Donald Savage
Headquarters, Washington, DC          February 23, 1995
(Phone:  202/358-1547)  

Tammy Jones
Goddard Space Flight Center, Greenbelt, MD
(Phone:  301/286-5566)

Ray Villard
Space Telescope Science Institute, Baltimore, MD
(Phone:  410/338-4562)

RELEASE:  95-17

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.  

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       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.

                       - end -

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