Infrared image of Titan with the Sun shining on some of its lakes. NASA / JPL

The presence of large liquid masses on Titan was well known, but highly reflective patches at its equator have intrigued scientists for two decades.

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

MEGALUNA WITH SPECULAR REFLEXES
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, excluding 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. The study of Titan is very important because it is a world rich in organic compounds that has many similarities to the early Earth.

The large radio telescopes at 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 studied how the waves are reflected. These radio telescopes thus detected a dozen places, located near the equator of Titan, that reflect the radar signals as if they were perfect mirrors. Scientists initially thought that such specular reflections were caused by seas or lakes with very smooth surfaces, without any waves or turbulence.

SEAS AND LAKES
When the Cassini probe visited Titan in 2004, we had the opportunity to see for the first time, and in great detail, the surface of this large moon. And the Huygens probe’s parachute descent over its surface offered us some of the most exciting moments in astrophysics so far this century. This revealed a relatively young surface, 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 intrigued scientists for the past two decades. Trying to give you an answer, Jason Hofgartner, an astronomer at NASA’s JPL (Jet Propulsion Laboratory) institute, has led a study in which they have re-examined all the available data on the equatorial regions of Titan, both those 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 be the dry beds of lakes or seas, similar to others observed in the more humid polar regions of Titan.
THE METHANE CYCLE
Let’s go to Titan. Its atmosphere is made mostly of nitrogen, but it also contains up to 6% methane and other hydrocarbons. Its climate has seasons similar to the terrestrial ones. There are winds, clouds and rain (but hydrocarbons), and geographical features such as dunes are created, as well as rivers that 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, one might think that they originated in methane rains themselves. But such rains, although periodic, are infrequent and cannot explain the stability of the spots. Reflective spots cannot be interpreted as large dunes either, since it is known that these 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 the polar regions, as one more effect of the methane cycle on Titan, could be the agents responsible for the desiccation of the liquid masses.

THINKING ABOUT 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 look for such liquids well, because 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 on nearby exoplanets, something that could give us clues about the possible presence of life on those worlds.