NASA Hubble Space Telescope Spots Rare Triple Eclipse on Jupiter

At first glance, Jupiter looks like it has a mild case of the measles.
Five spots — one colored white, one blue, and three black — are
scattered across the upper half of the planet. Closer inspection by
NASA’s Hubble Space Telescope reveals that these spots are actually a
rare alignment of three of Jupiter’s largest moons — Io, Ganymede,
and Callisto — across the planet’s face. In this image, the telltale
signatures of this alignment are the shadows [the three black circles]
cast by the moons. Io’s shadow is located just above center and to the
left; Ganymede’s on the planet’s left edge; and Callisto’s near the
right edge. Only two of the moons, however, are visible in this image.
Io is the white circle in the center of the image, and Ganymede is the
blue circle at upper right. Callisto is out of the image and to the
right.

On Earth, we witness a solar eclipse when our Moon’s shadow sweeps
across our planet’s face as it passes in front of our Sun. Jupiter,
however, has four moons roughly the same size as Earth’s Moon. The
shadows of three of them occasionally sweep simultaneously across
Jupiter. The image was taken March 28, 2004, with Hubble’s Near
Infrared Camera and Multi-Object Spectrometer.

Seeing three shadows on Jupiter happens only about once or twice a
decade. Why is this triple eclipse so unique? Io, Ganymede, and
Callisto orbit Jupiter at different rates. Their shadows likewise
cross Jupiter’s face at different rates. For example, the outermost
moon Callisto orbits the slowest of the three satellites. Callisto’s
shadow moves across the planet once for every 20 shadow crossings of
Io. Add the crossing rate of Ganymede’s shadow and the possibility of
a triple eclipse becomes even more rare. Viewing the triple shadows
in 2004 was even more special, because two of the moons were crossing
Jupiter’s face at the same time as the three shadows.

Jupiter appears in pastel colors in this photo because the observation
was taken in near-infrared light. Astronomers combined images taken in
three near-infrared wavelengths to make this color image. The photo
shows sunlight reflected from Jupiter’s clouds. In the near infrared,
methane gas in Jupiter’s atmosphere limits the penetration of sunlight,
which causes clouds to appear in different colors depending on their
altitude. Studying clouds in near-infrared light is very useful for
scientists studying the layers of clouds that make up Jupiter’s
atmosphere. Yellow colors indicate high clouds; red colors lower clouds;
and blue colors even lower clouds in Jupiter’s atmosphere. The green
color near the poles comes from a thin haze very high in the atmosphere.
Ganymede’s blue color comes from the absorption of water ice on its
surface at longer wavelengths. Io’s white color is from light reflected
off bright sulfur compounds on the satellite’s surface.

In viewing this rare alignment, astronomers also tested a new imaging
technique. To increase the sharpness of the near-infrared camera images,
astronomers speeded up Hubble’s tracking system so that Jupiter traveled
through the telescope’s field of view much faster than normal. This
technique allowed scientists to take rapid-fire snapshots of the planet
and its moons. They then combined the images into one single picture to
show more details of the planet and its moons.

Credit: NASA, ESA, and Erich Karkoschka (University of Arizona)

Electronic images, animation, and additional information are available
at: http://hubblesite.org/news/2004/30

For additional information, contact: Erich Karkoschka, Univ. of Arizona,
Lunar and Planetary Lab, Tucson, AZ 85721, (phone) 520-621-3994,
(e-mail) erich@pirl.lpl.arizona.edu

The Space Telescope Science Institute (STScI) is operated by
the Association of Universities for Research in Astronomy, Inc.
(AURA), for NASA, under contract with the Goddard Space Flight
Center, Greenbelt, MD. The Hubble Space Telescope is a project
of international cooperation between NASA and the European Space
Agency (ESA).