Paula Cleggett-Haleim
Headquarters, Washington, D.C.                                            
October 2, 1992
(Phone:  202/350-1547)

Jim Elliot
Goddard Space Flight Center, Greenbelt, Md.
(Phone:  301/286-6256)

Ray Villard
Space Telescope Science Institute, Baltimore, Md.
(Phone:  410/338-4514)


RELEASE: 92-163

NASA'S HUBBLE TELESCOPE EXPLORES JUPITER'S VOLCANIC MOON IO

   Extended observations by the Hubble Space Telescope indicate 
that Jupiter's moon Io has a smaller atmosphere than previously 
thought with very dense regions possibly over volcanoes and 
surface frost, NASA announced today.

   The observations also show that despite continual volcanic 
activity, Io's surface has remained largely unchanged since 
first photographed by the Voyager spacecraft when it flew past 
the moon in 1979.

   The Hubble observations mark the first time astronomers have 
been able to directly gauge the size of Io's tenuous atmosphere.  
Sulfur and oxygen emissions from the atmosphere indicate that it 
is at least three times smaller than previously thought -- 1.5 
Io diameters across instead of the previous upper limit value of 
5 Io diameters.

   These observations show that the atmosphere may be patchy, 
with very dense regions having 1000 times higher pressure than 
adjoining, low-density regions.  Likely sources for the 
atmospheric gas are sulfur dioxide from the volcanoes, 
evaporation of surface frost in sunlit areas or material knocked 
out of the surface ("sputtered") into the atmosphere.  The 
observations also confirm that the surface contains sulfur 
dioxide frost.


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   Hubble's observations reveal a new oxygen emission never 
before detected from the torus, a giant ring of high-temperature 
gas encircling Jupiter.  From these measurements, the density 
and the amount of oxygen relative to sulfur in the torus have 
been determined.  Oxygen is the most abundant component of the 
torus with about twice as much concentration as sulfur.  

Moon's Effects On Jupiter

   Though no larger than Earth's Moon, Io affects the immense 
planet Jupiter on a grand scale.  Io's atmosphere feeds material 
to the torus encircling Jupiter at Io's distance.  Understanding 
Io's atmosphere is essential to understanding the plasma torus, 
which in turn affects Jupiter's immense magnetosphere and 
aurorae.

   Sulfur from the volcanic plumes cannot escape directly into 
space to fuel the plasma torus.  Instead, sulfur and oxygen 
might be stripped from Io's atmosphere in a complicated 
interaction between the atmosphere and the plasma torus. 

   Observations of Io's atmosphere were made with Hubble's Faint 
Object Spectrograph (FOS) by Melissa McGrath of the Space 
Telescope Science Institute (STScI), Baltimore; John Clarke, 
University of Michigan and Darrell Strobel, Gilda Ballester, 
Warren Moos and Paul Feldman of The Johns Hopkins University, 
Baltimore.  

   Io images were taken by Francesco Paresce, European Space 
Agency/STScI; Paola Sartoretti, University of Padova and 
co-investigators with the Faint Object Camera (FOC).  Additional 
images were taken by amateur astronomer Jim Secosky with 
Hubble's Wide Field/Planetary Camera (WFPC). 

   Earlier space probes revealed that Io has surface sulfur 
dioxide frost appearing as bright white patches in visible 
images and that Io has a very tenuous atmosphere, composed 
primarily of sulfur dioxide, with a surface pressure 1 billionth 
that of Earth's atmosphere.

   Scientific progress on understanding Io has been slow since 
the 1979 Voyager observations.  Io is so small and one-half 
billion miles from Earth, preventing ground-based telescopes 
from distinguishing Io's surface features.

   Because Io's atmosphere is primarily sulfur dioxide, 
atmospheric studies can be undertaken best at ultraviolet 
wavelengths.  These wavelengths cannot penetrate Earth's 
atmosphere for studies by ground-based telescopes.


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Io's Surface Unchanged Despite Volcanism

   To look for possible surface changes, researchers compared 
FOC visible-light images to a "synthetic" Voyager image modified 
to match Hubble's resolution.  The astronomers concluded that 
Io's trailing hemisphere, known to be more geologically active, 
has not changed noticeably in the 13 years  between Voyager and 
Hubble observations. 

   Detailed analysis of the images is still being carried out to 
search for less obvious changes.  Two small areas roughly 200 
miles across seem to have undergone slight change.

   This lack of large-scale change is mysterious because Io's 
volcanism should resurface the moon at a rate of a few inches 
per year.  One possibility is that there is a constant 
equilibrium between volcanic eruptions and unknown processes 
which might remove or cover volcanic debris.  This would 
preserve the general appearance of Io's surface over long 
periods.

   Io's surface looks remarkably different in ultraviolet (UV) 
light.  Regions which look bright in visible light are dark in 
UV.  The most likely explanation is that large areas of Io are 
covered with a sulfur dioxide frost.  Because sulfur dioxide is 
a strong absorber of UV radiation, sulfur dioxide-rich areas are 
dark in the UV and bright in visible light.  

   Dr. Paresce points out that there also are regions that are 
bright or dark in images taken at both wavelengths.  This 
suggests that the size of sulfur dioxide grains also may play a 
role in brightness.  The reflectivity of sulfur dioxide is very 
sensitive to the grain size at ultraviolet wavelengths. 

   Amateur astronomer Jim Secosky made near-infrared images 
(7100 Angstroms) of Io which complement the FOC images by 
providing new constraints on Io's surface composition.  Some 
models predict the presence of basalts and polysulfur oxide on 
the surface.  But these dark compounds do not show up in 
Hubble's longer wavelength images.  This further supports the 
model for Io's surface being predominantly sulfur and sulfur 
dioxide. 

   Secosky took HST snapshots of Io emerging from Jupiter's 
shadow to look for evidence of frost evaporation which might 
have formed on Io while it was chilled behind Jupiter.  This 
would have been evident if Io was 10 percent brighter than while 
emerging from eclipse. 

   Secosky did not see any evidence of the "post-eclipse 
brightening" phenomena which have been reported occasionally by 
ground-based observers since 1964.  Secosky thinks his negative 
results mean that the post-eclipse brightening effect, if real, 
may be driven by sporadic volcanic activity.
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   The researchers continue developing models of Io's complex 
surface structure and composition to account for Hubble's 
imaging and simultaneous spectroscopic observations.  Because Io 
is the solar system's most dynamic and evolving moon, Hubble 
will continue to be used for detecting changes in Io's 
atmosphere and on its surface. 

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Editors Note:  Photographs and a video, "HST Studies Jovian Moon 
Io," are available to media representatives by calling 
202/453-8373.

HST Single Image                            HST Comparison 
Images
Color:  92-HC-671                            92-HC-672
B&W:   92-H-726                              92-H-727