Europa: as the human eye might see it (colors adjusted, Credit: NASA/JPL/Ted Stryk)

Jupiter's enigmatic moon Europa has long been thought to contain a huge ocean beneath its icy crust, but what is in that ocean and does it ever come to the surface?

Since the Voyager and Galileo probes explored the Jovian system, its moons have presented an extraordinary and fascinating puzzle. The largest of the 67 known moons are the ones that Galileo Galilei watched wend their way around Jupiter's bulk back in the early 1600's - Io, Europa, Ganymede, and Callisto. These bodies are remarkable both for their size (Ganymede is 8% larger in diameter than the planet Mercury, Callisto is 99% Mercury's size) and their diversity.

Io, orbiting closest to Jupiter out of these four Galilean moons, is the only body in the solar system other than the Earth with observable volcanism (as opposed to cryovolcanism). In fact it's thoroughly covered in pimples, with an estimated 400 active volcanic structures. Tidal flexing due to Io's elliptical orbit within Jupiter's enormous gravitational field keeps it hot.

The Galilean Moons - and their incredible variety (NASA)

Ganymede and Callisto are covered in filthy water ice, each conceivably containing deep liquid water oceans a hundred or more miles beneath their surfaces. Although Callisto is far less differentiated than Ganymede, with its internal layers less well defined, suggestive of a colder history where material has still not settled fully in the moon's own gravity well.

But Europa is perhaps the biggest mystery. Spacecraft imagery reveals a relatively smooth surface, criss-crossed by remarkable structures - cracks, plates, and signs of vast ice-floes that appear as if they once broke free and rolled into new positions. Some 'chaos terrain' suggests material atop of a much warmer, liquid water, body. Relatively few craters pock Europa's surface, also indicating that it's young and frequently replenished.

Measurements of magnetic fields, induced by passage through Jupiter's own powerful field, point to a conductive medium somewhere beneath Europa's frozen surface. Chemically rich water, in liquid form, is the best candidate. Perhaps an ocean descending to 60 or so miles, enough fluid to fill Earth's seas twice over and capped with thick ice.

This ocean may be maintained by a combination of the same kind of tidal flexure torturing Io, along with a base of warm rock - something that might conceivably parallel the deep ocean hydrothermal systems we find here on Earth.

One hypothesis for the interior of Europa (NASA)

An ocean in Europa has provided ample room for speculation about the existence of life there. Many of the conditions seem suitable; liquid water and chemical feedstuff from a rocky interior could be sufficient to sustain organisms. We also know that much of life on Earth doesn't require access to our planet's surface - at least not directly.

One of the biggest stumbling blocks for figuring out whether Europa could sustain a sub-surface biopshere is the need for geochemical recycling, and particularly a source of oxygen to help drive its chemical engines. The surface of the moon, exposed to the harsh radiation environment around Jupiter, could be an excellent resource - but not if it's sealed off from the interior.

Now a beautiful new piece of Earth-bound astronomical research seems to shed light on both the surface, and perhaps sub-surface, of Europa. Mike Brown and Kevin Hand have used the Keck observatory to produce a very fine spectral map of the surface of this moon - they do a 40 times better job at resolving the frequencies of light than the Galileo spacecraft managed up close.

What they have discovered, in a nutshell, is that a previously unseen 'salt' - magnesium sulfate, litters some of Europa's surface. But this magnesium compound is only on the 'trailing' face of the moon - the side that receives the worst dose of particle radiation from the potent nest of magnetic fields and plasma circulating in the Jovian system.

The suggestion is that this form of magnesium salt is produced as radiation pummels Europa's trailing side and catalyzes a reaction between preexisting sulfate formed on the surface (with sulfur originating from volcanic Io) and something containing magnesium. But we don't think that the magnesium is just floating around in space - so it must be coming from Europa itself.

The authors point out that sodium and potassium are also known to exist on Europa's crust, so now we can add magnesium to the mix. It's less easily 'sputtered' off the trailing face of the moon by radiation, while sodium sulfates and potassium sulfates tend to be removed, so the magnesium component gets enhanced.

But if magnesium wasn't always in the form of magnesium sulfate (seemingly produced only on the radiation heavy side of the moon), what was it? The best hypothesis is that it, together with sodium and potassium, was combined with chlorine - and it gets to the surface from the deep ocean.

In other words, this is indirect evidence that Europa's ocean is full of sodium chloride, potassium chloride, and magnesium dichloride. If those sound kind of familiar, well they should. As Mike Brown notes in an excellent series of explanatory posts, if you go and lick Europa it'll probably taste like a mouthful of Earth's oceans.

This is also not entirely unprecedented. Recent measurements of the water geysers from Saturn's moon Enceladus also suggest the presence of sodium and potassium salts (although whether chlorine salts or sulfate salts is not known).

The next steps will be to look for the presence of chlorine - which is tricky when they're in solids, less so when they evaporate into space. Long term this all adds to a body of evidence that not only does Europa indeed have an interior liquid ocean, but that ocean somehow gets onto the surface, bringing salts and perhaps whatever else is down there.

This would also offer a chance for surface material to get back into the ocean, carrying chemical energy. It's possible therefore that we're beginning to see the intimate workings of a truly alien world, one that might yield even more surprises in the years to come.