polarized (RCP) or Left Circularly Polarized (LCP). Image Credit: Poggiali et al. 2024.

The signals that reach the DNS are polarized, which reveals more information about the hydrocarbon seas on Titan. While Cassini’s radar instrument revealed how deep the seas are, the bistatic radar data tells researchers about both their compositions and surface textures.

This image of the hydrocarbon seas on Titan is well-known and was radar-imaged by Cassini. That radar data told us how deep the seas are. New bistatic radar data can reveal more about the composition and surface texture of the seas. Image Credit: [JPL-CALTECH/NASA, ASI, USGS]

“The main difference,” Poggiali said, “is that the bistatic information is a more complete dataset, and is sensitive to both the composition of the reflecting surface and to its roughness.”

“It’s like on Earth, when fresh-water rivers flow into and mix with the salty water of the oceans.”

The experimental bistatic radar required meticulous cooperation.

Philip Nicholson, a professor in the Department of Astronomy at Cornell, is one of the study’s co-authors. “The successful execution of a bistatic radar experiment requires exquisite choreography between the scientists who design it, Cassini mission planners and navigators, and the team who collects the data at the receiving station,” Nicholson said.

These results are based on bistatic radar data from four Cassini flybys from 2014 to 2016. In this work, the researchers focused on three large seas on the surface of Titan’s polar regions: Kraken Mare, Ligeia Mare and Punga Mare.

The bistatic radar data revealed new information about the three seas. Though they’re all hydrocarbon seas, their composition varies based on latitude and their proximity to other features like estuaries and rivers. The bistatic radar measured the dielectric constant of Titan’s seas. The dielectric constant is a material’s capacity to store electrical energy. In practical terms, it’s a measure of a surface’s reflectivity, so it reveals the composition. Earth’s water has a dielectric constant of about 80. Titan’s methane and ethane seas have a dielectric constant of only about 1.7. Kraken Mare’s southernmost region had the highest dielectric constant.

This figure from the study shows Titan’s polar regions with the three large seas labelled. The colour key on the right and the text on the image show the dielectric constants of different regions. The white lines labelled T101, T102, T106, and T124 are the four flybys. Image Credit: Poggiali et al. 2024.

Bistatic radar data also showed all three seas had calm surfaces during the four flybys. Waves were no more than 3.3 mm, about 0.13 of an inch. Near estuaries, straits, and coastal areas, the waves were slightly larger: 5.2 mm or 0.2 of an inch. So small they barely merit the name ‘wave.’

This figure from the study is similar to the previous image but shows wave height instead of dielectric constant. Image Credit: Poggiali et al. 2024.

The bistatic radar data also revealed the composition of some of the rivers that flow into the seas.

“We also have indications that the rivers feeding the seas are pure methane,” Poggiali said, “until they flow into the open liquid seas, which are more ethane-rich. It’s like on Earth, when fresh-water rivers flow into and mix with the salty water of the oceans.”

These results agree with scientific models of Titan’s hydrocarbon seas and thick atmosphere. Models show that methane rains down from Titan’s atmosphere and then flows into its lakes and seas. They also show that the rain contains only tiny amounts of ethane and other hydrocarbons and almost completely consists of methane.

“This fits nicely with meteorological models for Titan,” Nicholson said, “which predict that the ‘rain’ that falls from its skies is likely to be almost pure methane, but with trace amounts of ethane and other hydrocarbons.”

The Cassini mission is very instructive for future missions. Though it ended its mission when it plunged into Saturn in 2017, scientists are still making new discoveries with its vast trove of data. The same will be true of missions like Juno when they end.

The researchers behind this work say there’s lots left to learn from all of Cassini’s data.

“There is a mine of data that still waits to be fully analyzed in ways that should yield more discoveries,” Poggiali said. “This is only the first step.”