Although water is a very common substance in the universe, our planet is the only place where we can find it in all three states in a stable way: solid, liquid and gas. That is for now, because although at the end of the 20th century it was thought that it was very difficult to find liquid water in some body of the universe, now it is no longer so clear. We had always believed that this required a continuous supply of energy from the star, which meant that the planet should be in what is called the Habitable Zone , the strip of space around the star where the energy provided allows , at least theoretically, the existence of liquid water on its surface. However, space missions have been finding liquid water where no one suspected: on satellites around planets far away from the Sun.
However, there were already suspicions that something like this could happen. Europa, the smallest of the four large moons of Jupiter, is an icy kingdom, with temperatures of 220 degrees below zero and has a saltwater ocean under a 100 km layer of ice. And not only that, but thanks to the 15 months that the Hubble Space Telescope spent observing it as it repeatedly passed in front of Jupiter, we know that columns of water reaching 200 km come out through surface fractures. All because of the action of Jupiter: the gravitational tidal forces are so intense under the ice cap that the difference between ebb and flow is 200 meters.
Similarly, the largest moon in the Solar System, Ganymede – it is so large that it has its own magnetic field – also has a sea of salt water under its frozen surface . The same thing happens with Saturn’s moon Enceladus , as jets of water rise at 1,800 km/h from the south pole and reach 500 km in height. In addition, the analysis of the photographs taken by the Cassini probe show that this moon has a peculiar oscillatory movement that can only be explained if there is a large ocean below the surface. The same thing happens in the great hope of astrobiology, Titan, only that the ocean is made of water and ammonia. It is also the only body in the Solar System (with the exception of Earth) that has lakes and liquid seas on its surface. Titan’s seas, however, are not composed of water either, but of liquid hydrocarbons. What is below its surface? Finding out is one of the goals of a project presented in 2014 to NASA: sending a submarine to explore Titan’s largest northern sea, Kraken Mare .
In 2016 the universe gave us another surprise. Pluto, the planet that is no longer a planet, may also have an ocean . The images taken by the New Horizons probe in 2015 showed certain tectonic characteristics that indicate that this icy world -with an average temperature of 229 degrees below zero- has expanded, and that only happens if there is an ocean under its surface that is freezing. The water would be mixed with ammonia , which acts as if a kind of antifreeze. Specifically, in the region of the Virgilian Trench, recent cryovolcanic eruptions have been found that seem to emanate from a source of liquid water located below the surface. On the other hand, latest research suggests that the Sputnik Planitia , a large area of ice containing nitrogen, methane and carbon monoxide, is an impact basin. Simulations of the cataclysm indicate that powerful seismic waves must have propagated around and through Pluto, ripping apart the surface on the far side of the dwarf planet. The most striking thing is that for this transmission to take place -and the creation of Sputnik Planitia- it is necessary for Pluto to host an underground ocean of liquid water 150 kilometers thick. Planetary scientists suspect that this ocean may have formed 4.5 billion years ago, at the dawn of the Solar System.
That water is so abundant in our cosmic neighborhood prompted NASA to create the Ocean Words Exploration Program in 2015 to “assess habitability and search for biosignatures of simple extraterrestrial life.” Astrobiologists have it clear: their mantra to search for life is “follow the water”.
An example is the red planet. Ancient Mars was a very different place than today, dry and arid. There is no doubt that water ran off the surface of Mars, which brings us to a warmer planet with a denser atmosphere 3.8 billion years ago. The maximum flows of some Martian channels were much greater than the maximum flows that have flowed through our planet. A flow of one billion cubic meters per second circulated through the Ares Vallis – the area where the Sojourner was passing in the summer of 1997. 300,000 circulate through the Amazon, and the great waterfall that filled the Mediterranean through Gibraltar when it dried up 5.5 million years ago had only 60 million.
On the red planet it has not happened like on Earth, where liquid water has been present since its inception, 4,000 million years ago. There has been water on Mars sporadically, probably repeatedly but perhaps short-lived. Was there ever an ocean? It is not very clear, but scientists have given it a name: it is the Oceanus Borealis, located in the plains of the northern third of the planet with 65 million cubic meters and an average depth of 1,700 m, which would have coexisted in time with a large South Pole.
And is there water outside our Solar System?
In June 2020, planetary scientist Lynnae Quick of NASA’s Goddard Space Flight Center decided to explore the possibility of liquid water on any of the discovered extrasolar planets ; in particular, 53 Earth-like planets. His analysis, which time will confirm or refute, predicts that a quarter of the planets studied could have an ocean, either on the surface or under a thick layer of ice.
The same conclusion has been reached by Li Zeng, from Harvard University, who in his study of more than 4,000 exoplanets has reached the conclusion that 35% of them could have up to half their planetary mass in the form of water . An overwhelming amount considering that on Earth the mass of water is less than 0.02%. For now the most promising candidates to be ‘ocean planets’ are those of the style of the Earth that are within the Habitable Zone of their star: the one that orbits the red dwarf of the triple system Gliese 667, the one that does it around Kepler-22 (a Sun-like star) and that of the ultracool dwarf TRAPPIST-1. But determining that there is water on its surface uses techniques such as rotational mapping or specular reflection (see the flash of light reflecting off the planet’s ocean). And that is not an easy task.
