Saturn storm rages for weeks
For nearly a month, a huge storm with massive lightning has been raging on Saturn under the watchful eye of the Cassini spacecraft.
It was a Cassini ear, a radio receiver, that first detected the storm on 23 January. And this week, scientists finally coaxed an image from Cassini's camera.
Sunlight reflecting off the planet's rings provided enough light during the Saturn night for Cassini's camera to pick out the storm and cloud features.
Researchers now are working hard to identify spots within the storm that could be lightning.
The only other place in the solar system besides Earth where lightning has been positively identified is on Jupiter.
"Cassini's radio detector is on all the time," says planetary meteorologist Professor Andrew Ingersoll, with the California Institute of Technology and a member of the Cassini science team.
"Months go by and there is no radio noise from Saturn. Then all of a sudden, in the space of a day, they get it. That's what happened on 23 January."
The radio emissions are similar to the popping and crackling that you might hear if you were driving through a thunderstorm with an AM radio on.
What is generally thought of as radio interference is actually radio waves generated by lightning.
Behind the storm
Scientists are not sure what causes the lighting on Saturn, but they believe it is triggered by the same process that creates lightning on Earth.
"On Earth, we think the convection in thunderstorms is important in the process of separating positive charges from negative charges. And if you separate enough charges, you build up a very large potential field between those separated charges," says University of Iowa's Dr Bill Kurth, the deputy principal investigator of Cassini's radio and plasma wave science instrument.
"Nature doesn't like that, and it tries to correct the situation by running a current between the two separated charges. That forms a very large spark, there's an awful lot of energy that's released, and it's meant to basically allow the negative charges and the positive charges to get back together again," Kurth says.
On Saturn, the phenomena manifests itself on a gigantic scale, with lighting bolts more than 1000 times stronger than strikes on Earth, the radio data reveals. The storm itself spans an area larger than continental US.
Why Saturn should have such massive storms is a mystery. The planet gets only about 1% of the sunlight that reaches Earth, so there is much less energy to drive storms, Ingersoll says.
"Given that, there are two possibilities: one is that you have the same number of storms that you have on Earth, but they are weaker; or two, they are just as strong but there are fewer."
While more studies are needed, Ingersoll favours the second explanation.
"This storm certainly looks more vigorous and the radio emissions are more powerful deep down, [so] there may be more water in the atmosphere than on Earth," Ingersoll says.
Amateurs beat the professionals
The first images of the storm actually came from two ground-based amateur astronomers, not the Cassini spacecraft.
From its present orbital perch, Cassini has been unable to image the sunlit side of Saturn.
The team studying the radio emissions, however, decided to ask the amateur astronomy community if anyone had any pictures of Saturn that corresponded to where radio emissions were coming from.
Within hours, two astronomers near Paris submitted the first visual confirmation of the storm.
Their findings were confirmed by the Cassini images shot in darkness with illumination from the planet's rings.