Date: 24 February 1995
Subject: Hubble Finds Oxygen Atmosphere on Jupiter's Moon Europa
From: Jet Propulsion Laboratory
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 second moon, Europa. The planets Mars and Venus are the only
two other solar system objects beyond Earth known to 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,
and is reported in the Feb. 23 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," said Principal
Investigator Doyle Hall, of Johns Hopkins. "If all the oxygen on Europa
were compressed to the surface pressure of Earth's atmosphere, it would
fill only about a dozen Houston Astrodomes. It is truly amazing that
the Hubble Space Telescope can detect such a tenuous trace of gas so far
away."
Scientists had predicted previously that Europa might have an atmosphere
containing gaseous oxygen, but had to wait for Hubble's sensitive
instruments for confirmation. The HST researchers caution that the
detection should not be misinterpreted as evidence for the presence of
life on the small, frigid moon. Located 490 million miles (780 million
kilometers) from the Sun, Europa's surface 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).
The definitive detection of Europa's tenuous atmospheric oxygen was made
possible by 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 then 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 Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency.
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- T o d d E. V a n H o o s e a r -
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(_) Computer Laboratory - Department of Communication
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