Oxygen atmosphere found on Jupiter's 'ice moon'

THE BALTIMORE SUN

A Johns Hopkins University astrophysicist has discovered an oxygen atmosphere on Europa, the "ice-moon" of Jupiter. But don't pack your bags.

It's 230 degrees below zero out there, 483 million miles from the sun. And the atmosphere at Europa's surface is as wispy as Earth's 150 miles above the ground, near where the space shuttle flies.

"From an existential point of view, it's surprising to think there is another place with substantial quantities of the stuff we breathe," said Dr. Doyle Hall, 32, the Hopkins research associate who discovered the Europan oxygen in June using the Hubble Space Telescope's Goddard High Resolution Spectrograph.

But the find really has no significance in the search for extraterrestrial life. "It's just a coincidence," he said.

Europa's oxygen is believed to come from the breakdown, under bombardment from space, of water ice that covers its surface. Earth's green plants are credited for most of the oxygen that makes up 21 percent of its atmosphere.

"The most important thing is that this [discovery] tests out atmospheric theories. It pushes the envelope of our understanding of how atmospheres work," he said. "This is an interesting atmospheric find, and a lot of atmospheric scientists are going to be very excited about it."

The discovery was to be reported today in the science journal Nature.

Dr. Donald E. Shemansky, director of space and planetary studies at the University of Southern California, said that knowing how bombardment from space by charged particles breaks down water ice on Europa can help scientists understand how fast similar bombardment might erode solids elsewhere, such as the rings of Saturn.

"From that point of view," he said, "this is a rather important observation."

He said there may be some questions raised about whether the observed oxygen has been "dumped into an atmospheric environment, or whether it is something that is still included in the solid. Either way, it is still a very, very interesting observation."

Europa is one of Jupiter's 16 known moons, one of the four discovered by Galileo in 1610. It is nearly 2,000 miles in diameter, about the size of Earth's moon.

Dr. Hall said scientists have theorized for decades that molecular oxygen might accumulate above icy surfaces in space. Europa became a candidate in 1980, after the Voyager spacecraft revealed it to be covered by water ice, laced with dark lines believed to be cracks. No atmosphere was detected then.

Dr. Hall's discovery makes it only the fourth moon in the solar system known to have an atmosphere. The others are:

* Io, another moon of Jupiter, with an atmosphere of sulfur dioxide generated by volcanic activity.

* Titan, a moon of Saturn, with a dense atmosphere of nitrogen and methane.

* Triton, a moon of Neptune with a very thin atmosphere of nitrogen and methane.

Dr. Hall believes Europa's oxygen originates in water vapor liberated by the bombardment of its surface ice by electrons and other charged particles flowing through Jupiter's powerful magnetic field, and by micrometeorites. Some ice also may be converted directly to water vapor in a process called sublimation.

The water vapor is then broken down by sunlight and other charged particles into its constituent elements -- hydrogen and oxygen. The hydrogen, being lighter, flies off into space. Heavier oxygen molecules, however, remain and accumulate above the surface, forming a tenuous atmosphere.

"We can't rule out the existence of other constituents, but we have only detected oxygen," Dr. Hall said.

Finding an oxygen atmosphere on Europa "tells us a little about how much water is out there in the outer solar system," Dr. Hall said. "It might tell us that some oxygen [atmospheres] can be created through non-biological processes."

But greater significance may lie closer to home. Faced with evidence of atmospheric changes triggered by man's own activities, scientists are working hard to test their theories about how atmospheres really work.

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