Johns Hopkins Gazette: February 27, 1995

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
     "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
     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
     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
     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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