Experimental radar technology reveals composition of Titan's seas

This color mosaic from NASA's Cassini spacecraft shows the most complete view yet of Titan's northern lake-and-sea region. Saturn's moon Titan is the only world in our solar system other than Earth with stable liquid on its surface. The liquid in Titan's lakes and seas is mostly methane and ethane. New data from Cassini's bistatic radar reveals even more details about Titan's hydrocarbon seas.

The Cassini-Huygens mission to Saturn collected so much data that it’s impossible to assess its significance. Suffice it to say that the volume is enormous, and that it was generated by numerous scientific instruments. One of these instruments was radar, designed to see through Titan’s thick atmosphere and get a scientific view of the moon’s unusual surface.

Scientists are still making new discoveries using all this data.

Although Saturn has nearly 150 known moons, Titan gets almost all the attention from scientists. It is Saturn's largest moon and the second largest in the solar system. But it is Titan's surface that sets it apart. It is the only object in the solar system other than Earth that has liquid on its surface.

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

But Cassini had a third mode, called bistatic, which was used more limited. It was experimental and used the Radio Science Subsystem (RSS) to bounce signals off Titan’s surface. Instead of bouncing back to the spacecraft’s sensors, the signals were reflected back to Earth, where they were picked up by one of NASA’s Deep Space Network (DNS) stations. Crucially, after bouncing off Titan’s surface, the signal was split into two, hence the name “bistatic.”

A team of researchers used Cassini’s bistatic data to learn more about Titan’s hydrocarbon seas. Their paper, “Surface Properties of Titan’s Seas Revealed by Cassini’s Bistatic Radar Experiments,” is published in the journal Nature Communications. Valerio Poggiali, a research scientist at the Cornell Center for Astrophysics and Planetary Science, is the lead author.

This diagram shows how the Cassini bistatic radar experiment works. The orbiter used its radio subsystem to transmit signals to the surface of Titan. The signals were then reflected from Titan to Earth, where they were received by one of the DNS receivers in Canberra, Goldstone, or Madrid. The signals are either right-handed circularly polarized (RCP) or left-handed circularly polarized (LCP).

The signals that reach DNS are polarized, allowing researchers to learn more about Titan's hydrocarbon seas. While Cassini's radar instrument shows the depth of the seas, bistatic radar data tells researchers both their composition and the texture of the surface.

This well-known image of Titan's hydrocarbon seas was taken by Cassini's radar. The radar data allowed us to determine the depth of the seas. The new bistatic radar data can reveal more about the composition and surface texture of the seas.

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

The experimental bistatic radar required careful collaboration between scientists.

Philip Nicholson, a professor in the Cornell Department of Astronomy, is a co-author of the study. “Successfully conducting a bistatic radar experiment requires delicate choreography between the scientists who design it, the Cassini mission planners and navigators, and the team collecting the data at the receiving station,” Nicholson says.

These results are based on bistatic radar data obtained during four Cassini flybys between 2014 and 2016. In this work, the researchers focused on three large seas on the surface of Titan's polar regions: Mare Kraken, Mare Ligeia, and Mare Pungi.

Bistatic radar data provided new information about the three seas. Although they are all hydrocarbon seas, their composition varies with latitude and proximity to other features such as estuaries and rivers. Bistatic radar measured the permittivity of Titan's seas. Permittivity is the ability of a material to store electrical energy. In practical terms, it is an indicator of the reflectivity of a surface, which allows us to determine its composition. The permittivity of Earth's water is about 80. Titan's methane and ethane seas have permittivity of only 1.7. The southernmost region of Kraken Mare has the highest permittivity.

This image from the study shows Titan's polar regions, with three major seas labeled. The color key on the right and text in the image show the permittivity of the different regions. The white lines labeled T101, T102, T106, and T124 are the four flybys.

Bistatic radar data also showed that during the four passes, the surface of all three seas was calm. Waves did not exceed 3.3 mm, about 0.13 inches. Near estuaries, straits and coastal areas, the waves were slightly larger: 5.2 mm, or 0.2 inches. They are so small that they hardly deserve the name “wave.”

This image from the study is similar to the previous one, but instead of permittivity, it shows the wave height.

Bistatic radar data also revealed the composition of some rivers flowing 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 richer in ethane. This is similar to how freshwater rivers on Earth flow into the salty oceans and mix with it.”

These results are consistent with scientific models of Titan's hydrocarbon seas and thick atmosphere. The models show that methane rains out of Titan's atmosphere and then flows into its lakes and seas. They also show that the rain contains only trace amounts of ethane and other hydrocarbons and is almost entirely methane.

“This is consistent with meteorological models of Titan,” Nicholson said, “which predict that the 'rain' falling from its skies is likely to be almost pure methane, but with traces of ethane and other hydrocarbons.”

The Cassini mission is very instructive for future missions. Even though it ended its mission by diving into Saturn in 2017, scientists continue to make new discoveries thanks to the huge amount of data. The same will happen with missions like Juno when they end.

The researchers behind the work say there's much more to learn from all the Cassini data.

“There is a treasure trove of data that still needs to be fully analyzed to make even more discoveries,” Poggiali says. “This is just the first step.”