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Europa's Churning Ice Shell (labeled version)
Europa's bizarre surface features suggest an actively churning ice shell above a salty liquid water ocean. This artistic composite illustrates Europa's many different types of surface features and the interior processes that might form them.
Heat is created within the floating ice shell by flexing and squeezing due to rising and falling tides as Europa orbits the giant planet Jupiter. It is this tidal heat that keeps the ocean liquid at a temperature near 0°C (32°F), even though the surface is frigidly cold at a temperature of about -170°C (-275°F). The cold near-surface ice can crack or crumble, while the tidal heat keeps the base of the ice shell close to the ice melting temperature, so the ice there can slowly flow, like a glacier.
Blobs, or diapirs, of relatively warm ice may slowly rise from the base of the floating ice shell to the cold near-surface ice, and they might melt and expel salts in their path. Diapirs might push the surface upward to create domes, or crumble the surface and erupt to form spot. Diapirs might merge and tidal heat can concentrate within the ice shell, perhaps partially melting it, to create the jumbled blocks of chaos terrain. Ice diapirs could be a sign of convection within the ice shell, like the rise of warm rock within the Earth's mantle.
Ridges criss-cross the surface, and are places where warm ice or water has pushed up the surface or erupted. Some ridges are cycloidal in shape, arcing across the surface: these probably formed along cracks that were influenced by the rising and falling tides. Bands are places where the icy surface has pulled apart, with new warm ice rising up, chilling, and cracking. Some water-filled cracks, or dikes, could shoot upward from the base of Europa's ice.
Europas surface shows few large impact craters, indicating that vigorous geological activity has wiped out the older ones. A large impact with bulls-eye rings may have once punched all the way through the ice shell to the watery ocean, though the icy hole has since healed.
Europa's churning ice shell could allow surface contaminants to move downward into the ocean. It can also dredge up deep slushy ice, allowing material from the ocean--including life, if it exists there--to be carried to the surface.
An unlabeled version of this image is also available in this gallery.
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