EUROPA: VOLCANISM AND TECTONISM ON AN ICY WORLD

Jupiter’s moon Europa has an extremely youthful surface (40-70 Ma), likely the result of tectonic and volcanic resurfacing. I here review the major forms of cryovolcanism and tectonism that have shaped Europa’s surface. Cryovolcanism primarily appears in the form of chaotic terrain, in which large swaths of the surface are disrupted by fractured plates of ice that have been shifted into new positions within a background matrix. Some chaos areas stand higher than the surrounding terrain, and are inferred to have formed from compositionally or thermally buoyant ice rising to the surface. Chaos regions commonly have associated dark, smooth material showing evidence of embayment, thought to be evidence of melting of subsurface material. Recent Hubble Space Telescope observations inferred plume activity in Europa’s southern hemisphere, which may represent the first indication of explosive volcanism on the satellite.

Europa’s surface is dominated by tectonic features in the form of linear ridges, bands, and fractures. Fractures appear as simple troughs or scarps and are generally the youngest features. Ridges, consisting of two parallel ridge crests with an intervening trough, are the most common landform. Cycloidal ridges are similar to double ridges, but form chains of linked arcs. The process of ridge formation is unknown, but most models of their formation include fracturing in response to processes within the ice shell. Some features, such as cycloidal ridges, appear to initiate as a direct result of Europa’s tidal cycle. Bands are linear, wedge-shaped features that cut swaths through preexisting terrain at a variety of different scales. Their surfaces vary from relatively smooth to heavily fractured, and they generally display bilateral symmetry. Band morphology reflects fracturing and lithospheric separation with fresh material filling the newly created gap, much like sea-floor spreading on Earth.

The accommodation of extensional features has, until recently, been a significant outstanding question, with evidence only of minor contractional features. Recent results propose the existence of a subduction zone in the northern hemisphere, which has apparently accommodated ~20,000 km² of material, suggesting that Europa’s surface may undergo plate tectonics.