If you think of outer space or its planets, moons, and asteroids, you probably think of a desert wasteland. Desert partly due to the absence of life, but also due to the absence of water. And if you think of the Earth , especially if you compare it with other bodies in the solar system, you will think of jungles, oceans and rivers . To date, there is no unquestionable evidence of extraterrestrial life, but there is evidence that the universe, or at least the solar system, has plenty of water . Before exploring beyond our planet, how much water is there on Earth?
During the formation of the Earth and of any planet in general, the heavier, denser elements end up sinking and occupying the interior of the planet, while the lighter elements float on top of them. This differentiation is not perfectly efficient, but it is the reason why we have a terrestrial core made up of iron, nickel and other dense metals and a surface where water abounds and on which an atmosphere rests .
In short, since the formation of the planet, most of the water has been concentrated near the surface . The small deposits that remain in the depths have ended up evaporating due to the high temperatures inside the planet and escaping over thousands and millions of years to the surface. Therefore, between oceans, seas, lakes and rivers, we estimate that there are about 1,300 million cubic kilometers of liquid water on Earth . If we gathered all that water into a sphere, it would be about 1,350 kilometers in diameter, about ten times smaller than the planet. But is this too much or too little? Let’s compare it with the water present in other bodies of the solar system.
We’ll start with Mars. Mercury is too close to the Sun to have water of any kind, and although we think that Venus may have had oceans hundreds of millions of years ago, water has now only been detected in its atmosphere , and in very small amounts. Liquid water on Mars there is not much. Mainly because on its surface, as on the surface of any body beyond the Earth, it is too cold, but below it the necessary conditions can be met.
This is what happens in the lakes that were found in 2018 by a team from the Italian space agency . Although it is believed that there may be other lakes of this type yet to be discovered, most of the water on Mars is in a solid state . Currently, between the ice observed on the Martian surface and what we estimate there should be at shallow depths, it is estimated that there are about 5 million cubic kilometers of ice , about 260 times less than on Earth. It is a small amount, but if all this ice melted it would be able to cover the entire surface of Mars with a layer of about thirty meters of water.
On the other hand, it might seem that the further we get from the Sun, the harder it is to find liquid water and the easier it is to find ice, and yet most of the liquid water in the Solar System lies beyond the orbit of Mars . Several moons of Jupiter and Saturn, such as Ganymede and Europa on the one hand and Enceladus on the other, show clear signs of containing oceans of liquid water under thick layers of ice .
These oceans are made possible by the friction generated by the tidal forces that the gas giants exert on their satellites . In the same way that our Moon is able to raise and lower the sea level with the tides, Jupiter and Saturn would be able to remove the very core of their moons , keeping their interiors warmer than they would be without these processes.
It is estimated that on Enceladus there could be an ocean of water between ten and thirty kilometers deep that would completely surround the core of the moon and that both on Europa and on Ganymede there could be two oceans up to 100 kilometers deep each, both under miles of ice. By comparison, Earth’s oceans have an average depth of about 3.7 kilometers, so these satellites, which are considerably smaller than our planet, could hold several times the water of Earth .
If we go now to the ice giants, to Uranus and Neptune , things get complicated. These two planets are known to contain large amounts of ammonia, methane, and water , which together could have masses several times that of planet Earth , but this water would not be exactly drinkable. Beyond the fact that it is mixed with other substances, the high temperatures inside these planets and the very high pressures, hundreds of thousands of times the pressure on the surface of our planet, would mean that this “water” was not made up of H molecules. 2 O, if not by a soup of hydrogen ions and oxygen ions .
The presence of liquid water in other bodies of the solar system beyond those that we have already commented on is not so clear, due to insufficient data or because they do not contain enough water to be detected by our instruments. However, it is estimated that most asteroids, comets and dwarf planets beyond Neptune would be made up mostly of water ice .
It is also believed that some of them would have small reserves of liquid water inside , such as the one that Pluto is believed to harbor. The dwarf planet, under a thick layer of ice, could have almost as much liquid water as Earth . In total, it is estimated that in the solar system, in addition to the water on Earth and without taking into account the ionic water of Uranus and Neptune, there could be between twenty-five and fifty times more liquid water than on our planet .
References:
Dyches, F. Chou, 2015, The Solar System and Beyond is Awash in Water, NASA, https://www.nasa.gov/jpl/the-solar-system-and-beyond-is-awash-in-water
