By Jove, Water on Europa?
by Kenneth Chang
Beneath the icy surface of Jupiter's moon Europa, something appears to be flowing. And if that something is liquid water, could there be any extraterrestrial fish swimming in there? Science writer Richard Hoagland first suggested the idea of oceans on Europa and life within them in a 1979 Sky and Telescope article, which in turn inspired a major portion of Arthur C. Clarke's 2010, the sequel to 2001: A Space Odyssey. Hoagland has since graduated to greater notoriety championing theories about the supposed face on Mars and conspiratorial NASA cover-ups. Meanwhile, the ocean half of Hoagland's Europa hypothesis has entered the mainstream of scientific debate. This Thursday's issue of the journal Nature includes a quartet of articles that examine images taken in the past year by NASA's Galileo spacecraft.
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.