A 20-year-old mystery about one of Jupiter's moons may lead mankind closer to the first ocean of liquid water outside Earth and perhaps to extraterrestrial life.
The mystery concerns hundreds of linked arcs and curved lines that crosshatch the icy surface of Europa, Jupiter's second moon. Called "flexi," these formations are found nowhere else in the solar system and have baffled scientists since Voyager sent photos of the frozen moon in 1979.
But a team from the University of Arizona has proposed a novel explanation: The markings are the result of massive tidal swings in a liquid ocean of water beneath the surface. In an article published today in the journal Science, the researchers outline how 98-foot tidal swings, could exert enough force to crack Europa's ice sheath in arcing patterns.
If the tidal theory bears out, it would be the strongest evidence yet that Europa harbors liquid water. More important, it would provide further evidence that Europa has all the crucial preconditions for the development of life. "You have water. You have energy sources. You have an influx of carbonaceous materials from meteorites that could bring about life somewhere in the subsurface. It's a tantalizing prospect," says David Senske, a scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., who works with images of Europa.
These days, finding liquid water is all the rage among astronomers who hope to learn about everything from how water affects plate tectonics and planetary formation to the role water plays in the evolution of life.
This summer, NASA researchers shot a probe into the moon in a fruitless search for lunar water. Past missions to Mars have likewise sought out liquid water. And one of the primary purposes of the Galileo mission is to seek out evidence of water among other planets - in particular, among Jupiter's four major moons.
But evidence is mounting that Europa is indeed blessed with such liquid assets. Precise measurements of how Europa's gravity affects Galileo's trajectory have given scientists reason to believe the orb consists of a dense center surrounded by a lighter band of matter that may be liquid.
Spectroscopic readings of Europa's material composition indicate large quantities of water, including regions of salinity on the surface. These saline spots are most prevalent near iced-over ruptures that could be the remnants of meteor impacts. This concentrated salinity implies that a briny substance - like salt water - was thrown to the surface by some sort of explosive force.
Furthermore, some ice formations look like the polar caps of Earth, complete with what appear to be icebergs and tilted blocks of ice that appear to be the result of collisions between ice floes.
So the University of Arizona scientists had good reason to suspect there is liquid water on Europa when they began trying to explain the flexi.
"We made a few assumptions as far as what type of stress it would take to break the ice and the speed that crack would have to propagate," says Gregory Hoppa, one of the authors of the paper. "We plugged those parameters into our model and it was pretty amazing how a cycloidal shape just popped right out of it."
Further computer modeling by Dr. Hoppa and his colleagues confirmed that many of the icy lines could theoretically be explained by tidal shifts brought about by Jupiter's strong gravitational pull.
Questions about the veracity of the tidal theory still remain. Researchers have not yet been able to document the development of any new flexi patterns over time. New patterns would indicate that tides are still active. A lack of new flexi might suggest that a liquid-water ocean was once there but is now frozen solid.
"There is that nagging feeling that we really should be seeing changes," says Jeffrey Kargel, a US Geological Survey geologist who calls the tidal hypothesis intriguing but not yet compelling.
Even should there be a liquid ocean, there is no guarantee that life exists beneath the ice cover, which is likely at least a half mile thick. Moreover, the Europa environment has little or no oxygen and is exposed to sizzling radiation from Jupiter that would fry an astronaut.
"On Earth, the biosphere at the surface of the planet completely relies on photosynthesis. That's almost certainly not available on Europa," says Chris Chyba, a planetary scientist at the Search for Extraterrestrial Intelligence Institute, based in Mountain View, Calif. "Even in pure ice, the sort of wavelengths you need to drive photosynthesis will not penetrate 15 meters of ice,"
These questions will likely be answered more conclusively as researchers study images of Europa further to try to pinpoint precise changes in the surface patterns over time. Failing that, the question will be definitively settled when a planned NASA orbiter reaches Europa in 2008, equipped with a laser altimeter that can measure the exact rise and fall of the moon's surface. Should the level of Europa's surface rise much less than 98 feet, that would indicate that most of the subsurface is frozen.
But prospects are looking brighter than ever for an oceanic first. "We think there is a fairly good chance there is liquid water under the surface," says Hoppa.
Should the existence of a liquid ocean be confirmed, then NASA will likely send a landing craft to take an even closer look. "It sure would be exciting if we could go into that ocean if the ocean exists," says Dr. Kargel. "Who knows what we would find there? Maybe an organism."
(c) Copyright 1999. The Christian Science Publishing Society