Does Europa Have Liquid Water?

Do you think there is enough evidence to support the theory that Europa may have liquid water beneath its surface? - Question posed by Dyana in the comments for this post.

Europa is Jupiter's 4th largest and 6th closest moon. It's also one of the most famous Jovian satellites due to its status as one of our solar system's most interesting possibilities for harbouring life beyond Earth.

Our studies of life lead us to believe that one of the most important environmental ingredients for the creation of life is the presence of liquid water. It is for this reason hat many people find Europa so interesting: it is widely believed, though not yet confirmed, that the icy surface of the moon could conceal a vast ocean of liquid water.


Here's the evidence.
Observations of Europa show it to have a relatively high albedo (that means it's quite reflective), which has hinted at the possibility of water ice for some decades now. Observations of Europa from observatories based both on the ground and in the air have recorded spectra with tell-tale gaps that correspond to what we'd expect from a body covered in water ice. The details of both of these sources of information suggest that the water ice itself is relatively uncontaminated.

So we're fairly convinced that Europa contains some water at least, even if that water is frozen. What we'd really be interested in is the presence of liquid water, and we've seen no evidence for that on the surface. Many people are certain, however, that there's at least the possibility of liquid water hidden underneath the icy crust.


Why?

The surface of Europa, much like the surface of Earth, seems to be broken up into large plates, which show evidence of having moved around in the moon's past. For these plates to be moving around, there must be something for them to move around on; something with relatively low friction that is able to flow. Earth's tectonic plates move around on top of an enormous ocean of molten rock. Measurements of Europa's density don't allow for such a heavy material to take up so much of its interior, but liquid water or more slushy ice would do the job nicely.

This evidence, though, only suggests that there may have been liquid water under Europa's surface in the past. How can we be sure that it's still there? Well, Europa's surface is relatively smooth (i.e. it doesn't have so many craters) compared to other bodies of the solar system, including Jupiter's other satellites. This suggests hat some process has re-surfaced much of Europa, and has done so recently. In the same way that plate tectonics on Earth has wiped its surface of many of history's impact scars, Europa's icy plates have moved and allowed water-based relatives of volcanoes and lava flows to smooth out the scars. Estimates place much of the surface of Europa to be somewhere between a million and a billion years old, which is quite young in geological terms.

More recently, space probe Galileo flew past Europa and made measurements of Jupiter's magnetic field in its vicinity, and found that the field around the moon changed direction every 5 hours. This suggests that an electric current is flowing through Europa, which suggests in turn that there is a substance in Europa through which an electric current can flow. Water ice is a poor conductor of electricity, but salty liquid water would do a much better job.


What keeps the water from freezing, though?

Way out at the orbit of Jupiter the Sun's heating effect is much lower than it is where we are: the equilibrium temperature due to the Sun five astronomical units away is much lower than the freezing point if water. So there must be some other mechanism keeping Europa warm, and as its surface is frozen, this heat must be coming from inside the planet.

Europa and its sister moons, Io and Ganymede, are in orbital resonance with each other. This means that their orbits are gravitationally connected, and has the effect that each moon gives Europa a little tug away from Jupiter at the same point in their orbits. This keeps Europa in a slightly eccentric orbit around Jupiter. The effect of a body's gravity increases as you get closer to it and decreases as you move away. Being in an eccentric (i.e. not completely circular) orbit means that Europa is squeezed and stretched over the course of an orbit by tidal forces from Jupiter. This effect is more obviously seen with the closer, more eccentric orbit of Io, and it's consequently violent volcanic activity. Europa's more gentle massaging, however, may cause its core to heat just enough to keep an ocean of liquid water surrounding its rocky innards.

There's one question left to ask:


Just how deep is this possible ocean?

Measurements of Europa's density suggest that much of the body is made of of rock, leaving around 100 km or so of its 1500 km radius for both the liquid water and the icy crust. Put into some kind of perspective, the deepest oceans on Earth are only about 11km deep.

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