for National Geographic News
Scientists say Jupiter's moon Europa rivals Mars as a potential refuge for life. Some of them are now urging NASA to explore the ice-covered satellite.
"It takes longer to get there [than to get to Mars], it's more expensive, and a bigger deal to plan a mission. But if I had a choice, I'd go for Europa," said Lynn Rothschild, an astrobiologist with the NASA Ames Research Center in Moffett Field, California.
Rothschild studies the origins of life on Earth and other planets. She's intrigued by Europa because it appears to contain likely key ingredients for lifewater, an energy source, organic compounds, and billions of years of development.
Taken together, these ingredients are sufficient to support life, scientists say. To answer the question of whether life actually exists on Europa, however, requires further exploration with orbiters and landers like those currently exploring Mars. (See picture of and news about Mars's newfound "frozen sea.")
"The big unknown is what's needed for life to originate," said Robert Pappalardo, a planetary scientist and expert on Europa at the University of Colorado at Boulder.
Europa is roughly the size of Earth's moon but is otherwise markedly different. To begin with, at 490 million miles (790 million kilometers) from the sun, Europa's surface is a bone-chilling 230° Fahrenheit (145° Celsius). That's much too cold to support life as we know it.
But scientists believe the interior of Europa is heated by tidal flexing, a process that results from the gravitational tug-of-war among Jupiter and its moons. The heating may be sufficient to keep the inner layers liquid.
The Galileo spacecraft beamed close-up images of Europa's surface to Earth in the late 1990s and early 2000s. The images revealed patterns of ridges and cracks in the crust that are suggestive of an icy shell moving over a liquid ocean, scientists say.
Scientists are uncertain if Europa has organic compounds. Galileo, though, did detect carbon dioxide, sulfur dioxide, cyanogens (colorless, poisonous, flammable gases), and hydrocarbons (hydrogen-carbon compounds often found in natural gas and petroleum) on neighboring moons Callisto and Ganymede.
"It all suggests there is energy, water, and possibly organics, and so it starts to get very exciting," Rothschild, the NASA astrobiologist, said.
Another possibility is that hydrothermal vents, like those at the bottom of the Earth's oceans, are spewing energy and chemicals into Europa's ocean. If so, such vents could be a refuge for life, Pappalardo said.
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