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."
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