If the icy surface of Europa conceals a liquid ocean, which seems increasingly likely, then the Jovian moon will become one of the hottest spots in the solar system to look for alien life.
Europa Orbiter, a NASA mission in the early planning stages, that is scheduled for launch in 2003, is being designed specifically to look for evidence of a Europan ocean. If one is found, Europa and Earth would be the only two worlds in the solar system where liquid water is known to exist. And liquid water is thought to be essential for the development of life.
Christopher Chyba ‚ the Carl Sagan Chair for the Study of Life in the Universe at the SETI Institute and a consulting professor of geological and environmental sciences at Stanford ‚ chairs the science definition team for the mission. At the American Geophysical Union meeting in San Francisco, he summarized the current evidence for an ocean on Europa and described the instrument package that his team has proposed for the next Europa mission.
Europa looks something like a cracked cue ball. The possibility that a liquid water ocean may lurk beneath its ice crust was first raised at the time of the Voyager missions in the late 1970s, but was reinforced in 1996 when images of Europa's surface were beamed to Earth by the Galileo spacecraft. The images showed areas where the surface ice has been broken up and shifted around like pieces of a jigsaw puzzle, leading Ronald Greeley from Arizona State University to propose that the Europan icebergs must be lubricated from below by warm ice or liquid water.
Since then, "there has been a convergence of evidence that supports the existence of a liquid ocean on Europa," Chyba said.
• In addition to the iceberg-like areas, Galileo imagery has revealed an impact crater that appears to have been filled in at the bottom, areas that appear to show localized melting near the surface, and other features consistent with a liquid layer below the ice;
• Galileo's onboard magnetometer, which measures magnetic fields, has measured fluctuations that are consistent with the magnetic effects of currents flowing in a salty ocean;
• Lack of cratering on Europa's surface indicates that it is very young ‚ less than 10 million years ‚ which suggests that it is being continually resurfaced, possibly by frost falling from liquid water geysers encountering Europa's frigid surface temperatures, which hover at -170 degrees Celsius;
• Theoretical estimates of the amount of heat produced by the gravitational push and pull exerted on Europa by the other Jovian moons indicate that it should be adequate to warm the moon's interior enough to sustain a liquid ocean.
"All these lines of evidence point to a liquid water ocean," Chyba said.
The investigations that the science definition team has suggested for the proposed $250 million Europa Orbiter include imaging, altimetry, gravity measurements and subsurface radar soundings.
"The most decisive measurements are likely to come from the altimetry and gravity measurements," Chyba said.
As Europa travels in a slightly eccentric orbit around Jupiter, tides are raised, similar to the lunar tides on Earth. If the distant satellite contains a deep ocean covered by the thin ice crust, then the tidal movements should be fairly large, producing a 30-meter rise and fall each 3.5 days. But if the moon is solid ice the deformation would be only a meter or so. The altimeter and gravity measurements independently would measure this effect.
These measurements should be definitive for the case of a global ocean, but would be more difficult to interpret if the liquid layer takes the form of a number of discontinuous seas, Chyba said.
In that case, a radar sounder might provide the needed data. Radar is routinely used to sound ice on Earth. That is how Lake Vostok ‚ a body of water about the size of Lake Ontario buried under 3,700 meters of ice in Antarctica ‚ was discovered. A clean interface between water and ice can be seen clearly in radar reflections. Depending on the consistency of the Europan ice, a radar sounder should be capable of penetrating somewhere between a kilometer and several kilometers into the crust.
"Even if the radar sounder did not find clear evidence of liquid water, it would still provide us with extremely valuable information about the subsurface geological features," Chyba said.
Another possible instrument is an infrared spectrometer. Such a device could provide information about the chemical composition of Europa's surface, including the presence of organic molecules.
The decision on which instruments the orbiter will carry will be made next year. The selection will be particularly difficult because the spacecraft will have an extremely small payload of about 20 kilograms, he said.
"If the orbiter confirms that Europa has a liquid ocean, then it will become one of hottest places in the solar system, along with Mars, to search for life. In this case there will be an entire program of exploration, likely involving a series of spacecraft to Europa." But if the Moon does not conceal such an ocean, then it will move down significantly on the space agency's priority list, he said.
By David F. Salisbury
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