A powerful, five-month-long storm continues to rage on Saturn, making it the planetās longest continuously observed tempest. Scientists suspect the storm ultimately draws its amazing power ā producing lightning bolts 10,000 times as strong as those on Earth ā from the planetās own internal heat.
The Cassini spacecraft has previously observed lightning storms on Saturn in 2004 and 2006, but those lasted for less than a month.
Before that, NASAās two Voyager spacecraft each spent a few days listening to radio emissions from lightning storms during flybys of Saturn in 1980 and 1981. The flybys were spaced about nine months apart, so it is possible that the emissions came from a single storm that seethed for nearly a year.
Advertisement
But because the Voyagers did not track the storms over time, it is impossible to say for sure. āFrom this point of view, [this new storm] is the longest lightning storm weāve ever seen,ā says Georg Fischer, a member of Cassiniās radio and plasma wave science team at the University of Iowa in Iowa City, US. āIt hasnāt stopped ā I look at it every day.ā
Radio emission from the storm was first observed on 27 November 2007, and about a week later, Cassiniās cameras spotted complex, rotating storm clouds ā thought to be made of water ā that had risen above the planetās ammonia cloudtops.
The electrical storm takes up the same volume of space as the Earthās troposphere, the region of the atmosphere from the surface to an altitude of about 15 km, where weather occurs. āItās absolutely huge,ā Fischer told Ā鶹“«Ć½. āImagine you have lightning on Earth everywhere ā thatās how you could compare it.ā
The storm has continued for five months, though with varying strength. āIt waxes and wanes,ā Fischer says. āIt seems that every 10 to 14 days thereās a kind of enhancement, then for one week to 10 days, it is less intense.ā
Violent updrafts
The reasons for this variation are not clear ā the storm actually originates too far below the planetās gaseous āsurfaceā to observe. Modelling suggests that temperature differences at a depth of about 200 to 300 kilometres might provide the trigger. Sunlight does not penetrate to such depths, suggesting it is the planetās own internal heat that is somehow responsible for the temperature differences.
The stormās location may aid and abet its survival. It occurs at a latitude of about 35Ā° in the planetās southern hemisphere ā a region dubbed āStorm Alley.ā At that location, Saturnās atmosphere circles the planet more slowly than in other bands. That may allow the storm clouds to remain intact as they ride violent vertical winds, or āupdraftsā, from their source deep within the planetās atmosphere up to the surface.
āItās a very unique location ā it seems at Saturn, lightning storms cannot actually happen everywhere,ā says Fischer.
Shaded belt
The only other place on Saturn that similar storms have been observed is the equator, which is where the Voyager spacecraft observed them. In that case, sunlight may have played a role in powering the storms.
Thatās because at that time, the Sun hit the rings nearly edge-on, keeping Saturnās equator constantly shaded. The drastic temperature difference between the cool, shaded equatorial belt and the surrounding Sun-kissed areas may have fuelled regular convection patterns that drove the storms.
In September 2009, the planetās orientation will be the same as it was during the Voyager flybys, with the rings casting a shadow over the equator. āWeāll see if we have the same storms,ā says Fischer. āThatās why itās important to have an extension of the Cassini mission, which was just now approved. You can only watch these things when you are there for a long time.ā
Cassini: Mission to Saturn ā Learn more in our continually updated .
