NASA's
Hubble Space Telescope has caught Jupiter's moon Ganymede playing a game of
"peekaboo." The new image, released today, shows Ganymede just before
the seemingly tiny orb ducks behind its giant host.
Ganymede
orbits Jupiter every seven days. And since the moon's orbit is tilted nearly
edge-on to Earth, astronomers often spot Ganymede passing in front of and
disappearing behind Jupiter, only to reemerge later.
Ganymede is
the largest of Jupiter's four moons called the Galilean
moons for their discoverer. In fact, this ball of rock and ice is the
largest moon in our solar system, with a diameter of about 3,270 miles (5,262
km).
Even so,
the mega moon appears as a small marble next to Jupiter, the largest planet in
the solar system. Jupiter is so big that only part of its southern hemisphere
can be seen in the Hubble image.
Unlike the
gas giant planet, Ganymede has a hard shell. The image shows features on Ganymede's
surface, including the white impact crater called 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 (a
large eye-shaped feature at the image's upper left). A storm the size of two
Earths, the Great Red Spot has been raging for more than 300 years.
In addition
to wowing viewers, images like this provide astronomers with information about
Jupiter's upper atmosphere. That's because 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.
The new
photo is based on imagery recorded by Hubble on April 9, 2007. Hubble takes so
many photographs of the cosmos that officials often process and release some of
them long after the photos are actually captured.
Hubble
images arrive as ones and zeros representing grayscale, digital photos taken
with filters to block all but one color of light. The colors
are added later with the popular Photoshop program.
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