http://www.digitaljournal.com/science/tectonics-on-jupiter-s-moon-europa-another-similarity-to-earth/article/402075

Tectonics on Jupiter’s moon Europa — another similarity to Earth

Posted Sep 9, 2014 by Robert Myles
Planetary geologists report evidence that Jupiter’s moon Europa shows signs of plate tectonics, shaping and moulding the moon’s surface similar to geological activity on Earth.
Artist s concept of a plume of water vapor thought to be ejected off the frigid  icy surface of the ...
Artist's concept of a plume of water vapor thought to be ejected off the frigid, icy surface of the Jovian moon Europa, located about 500 million miles (800 million km) from the sun. Spectroscopic measurements from NASA's Hubble Space Telescope led scientists to calculate that the plume rises to an altitude of 125 miles (201 km) and then it probably rains frost back onto the moon's surface.
NASA/ESA/K. Retherford/SWRI
Europa, say the geologists, is the first planetary body so far discovered, other than Earth, to possess such a feature. Scientists were previously aware that Europa’s icy crust displayed clear evidence of expansion. But what remained a mystery was what happened to areas where the old crust was destroyed to make way for the new.
Observations were made by planetary geologists Simon Kattenhorn, of the University of Idaho, Moscow, and Louise Prockter, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. As part of their research, they examined images of Europa taken by NASA’s Galileo orbiter in the early part of the new millennium.
Launched in 1989, Galileo’s mission was to explore Jupiter and its moons. Arriving in Jupiter orbit in 1995, Galileo made around a dozen passes of Europa during a mission lasting till September 2003 when the probe’s orbit was allowed to decay.
Plate tectonics theory holds that Earth’s outer layer, its crust, is made up of plates of land (both above and below the oceans) that ‘float’ on the next layer down, the mantle. As the plates move, they grind against each other, sometimes pushing up to form mountain ranges. At other times plates push, one against the other, till one plate slips below the other causing earthquakes, California’s San Andreas Fault being a prime example.
Five facts about Europa:
• Europa is the smallest of Jupiter’s four largest moons, called the Galilean satellites, discovered by Galileo Galilei in 1610. The others are Io, Ganymede and Callisto.
• Europa’s radius of 1561 kilometers means it’s slightly smaller than our own Moon (1737 km).
• Europa is extremely geologically active, to the extent that its surface is comparatively young, estimated at between 40 and 90 million years on average.
• Europa is believed to have a liquid water ocean under the moon’s icy crust. Europa is entirely covered by this one ocean which contains more liquid water than all of Earth’s oceans combined.
• Europa whizzes round Jupiter with an orbital period of just 3.55 days. Like our own Moon Europa is tidally locked so that one side always faces its parent planet.
Europa’s surface is scarred with cracks and ridges. Much of Europa’s surface shows signs of extension, the process whereby bands of crust, often miles wide, have formed as the moon’s surface gaped apart and new icy material from the layer below moved up and in to fill the gap.
That process is similar to what takes place on Earth’s seafloor. Here on Earth, new surface material bubbles up from the mantle below to form mid-ocean ridges — what might be termed the engine of plate tectonics. As new material forces tectonic plates to move, older material is destroyed at the outer edges where two plates meet — called subduction zones — and forced back down into the melting pot of the mantle.
Scientists have found evidence of plate tectonics on Jupiter s moon Europa. This conceptual illustra...
Scientists have found evidence of plate tectonics on Jupiter's moon Europa. This conceptual illustration of the subduction process, where one plate is forced under another, shows how a cold, brittle, outer portion of Europa's 20-30 kilometer-thick ice shell moved into the warmer shell interior and was ultimately subsumed.
NASA
Up till now, although Europa exhibited evidence of abundant new material on the surface, hadn’t been able to figure out how the moon’s surface could accommodate all the new material, i.e. where did all the old crust go?
How Europa looked before any tectonic disruption could be gleaned from reconstructing Galileo’s photo records, putting together image blocks of Europa’s surface much in the same way as a jigsaw puzzle might be assembled. But when Kattenhorn and Prockter rearranged the icy terrain in the images, they found more than 12,000 square miles (almost 20,000 square kilometers) of Europa’s surface were missing in the moon’s high northern latitudes.
A number of factors pointed to Europa’s missing terrain having moved under a second surface plate, similar to subduction on Earth. The planetary geologists found ice volcanoes on the overriding plate. They speculated that these could be formed by melting and absorption of the tectonic slab as it was forced below the surface. The absence of mountains at the subduction zone — the point of impact — implied that ‘old’ surface material was being pushed into the moon’s interior rather than concertinaed upwards as two plates went head to head.
The scientists theorize that Europa’s ice shell (estimated at about 20 miles (30 km) thick) absorbed the subducted area rather than the ‘old’ surface penetrating though to Europa’s moon-wide underlying ocean. To date, although planetary geologists have seen evidence of material moving up from under the shell, until now they haven’t been able to identify any mechanism that moves material back into the moon’s outer shell or, possibly, into the large ocean below the icy outer crust.
“Europa may be more Earth-like than we imagined, if it has a global plate tectonic system,” Kattenhorn says, “Not only does this discovery make it one of the most geologically interesting bodies in the solar system, it also implies two-way communication between the exterior and interior -- a way to move material from the surface into the ocean -- a process which has significant implications for Europa’s potential as a habitable world.”
The new research is likely to stir debate as to what scientific instruments should be aboard any future probe to unlock Europa’s mysteries. In July 2014, NASA issued an Announcement of Opportunity inviting proposals for just such a mission.
Commenting on the findings, Curt Niebur, Outer Planets program scientist at NASA HQ, Washington, said, “Europa continues to reveal itself as a dynamic world with compelling similarities to our own planet Earth. Studying Europa addresses fundamental questions about this potentially habitable icy moon and the search for life beyond Earth.”
The research team’s analysis is published online in the journal Nature Geoscience.