Jovian Moon Yields Oxygen But Not Life By Emil Venere Scientists at Hopkins and the Space Telescope Science Institute have discovered that the Earth isn't the only body in the solar system with oxygen molecules in its atmosphere. Jupiter's moon Europa has a thin atmosphere containing oxygen, according to a paper published Feb. 23 in the journal 'Nature.' The discovery has important implications for both astronomy research and studies of Earth's atmosphere. But the revelation does not suggest the presence of life on the frigid, ice-covered Jovian satellite. Europa is far too cold, at 230 degrees Fahrenheit below zero, 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 Doyle Hall, a Hopkins planetary scientist who led the team of researchers making the discovery. 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," said Dr. Hall, a research associate in the Department of Physics and Astronomy. However, the finding is immensely important for research purposes. "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," Dr. Hall said. "We want to understand the Earth's atmosphere in detail for the obvious reasons." 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 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. 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. Detecting the oxygen was 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 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. 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. Scientists can't tell whether the atmosphere contains other gases but more detailed observations are planned with NASA's Galileo space probe, which will rendezvous with Jupiter and its moons in December 1995. The observations were conducted in June 1994, over a period during which the 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. Up until now, no ultraviolet instruments in space have been sensitive enough to detect Europa's wispy gas.
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