Beyond Earth, the only known seas in the Solar System are on Titan, Saturn's largest moon.
Using data previously collected by the Cassini probe, an international scientific team, including a researcher from the Paris Observatory - PSL, provides new insights into their composition and roughness. The results were published in the journal Nature Communications on July 16, 2024.
The great hydrocarbon sea, Ligeia Mare, on Titan
The Cassini mission (NASA/ESA) explored the Saturn system for thirteen years. Particularly on Titan, it revealed a fascinating diversity of landscapes: plains, mountains, vast dune fields, labyrinthine terrains, rivers... And in 2007, it unveiled, in the northern hemisphere's polar regions, the existence of three great liquid hydrocarbon seas - named respectively Kraken Mare (the largest, with an area greater than the Caspian Sea), Ligeia Mare, and Punga Mare.
The great hydrocarbon sea, Ligeia Mare, on Titan, Image taken by NASA's Cassini probe Radar instrument shows the evolution of a transient feature in the great hydrocarbon sea, Ligeia Mare, on Titan. NASAT hese seas are accompanied by a multitude of smaller lakes. The long-term exploration by Cassini has allowed for 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 - 294°F (-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 utilized. Many questions remain, particularly regarding the properties of the methane cycle on Titan and the interactions between its polar seas and its atmosphere.
An international scientific team, led by a researcher from Cornell University and including an astrophysicist from the Paris Observatory - PSL, analyzed a set of data collected between 2014 and 2016 through a bistatic radar experiment conducted by the Cassini mission.
During its observations, the Cassini probe sent a radio signal to Titan using the antenna usually used for communicating collected data to Earth. The liquid surfaces of the seas then acted like a mirror, reflecting this signal back to Earth. By comparing the received signal with the sent signal, the reflective properties (composition and roughness) of Titan's seas can be determined. This experiment provides a glimpse into the composition and roughness of Titan's liquid hydrocarbon seas, as well as some of its estuaries.
These seas exhibit different compositions, active tidal currents, ripples, and increased 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, without major disturbances. The researchers noticed variations in the liquid's composition between the different seas, corresponding to differences in the methane-ethane mixing ratio.
The data related to the estuaries suggest that methane-rich rivers may have lower ethane levels than the open seas. Since precipitation is mainly composed of methane, rivers contain little ethane, which is transported and accumulated in the seas, similar to how fresh water rivers on Earth accumulate salt in the seas.
The authors estimate the small-scale roughness (a few millimeters) from the diffusion at the sea surface, which indicates the presence of small surface waves. Greater roughness was concentrated in coastal areas near estuaries and inter-basin straits, possibly indicating the presence of active tidal currents.
"This is the first time that bistatic radar data from the Cassini mission, acquired about ten years ago, has been exploited. Beyond its new scientific conclusions, this study also highlights the significant discoveries that can be made with the abundance of still-unexploited data from past space missions. Given the ecological issues facing society today, it's urgent to reduce the number of new space missions and finally look at the data we already have," emphasizes Léa Bonnefoy, co-author and postdoctoral researcher at the Paris Observatory - PSL during this study, concerned about the carbon footprint of astronomical instrumentation.
Reference:
Article titled "
Surface properties of the seas of Titan as revealed by Cassini mission bistatic radar experiments", by Valerio Poggiali (Cornell University) et al., published on July 16, 2024, in Nature Communications.