NASA’s Cassini-Huygens spacecraft may have ended its 20-year mission exploring Saturn’s neighborhood in dramatic fashion seven years ago, when it plunged into the gas giant, but it’s still delivering the scientific goods.
Using radar data collected by Cassini, Cornell astronomers have been able to gather new information about the liquid ocean of Saturn’s largest moon, Titan, which is made up of hydrocarbons, a class of organic chemicals made of carbon and hydrogen. For example, this class includes chemicals like methane and ethane.
The team was able to analyze the composition and “roughness” of Titan’s sea, which lies near Earth’s North Pole. The researchers found calm methane seas with gentle tidal currents. Not only is this something that previous examinations of Titan’s seas have failed to detect, it also lays the groundwork for future investigations of ocean moons in the solar system.
The Cassini data used to reach these new findings were collected using “ballistic radar,” which involved the spacecraft pointing a radio beam at Titan that was then reflected back toward Earth.
Related: Saturn’s ocean moon Titan may not be able to support life after all
The effect of this is to polarize the surface reflection from Titan, which provides views from two different perspectives. The standard radar that saw the reflected signal back to Cassini provided only one perspective.
“The main difference is that the 2D information is a more complete data set and is sensitive to both the composition and roughness of the reflective surface,” says Valerio Poggiali, a team member and researcher at the Cornell Center for Astrophysics and Planetary Sciences (CCAPS). He said in a statement:
Cassini launched on October 15, 1997, and then spent seven years on its journey to the Saturn system. NASA crashed Cassini into Saturn in 2017 to prevent the spacecraft from eventually colliding with one of the gas giant’s 146 known moons.
The ballistic radar data used by Poggiali and his colleagues were collected by Cassini during four flybys on May 17, June 18, and October 24, 2014, and again on November 14, 2016. For each of these ballistic radar data sets, surface reflections were seen when Cassini approached Titan at its closest point, and then again as it moved away from the moon.
The researchers examined observations of three of Titan’s polar seas: the Kraken Sea, the Ligeia Sea, and the Ponga Sea. They found that the composition of the surface layers of the hydrocarbon seas depended on location and latitude. In particular, the materials on the surface of the southern part of the Kraken Sea were the most efficient at reflecting radar signals.
Titan’s three seas appeared calm when observed by Cassini, with waves measuring about 3.3 millimeters high. Where the hydrocarbon seas met the shoreline, the waves rose to just 5.2 millimeters, indicating weak tidal currents.
“We also have indications that rivers that feed the seas contain pure methane until they flow into the open liquid seas, which are richer in ethane,” Poggiali added. “It’s similar to what happens on Earth when freshwater rivers flow into and mix with salty ocean water.”
The team said the discovery is consistent with atmospheric models of Saturn’s moon, which predicted that rain falling on Titan is mostly methane, with small amounts of ethane and other hydrocarbons.
Poggiali added that the team is continuing to work with the data Cassini has collected during its 13-year study of Titan. “There is a mine of data still waiting to be fully analyzed in ways that will lead to more discoveries,” he concluded. “This is just the first step.”
The team’s research was published Tuesday (July 16) in the journal Nature Communications.
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