Jupiter's intriguing moon Europa may have a very Earth-like quality - its thick, icy crust appears to act like Earth's tectonic plates. And that could be a very positive thing for the little world's life-giving potential.
During studies of photographs taken by NASA's Galileo spacecraft that orbited the gas giant from 1995 to 2003, planetary geologists have found it hard to explain why most of the crust was relatively new ice (on average, the icy surface is 40-90 million years old) and yet there was little evidence of old ice that had been crushed up on the surface to make way for the new material.
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"We have been puzzled for years as to how all this new terrain could be formed, but we couldn't figure out how it was accommodated," said Louise Prockter, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. "We finally think we've found the answer."
Prockter and co-investigator Simon Kattenhorn, of the University of Idaho, Moscow, were able to seek out some curious features in the Galileo imagery that may explain what is going on. Their research was published Sunday in the journal Nature Geoscience.
Their conclusion is that, like Earth's rocky crust, there must be subduction zones where old material is pushed against plate boundaries, making the ice sink into the subsurface ocean, where it melts and gets cycled.
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On Earth, subduction zones are associated with mountain ranges, volcanoes and earthquakes - e.g. the Andean Volcanic Belt that stretches down the west coastal region of South America where the Nazca Plate and Antarctic Plate is continually being subducted under the South American Plate. One plate edge is pushed under a neighboring plate, causing the rock to be melted by the Earth's interior and recycled. The motion of the two plates rubbing along one another triggers quakes and provides energy for volcanic eruptions.
Like the Earth example, the researchers discovered features that provide evidence for subduction zones on Europa, including subsumption bands (where the ice gets bunched up, much like a mountain range) and "ice volcanoes" (where melt-water from the diving ice plate is forced to the surface to create "cryolava"):