Light, bright polar caps: One aspect of Ganymede that has puzzled scientists is that its surface is brightest at high latitudes, near to the poles.

Credit: NASA

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NEW YORK: A mechanism similar to the one behind Earth's northern lights, or aurora borealis, might be brightening the polar caps of Ganymede, Jupiter's largest moon.

Ganymede has a thick outer ice layer and a mantle of ice and rock. It is the largest satellite in the Solar System, and though it has less mass, it has a greater diameter than the planet Mercury.

The moon is unique in that it is the only one known to have a magnetic field. Another aspect of Ganymede that has long puzzled scientists is that its surface is brightest at high latitudes (greater than 40 degrees), near to the poles.

Now, a new computer model detailed in the journal Icarus suggests the two anomalies are linked.

Magnetic attraction

According to that model, the moon's magnetic field is funneling charged particles, or plasma, that are swirling around its parent planet, Jupiter. The magnetic field drives this plasma to Ganymede's poles, while simultaneously diverting it away from the moon's midsection.

U.S. experts behind the model have found that the regions predicted by it to be unprotected by the magnetic field precisely matched those areas observed to be brightest.

"The magnetic field of Ganymede is strong enough that it can protect the equatorial region… from Jupiter's plasma," said study co-author Krishan Khurana at the University of California, Los Angeles.

On Earth, plasma particles from the Sun are funneled to the poles, where they interact with the atmosphere to create the aurora borealis and aurora australis, the Northern and Southern lights, respectively. Auroras have also been observed on Ganymede.

Jupiter's plasma particles are actually debris spewed by the volcanic moon Io that have been accelerated by the planet's magnetic field to speeds of nearly 650,000 kilometers per hour. When they crash into the ice layer at Ganymede's poles, they create a spray of water molecules that fall back to the moon's surface as a very fine snow, which shimmers more brightly than the normal ice coating at the equator.

"Striking results"

Adam Showman, an astronomer at the University of Arizona, in Tucson, who was not involved in this research, called the model results "striking."

"This implicates an ion-bombardment mechanism for forming the bright polar caps," Showman added. "The details of exactly how the incoming ions may cause the brightening remain somewhat uncertain, and I think this study will help motivate additional research on that question."

Khurana thinks the new model could also explain why the surface of Europa, another icy Jovian moon, has one of the most reflective surfaces in the Solar System. "Because Europa does not have a [magnetic] field, it is not able to protect its surface from the swirling plasma of Jupiter," he said. "We believe that most of its surface is bombarded by the Jovian plasma which creates a layer of fine frost all over its surface."