Jupiter's giant moon Europa is one of the few places in our solar system where scientists believe there is a reasonable chance that life has made a home. An ice-covered world with a vast frigid ocean beneath, Europa will not be an easy place to explore.

If there is life there, it's likely to be in the ocean, and although the moon's surface may hold clues to what lies below, making a comprehensive plan to search for life on Europa means figuring out how to probe its watery depths.

NASA's ENDURANCE project - the acronym stands for Environmentally Non-Disturbing Underwater Robotic Antarctic Explorer - is a step in that direction.

Funded by the agency's ASTEP (Astrobiology Science and Technology for Exploring Planets) program and headed by Peter Doran, a professor of earth and environmental sciences at the University of Illinois at Chicago, ENDURANCE has completed the first of two field seasons exploring the ice-covered Lake Bonney in Antarctica's McMurdo Dry Valleys. Antarctica's ice-covered lakes, said Doran, "can be used as models of an ice-covered ocean on Europa on a much smaller scale."

Although scientists have studied Lake Bonney for years, their efforts have consisted of annually boring a small hole in the ice at various points in the lake and dropping a sensor or two into the water below. The lake stretches for 1.5 kilometers (just under 1 mile); at its widest point it is about half a kilometer (a third of a mile) wide.

With so little data about such a large lake, Doran said, it was all but impossible to develop a dynamic understanding of its underwater environment.

The ENDURANCE autonomous underwater vehicle (AUV), designed and built by Bill Stone and his colleagues at Stone Aerospace in Del Valle, Texas, has enabled scientists to begin to construct a more-comprehensive, three-dimensional understanding of the lake.

"The ENDURANCE vehicle was unbelievably useful in seeing parts of the lake we'd never seen before and looking at it in a completely different way," Doran said.

Each day for a month, ENDURANCE dipped down through a hole in the ice in the middle of the lake, and following preprogrammed instructions, traveled to a different part of the lake.

At regular intervals it would stop, lower its instrument package of temperature, chemical and biological sensors to various depths and record the results. At the end of the day, it would return to the hole and rise to the surface, where it would be fished out by a crane and plugged in for an overnight battery recharge.

Although engineers maintained a fiber-optic connection to the vehicle that enabled them to "watch what it's doing and watch the decisions it's making," Doran said, the vehicle operated entirely on its own, without human intervention. "When it got close to the hole was the most impressive part to me.

It had an upward-looking camera that would find the hole. It would center itself on the hole, and then ride up without touching the edges at all."

Logistics were a major challenge for the ENDURANCE team. The nearly spherical submersible is about 2 meters (6.5 feet) in diameter and in air weighs more than 1300 kilograms (about 1.5 tons). Before Doran's team could deploy it, they first had to melt a hole in the lake's 4- to 5-meter-thick ice cover, a hole large enough to lower ENDURANCE through.

That task alone took 3 days. Then, centered around the hole, they had to construct the "Bot House," a fabric-covered temperature-controlled workshop, complete with doors and windows, an electrical system and a floor strong enough to support both ENDURANCE and the crane used to lift it.

Nature also threw a couple of curve balls at the team. Unseasonably warm weather was one problem. Taylor Glacier rises steeply from the west end of Lake Bonney, the tip of the glacier extending down below the surface of the lake.

One of the goals of the project is to create a photomosaic of the underwater face of the glacier, which has never before been studied. But the warm weather caused a torrent of water to gush from the face of the glacier into the lake, causing the lake water to turn from crystal clear to nearly opaque.

"Because of the murkiness we couldn't put together the photomosaic," Doran said. He plans to begin the underwater glacier-imaging work earlier in the next field season, before the glacial runoff begins.

Another problem was bubbles. High levels of saturated gases in the lake water caused "microbubbles [to] form on surface on the AUV, and they change over time as it's swimming, and so that really wreaks havoc with the buoyancy of the vehicle," Doran said.

"The bubbles would make it lighter," so EDURANCE "had to use a lot of battery power to thrust itself down."

Doran and his colleagues had some familiarity with the Lake Bonney environment from previous work, but "as smart as we all were, we didn't predict this microbubble effect, and that we would have to come up with some way to overcome buoyancy," Doran said.

"You can't predict everything. It really has impressed upon me how difficult and risky it is."

Doran said the "take-home lesson" for any future mission to Europa is, "You need adaptability." Practicing in Antarctica, where engineers are standing by, ready to make needed tweaks to hardware and software, is one thing. But a robotic probe, hundreds of millions of miles from Earth, submersed deep within the ice-covered ocean of another world, Doran concluded, "is only as good as what you put on it."