NASA’s Cassini-Huygens spacecraft may have dramatically ended its 20-year mission to explore Saturn’s surroundings seven years ago when it plunged into the gas giant, but it’s still delivering the science goods.
Using radar data collected by the Cassini probe, Cornell University astronomers have gathered fresh information about the liquid ocean on Saturn’s largest moon, Titan, which is composed of hydrocarbons, a class of organic chemicals composed of carbon and hydrogen. For example, this 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. Scientists found calm seas of methane with a gentle tidal current. Not only is this something previous surveys of Titan’s seas have failed to detect, but it also lays the groundwork for future explorations of the solar system’s oceanic moons.
The Cassini probe data used for these new findings was collected using “ballistic radar,” which involved the spacecraft aiming a radio beam at Titan, which then bounced back toward Earth.
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The consequence of this is the polarization of the surface reflection from Titan, which offered views from two different perspectives. The standard radar that saw the signal reflected back to Cassini offered 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 its roughness,” Valerio Poggiali, a team member and researcher at the Cornell Center for Astrophysics and Planetary Science (CCAPS), said in a statement .
Cassini launched on October 15, 1997, then took seven years to sail into the Saturn system. NASA collided Cassini with Saturn in 2017 to prevent the spacecraft from eventually crashing into one of the gas giant’s 146 known moons.
The ballistic radar data used by Poggiali and colleagues was 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 sets of ballistic radar data, surface reflections were seen when Cassini it made its closest approach to Titan, and then once more as it moved away from the Moon.
Scientists examined observations of Titan’s three polar seas: Kraken Mare, Ligeia Mare and Punga Mare. They found that the composition of the surface layers of hydrocarbon seas depends on location and latitude. In particular, the material on the surface of the southernmost part of Kraken Mare was most effective at reflecting radar signals.
All three of Titan’s seas appeared calm when Cassini observed them, with the probe seeing waves 3.3 millimeters in diameter. Where the hydrocarbon seas met the coast, wave heights rose to just 5.2 millimeters, indicating the existence of weak tidal currents.
“We also have indications that the rivers feeding the sea are pure methane until they flow into the open liquid seas, which are richer in ethane,” Poggiali added. “It’s like on Earth when freshwater rivers flow into and mix with the salt water of the oceans.”
The team said the discovery is consistent with meteorological models of Saturn’s moon, which predicted 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 generated by Cassini during its 13 years of studying Titan. “There is a mine of data still waiting to be fully analyzed in a way that should yield further discoveries,” he concluded. “This is just the first step.
The team’s research was published Tuesday (July 16) in the journal Nature Communications.