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February 18, 1995
CONTACT: Emil Venere
FEBRUARY 22, 1995

Scientists Find Jupiter Moon Has Oxygen Atmosphere

Scientists using the Hubble Space Telescope have detected a thin atmosphere of molecular oxygen on Jupiter's moon Europa, a finding that has important implications both for astronomy research and studies of Earth's atmosphere, which also contains molecular oxygen.

"Discovering another body in the solar system that contains oxygen in its atmosphere provides another way for us to learn more about our own atmosphere," said Doyle Hall, a Johns Hopkins University planetary scientist who led the team of researchers making the discovery.

"We want to understand the Earth's atmosphere in detail for the obvious reasons," he said. For example, scientists are monitoring the progress of ozone depletion and the effects of pollutants, including so-called greenhouse gases. "So we need to develop our atmospheric theories as much as we can. This gives us another atmosphere against which we can test those theories."

The research findings will be outlined in a scientific paper to be published in the Feb. 23 issue of the journal Nature. The observations indicate that Europa, which is about the size of Earth's moon, has an atmosphere with a surface pressure that is roughly one hundred billionth the surface pressure of the Earth's. But the wispy gas is still thick enough to be called an atmosphere.

Molecular oxygen is the same form of oxygen that we breathe on Earth. However, Dr. Hall stressed that the presence of molecular oxygen in the atmosphere of the frigid, ice-covered moon is not an indication that life exists there. Europa is far too cold, at 230 degrees Fahrenheight below zero (minus 145 degrees Centigrade), to foster the development of organisms.

"According to prevailing theories, for life to evolve liquid water must be present for a very long period, perhaps more than hundreds of millions of years," said Dr. Hall, a research associate in the Johns Hopkins Department of Physics and Astronomy.

Europa's surface is so cold that all of the surface water there is frozen as hard as rock. "If you tried to chip that ice with an ice pick you probably couldn't do it; it would be as hard as granite because it's so cold," he said.

Europa is only the fourth moon in the solar system known to have an atmosphere. None of the moons of the inner planets could be said to have true atmospheres, although most bodies in space, including Earth's moon, have thin envelopes of gas around them. If gas molecules near the surface of a body collide with each other many times before escaping into space, the gas is said to be "collisionally thick," and it is considered to be a true atmosphere.

The three other moons that possess atmospheres fitting that description are Jupiter's first moon, Io, Saturn's largest moon, Titan, and Neptune's largest moon Triton. But none of them contains detectable quantities of molecular oxygen. Io's atmosphere is driven in part by volcanic eruptions and is largely sulfur dioxide. The atmospheres of Titan and Triton are mostly nitrogen, with some methane.

The discovery ends a 23-year effort by planetary scientists to find oxygen gas above large icy bodies in space, Dr. Hall said. Just finding oxygen at all is no easy task, considering that Europa is about 490 million miles (780 million kilometers) from the sun, about five times farther than the Earth is from the sun.

"I consider myself lucky because, in many of these kinds of astronomical search programs, you just don't see anything at all," Dr. Hall said. "That's just the way science is."

Other members of the research team are Paul Feldman, a professor in the Johns Hopkins Department of Physics and Astronomy; Darrell Strobel, a professor in the Department of Earth and Planetary Sciences; and astronomers Melissa McGrath and Hal Weaver, at the Space Telescope Science Institute in Baltimore.

The team used the Hubble Space Telescope's Goddard High Resolution Spectrograph to observe Europa's atmosphere. The spectrograph detected two wavelengths of ultraviolet emission in a specific ratio bearing the fingerprint of molecular oxygen. The observations were conducted in June 1994, over a period during which the Hubble Space Telescope orbited around the Earth six times -- a time span that is roughly equivalent to one night's observing with a ground-based telescope.

But no ground-based telescope could have made such observations since ultraviolet light is absorbed by Earth's atmosphere before it reaches the ground, and the oxygen gas in Earth's atmosphere would have interfered with attempts to detect it anywhere else in space. Previously, no ultraviolet instruments in space have been sensitive enough to detect Europa's wispy gas, but scientists are planning to make detailed observations using instruments aboard NASA's Galileo space probe, which will rendezvous with Jupiter and its moons in December 1995.


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