In September 2003, after eight years in orbit, the Galileo spacecraft was purposely sent into Jupiter's atmosphere, where it would burn up. The maneuver prevented the spacecraft from crashing into Europa and potentially contaminating the icy moon with microbes that might have hitchhiked from Earth.
"I'm delighted I'm a biologist and not a planetary scientist," Rothschild said. "There's not a whole lot of new data since Galileo. People are combing over the past data, but what we need is a mission."
Pappalardo said he and his colleagues have recently used Galileo's data to build better models of Europa. These models demonstrate the evidence for a salty ocean, the active surface geology, and even the estimated maximum thickness of the icy surfaceabout 12 miles (20 kilometers).
But Pappalardo is yearning for new data. He was one of 80 U.S. planetary scientists who signed a report in January urging NASA to make a mission to Europa a priority.
The U.S. space agency has tentative plans to launch the nuclear-powered Jupiter Icy Moons Orbiter in 2015, four years later than originally planned. Technical challenges may further delay or entirely scrap the mission.
"Europa is such a high priority for exploration that it shouldn't wait," Pappalardo said.
Putting off a mission to Europa, he said, would be as if NASA had ruled out all Mars exploration until the agency had developed the perfect technology for bringing back a Martian soil sampleinstead of proceeding with other types of Mars exploration in the meantime, as the space agency has done.
"It's the same with Europa. It's a high priority, and it doesn't seem prudent to wait for the ultimate mission when we should be doing that reconnaissance exploration in the shorter term," the planetary scientist said.
As a first step, Pappalardo said, NASA should send an orbiter to Europa to determine the characteristics of its ice shell, confirm the existence of an ocean, and analyze the chemistry of what appears to be dark organic matter on the moon's surface.
Later missions could include landers to search for life and potentially an underwater robot that could melt through surface ice to sample water below. "But that's hard to do, and it's a long time off," he said.
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