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Release No. STScI-PRC08-42
Hubble Catches Jupiter's Largest Moon Going to the 'Dark Side'
NASA, ESA, and E. Karkoschka (University of Arizona)
NASA's Hubble Space Telescope has caught Jupiter's moon, Ganymede, playing a game of
"peek-a-boo." In this crisp Hubble image, Ganymede is shown just before it ducks behind
the giant planet.
Ganymede completes an orbit around Jupiter every seven days. Because Ganymede's orbit
is tilted nearly edge-on to Earth, it routinely can be seen passing in front of and disappearing
behind its giant host, only to reemerge later.
Composed of rock and ice, Ganymede is the largest moon in our solar system. It is even
larger than the planet Mercury. But Ganymede looks like a dirty snowball next to Jupiter, the
largest planet in our solar system. Jupiter is so big that only part of its Southern Hemisphere
can be seen in this image.
Hubble's view is so sharp that astronomers can see features on Ganymede's surface, most
notably the white impact crater, Tros, and its system of rays, bright streaks of material
blasted from the crater. Tros and its ray system are roughly the width of Arizona.
The image also shows Jupiter's Great Red Spot, the large eye-shaped feature at upper left.
A storm the size of two Earths, the Great Red Spot has been raging for more than 300
years. Hubble's sharp view of the gas giant planet also reveals the texture of the clouds in
the Jovian atmosphere as well as various other storms and vortices.
Astronomers use these images to study Jupiter's upper atmosphere. As Ganymede passes
behind the giant planet, it reflects sunlight which then passes through Jupiter's atmosphere.
Imprinted on that light is information about the gas giant's atmosphere, which yields clues
about the properties of Jupiter's high-altitude haze above the cloud tops.
This color image was made from three images taken on April 9, 2007, with the Wide Field
Planetary Camera 2 in red, green and blue filters. The image shows Jupiter and Ganymede
in close to natural colors.
The Hubble Space Telescope is a project of international cooperation between NASA and
the European Space Agency (ESA) and is managed by NASA's Goddard Space Flight
Center (GSFC) in Greenbelt, Md. The Space Telescope Science Institute (STScI) conducts
Hubble science operations. The institute is operated for NASA by the Association of
Universities for Research in Astronomy, Inc., Washington, D.C.
STScI is an International Year of Astronomy 2009 (IYA 2009) program partner.