Plate tectonics on Europa

Plate tectonics on Europa

Europa, named after a lover of Zeus, is the fourth largest moon of Jupiter. Europa is more massive than all moons smaller than itself combined. Still, it is the smallest of the four Galilean satellites, trailing behind Ganymede, Callisto, and Io. Our moon also carries slightly more mass than Europa.

Jupiter and the Galilean satellites, Io, Europa, Ganymede, and Callisto
Montage of photographs by Voyager 1

This distant icy moon remains a fascinating place. Europa has the smoothest face of all solid body objects in the Solar System. The smoothness of Europa supports evidence that beneath the surface an ocean of liquid water flows. Intermittently, the surface cracks or rifts apart allowing liquid water to flow on the surface. The formation and later processing of these cracks has shown Europa to be the first place, other than Earth, where plate tectonics play a role in shaping surface geography.

Earth, Moon, and Europa shown to scale
Image credits: NASA/JPL

The face of Europa is smooth, but highly distinctive. Europa has character; striking and dramatic dark lines crisscross the entire globe. These lines, or lineae, measure up to 20 km (12 mi) across. Along the edges, these bands can run a dark umber, while the center of the stripes shows some clearing. The borders of these dark streaks at times blush, tending to blend into the surrounding surface features. The smoothed over, bluish regions contain predominately water ice, while the darker strikes are enriched in other minerals.

Mid-Atlantic Rift in Iceland
Credit: Debivort CC BY-SA

Detailed images returned from the Voyager and Galileo spacecraft reveal that opposite edges of these rifts have moved laterally over time. Thus, it seems that the surface of Europa is cracking and separating, driven by pressures beneath. As this is happening the emerging plates slide along next to one another. 

The tidal forces wrenching on Europa from the immensity of Jupiter account for these incipient cracks along the face. Since Europa is locked in position with Jupiter, the same side always facing the planet, it could be expected that the cracks would take on predictable patterns. However, that is not what the images show.

The reason may be that the crust of Europa appears to be rotating slightly faster than the moon's interior. It takes a long time for these effects to be noticeable. Older fractures crossing the face of Europa stray farther in their orientation than fresh fractures. Astronomers have now compared back-logged images or Europa from the Voyager spacecraft with more recent images from Galileo. They estimate that it takes at least 12,000 years for the outer crust of Europa to make a complete revolution over the interior. Additional images demonstrate evidence for subduction of surface ice. As plates cross over each other, material is forced beneath the surface for recycling.

Complex surface geology of Europa
Composite images from Galileo spacecraft 

All of this offers proof that Europa is the first world beyond our own with a surface governed by plate tectonics. Since Galileo Galilei discovered Europa in 1610, it remains a place of wonder. We now see that beneath Europa, a subsurface ocean of liquid water flows. What that ocean may hold and how it relates to possible life beyond Earth, is a big question right now. The upcoming Europa Clipper mission will hopefully provide answers.