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Auroras Shed Light on Jupiter's Moon Ganymede
By Kenneth Silber
Staff Writer
posted: 06:07 pm ET
17 November 1999
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ganymede_aurorasObservations of auroras on Jupiter's moon Ganymede are providing clues to the atmosphere and magnetic field of the jovian satellite. Ganymede is one of a handful of bodies in the solar system known to have auroras, which are luminous phenomena that emerge from the interaction of charged particles with oxygen or other gases. On Earth, auroras are known by such names as the "northern lights" and the "southern lights." Astronomers used the Hubble Space Telescope in late 1998 to capture ultraviolet images showing faint auroras on Ganymede. An analysis of the findings was recently submitted to Astrophysical Journal. The auroras are "a signature of the presence of a tenuous atmosphere on Ganymede," says Paul D. Feldman, an astronomer at Johns Hopkins University and lead author of the submitted paper. The findings, Feldman tells space.com, confirm that oxygen is present on Ganymede, something that had been inferred from earlier spectroscopy data. The images also confirm the presence of a magnetic field around Ganymede, first detected by the Galileo spacecraft in 1996. Ganymede's auroras are different in various respects from auroras on Earth, says Melissa McGrath, an astronomer at the Space Telescope Science Institute and member of the team conducting the research. Ganymede's auroras extend further down from the poles than auroras typically do on Earth, for instance. Also, McGrath observes, the charged particles on Ganymede sometimes hit the jovian moon's surface, something that rarely happens on Earth, where most such particles are stopped by the atmosphere. 
This Hubble image shows Ganymede's auroral glow. The graph of the moon's globe has been superimposed to give perspective. Credit: NASA and Space Telescope Science Institute.
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