The Oceans of Europa
On the Europa Clipper, the oceans of Europa and the possibility of life beyond Earth
The Earth, Carl Sagan reported in 1993, showed abundant signs of life. Three years earlier the Galileo space probe had detected oxygen in its atmosphere, picked up an odd amount of methane, and spotted strange radio signals emanating from the planet. Life - at least life involving water, sunlight and oxygen - seemed a likely explanation.
Of course, the question was slightly tongue in cheek. We know Earth is inhabited - we are here, after all. But how, Sagan was really asking, could we find out if another world were inhabited? How, simply by looking at it from space, could we tell if an odd set of properties was driven by biology or mere chemistry?
Later, after Galileo had reached and studied Jupiter, researchers turned its attention to the icy moon Europa. They already knew it to be an odd place. Telescopes had noticed how brightly it shined, especially in comparison to the other moons of Jupiter. Voyager, when it had passed by, had seen a surface laced by tangled cracks and surprisingly smooth, as though something had recently resurfaced it.
The nearby moon of Io, the innermost of Jupiter’s four big moons, was riven by volcanic eruptions, the result of extreme tides induced on that world by Jupiter. Europa, scientists thought, might experience something similar, though on a less violent scale. Its core, they speculated, could be hot, and under that icy surface might be a warm and liquid ocean.
Sadly, the Galileo probe was not designed to directly look for that ocean. But indirectly it saw signs that one might exist. By measuring Europa’s gravitational field it determined the density of the moon’s layers, and so found it to have an inner core of iron, a rocky mantle and an outer layer of water, though whether this layer was solid or liquid it could not tell.
Europa’s magnetic field, however, strongly suggested the presence of at least some liquid inside the moon. An ocean, then, and probably one filled with salty water, like those on Earth. More recent models hint at volcanic vents smouldering at the bottom of this ocean, similar to those found in the depths of the Atlantic and Pacific. And studies of its surface suggest the ocean occasionally bursts outwards, erupting in geysers and bringing salty water to the moon’s surface.
It seems, then, that Europa is an active world with a hidden ocean under its icy exterior. But is it inhabited? Does it have, even, the necessary ingredients for life, or at least life as we would know it? And how, assuming we go to look, would we find out?
