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1996-01-12
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Contact: Ray Villard/STScI FOR RELEASE: February 23, 1995
(410) 338-4514
PRESS RELEASE NO.: STScI-PR95-12
Dr. Doyle Hall/JHU
(410) 516-7338
HUBBLE FINDS OXYGEN ATMOSPHERE
ON JUPITER'S MOON EUROPA
Astronomers using NASA's Hubble Space Telescope (HST) have identified
the presence of an extremely tenuous atmosphere of molecular oxygen
around Jupiter's moon, Europa. This makes Europa the first satellite
ever found to have an oxygen atmosphere, and only the third such solar
system object beyond Earth (the planets Mars and Venus have traces of
molecular oxygen in their atmospheres).
This detection was made by a team of researchers at the Johns Hopkins
University and the Space Telescope Science Institute, both in
Baltimore, Maryland, and is reported in the February 23, 1995 issue of
the journal Nature.
"Europa's oxygen atmosphere is so tenuous that its surface pressure is
barely one hundred billionth that of the Earth," says principal
investigator Doyle Hall, of Johns Hopkins. "It is truly amazing that
the Hubble Space Telescope can detect such a wispy gas so far away." If
all the oxygen on Europa were compressed to the surface pressure of
Earth's atmosphere, it would fill only about a dozen Astrodome-sized
stadiums.
The HST researchers caution that the detection should not be
misinterpreted as evidence for the presence of life on the small,
frigid moon. Located 5 times farther from the Sun than Earth, Europa
is too cold, measured at -230 degrees Fahrenheit (-145 degrees
Celsius), to support life as we know it.
Unlike Earth, where organisms generate and maintain a 21% oxygen
atmosphere, Europa's oxygen atmosphere is produced by purely
non-biological processes. Europa's icy surface is exposed to sunlight
and is impacted by dust and charged particles trapped within Jupiter's
intense magnetic field. Combined, these processes cause the frozen
water ice on the surface to produce water vapor as well as gaseous
fragments of water molecules.
After the gas molecules are produced, they undergo a series of chemical
reactions that ultimately form molecular hydrogen and oxygen. The
relatively lightweight hydrogen gas escapes into space, while the
heavier oxygen molecules accumulate to form an atmosphere which may
extend 125 miles (200 kilometers) above the surface. The oxygen gas
slowly leaks into space and must be replenished continuously.
Europa is approximately the size of Earth's Moon, but its appearance
and composition are markedly different. The satellite has an unusually
smooth and nearly craterless surface of solid water ice. Mysterious
dark markings crisscross the surface, giving the moon a "cracked
eggshell" appearance. Under the apparently fragmented icy crust, tidal
heating by Jupiter might heat the icy material enough to maintain a
subsurface ocean of liquid water.
Of the 61 identified moons in the solar system, only three other
satellites are known to have atmospheres: Jupiter's volcanically
active moon Io (sulfur dioxide), Saturn's largest moon Titan
(nitrogen/methane) and Neptune's largest moon Triton
(nitrogen/methane).
Although scientists had predicted previously that Europa might have
gaseous oxygen, a definitive detection had to wait for the ultraviolet
sensitivity provided by HST's Goddard High Resolution Spectrograph
(GHRS) instrument. The GHRS recorded the spectral signature of
molecular oxygen (O2) on Europa in ultraviolet light during
observations made on June 2, 1994, over a period of six Hubble
orbits. Europa was at a distance of 425 million miles (684 million
kilometers) from Earth.
The Hubble observations will be invaluable for scientists who are
planning close-up observations of Europa as part of NASA's Galileo
mission, which will arrive at Jupiter in December 1995. During its
initial entry into the Jovian system on Dec. 7, Galileo will fly by
Europa at a distance of less than 22,000 miles (35,000 kilometers).
* * * * * * * *
The Space Telescope Science Institute is operated by the Association
of Universities for Research in Astronomy, Inc. (AURA) for NASA, under
contract with the Goddard Space Flight Center, Greenbelt, MD. The
Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency (ESA).