Putting the Pieces Together
"What we find in these pictures," says Michael Carr of the U.S.
Geological Survey in Menlo Park, Calif. and lead author of one of
the articles, "is pretty good evidence there is water at fairly
shallow depths."
From various
observations, both from
Earth and passing
spacecraft, scientists have
concluded that Europa is
covered with a layer of
water probably 50 to 100
miles thick, surrounding a
rocky core. The surface is
undeniably solid ice, though
fractured and blistered into a
ball-of-string appearance.
The question: is there,
beneath the ice, any liquid
water? The friction caused
by the gravitational pull of
Jupiter, as well its sibling
moons of Io and Ganymede,
might be enough to heat
Europa's inside.
Another tantalizing
observation: Europa's
surface, unlike the other
moons of Jupiter, is relatively
clean of craters. Perhaps
eruptions of water from
below had filled them in.
Carr's group examined in
detail one particular slice of
Europa's surface. Galileo's
images, some 40 times more detailed than the Voyager
spacecraft had taken 17 years earlier, allowed the researchers to
piece together the fractured patterns almost like a jigsaw puzzle,
seeing how the pieces used to fit together.
"These icy crusts have broken apart into rafts that have
moved, rotated and tilted," Carr says. "Everywhere we look,
we're finding evidence for this kind of breakup. This is a pretty
good indication there is a mobile layer down there."
Ice Flows, Too
Another group, headed by Robert Pappalardo and James Head
of Brown University in Providence, R.I., assert in a separate
article in Nature that while there may once have been water
below, it has now frozen. As evidence, they point to dome-like
structures, some 5 miles wide, that dot the surface of Europa.
These domes, they say, are much younger than the cracks and
fissures.
"We see evidence," Head says, "that Europa has undergone a
change in its tectonic evolution."
In this picture, beneath the icy crust is yet more ice, but
warmer and able to stretch, twist and flow much as glaciers do
on Earth. The warmer ice (still heated by tidal friction), would
expand and push upwards towards to the surface. "Rising like a
lava lamp, essentially," Head says.
A Sliding Surface
Paul Geissler, a senior research associate at University of
Arizona's Lunar and Planetary Laboratory, and his colleagues
find indirect evidence that the outer crust is rotating slightly
faster than the rocky interior. Europa circles Jupiter every 3½
days and rotates at the same rate, appearing to keep the same
side facing Jupiter at all times.
However, looking at how the direction, number and age of the
fractures on Europa's surface, Geissler's group concluded that
the crust is slightly out of sync. The side
facing Jupiter changes
very slowly, a revolution taking somewhere between 10,000
years and several tens of millions of years.
Other measurements of Europa's gravity hint that the
rotation
of the rocky interior is precisely locked into the
3½-day orbit. A
completely solid Europa would rotate as one piece. For the icy
crust to slide over the inner core requires the equivalent of
planetary motor oil in between.
"It doesn't actually prove there is an ocean on Europa,"
Geissler says, "but it is consistent with that." But,
he adds, it's
also consistent with the ice-flow model of Head and Pappalardo.
"I think there is absolutely compelling evidence
that beneath
the surface there is a material easily deformed," comments
Steven Sqyres, an astronomer at Cornell University in Ithaca,
N.Y. "It is stuff that flows."
Galileo is now beaming back its latest Europa photos taken
during a December encounter, but conclusive evidence about
what lies below the surface will probably have to await
follow-up spacecraft carrying ice-penetrating radar or a laser
altimeter to precisely measure the size of the tidal
bulges. (Water
would bulge more than ice.) Planetary scientists hope that one
day a probe will land on the surface and poke into the ice. Such
missions have been discussed, but not yet approved.
Search for Life
If the answer is indeed liquid water, then comes the second half
of Hoagland's assertion: life in the ocean. Europa
is much too far
away for sunlight to warm it. But it is conceivable that
the tides
could rub the rocks hot enough to give rise to underwater
volcanoes.
And that environment would resemble the volcanic rifts that
lie at the bottom of the Earth's ocean, an environment now
known to teem with exotic organisms.
"That's the big question," Sqyres says. "We don't know the
answer to that. The answer is `maybe.'"
NASA is now drawing up follow-up missions to Europa,
probably first an orbiting spacecraft followed by
another that will
land and poke directly into the ice.
Few listen when Hoagland talks about the face on Mars. But
fish on Europa? That's a creditable, if fanciful, possibility.