Original Source: University of Arizona

February 08, 2002

The biosphere of Jupiter's moon Europa might offer several niches for the evolution of life

Credit: NASA/JPL

Jupiter's moon Europa might not only sustain but foster life according to the research of a University of Arizona professor.

Richard Greenberg, a professor of planetary sciences and member of the Imaging Team for NASA's Galileo Jupiter-orbiter spacecraft, reported in the February issue of American Scientist that a combination of several factors could create habitable niches. The combination of tidal processes, warm waters and periodic surface exposure may be enough to not only warrant life but also encourage evolution.

Since late 1997 Greenberg and his team have been studying the images sent from Galileo. High-resolution images of the surface along with knowledge of Earth's geology have helped to reveal the environment of Europa.

Europa, similar in size to Earth's moon, has been imaged by Galileo for the last 4 years. Its surface, a frozen crust of water, was previously thought to be tens of kilometers thick, denying the oceans below any exposure.

One factor contributing to a habitable environment is the presence of liquid water. With Jupiter being the largest planet in the solar system, its tidal stresses on Europa create enough heat to keep the water on Europa in a liquid state.

However, Greenberg points out that more than just water is needed to support life. Tides also play a role in providing for life. Ocean tides on Europa are much greater in size than Earth's with heights reaching 500 meters. Even the shape of the moon is stretched along the equator due to Jupiter's pull on the waters below the icy surface. "Everything on and under the surface is driven by the tides. The mixing of substances needed to support life is also driven by tides," Greenberg said.

Ice rafts on Europa as seen by Galileo Credit: NASA/JPL

Tides have created the two types of surface features seen on Europa: cracks/ridges and chaotic areas. The ridges are thought to be built over thousands of years by water seeping up the edges of cracks and refreezing to form higher and higher edges until the cracks close to form a new ridge. The chaotic areas are thought to be evidence of the melt-through necessary for exposure to the oceans. The tidal heat, created by internal friction, could be enough to melt the ice, undersea volcanoes are also a possible explanation for large melt-throughs.

"Stable environments are also necessary for life to flourish," Greenberg said. Europa, whose orbit around Jupiter is in-synch with its rotation, is able to keep the same face towards the gas giant for thousands of years. But over longer periods of time, any given niche freezes. That would require an organism to adapt in some way.

The surface of Europa was previously thought to be tens kilometers thick, never exposing the oceans. Greenberg said, the geologic structures are evidence that exposure occurs more commonly than ever thought. "The ocean is interacting with the surface. There is a possible biosphere that extends from way below the surface to just above the crust."

Credit: American Scientist/Greenberg

Greenberg said, the combination of factors would give organisms a stable but changing environment -exactly the type that would encourage evolution. The melted-through ice provides light and surface chemicals to the oceans. Life on Europa could resemble that of simple sea-dwelling organisms of Earth, possibly utilizing photosynthesis for energy. "Plenty of Earth's organisms live at 32 degrees (Fahrenheit) or below." Microbes, recently discovered in the Antarctic, can hibernate for up to a million years in the ice. Europan organisms, trapped in the ice, could be thawed out when the next warm tide flowed through, effectively releasing them, Greenberg said.

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see also:

• Article abstract - American Scientist, Jan/Feb 2002


• Biosphere image with caption - American Scientist, Jan/Feb 2002


• Europa images from Galileo

Other news about Europa:

• Jan 17, 2002 - Hydrothermal vent plumes may influence the icy surface of Jupiter's moon Europa
• Nov 08, 2001 - Simulations of impact craters indicate that Europa's ice crust must be deeper than 3 kilometers
• Aug 25, 2000 - Strongest evidence yet for an ocean of water under Europa's icy crust
• Jan 10, 2000 - Galileo finds new evidence for a liquid ocean beneath Europa's icy crust