At NASA, they like to use the slogan “follow the water” on their missions. By this they mean that habitable places can be found where liquid water exists. So they should better say “follow the liquid water”. Because that’s what it’s all about. Indeed, water in the form of ice and sometimes in the form of vapour is very abundant in our Solar System and beyond. In the outer solar system (further from the Sun), there are numerous objects that contain very much water, or even consist almost entirely of water. This is the case of comets, for example, some icy moons around giant gas planets, or a dwarf planet like Pluto. But all these objects have surface temperatures of -170°C or colder. Their water mantle is therefore frozen so hard that it feels like granite.
Yet liquid water also occurs there. In fact, some of these objects have a warm core or subsurface. Between the warm rock core of a moon and the frozen water crust, you can sometimes find a kilometre-deep ocean of liquid water. Magnetic field measurements showed that such ocean on the moon Europa contains salty water, just like the oceans on Earth. Chances are that there are deep-sea warm-water vents at the bottom. It would therefore be the best place in our Solar System where alien life could arise and thrive. We will explore this in further chapters.
Asteroids between Mars and Jupiter also sometimes contain quite a lot of water, although you cannot see it at first sight. The best-known example of this comes from the very largest asteroid, the dwarf planet Ceres. In the middle of the Occator crater are bright white spots called “faculae” by the researchers. The white spots are said to consist of dried-up salt because brine water would be coming out of the soil at that location. When the crater was formed some 20 million years ago, cracks and fissures must have formed through which the brine water could exit. So in the outer Solar System, the presence of water is quite common. Yet there is no habitat like Earth. The main differences are the very low temperature at the surface, and the fact that oceans, lakes and rivers are never found above ground.
Venus too most probably had liquid water in its early stages, but we know much less about it.
There used to be a lot of liquid water on Mars. This is covered in part 4 of this course. Today, that liquid water has disappeared from the surface, but it can still be found in the deeper subsurface. Furthermore, water on Mars can be found a lot in the form of ice in the ground, both deep and shallow. In the colder polar regions, even a lot. Water vapour is also found to a much lesser extent. Cloud formation on Mars is common, though not only with water but also with frozenCO2. All this is therefore a reason why the search for life within our Solar System is partly focused on this planet. Venus too presumably had floatable water on its surface as a young planet, but we know much less about it.
In summary, we can answer the question “Where do we find liquid water in the Solar System” (in order of importance):
- On Earth, of course!
- Under the ice crust of a lot of distant moons and other water ice objects.
- In the subsurface on Mars, mainly at greater depths.
