Washington, March 18 : Scientists have observed unexpected luminous spots on Jupiter, which are believed to be caused by its moon Io.
Besides displaying the most spectacular volcanic activity in the solar system, Io causes auroras on its mother planet that are similar to the Northern Lights on Earth.
The auroral emissions linked to the volcanic moon are called the Io footprint.
From previous studies, researchers had found the Io footprint to be a bright spot that is often followed by other auroral spots. Those spots are typically located downstream relative to a flow of charged particles around the giant planet.
Now, a team of planetologists from Belgium and Germany have discovered that Io's footprint can include a faint spot unexpectedly upstream of the main spot.
According to the scientists, each appearance of such a "leading spot" occurs in a distinctive pattern: When a leading spot in the northern or southern hemisphere of Jupiter precedes the main footprint, downstream spots in the opposite hemisphere also follow it.
"Previously, we only observed downstream spots, but only half of the configurations of Io in the Jovian magnetic field had been studied," said Bertrand Bonfond of the University of Li¨ge in Belgium, who is a member of the team that found the new type of spot.
"Now we have the complete picture. The results are surprising because no theory predicted upstream spots," he added.
Like a rock in a stream, Io obstructs the flow of charged particles, or plasma, around Jupiter. As the moon disrupts the flow, it generates powerful plasma waves that blast electrons into Jupiter's atmosphere, creating the auroral spots.
"The finding of the leading spot puts all the previous models of the Io footprint into question," said Bonfond.
He and his colleagues have proposed a new interpretation in which beams of electrons travel from one Jovian hemisphere to the other.
For this latest Io-footprint analysis, Bonfond and his colleagues at Li¨ge and at the University of Cologne in Germany used the Hubble Space Telescope to observe Jupiter in ultraviolet wavelengths.
"New insights regarding Io-Jupiter interactions could apply to other situations in which an electrically conductive body-in this case, Io-orbits near a magnetised body," said Bonfond.
In order to test their new theory of how leading and downstream spots form, Bonfond and his colleagues plan further observations of Io's footprint after August 2008. That's when repairs and improvements to the Hubble Space Telescope are scheduled to occur.