NASA’s Cassini-Huygens spacecraft may have ended its 20-year mission exploring Saturn’s neighborhood dramatically seven years ago when it crashed into the gas giant, but it’s still delivering science goods.
Using radar data collected by Cassini, Cornell University astronomers have gleaned new information about the liquid ocean of Saturn’s largest moon, Titan, which is made up of hydrocarbons, a class of organic chemicals made up of carbon and hydrogen. That class includes chemicals such as methane and ethane.
The team was able to analyze the composition and “roughness” of Titan’s sea, which is located near the world’s north pole. The researchers found calm seas of methane with a gentle tidal flow. Not only is this something that previous studies of Titan’s seas have failed to reveal, but it also lays a foundation for future studies of the solar system’s ocean moons.
The Cassini data used for these new findings were collected using “ballistic radar,” in which the spacecraft aimed a radio beam at Titan, which was then reflected back to Earth.
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The effect of this is to polarize Titan’s surface reflectivity, providing two different perspectives. Standard radar that saw the signal reflected back to Cassini provided only one perspective.
“The main difference 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,” said Valerio Poggiali, a team member and researcher at the Cornell Center for Astrophysics and Planetary Science (CCAPS), in a statement.
Cassini was launched on October 15, 1997, and took seven years to reach the Saturnian system. NASA sent Cassini into Saturn in 2017 to prevent the spacecraft from ultimately crashing into one of the gas giant’s 146 known moons.
The ballistic radar data that Poggiali and colleagues used were collected by Cassini during four flybys on May 17, June 18, and October 24, 2014, and then again on November 14, 2016. For each of these ballistic radar datasets, the surface reflections were seen when Cassini made its closest approach to Titan, and then again as it moved away from the moon.
The researchers studied observations of three of Titan’s polar seas: Kraken Mare, Ligeia Mare, and Punga Mare. They found that the composition of the surface layers of the hydrocarbon seas depended on location and latitude. In particular, the surface material of the southernmost part of Kraken Mare was the most efficient at reflecting radar signals.
All three of Titan’s seas appeared calm when Cassini observed them, with the spacecraft seeing waves of about 3.3 millimeters (0.13 in). Where the hydrocarbon seas hit the shore, the waves rose to just 5.2 millimeters (0.2 in), suggesting the existence of weak tidal currents.
“We also have evidence that the rivers that feed the seas are pure methane until they flow into the open liquid seas, which are more ethane-rich,” Poggiali added. “It’s just like on Earth when freshwater rivers flow into the saltwater of the oceans and mix with it.”
The team says the discovery fits with meteorological models of Saturn’s moon, which predict that the rain that falls on Titan is mostly methane, with small amounts of ethane and other hydrocarbons.
Poggiali added that the team continues to work with data that Cassini has generated during its 13 years of studying Titan. “There is a mine of data that is still waiting to be fully analyzed in ways that should yield more discoveries,” he concluded. “This is just the first step.”
The team’s research was published Tuesday (July 16) in the journal Nature Communications.