Ganymede may have an aurora of its own

 Auroras aren't just for planets anymore.

 Typically generated when a magnetic field sends charged particles crashing into a planet's atmosphere, the shimmering lights of an aurora grace the north and south polar skies of Earth, Jupiter, Saturn, Uranus, and Neptune.

 It now seems that Jupiter's moon Ganymede joins the ranks of the big guys. Ultraviolet emissions recorded by the Hubble Space Telescope suggest that this satellite has its own polar light show.

 If the finding is confirmed, Ganymede would be the first moon known to possess an aurora.

 Hubble has tracked Jupiter and its moons for over a year, first as a scout for the Galileo spacecraft, then as an occasional collaborator during the craft's 2-year Jovian tour, which began last December. In June, just before Galileo's first rendezvous with Ganymede, Hubble's Goddard high-resolution spectrograph measured ultraviolet light emitted by the moon. The spectra revealed that Ganymede, like the Jovian moon Europa, has a thin atmosphere rich in oxygen.

 Ganymede's spectra show two distinct peaks caused by oxygen, indicating that such emissions do not arise uniformly throughout the moon's atmosphere. Judging from the orientation of Ganymede as it passed across the aperture of the spectrograph, Doyle T. Hall of Johns Hopkins University in Baltimore and his colleagues find that the peaks come from Ganymede's north and south poles.

 That pattern is just what an aurora would produce, says study collaborator Melissa McGrath of the Space Telescope Science Institute in Baltimore. She reported the details last week at a workshop on the Goddard spectrograph at NASA's Goddard Space Flight Center in Greenbelt, Md.

 The findings are all the more intriguing, notes Hall, because during its June rendezvous, Galileo found evidence that Ganymede has its own magnetic field and charged particles nearby--prerequisites for generating an aurora. A Sept. 6 flyby has now confirmed the magnetic field, says Margaret G. Kivelson of the University of California, Los Angeles.

 Ganymede lies within the vast region dominated by Jupiter's huge magnetic field. Jupiter may supply the charged particles needed to create Ganymede's aurora, says John T. Clarke of the University of Michigan in Ann Arbor, much as the sun's wind supplies the charged particles that generate Earth's auroras.

 Astronauts are scheduled to replace Hubble's spectrographs next February with a device that can simultaneously record images and spectra. If Ganymede has an aurora, this device should detect it directly, McGrath says.