Europa’s fascinated scientists for a long time as a possible home for extra-terrestrial life. When the Voyager mission returned the first pictures, it was clearly a geologically active world — there were few craters, and the surface was covered in striations that looked like obvious signs of a crust shifting over a molten subsurface. The spectroscopy indicated a surface of water ice, and the density (determined from careful measures of Europa’s deflection of Voyagers 1 and 2) was pretty consistent with water ice as well. Plus, just doing the math tells us that it experiences enormous tidal strain. This was so compelling, it became a major feature of Arthur C. Clarke’s novel “2010: The Year We Made Contact”, his long-awaited sequel to “2001: A Space Odyssey”.
Then, the Galileo probe arrived at Jupiter, after a much longer cruise than originally intended. Unlike the Voyagers, Galileo was to slip into orbit around the giant planet, allowing it to linger and study the moons in far greater detail than had ever been possible before. Although the mission was marred by a crippled high gain antenna, it nevertheless returned a gigantic wealth of data, including magnetometer readings proving what had already been suspected from Voyager data: Europa has its own magnetic field.
To produce a magnetic field, a celestial body requires electrically conductive material that is in motion. On Earth, this is our molten iron core. On the Sun, it’s the seething mass of unfathomable megatons (exatons, even!) of roiling plasma. On Jupiter, it’s believed to be an exotic, super-compressed form of liquid metallic hydrogen, which would be superconductive in that environment. But what could it be on Europa?
Observations of Europa by Galileo revealed that the linea (the striations across the surface) are indeed cracks where material has welled up from beneath, and frozen, and many clear geological features have been seen that show the crust is floating on a liquid or semi-liquid material. (Our own crust floats on the mantle, which is actually not truly liquid, but does still flow. So this doesn’t prove Europa has a liquid ocean, in and of itself.) And they revealed the presence of interesting contaminants on the surface. This image of the sub-Jupiter point on Europa contains an inset from Galileo’s mapping spectrometer. There is still debate over the exact composition of the mottled surface, but the most likely explanations are sulfuric acid or mineral salts that have welled up from below. Saltwater, if there are enough electrolytes in it, is famously conductive, and a deep subsurface ocean would definitely account for Europa’s magnetic field.
Then, in 2005, Cassini arrived in the Saturn system. Saturn has another moon with haunting similarities to Europa: Enceladus. Enceladus also has a density consistent with water ice, a surface spectrum also consistent with water ice, and a suspiciously young surface. But Cassini found something more: its particle sensors picked up saltwater as it flew past the cracked southern hemisphere of Enceladus, directly detecting geysers erupting from the moon and marking the first direct observation of liquid water elsewhere in the solar system. Subsequent observations were able to photograph Enceladus’ faint plumes, which, rather like Io’s volcanoes, extend hundreds of kilometers into space. So that raised the question: could the same thing be found on Europa?
It would be years before a new spacecraft could be sent to Jupiter. That spacecraft, Juno, has now arrived, but has only just barely begun its mission. So scientists have been working instead with the Hubble Space Telescope. Although Hubble has been orbiting since just six months after Galileo was launched, it has benefitted enormously from crewed servicing missions to upgrade its sensors. Today, it is capable of work far beyond its original design. Using this instrumentation, scientists have detected water vapor near the south pole of Europa. This graphic indicates the apparent position of the vapor; note, however, that the image of Europa is a simulation. The vapor cloud is not visible to the eye.
The next step is to rule out other explanations. They haven’t proven that the plume is actually associated with Europa. one problem with observing from Earth is that it can be difficult to tell whether something is actually on the object you’re looking at, or just floating right in between you and the target. They have conclusively determined it *is* water vapor, but more work is needed to prove for certain that it is a plume from Europan geysers.
Perhaps soon we will get a better answer. The Juno spacecraft newly arrived at Jupiter isn’t well equipped to answer the question, as it carries equipment better suited to studying the giant planet itself. But ESA’s planned JUICE mission (JUpiter ICy moons Explorer — yeah, scientists can get a little desperate in their acronyms sometimes) would definitely be able to answer the question. Planned for launch in 2022, this mission would be dedicated entirely to Jupiter’s large icy moons: Europa, Ganymede, and Callisto. And for good reason: while Europa definitely has a large subsurface ocean, and copious energy sources in the form of Jovian tides and radiation, it isn’t alone. Ganymede and Callisto have older, more battered surfaces, but they too are made mostly of ice, and they too have magnetic fields. They may not be as active as Europa, but there’s something going on there too.
Oceans may be more common than we know.