Here Be Dragons

THE INCREDIBLE SHRINKING MARTIANS
searchers, could have generated the current surface topography on
Europa. What is not quite so certain is whether that ocean still exists
today, or whether we are observing the petrified remains of a body of
water that froze solid long ago. Because the surface generally looks
very pristine, with very few impact craters, the majority opinion is that
the ocean still exists.
Evidence in support of this conclusion was provided in 1998 by a
uciA/jpL/Caltech group that analyzed magnetometer readings obtained
by the Galileo spacecraft. The researchers found that Europa, as
well as another Jovian moon, Callisto, distorted Jupiter's magnetic
field in a fashion best explained by the presence of liquid saltwater
oceans on both moons. 31 Current estimates are that Europa's ocean is
about 200 kilometers deep and is capped by i to 10 kilometers of ice—
about the same thickness of ice that covers Lake Vostok in Antarctica
(see Chapter 2).
Several methods could be used to further study these potential
oceans. Ground-penetrating radar could detect liquid water beneath
the ice. Repeated imaging could detect changes in the seascape as the
ice rafts move. And laser altimetry could detect the up-and-down motion
of the icy surface with the ebb and flow of tides in the ocean—
tides that should be raised by Jupiter's enormous gravity. Future missions
will use some or all of these techniques to provide an answer.
Europa's ocean remains liquid for the same reason that Lake Vostok
remains liquid. The water is insulated by the thick layer of overlying
ice, so it can only very slowly lose heat to the surface. This loss is compensated
by geothermal heat coming from the interior. In Europa's
case, the geothermal heat is generated mainly by tidal heating, as massive
Jupiter flexes the body of its spinning satellite. It is quite possible
that some of this heat passes into the ocean via hydrothermal systems
similar to the deep-sea vents on Earth.
Of course, just possessing a water ocean doesn't mean that Europa
is actually inhabited. Chris McKay, for one, is quite skeptical. "It's the
next best bet to Mars," he says, "but it's not nearly as good a bet.
There's no light under that ice. The only mechanism that could work
is the chemosynthetic, hydrothermal one. But the good thing about
Europa is that it's so far away. It's probably never been contaminated
by life from Earth, or vice versa. If there's life there, it's probably a separate
Life."
The long-term goal is to explore Europa's ocean directly (see Color
Plate 3). Frank Carsey of JPL, who is one of the planners for this still-
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