CICLOPS: Cassini Imaging Central Laboratory for OPerationS
Io Eclipse In 'True' Color

During the eclipse of the moon, Io, on January 1, 2001, Cassini recorded images in several colors ranging from the near-ultraviolet to the near-infrared. Two of these colors have been added to the clear-filter "movie" of this eclipse (released on February 5, 2001) to make visual the evidence used by imaging scientists in determining the source of Io's auroral glows. The color pictures were taken at lower resolution (120 versus 60 kilometers per pixel) and less frequently than the clear filter images, though they still span the entire two hour duration of the movie. The white dots near the equator are volcanoes, the brightest being Pele, that are often much brighter than the faint atmospheric glows.

Emissions in the 595 to 645 nanometer wavelength range likely arise from a tenuous atmosphere of atomic oxygen; they would appear red to the eye and are consequently colored red in the movie. Emissions in the near-ultraviolet, between 300 and 380 nanometers, correspond in wavelength to the bright blue visible glows one would expect from molecular sulfur dioxide; they have been colored blue in the movie. The blue glows are restricted to areas deep down in the atmosphere near the surface of Io, whereas the red glows are much more extensive (reaching heights of up to 900 km). This would be expected if the blue glows are produced by sulfur dioxide, which is heavier than atomic oxygen and more closely bound to the surface by the moon's gravity. The prominent blue and red regions near the equator of Io dance across the moon with the changing orientation of Jupiter's magnetic field, dramatically illustrating the relationship between Io's aurorae and the magnetic-field-aligned electrical currents which excite them.

A faint but localized blue emission is visible near the north pole of Io, possibly due to a volcanic plume erupting from the volcano Tvashtar at high northern latitude on the side of Io opposite Cassini. This eruption, observed by both Galileo and Cassini, left an enormous red ring around Tvashtar. (See the images released on March 29, 2001).

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Office of Space Science, Washington, D.C. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission, visit and the Cassini imaging team home page,

Credit: NASA/JPL/University of Arizona
Released: May 31, 2001
Image/Caption Information
  Io Eclipse In 'True' Color
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256x256 50 frames:
GIF 2.5 MB
MP4 movie 1.1 MB

Alliance Member Comments
carolyn (CICLOPS) (Mar 24, 2007 at 6:30 AM):
The images were taken by Cassini from a distance of only 10 million kilometers during the Jupiter flyby of 2000/2001. An AU is ~ 148 million kilometers.
Moonsister (Mar 23, 2007 at 4:40 PM):
It is just amazing we can see eclipses like this from so far away. How many AUs is this 3 or 4?