More Signs That Europa Could Be Habitable

Researchers see plate tectonic activity on a distant moon.

Europa ridges.jpg
Europa’s surface as seen by NASA’s Galileo probe, with a prominent double-ridge system running from the lower left to the upper right. The feature could function similarly to a mid-oceanic ridge on Earth.

Evidence of plate tectonics on Jupiter’s moon Europa—in particular the process of subduction, where one slab of ice is pushed under another—was reported in the journal Nature Geoscience this week by Simon Kattenhorn from the University of Idaho, Moscow, and Louise Prockter from the Johns Hopkins University Applied Physics Laboratory. The slab we’re speaking of is very large—about 12,000 square miles (nearly 20,000 square kilometers). The researchers noticed that it was missing from the surface in the moon’s high northern latitudes. Even more tantalizing, they saw what looked like fresh ice flows on the overriding plate.

Based on what we know of our own planet, this finding has huge implications. On Earth, plate tectonics is the recycling mechanism by which nutrients move from the planet’s interior to the surface and vice versa, producing a diverse ecosystem with a substantial amount of biomass. Earth has mid-oceanic ridges where new lava spills out, and subduction zones where old crustal material is pushed beneath the crust. The Pacific oceanic plate, for example, is subducted underneath the South American continent, resulting in a volcanic mountain chain (the Andes), from which gaseous contents of the subducted plate are released, along with surface lava flows.

Kattenhorn and Prockter saw a very similar pattern on Europa. Since the plate is made of ice rather than rock, we would not expect a high mountain chain. In fact, mountains in this case would probably imply that the two plates have crumpled together, rather than one being subducted. The fresh ice flows seen on the overriding plate are exactly what you would expect if plate tectonics occurs on Europa.

We’ve known for a long time about the extended double ridge systems on this moon, which resemble mid-oceanic ridges on Earth. These had been suggested as outlets of liquid water from the interior—water that later froze. What wasn’t known until now is where the ice disappeared to. Based on this new finding, it appears to be subducting.

If later research confirms the existence of an active plate tectonic system on Europa (and perhaps the recently discovered erupting water plumes are related), the consequences for life could be far-reaching. Oxygenated compounds could be carried into the deeper, chemically reduced ocean, and these compounds could be utilized by life for metabolic processes, with the result that ecosystems with simple multicellular organisms, as envisioned by some of us in the field of astrobiology, could exist.

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