Europa – what lies beneath
Travel one moon further out from Io and you’ll go from fire to ice: Europa. All of the moons of the Solar System beyond Io are icy, but Europa is special: its frozen layer may be comparatively thin, and beneath it lies a thick global water ocean. As on early Mars, conditions there may be conducive to the development of life, and unlike on the Red Planet there’s no reason to say that’s all in the past.
The ocean within
The submarine heating to keep the water liquid has the same cause as on Io: tidal heating due to Jupiter’s gravitational field. Europa’s orbit is less elliptical than Io’s and it’s further away from the gas giant, but the heating effect is still enough to give it a layer of water or mushy ice 100km thick.
Calculations based on its density distribution suggest Europa has three times as much water (in actual volume, not percentage) than Earth. That may sound like a lot, especially considering the lack of water on its sister Io, but it’s actually what we’d expect in this neighbourhood. It formed beyond the ‘snow line’ – the distance from the early sun where it was cool enough for volatiles like water to condense into solid ice grains and be incorporated into planets. In fact it’s more of a problem to account for the water on Earth than that on Europa – our planet’s water may have come from comets and asteroids which impacted it early in its history. The main reason Io is anhydrous is that intense volcanic activity has depleted its original water over the millennia, though it may also have started out with less than Europa due to forming nearer to the centre of the proto-jovian nebula.
Exactly how much liquid water and how much ice encircles Europa is very much a matter of debate. Opinion generally falls into two camps: the thin ice/thick water model, and the hard ice/mushy ice/thin water model as shown in the diagram to the right. Surface features are often used to argue for one model or the other, though often the same feature is pointed to as evidence by both camps – such is science when you’re going on very little data.
The surface: a nest of fractures
The surface of Europa resembles a bird’s nest: a host of brown lineations cross-cut each other, obscuring the cleaner water ice background. Some of these lineations have positive relief often forming double-ridges with a trough between them. Others are bright bands with little relief.
There are diverse explanations for these. A popular group of theories say that the cracks form through the varying daily tidal forces. Extension during part of the day pulls the surface apart and brown brines come up from beneath. These freeze and are pushed up to form the ridges as the cracks try to close up again. The reason the orientations of the cracks change through time, making the basket-weave we see, is said to be that the liquid subsurface ocean decouples the outer icy crust from the rotation of the silicate part of the moon. This leads to non-synchronous rotation of the crust, which means it’s at different orientations relative to the tidal forces through time1.
Whether the cracks reach through to the liquid ocean, however, is far from proven. It’s difficult to achieve cracking of the surface to such depths, especially if the thicker ice model above is correct2. One theory is that movement at the cracks is sideways (strike-slip faulting like at the San Andreas Fault, CA, USA). That could cause shear heating of the ice near the surface, which would provide the ridge and band material3.
As well as the lineations, whole sections of the lineated surface are sometimes broken up, forming a terrain referred to as chaos. Many researchers say this proves the ice layer is thin, but since a hard ice layer on mush could also break up in this manner, it’s not conclusive.
The rise of brines from a semi-molten interior is much the same as the eruption of lava through rock on Earth. This is termed cryovolcanism. Cryovolcanism certainly occurs on other icy moons, but it’s debated whether it does on Europa. For example the shear heating explanation above shows how liquid water can be released from shallow depths, with no connection to the moon’s interior.
One seemingly strong piece of evidence is the smooth ‘pools’. These are localised areas within the lineations, sometimes overlying them and sometimes confined by them. They often have associated domes and pits which could be small ice volcanoes. Strong as this evidence may look, they could also be formed by sublimation of ice at the surface, so the jury’s still out4.
The problem is that cryovolcanism is difficult. Unlike rocks, where molten rock is buoyant in solid rock, water is denser than ice. Under normal conditions, it won’t erupt upwards. Various models have been proposed for how this could be overcome, for instance the addition of other substances to water to make it less dense – awkwardly, the kind of compounds like sulphates which could account for the brown colour in the lineations would actually make it heavier. Alternatively, some kind of overpressure situation could occur in the subsurface, because ice has a higher volume than water and its growth would put any pockets of liquid water under pressure. That pressure would be released as soon as it was erupted, but it may cause short eruptions immediately after cracking of the surface occurred.
Importantly, the surface of Europa is young: as little as 20Ma. Cracking alone would not resurface the moon, so some kind of material deposition must be occurring. If not cryovolcanism, we’ll need to work out what that is.
Life in the deeps?
To thrive, life needs water and an energy source. Both are available beneath the ice on Europa. The heat that keeps the water liquid down there comes from underwater volcanism in the silicate centre of the moon. On Earth, that kind of environment has been shown to support whole ecosystems – the weird and wonderful creatures of the ‘black smokers’ around our submarine volcanic vents.
We’re less certain though about what kind of organisms could live there: complex creatures or just extremephile microbes? This all depends on the conditions down there. Many say the ocean is oxidised and so can support complex life. The oxygen would come down from the surface where it is released when the ice is irradiated by energetic particles from Jupiter’s magnetosphere. That very oxidation, however, may set up conditions hostile to life: oxidants may react with the sulphides in the ice and water to form sulphuric acid. The ocean’s pH could be as low as 2.65.
Whatever the real conditions down there, unfortunately it may be a long while before we can take a look: the radiation at the surface would probably kill anything up to a few meters down, so we’d need something very powerful to drill a lot of rock-hard ice. Here’s hoping someone with the bucks and curiosity to do it does so within our lifetimes!