Mars is the planet with which extraterrestrials identify, but there are other nearby worlds, within the Solar System, where life is possible. Covered by tens or hundreds of kilometers of rock and ice, the underground oceans of some moons of Jupiter or Saturn remain warm enough to have liquid water and have chemical conditions where some terrestrial microorganisms could survive. When the probe Voyager 2 passed by Jupiter's moon Europa in 1979, observed grooves and fractures in the frozen surface that raised suspicions about what was hidden inside. After decades of observation, it is believed that, in addition to Europa, there are at least seas inside Ganymede, the largest moon in the Solar System, on Saturn's satellite Enceladus, from which plumes emerge into space, or on Titan, which has its surface covered with methane lakes. Today, an article that is published in the magazine Naturesuggests that there is also an underground sea on the Saturnian moon Mimas.
The existence of a sea under a moon is usually intuited by changes in its surface, such as the Europa faults, caused by volume changes in the water when it freezes or melts. But Mimas, a world that seems geologically dead for a long time, does not have them. However, the authors of the work published in Nature, with Valery Lainey, from the Paris Observatory (France), at the helm, found it thanks to a detailed analysis of its movements around Saturn. This small moon, only 400 kilometers in diameter, would have its liquid ocean under a layer of ice 20 or 30 kilometers thick. The simulations suggest that the sea appeared recently, between 25 and two million years ago, an insufficient time to cause visible effects on its surface.
Olga Prieto, head of the Department of Planetology and Habitability at the Center for Astrobiology, in Madrid, considers that the most interesting thing about this work “is that it shows that in worlds where there is no external manifestation of their existence, there may also be oceans.” This makes it possible that underground seas in the Solar System are more the norm than the exception. In addition to the moons of Jupiter or Saturn, other bodies such as Vesta, in the asteroid belt, several satellites of Uranus and even the dwarf planet Pluto, could have large amounts of water under their surface.
In some celestial bodies, the disintegration of radioactive elements can explain the origin of the heat necessary to have liquid water so far from the Sun. In others, such as Mimas, the gravitational effects of Saturn and other moons can shake the satellite's core, with the same mechanism that produces tides on Earth, but much more intense, and causing the temperature inside to rise. This phenomenon raises the possibility that crossings with the orbits of other objects could generate the conditions for the ice to melt and raises questions about the stability of the habitats that would make life on these worlds possible.
The astrobiologist Alfonso Davila, from NASA's Ames Research Center, explains that, although the conditions for life may now exist on some of these moons, it is not clear that life could have emerged in these environments as it did on Earth. “We don't know the conditions under which life originated here. There are models that place this origin on the surface, with an important role of ultraviolet radiation and light, with episodes of flooding and drying that are important for organic chemistry and geochemical evolution, and there are oceanic models, in which they talk of hydrothermal chimneys where the conditions for life have been created. Furthermore, some of the oldest organisms are thermophiles, they like high temperatures,” explains Dávila. However, if life could not have originated in the ocean, moons like Europa or Enceladus would be habitable, but sterile. “On Earth we don't have those types of environments, because life colonizes all habitable places, so it would also be something interesting to study,” he says.
Despite the proximity of these aquatic worlds and the abundance suggested by the article published today Nature, many of them are as inaccessible as planets orbiting stars light years away. With current technology, it seems like science fiction to drill tens of kilometers of ice, but according to Prieto, there are already crazy ideas that space agencies are listening to as possible long-term plans. He Exobiology Extant Life Surveyorfor example, is a robotic worm that could sneak through the cracks through which the plumes of Enceladus emerge and reach its ocean, dozens of kilometers deep, by crawling.
It seems even more difficult to reach the newly discovered Mimas Sea, hidden by a surface that seems to be from an inert world. As a speculation, if one day it were possible to reach its interior, Dávila believes that it would not be possible to find life there. “The models tell us that life on Earth appeared relatively quickly in geological terms, but perhaps it took 200 or 300 million years,” he says. In Mimas, where the ocean is only 25 million years old, there would not have been time for life to develop yet, but an environment could be found in which simple molecules begin to combine to form molecules such as DNA that later made possible life. “There we could study that very interesting moment in the phases prior to the origin of life that we will not have on Earth, because the geological record has been destroyed,” says Dávila. For now, the Mimas ocean is a new surprise that changes expectations about our cosmic neighborhood.
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