Ask Ethan: Where does the liquid water come from on the frozen planets?

The high reflectivity of the surface of the ice moon of Saturn, Enceladus , indicates the presence of a large number of constantly updated ice, which is not observed on any other moon in the solar system.

The solar system does not tire of surprising us, and perhaps one of the biggest surprises was the fact that the Earth is not the only world on the surface of which there is liquid water. Yes, of course, some water sometimes appears on Mars, but worlds such as the moon of Jupiter Europe, the moon of Saturn Enceladus and even distant Pluto have enormous subsurface oceans, and even more water on some of these worlds than on Earth. However, unlike Earth or even Mars, these worlds are so far from the Sun and it is so cold that even the highest temperatures on the surface do not reach the melting point of ice. So how is liquid water stored on them? This is what our reader wants to know:

I read about the moon of Saturn Enceladus and that scientists believe that there are oceans of liquid water on it under its icy crust. However, I also read that the maximum surface temperature is -90 ° C. How can this moon have liquid water? At such low temperatures and pressures, Enceladus should have only water ice and steam, but not water.

Let's start with how the water behaves on Earth.


Water in three states: liquid, solid (ice) and gaseous (invisible water vapor in the air). Clouds are a collection of water droplets condensed from the saturated air.

On Earth, water can exist in three states: solid, liquid, and gaseous, depending on temperature. Below 0 ° C, the water freezes and turns into ice; above this point and below 100 ° C water is liquid; above 100 ° C water exists in the form of gaseous vapor. This is how we are taught in school, and for the most part this is true. But there are some conditions under which water can begin to behave quite differently. For example, if you live at a high altitude, for example, in Bogota (Colombia), Quito (Ecuador), El Alto (Bolivia) - and more than a million people live in each of these cities - then the water boils at a much lower temperature .


Phase diagram of water, indicating different types of ice, liquid and gaseous states, and the conditions under which they occur. Note that below -22 ° C liquid water cannot exist at any pressure.

This is because pressure affects both the boiling point and the freezing point. In the depths of space, without atmosphere, liquid water cannot exist; it can exist either in the solid or in the gaseous phases. But on Earth, under reduced pressure, water boils at a lower temperature, and if a sufficiently high pressure is applied, ice melts and becomes liquid. The latter fact often surprises people, until you ask them to remember skating. Without skates on ice it is very slippery and it is difficult for you to control your movements or achieve friction; your shoes slide over the frozen surface of the ice. But with skates, all the pressure of your weight concentrates on the blade, which increases the pressure on the ice and makes it temporarily melt.


Skaters leave marks on the ice, as their skates, sliding on the surface, exert enough pressure to turn ice into water

Another fact should be taken into account: the freezing point of water varies depending on what is dissolved in it. If you ever put vodka in the freezer, you know that a mixture of water and 40% alcohol does not freeze at the freezing point of water, it needs a temperature much lower. Our ocean with salt dissolved in it also has a lower freezing point compared to pure water: about -2 ° C with about 4% salinity. Therefore, it is possible to lower the temperature below the freezing of water and still remain with liquid water, depending on what is still in it. This is one of the most amazing features of Mars, where pure liquid water should not exist at all.


Water streams on the slopes, like these - on the southern slope of the crater at the bottom of the canyon Melass - first gradually grow and then disappear, filling with dust from the Martian landscape. They are known to be the result of streams of liquid salt water.

At the pressures and temperatures existing on the surface of Mars, there should not be physically liquid water. But due to the high salt content in some types of Martian soil, water, condensing on the surface, can exist in the liquid phase. The streams of water going down the slopes of the walls of the craters were the first direct evidence of the presence of liquid water outside the Earth.

But if you look even further into the solar system, look at worlds like Europe, Enceladus, or even Pluto - there we will not find water on the surface.


Europe, one of the largest moons in the solar system, orbits around Jupiter. Under its frozen ice surface is a liquid ocean, heated by the tidal forces of Jupiter

A close study of these worlds reveals only ice. Yes, it is water ice, which gives us hope, but the temperatures on these worlds, located several times farther than the Earth from the Sun, not only never approach the 0 ° C index - which is necessary for the appearance of liquid water on the Earth's surface - but they never even approach the temperature that would allow liquid water to exist at any pressure. And yet, if on these worlds to go deeper under the surface of the ice, we will approach it, since there is tremendous pressure under all this ice.


Pluto and Charon in edited color; images obtained from the camera of the interplanetary station " New Horizons ". The frozen surface of Pluto is not all; at great depths it has a subsurface ocean of liquid water.

An atmosphere 100 kilometers thick is required to create the atmospheric pressure that we feel at sea level — but to double this pressure, you only need 10 meters of water. On another world, ice can easily reach thousands of meters in thickness, and create enormous pressures that bring us closer to the liquid phase of water. But even with the presence of salts in the ice, liquid water still does not appear without another additional factor: a source of heat. Fortunately, each of these worlds has a source of heat: a closely located massive companion planet.


"Plain Sputnik" on Pluto. Geological features identified by the station "New Horizons" indicate the presence of a subsurface ocean under a vast and deep ice crust on the surface of Pluto, stretching across the entire dwarf planet.

Europe has Jupiter, Enceladus has Saturn. Pluto has a moon Charon. All this trinity, combining a large mass and relatively close location, has a very serious tidal effect on these worlds. And these forces do not just lead to small deformations of the outer layers - they stretch, compress and split the insides of these worlds, because of which they warm up. If we take into account the amount of tidal heat and add the pressure exerted by the ice and the salt that exists under the outer layers of ice, you can get the desired: liquid ocean under the ice surface.


The tidal forces acting on Saturn's moon Enceladus are enough to break the ice crust and warm up the insides, which causes the subsurface ocean to spew water into space to a height of hundreds of kilometers.

Europe shows huge cracks on the surface, evidence of those moments when ice broke there and water protruded to the surface. The subsurface ocean of Enceladus - the most spectacular, liquid water erupts from it and rises into space hundreds of kilometers above the surface. These water pillars of Enceladus are so strong that they are responsible for the formation of one of the rings of Saturn - the E ring . Finally, under the frozen surface of Pluto, which may have turned out to be one of the most unexpected surprises, there is a liquid water ocean. And if there is water, heat and dissolved chemical compounds, then it is quite possible - although only hypothetically so far - that under the surface of these worlds one can find something more interesting than plain water.


An illustration of the insides of Saturn's moon Enceladus, which shows the global liquid water ocean, located between the rocky core and ice crust. Layer thickness is not to scale.

Can life exist on a world where sunlight never reaches a liquid ocean that can serve as a home for this life? It is possible, and it will be possible to test this hypothesis first on Enceladus. The presence of geysers gives the actual opportunity for sunlight to catalyze some of the biochemical molecules that can produce life before they fall again on the icy surface of the moon. For quite a long time, enough ice can accumulate over them so that the pressure can cause the ice to melt - and this process in principle can create a long-term cycle of life on this world. And in order to find out, we don’t have to dig this world or stick a probe into it to a great depth - you just need to send a spaceship past one of the geysers of Enceladus and take a sample from it. Can life outside the Earth be so easily accessible to us inside the solar system? Perhaps, if we are lucky, someday we will find out about it.

Ethan Siegel - astrophysicist, popularizer of science, blog Starts With A Bang! He wrote the books Beyond The Galaxy , and Treknologiya: Star Trek Science [ Treknology ].

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