New evidence from a fly-by of Jupiter's moon Europa has yielded the most compelling evidence yet that a vast ocean of water lies beneath the moon's ice-covered surface, UCLA researchers report today.
Using data from the Galileo space probe, the scientists detected a strong magnetic field under the moon's surface, which they said can be accounted for only by the presence of salt water.
Space scientists have been searching fervently for evidence of water on other planets and moons because they believe life can exist only in its presence. Finding water, as opposed to ice, on Europa is "a good first step" toward showing the existence of life there, said geophysicist Margaret G. Kivelson of UCLA, lead author of the paper appearing in today's Science.
"It's a long way from finding water to finding life, but it certainly makes it a more interesting possibility," she said.
The finding also increases the urgency of returning to the Jupiter system with a more sophisticated spacecraft.
"We now have a strong case that Europa has an ocean, and that supports the value of a future mission to Europa, which is planned but not likely to happen soon," said geologist David J. Stevenson of Caltech. He added that "it's still a longshot" that life exists on the icy satellite of Jupiter.
San Diego researchers reported in June that pictures from Mars hinted at the possibility that water may still exist below the surface of that planet, and space scientists have suspected the existence of water on Europa. But to date, the only place in the solar system where water (as opposed to ice) is definitely known to exist is on Earth.
The best previous evidence that water exists on Europa is photographs of its icy surface, which show cracks, breakage, folding of ice sheets and other characteristics of a highly active surface. Those features could be caused by tides in an ocean under the ice.
Those features "are consistent with an ocean," said Stevenson, "but they could also be explained by [extremely] cold, brittle ice sitting on top of [somewhat warmer] ice."
The new data point far more strongly to water.
The data come from a magnetometer on the Galileo probe, which flew past Europa in January. Previous fly-bys with the magnetometer had hinted at the presence of an electrical current flowing beneath the moon's surface, generated by changes in Jupiter's magnetic field.
If there was, in fact, such a current, Kivelson said, theory predicted that the location of Europa's north magnetic pole--normally on the moon's equator--would move around.
"Working with JPL, we were able to arrange another pass by Europa" at a time when the pole should have been in a different location if the theory were correct, Kivelson said.
"The observations were completely consistent with the prediction," she added.
The results showed that there is a layer under the surface of the moon conducting an electrical current and stretching all around it. Ice itself is not sufficiently conductive to account for the results.
"The only reasonable explanation for a layer of the required conductivity is salty water," Stevenson noted. "A global layer of water with a composition similar to Earth sea water and a thickness greater than about [6.2 miles] could explain the data."
Stevenson added that "if you are willing to speculate wildly, you could come up with some other possibilities," such as ice grains coated with graphite or a thin sheet of copper. "But they don't seem to make much sense."
Kivelson's group estimates that the water, if present, is nearly five miles under the moon's icy surface.
Though compelling, the results are not conclusive, Kivelson said. Researchers will not be sure until they are able to send another probe that will orbit Europa and provide a much greater amount of evidence. Such a craft could, for example, measure how much the level of the surface is altered by tidal forces, which would provide a good indication of whether water or warm ice lies beneath it.
Such a mission has been in NASA's plans for several years, but budget problems have moved it back. Now, the earliest the probe could be launched is 2008, Kivelson said, and another two years would be required to reach its destination.
That's a long time to wait for confirmation, she conceded. "But I started working on Galileo in 1977. I tend to take the long view."