Robert Pappalardo, an assistant professor in the astrophysical and planetary sciences department and one of the world's foremost Europa experts, said the icy moon is believed to contain an ocean some 13 miles under its icy surface. Satellite images appear to indicate surface warping -- including domes and reddish spots -- showing that "elevators" of sorts transport material up and down from the ocean to the surface, said the planetary scientist.

"Europa acts like a planetary lava lamp, carrying material from near the surface down to the ocean, and, if they exist, potentially transporting organisms from the ocean up toward the surface," he said. "Just a mile or two beneath the surface, the conditions may be warm enough to allow organisms to survive the journey."

The "thick shell" model of Europa has implications for the future exploration of the moon and whether the existence of life is possible in the lightless depths beneath the planet's surface, said Pappalardo. "It would be very difficult for a future spacecraft to drill all the way through a 13-mile-deep ice shell to search for life in the underlying ocean. But the motions of glacial ice may transport ocean material, and any life it might contain, to the surface."

Pappalardo and his research group at CU-Boulder's Laboratory for Atmospheric and Space Physics are attempting to tie together pieces of an elaborate puzzle to assemble a comprehensive model of how Europa functions. The results are being reported at the Geological Society of America meeting in Denver Oct. 27 to Nov. 1.

Under similar conditions in Arctic ice on Earth, organisms can remain in a state of hibernation until exposed to warmer and wetter conditions, he said. "If life exists in Europa's ocean, organisms might be carried on a slow ride from the bottom to the top of Europa's icy crust. Sampling the surface composition may provide direct insights into the nature of the ocean deep below, and could plausibly reveal dormant organisms if they exist within Europa."

CU-Boulder graduate student Amy Barr is developing a computer model to illustrate the Europa ice motions, said Pappalardo. She is modifying a computer model that has been used to understand Earth's plate tectonics and to better understand Europa's geology, including how nutrients created by ice irradiation at Europa's surface might be transported down to the moon's oceans.

Barr's ice-convection model, the most sophisticated yet applied to Europa, may show that organisms could thrive below the thick cap of ice, Pappalardo said. It incorporates information on how the satellite's thick ice shell is heated and how it flows as it is squeezed by the gravity of Jupiter, which raises huge tides on Europa.

CU undergraduate Michelle Stempel is analyzing Europa's pattern of cracks and ridges to understand how the Jupiter tides have fractured the surface, and over what time scales the cracking has occurred. By matching stress patterns to surface geological features, she is studying where and how the surface cracks are created in response to short- and long-term deformation of the thick icy shell overlying an ocean.

Pappalardo also has teamed with Francis Nimmo of University College, London, to understand the similarities and differences between Europa and its sibling Jovian moon, Ganymede. Ganymede may hide an ocean beneath its icy crust much deeper than Europa's, although Ganymede's era of geological activity has likely long ceased. By analyzing the topography of fractures on Ganymede, the two scientists have determined that Ganymede was once nearly as warm inside as Europa is today.

"This has important implications for the history of Ganymede, and also for how Europa's surface is shaped today," Pappalardo said. "Ganymede may be a fossil version of Europa." The two scientists found similar internal and external forces that probably have influenced the two moons, but with different geological expressions.

In addition, Pappalardo is working with Nick Makris of the Massachusetts Institute of Technology to study how a future Europa lander could precisely determine the depth and thickness of Europa's ocean, using the same techniques routinely used by the Navy to measure the depth and composition of Earth's oceans. The two are presenting back-to-back talks at Denver's GSA meeting to illustrate how the proven terrestrial technique can apply to the exotic environment of Europa.

Pappalardo recently served on a National Research Council panel that reaffirmed a spacecraft should be launched in the coming decade with the goal of orbiting Europa. The Europa Geophysical Explorer would have scientific objectives that include confirming the presence of an ocean, remotely measuring the composition of the surface and scouting out potential landing sites for a follow-on lander mission.

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