Water Plumes on Europa: What Lies Beneath?

Newly discovered geysers may make the exploration of Jupiter’s icy moon much easier

water plumes
Scientists have found evidence of two giant plumes of water vapor emanating from the surface of Jupiter’s moon Europa, according to a new report in the journal Science.

Lorenz Roth, Joachim Saur and colleagues used the Hubble Space Telescope to measure ultraviolet emissions from Europa, and saw hydrogen and oxygen concentrations consistent with 120-mile-high water vapor plumes. The density of the water vapor represents an intermediate case between plumes previously observed on the nearby moon Io and the low-gravity outpouring of water vapor at Saturn’s small moon Enceladus. The water content in Europa’s plumes is 10 to 100 times higher than what exists in the moon’s tenuous atmosphere, and seems to be located in regions where tidal stress is strongest. The height of the plumes suggests the kind of violent supersonic eruption velocities that scientists have seen on Enceladus. What’s significant, though, is that Europa is a much larger moon, and is one of the likeliest spots in the solar system to find life.

The tidal flexing of Europa’s icy crust is readily visible in its cracked-eggshell-like surface. Most scientists believe there’s a large liquid water ocean under that shell. If the new evidence of water plumes is confirmed by future studies, it would make an even stronger case for a large subsurface reservoir of water. Not that many scientists question it. Controversy in recent years has focused more on how thick the ice shell is above Europa’s ocean. The new Hubble results suggest that at least in some areas the ice crust is extremely thin—a notion already supported by the moon’s so-called Chaos Region (below), which looks like drifting icebergs in the Arctic ocean.

The most intriguing question, of course, is what might lurk below the ice: Is it a deep ocean with hydrothermal vents and black smokers inhabited by organisms similar to those found near Earth’s oceanic vents? Theoretical modeling suggests this might be a real possibility. Or would subsurface reservoirs of ammonia and water, or perhaps an ocean with high concentrations of sulfuric acid, preclude life?

For years, scientists have thought we would have to send a “cryobot” probe down through the thick ice shell to find out. If the new results are confirmed, this may no longer be necessary. Instead we would send a lander to the bottom of one of the plumes, and simply collect any erupting material that falls back to the surface. There we might find fresh fossilized life encased in ice, especially if we can sample it before Jupiter’s harsh radiation destroys it. We might even find preserved viable cells if we sample beneath the radiation-altered surface. This is a much easier engineering task than melting or drilling through miles of ice. With this new finding, the astrobiological exploration of Europa may have just gotten much easier.

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