“Everything on and under the surface is driven by the tides, even the mixing of substances needed to support life.” — Richard Greenberg, UA professor of planetary sciences

The presence of tides below the icy surface of Europa may foster a habitable environment for life according to Richard Greenberg, a professor of planetary sciences and member of the Imaging Team for NASA’s Galileo Jupiter-orbiter spacecraft.

He reports in the February 2002 issue of American Scientist that a combination of factors are involved. Tidal processes, liquid waters and periodic surface exposure may be enough to not only warrant life but also encourage evolution on the Jovian moon, he said.

“The implication is that these settings would actually be hospitable to life,” Greenberg says.

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 exposure to whatever was below.

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

Galileo data has shown that below the crust lies an enormous global ocean, complete with warmer waters and tides.

This “opened the door to speculation about life,” Greenberg says.

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. Greenberg points out that tides play a large 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, he says.

“Everything on and under the surface is driven by the tides,” Greenberg says, “even the mixing of substances needed to support life.”

Europa, whose orbit around Jupiter is nearly in synch with its rotation, is able to keep the same face towards the gas giant for thousands of years.

Greenberg pointed out an additional ingredient of stability necessary for life. The relatively slow movement of the tides and Europa’s synchronous rotation around Jupiter both help to maintain temperatures and conditions in smaller ocean niches, he said.

Richard Greenberg’s team of researchers study high-resolution images from the Galileo Jupiter-orbiter spacecraft. From the left are Research Scientist Paul Geissler, undergraduate Alyssa Sarid, Gwen Bart, Terry Hurford and David O’Brien, graduate students in planetary sciences and undergraduate Dereck Urbanowski.

However, longer periods of time would see even the most isolated areas freezing, Greenberg says, and “that would require an organism to adapt in some way.”

The images sent from Galileo show two distinct types of surface structures. Greenberg sees in them the evidence that exposure occurs more commonly that previously thought.

Tides, Greenberg says, have created the two types of surface features seen on Europa: cracks with ridges and chaotic terrain.

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 thin or melted ice could provide light and surface chemicals to the oceans, stimulating and sustaining the environment.

“We can see that the ocean is interacting with the surface,” Greenberg says, “There is a possible biosphere that extends from way below the surface to just above the crust.”

He says 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.

This combination of factors would give organisms a stable but changing environment exactly the type that would encourage evolution, he says. “Necessity drives change.”

Life on Europa could resemble that of simple sea-dwelling organisms of Earth, Greenberg said, possibly utilizing photosynthesis for energy.

“Plenty of Earth’s organisms live at 32 degrees (Fahrenheit) or below,” he said.

Microbes, recently discovered in the Antarctic, can hibernate for up to a million years in the ice.

Europa’s organisms that become trapped in the ice could be thawed out when the next warm tide flowed through, effectively reviving them, Greenberg said.

While the Galileo’s mission is near completion, Greenberg is already poised for future voyages. NASA is contemplating various missions to Europa sometime later in this decade, and Greenberg’s group hopes to participate.