|MadSci Network: Astronomy|
I answered a similar question last year on the Mad Scientist's Network: "Pressure and Composition of Europa's Atmosphere" (the question title is misleading). Europa may indeed have liquid water beneath its icy surface; the reasoning is given below.
Io, Europa, and Ganymede are in resonant orbits; the ratio of their orbital periods being exactly 1:2:4. This configuration arises for reasons similar to why the Moon's rotation rate and revolution rate are the same. The gravitational influence of the moons force each of them to have eccentric (not quite circular) orbits: as they get nearer and farther from Jupiter, they get stretched and squished by differences in its gravitational field.
This stretching and squishing causes frictional heating in the moons, the same way bending a paperclip causes it to heat up. Calculations of this "tidal heating" suggested that Io should be heated enough to have volcanoes: this prediction was confirmed when the Voyager missions photographed Io in 1979. Europa is also heated by this process, but to a much smaller degree.
The mass of Europa can be calculated using the amount of gravitational deflection a space probe making a flyby experiences. If we observe its radius, we can compute a density for Europa. This density is somewhat less than rock, but much more than ice; it's very likely that Europa is mostly rock with a thick layer of ice on top. More precise measurements can give details about the internal density structure of Europa. Results from the Galileo spacecraft showed that Europa almost definitely has a layer of ice between 100 and 200 km thick. This analysis (by Anderson et al., published in the May 23 1997 issue of Science) cannot tell the difference between ice and liquid water, though.
It's possible that the tidal heating of Europa is enough to melt the base of its ice layer. To find out, we need to build a thermal model of Europa, starting with a calcultion of the amount of tidal heating and calculating the heat flow via convection and conduction through the ice, giving us a temperature profile within the ice. If the temperature rises above freezing anywhere, we must have liquid water there. Several authors have published papers on this in the last few decades; the results are ambiguous and change from "no water" to "lots of water" every time someone adds a new bit of physics. The biggest problem is that we don't know the tidal heating rate very well, because it's hard to deduce the viscosity (amount of friction) of the Europa's interior.
We'll probably find the answer using other techniques. For example, planetary geomorphologists (people who study land-forms) are looking for volcanoes, frozen floods, or other structures which would require liquid water. Galileo has found some shapes on the surface which look a lot like icebergs floating in a frozen sea, but there are problems with this interpretation. Geochemists are using the NIMS (Near Infrared Mapping Spectrometer) on Galileo to search for salt on the surface. On Europa as on Earth, salts tend to be removed from rocks and dissolve in the ocean; ice is relatively fresh (unsalty). If Europa's ocean exists, it is probably very salty. If that liquid water reached the surface and evaporated, the salt would be left behind. McCord et al published a paper in the May 22 1998 issue of Science claiming to have observed salts on the surface of Galileo which are similar to what's left behind when ocean water evaporates on Earth. The salt was found in the dark lines on the surface which many have suggested are cracks where liquid water could escape to the surface. But is a liquid ocean the only explanation for the observed salt? Nobody's really sure.
Galileo is currently involved in the "Galileo Europa Mission", designed to study Europa in more detail. It will take more high-resolution photos of the planet, do more NIMS chemical detection, and get more fly-bys to nail down the internal structure of the planet. However, Galileo is probably unable to definitively answer the question of oceans on Europa.
Specialized space missions are currently being developed to answer this question. The most advanced is the Europa Ice Clipper, which would release a small impactor to slam into the surface. The Ice Clipper would fly through the cloud of ice and vapor raised by the impact, collecting samples to be returned to Earth. Daring, isn't it? A more sedate proposal (which I can't find a reference for) would place a spacecraft in orbit around Europa. This would use a laser rangefinder to precisely measure the shape of Europa and watch the planet get squished and pulled by Jupiter's tides. This would allow us to exactly determine the amount of tidal heating. A third proposal, the Europa Orbiter Mission, would use radar-sounding to try to detect a radar return from liquid water beneath the surface. But what lots of people really want to do is build an ocean explorer. The idea is to build a probe which melts its way through the ice using radioactive heating elements, and if it finds an ocean, starts sailing around looking for life!
And people claimed there was no adventure left in space exploration!
For more information on Europa generally, check out the Nine Planets website.
Try the links in the MadSci Library for more information on Astronomy.