Jupiter's Watery Moon to Get Closeup

Feb. 27, 2007 — It's been seven years since the Galileo space probe last flew by Jupiter's mysterious moon Europa, but what scientists learned still has them pining for more.

Beneath the moon's icy, flat and relatively crater-free surface lurks what is believed to be an enormous ocean, filled with more water than twice the amount on Earth.

Dedicated missions to Europa so far have been confined to PowerPoint presentations, but there is something in the offing that should temporarily appease researchers' appetites for more information.

On Wednesday, a Pluto-bound spacecraft will fly by Jupiter to slingshot itself off the giant planet's enormous gravity field, picking up an extra 9,000 mph of speed.

The flyby will position the half-ton probe to reach Pluto in another eight years — three years sooner than it could get there on its own. The spacecraft, called New Horizons, began its 3-billion-mile journey in January 2006.

Liquid or Ice?

During the Jupiter encounter, all the probe's science instruments will be collecting data, including possibly some details about suspected upwellings on Europa's frozen surface made by its hidden, subterranean sea.

"We know there's water there," Arizona State University planetary scientist Ronald Greeley said at the American Association for the Advancement of Science conference in San Francisco last week. "The question is, 'Is it liquid or not and where does it reside beneath the surface?'"

Scientists believe tidal flexing — a process triggered by the gravitational tug of Jupiter and neighboring moons — keeps Europa's interior far warmer than its frozen surface. The phenomenon is apparent on sister moon Io, which has active volcanoes.

On Europa, the process is more subtle, but there is ample evidence the moon's inner layers are liquid.

With liquid water, organic molecules and energy sources from tidal flexing and the Sun, Europa may be the most likely place in the solar system to find life beyond Earth, scientists say.

"If there is life, it probably evolved independent of life on Earth and it's probably carbon-based, but it may not be anything like what we have here," said Jere Lipps, a professor of integrative biology at the University of California at Berkeley.

Life in the Extreme

Lipps has made extensive studies of life in extreme environments on Earth, including beneath miles of frozen seas. Such habitats are believed to be similar to what exists on Europa.

Rather than tunneling down through what may be a thick ice covering, scientists should look for sediments that may have rafted up to the surface, Lipps said.

"Life loves ice," he said. "In ices live krill. Ices contain briny channels filled with frozen sea life."

New Horizons will look for deposits on the moon's surface during its Jupiter flyby. Although Galileo was considered a highly successful mission, a problem with its primary communications antenna forced scientists to miss much of the planned high-resolution imagery.

New Horizons has no such problem. As the spacecraft sails past Jupiter on Wednesday, scientists are counting on its infrared imager to flush out details of Europa's surface that Galileo missed.

"We don't know what's in the ocean besides water," said Bill McKinnon, a planetary scientist with Washington University in St. Louis. "I'm hopeful we will get some real clues about the surface of that ocean."

Missions Past

The strongest evidence for a liquid ocean on Europa came from an instrument on Galileo that detects magnetic fields.

The device found an electrically charged layer of some substance as close as 4.7 miles (7.5 kilometers) below the moon's icy crust. The most likely explanation is a liquid water ocean, say planetary scientists.

The ocean is believed to be as deep as 30 miles (50 kilometers).

New Horizons is expected to keep tabs on Jupiter for several months before going into hibernation for the rest of its cruise to Pluto.