Cassini Reveals Surface Properties of Titan’s Hydrocarbon Seas

Saturn’s moon Titan was explored by NASA’s Cassini spacecraft from 2004 to 2017. While Cassini revealed a lot about this Earth-like world, its radar observations could only provide limited information about Titan’s liquid hydrocarbons seas Kraken, Ligeia and Punga Mare. In a new paper in the journal Nature Communications, Cornell University researcher Valerio Poggiali and colleagues report the results of the analysis of the Cassini radar experiments data of Titan’s polar seas.

“The Cassini spacecraft explored Saturn’s largest moon, Titan, from 2004 to 2017, revealing an Earth-like world with a diverse set of strange, yet very familiar, surface morphologies shaped by a methane-based hydrologic system, operating in a dense nitrogen atmosphere,” Dr. Poggiali and co-authors said.

“Winds in the lower atmosphere move sediments and shape them into vast dune fields that encircle Titan’s equatorial latitudes.”

“In the mid-latitudes, flat and relatively featureless plains mark a transition between the eolian-dominated equator and lacustrine-dominated poles.”

“In the polar regions, large seas and small lakes of liquid hydrocarbons dominate the terrain.”

“Precipitation-fed channels flow into the seas creating estuaries, in some cases deltas, and other familiar coastal sedimentary deposits.”

“While Cassini has revealed much about Titan, these discoveries have prompted more questions.”

In the study, the scientists used four bistatic radar observations, collected by Cassini during four flybys in 2014 (May 17, June 18, October 24) and 2016 (November 14).

For each, surface reflections were observed as the spacecraft neared its closest approach to Titan (ingress), and again as it moved away (egress).

The authors analyzed data from the egress observations of Titan’s three large polar seas: Kraken Mare, Ligeia Mare and Punga Mare.

“A bistatic radar experiment involves aiming a radio beam from the spacecraft at the target — in this case Titan — where it is reflected toward the receiving antenna on Earth,” they explained.

“This surface reflection is polarized — meaning that it provides information collected from two independent perspectives, as opposed to the one provided by monostatic radar data, where the reflected signal returns to the spacecraft.”

“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.”

The team found differences in the composition of the hydrocarbon seas’ surface layers, dependent on latitude and location (near rivers and estuaries, for example).

Specifically, the southernmost portion of Kraken Mare shows the highest dielectric constant — a measure of a material’s ability to reflect a radio signal.

For example, water on Earth is very reflective, with a dielectric constant of around 80; the ethane and methane seas of Titan measure around 1.7.

The researchers also determined that all three seas were mostly calm at the time of the flybys, with surface waves no larger than 3.3 mm.

A slightly higher level of roughness — up to 5.2 mm — was detected near coastal areas, estuaries and interbasin straits, possible indications of tidal currents.

“We also have indications that the rivers feeding the seas are pure methane until they flow into the open liquid seas, which are more ethane-rich,” Dr. Poggiali said.

“It’s like on Earth, when fresh-water rivers flow into and mix with the salty water of the oceans.”

“This fits nicely with meteorological models for Titan, which predict that the ‘rain’ that falls from its skies is likely to be almost pure methane, but with trace amounts of ethane and other hydrocarbons,” said Cornell University’s Professor Philip Nicholson.

“More work is already underway on the data Cassini generated during its 13-year examination of Titan.”

“There is a mine of data that still waits to be fully analyzed in ways that should yield more discoveries. This is only the first step.”

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V. Poggiali et al. 2024. Surface properties of the seas of Titan as revealed by Cassini mission bistatic radar experiments. Nat Commun 15, 5454; doi: 10.1038/s41467-024-49837-2

This article is a version of a press-release provided by Cornell University.