A NASA’s Cassini spacecraftwhich explored Saturn and its icy moons, including majestic Titan, ended his mission with a fatal fall on the ringed giant planet in 2017. But some of the voluminous data collected by Cassini during its 13-year study of the Saturn system is only now available. being fully examined.
Cassini radar observations are providing intriguing new details about the seas of liquid hydrocarbons on the surface of Titan, the second-largest moon in our Solar System and a place of interest in the search for life beyond Earth.
Titan, shrouded in a smog-like orange haze, is the only known world besides Earth that has liquid seas on its surface — although they are not composed of water but rather nitrogen and the organic compounds methane and ethane, components of natural gas.
The study involved three seas near Titan’s north pole: Kraken Mare, the largest, covering an area comparable to the Eurasian Caspian Sea; Ligeia Mare, the second largest and similar in area to Lake Superior in North America; and Punga Mare, which is roughly equivalent to Africa’s Lake Victoria.
The chemical composition of these seas—methane-rich versus ethane-rich—was found to vary depending on latitude. The study also documented the extent and distribution of sea surface ripples, indicating active tidal currents and greater roughness near estuaries — the mouths of rivers.
Titan, at 5,150 km in diameter, is the second largest moon in our Solar System, behind Jupiter’s Ganymede, and is larger than the planet Mercury. Titan and Earth are the only worlds in the system where liquids fall from the clouds, flow like rivers into seas and lakes on the surface, and evaporate back into the sky to begin the hydrological process again.
On Earth, water falls from clouds. On Titan, clouds release methane — which is a gas on Earth — in liquid form due to the frigid climate.
“Titan is truly an Earth-like world with a diverse set of very familiar surface morphologies, shaped by a methane-based hydrological system operating in a dense nitrogen atmosphere,” said Valerio Poggiali, a Cornell University engineer and planetary scientist, main author of the study published this Tuesday (16) in the journal Nature Communications.
“Seas and lakes of liquid hydrocarbons dot the surface in polar regions, especially in the north. Precipitation-fed channels flow into these seas creating estuaries, in some cases deltas,” Poggiali added.
The Cassini data indicated that rivers carry pure liquid methane that then mixes with more ethane-rich liquids from the seas, in the same way that freshwater from Earth’s rivers mixes with saltwater oceans.
“Titan’s seas are pulled by Saturn’s enormous gravity, just like our seas, and the tidal range on some of its coasts can be around 30 cm. Because the tidal period — Titan’s day — is long, 16 Earth days, the tidal cycle is slow, so tidal currents are generally weak,” said Ralph Lorenz, planetary scientist at the Johns Hopkins University Applied Physics Laboratory. and co-author of the study.
The study used bistatic radar data collected during Cassini flybys of Titan, three in 2014 and one in 2016. Cassini aimed a radio beam at targets on Titan’s surface, which then reflected to a receiving antenna on Earth. This provided richer information about the composition of the reflecting surface and its roughness than Cassini’s ordinary monostatic radar, which bounces a radio signal from a target and returns it to the point of origin.
“This is probably the last untouched data set that the Cassini spacecraft left us,” Poggiali said.
Titan has environments with conditions considered potentially suitable for life. For example, Titan appears to house a vast subterranean ocean of liquid water.
“Are the heavy organic molecules produced in Titan’s atmosphere prebiotic in nature?” asked Poggiali, referring to the chemistry that could lead to the formation of life. “Has all this organic material ever been in contact with liquid water? We believe that similar interactions may have led to the origin of life on our planet, with the generation of molecules capable of producing energy or storing information.”
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