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The presence of large liquid masses on Titan was well known, but highly reflective spots on its equator have puzzled scientists for two decades. A new study now shows that it could be dry seas and lakes.

Megaluna with specular reflections

Slightly larger than the planet Mercury, Titan is the largest of Saturn's satellites, and the second largest of all the moons in the solar system. Furthermore, unlike Mercury, Titan has a very important atmosphere and, discounting Earth, it is the only object in the solar system that contains stable liquids , in the form of rivers, seas and lakes, on its surface. Titan's study is very important because it is a world rich in organic compounds that has many similarities to the early Earth.

The large radio telescopes in Arecibo (Puerto Rico) and Green Bank (West Virginia, USA) have been doing radar studies on Titan since 2000. In these experiments, a radio signal is sent over the satellite and studies how the waves are reflected. These radio telescopes thus detected a dozen places, located near Titan's equator, that reflect radar signals as if they were perfect mirrors . Scientists originally thought that such specular reflections were caused by seas or lakes with very smooth surfaces, without any swell or turbulence.

Seas and lakes

When the Cassini probe visited Titan , in 2004, we had the opportunity to see the surface of this large moon for the first time, and in great detail. And the parachute descent of the Huygens probe on its surface offered us one of the most exciting moments in astrophysics so far this century. Thus, a relatively young surface was revealed to us, with few craters and some cryovolcanoes. But undoubtedly the most fascinating result of this exploration was the discovery of great seas and lakes full of liquid hydrocarbons, mainly methane and ethane. But all of these liquid masses were observed near the satellite's polar regions, none near the equator .

What then were the specular reflections that radio telescopes had been observing for years? This question has puzzled scientists for the past two decades. Seeking to give him an answer, Jason Hofgartner, an astronomer at NASA's Jet Propulsion Laboratory (JPL) institute, has led a study that has re-examined all available data on Titan's equatorial regions, both from radio telescopes like Cassini-Huygens.

In this way, the scientists were able to locate that the specular reflections came from well-defined areas that are distinguished from the surrounding landscape by being softer and smoother, and by having a different composition. And, after examining various options, they concluded that these areas must have been the dry beds of lakes or seas , similar to others observed in the wetter polar regions of Titan.

The Vid Flumina river empties into the Ligeia Mare near the north pole of Titan.NASA / JPL

The methane cycle

Let's locate ourselves on Titan. Its atmosphere is mainly made of nitrogen, but it also contains up to 6% methane and other hydrocarbons. Its climate has seasons similar to terrestrial. There are winds, clouds and rain (but hydrocarbons), and geographical features such as dunes are created, in addition to rivers that will flow into the seas in large deltas. Thus, methane follows a cycle on Titan that is similar to the water cycle on Earth .

Due to all this, Hofgartner and his collaborators considered several options to explain the origin of the equatorial specular reflections. For example, it could be thought that they originated in the methane rains themselves. But such rains, although periodic, are infrequent and cannot explain the stability of the stains. Reflective spots cannot be interpreted as large dunes either, as it is known that they are not abundant in the equatorial regions.

Dry seas and lakes thus offer the most reasonable explanation . Solar radiation and the displacement of methane from the equatorial to polar regions, as another effect of the methane cycle on Titan, could be the agents responsible for the drying of the liquid masses.

Thinking of other worlds

This study illustrates the difficulties in identifying liquid masses such as lakes, seas, and oceans on other planets and moons. We must learn to search for such liquids well, since their presence (especially water) is very important in the context of the search for extraterrestrial life. Thus, this study of Titan can serve to guide the possible detection of oceans in nearby exoplanets, something that could give us clues about the possible presence of life on those worlds.

The paper by Hofgrtner et al., Entitled "The root of anomalously specular reflections from solid surfaces on Saturn's moon Titan," has been published in a recent issue of Nature Communications and is available here .

Rafael Bachiller is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain .

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