Nov. 14, 2008 -- Rather than shimmering rings, Saturn has auroras that blanket the planet's north pole like a beanie cap, a phenomena that scientists cannot explain.
"Our current ideas on what forms Saturn's aurora predict that this region should be empty," Tom Stallard, a scientist at the University of Leicester, England, said in a release. Stallard is the lead author of a paper in this week's Nature detailing new research from the science team of NASA's Cassini spacecraft.
Stallard headed a group that used infrared detectors aboard Cassini to probe Saturn's north pole -- the same region where colleagues previously found auroral emissions in ultraviolet light.
"The similarities are striking," said Larry Esposito, a planetary scientist with the University of Colorado in Boulder. "It's very consistent with our results."
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Auroras typically form as rings of light when charged particles travel along a planet's magnetic field lines and slam into its atmosphere. On Earth, the particles stream in from the sun, forming the beautiful northern and southern lights.
Jupiter's aurora are caused by the planet's own magnetic properties.
Saturn's main aurora, first imaged by the Hubble Space Telescope's ultraviolet camera, are, like Earth's, triggered by the solar wind.
Scientists don't know what causes Saturn's secondary aurora.
"We've been puzzled," Esposito said. "It doesn't seem consistent with auroral formation. It's hard to explain what we're seeing."
Cassini's infrared instrument studied the auroral display between October 2006 and June 2007. Images, released this week, show both a bright ring, which is visible from Earth, as well as infrared auroral emissions within the polar cap.
"This aurora appears to be unique to Saturn and cannot be explained using our current understanding of Saturn's magnetosphere," Stallard and colleagues wrote in Nature.
"There are periods when the polar aurora is relatively quiet, with the emission taking the form of localized bright spots, low levels of patchy emission across the pole or extended arcs," they said.
"However … they sometimes become as bright, or brighter, than the main auroral oval, filling the region between 82 degrees North and the pole with emission, unlike anything that has been observed in previous observations," the authors wrote.
Cassini found that the aurora can completely disappear in a period of time as short as 45 minutes.
"The bright polar aurora appears to be unique to Saturn," Stallard said.
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