Geotimes - January 2002 - Planetary Oceans Geotimes
News Notes

Planetary Geology
Oceans beyond Earth

The ocean hidden under Europa’s icy surface may be the most common type in the universe. Accumulated evidence shows “hidden oceans” are so pervasive in our solar system as to make them “almost unavoidable,” says David Stevenson of the California Institute of Technology’s Geological and Planetary Division.

[At right, Pwyll crater on Europa. Image produced by Elizabeth Turtle/PIRL/LPL/UA ]
 
Stevenson described how ice-covered oceans are probably much more common than Earth’s predominately open ocean,during his presentation at the Geological Society of America meeting in Boston on Nov. 7.
His talk, “Water’s Many Forms in the Solar System,” synthesized recent evidence for subsurface bodies of water in the outer satellites. Stevenson was also one of several planetary scientists who discussed evidence for oceans in the solar system during the December annual meeting of the American Geophysical Union (AGU) in San Francisco.


The evidence centers around Jupiter’s largest moons — particularly Europa, whose surface is completely covered with cracked water ice. During Galileo’s flyby of Europa in January 2000, the spacecraft’s magnetometer detected a change in the direction of the satellite’s electromagnetic poles from that detected on previous passes. The change in orientation — which Margaret Kivelson et al. reported in the Aug. 25, 2000, issue of Science — suggests that the electromagnetic field is not permanent, but instead is induced by Europa’s elliptical orbit through Jupiter’s powerful magnetosphere. An induced field would require some type of conducting layer underneath but near the surface of the satellite.
 
In this case, Stevenson says, “the most likely explanation is that Europa has a salty, global water ocean beneath its ice shell.” Others have offered explanations for the field, but Stevenson says the magnetic induction signal is “remarkably strong” evidence for an ocean, even though we can’t see it directly. “It is an argument by elimination — nobody has thought of a physically sensible alternative conductor.” He adds jokingly: “Maybe we just haven’t been imaginative enough.”
 
Such an ocean would be near the freezing point of water, and enclosed above by ice and below by ice or rock. It could be anywhere from 10 to 100 kilometers thick depending on its salinity. Like Earth’s ocean, it would be water-dominated and would contain an electrolyte that conducts the electromagnetic field, though this would not necessarily be the same type of salts as an ocean-snorkeler tastes on Earth.
 
Unlike Earth’s ocean, Europa’s is both global, encircling the entire satellite, and completely covered by the surface ice, which is itself probably 20 kilometers thick. The ocean is kept liquid in part by “tidal heating,” in which the satellite’s solid material is flexed by its irregular orbit, causing internal friction. “Europa’s case may be special,” Stevenson wrote in Science, “because the tidal heating may allow liquid water to get closer to the surface, possibly including occasional eruptions or flows.”
 
Odds are also good that hidden oceans exist elsewhere, even within our own solar system. Ganymede, Jupiter’s largest moon, and Callisto, its second largest, are two candidates. “They seem to show strong evidence for a liquid layer,” Torrence Johnson, a scientist at NASA’s Jet Propulsion Laboratory, said during the AGU meeting. These oceans would probably be thinner than Europa’s and kept liquid primarily by internal radioactive heat. Pluto, as well as Neptune’s moon Triton, could also house oceans. “Those perhaps are more likely candidates than we had thought for having liquid water,” Johnson said. Also during AGU, Ralph Lorenz of the University of Arizona’s Lunar and Planetary Science Lab presented work suggesting that Saturn’s moon Titan could host expanses of liquid hydrocarbon on its surface. The Cassini/Huygens mission planned for 2004 should collect more data about Titan.

 “Defined broadly enough, oceans may not be that rare,” Stevenson concludes, but odds are they’re covered with ice, making Earth’s surface oceans rare in comparison. Our oceans exist largely because Earth’s distance from the sun, combined with the greenhouse effect of our atmosphere, creates the ideal — and rare — surface temperature for water to exist as a liquid. “We have come to appreciate that a liquid-water-dominated layer is not that unusual,” Stevenson says, but that, relatively speaking, “the situation on Earth is. Earth itself may have had a frozen-over ocean at some point in the past.”
 
Since life as we know it requires liquid water, Stevenson wrote in Science, “after Mars, Europa remains the most attractive extraterrestrial environment within our solar system in which to seek evidence of past or present life.” Who knows what form it will take —cut off from solar radiation, it might depend on volcanism for energy. Regardless, Stevenson writes, “it is conceivable that these are the most common sites of life in the universe.”

“The possibility of a liquid water ocean at depth on these icy satellites is very exciting and intriguing and may help to explain a host of unusual properties that have been observed on their surfaces,” says Jim Head of the Department of Geological Sciences at Brown University. “We constantly have to broaden our horizons beyond our experiences here on Earth in order to overcome prejudices based on a terracentric view. By comparison, this new perception seems to be telling us that the environment that characterizes our own home planet is rather unusual.”


Julian Smith
Geotimes Contributing Writer


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