The Cassini mission (NASA/ESA) explored the Saturnian system for thirteen years. Particularly on Titan, it has revealed a fascinating diversity of landscapes : plains, mountains, large dune fields, labyrinthine terrain, rivers... And in 2007, it revealed, in the northern hemisphere, in the polar regions, the existence of three large seas of liquid hydrocarbons - named respectively Kraken Mare (the largest, with a surface area larger than that of the Caspian Sea), Ligeia Mare and Punga Mare -.
These seas are accompanied by a multitude of smaller lakes. Long-term exploration by the Cassini mission has led to a better understanding of Titan’s complex hydrological system, similar to Earth’s, but dominated by methane and ethane, rather than water, and with surface temperatures of -181°C.
Nearly seven years have passed since the end of the Cassini mission, but the colossal volume of its data has yet to be fully exploited. Many questions remain, notably concerning the properties of the methane cycle at work on Titan, and the interactions between its polar seas and atmosphere.
An international scientific team, led by a researcher from Cornell University and including an astrophysicist from Observatoire de Paris - PSL, has analyzed a dataset collected between 2014 and 2016 by a bistatic radar experiment conducted from the Cassini mission.
During its observations, the Cassini probe sent a radio signal to Titan, using the antenna normally used to communicate the data collected to Earth. The liquid surfaces of the seas acted as a mirror to reflect this signal back to Earth. By comparing the signal received with the signal sent, the reflective properties (composition and roughness) of Titan’s seas can be determined. This experiment provides an insight into the composition and roughness of Titan’s liquid hydrocarbon seas, as well as some of its estuaries.
These seas feature different compositions, active tidal currents, wavelets, and greater roughness near estuaries and inter-basin straits.
At the time of the bistatic observations, the surfaces of Ligeia Mare, Punga Mare and Kraken Mare were mostly smooth, with no major disturbances. The researchers found variations in liquid composition between the different seas, corresponding to differences in the methane-ethane mixing ratio. Estuary data suggest that methane-rich rivers may have lower ethane levels than open seas. As precipitation is mainly composed of methane, rivers contain little ethane, which is transported and accumulated in seas, much as rivers on Earth are composed of freshwater, with salt accumulating in seas.
The authors estimate small-scale roughness (a few millimetres) from sea surface scattering, suggesting the presence of small surface waves. Greater roughness was concentrated in coastal areas near estuaries and inter-basin straits, perhaps indicating the presence of active tidal currents.
"This is the first time that bistatic radar data from the Cassini mission, acquired some ten years ago, has been used. Beyond its new scientific conclusions, this study also testifies to the great discoveries that can be made with the multitude of as yet untapped data from past space missions. Given the ecological problems facing society today, we urgently need to reduce the number of new space missions, and finally take a look at the data we already have", stresses study co-author Léa Bonnefoy, a post-doctoral fellow at Observatoire de Paris - PSL at the time of the study, and concerned about the carbon impact of instrumental resources dedicated to astronomy.
Reference
Article entitled "Surface properties of the seas of Titan as revealed by Cassini mission bistatic radar experiments", by Valerio Poggiali (Cornell University) and all, published on July 16, 2024, in Nature Communications.