Saturn image taken on 24 December 2010 by the Cassini camera shows a storm with a latitudinal and longitudinal extent of 10,000 km and 17,000 km, respectively. The latitudinal extent of the storm’s head is approximately the distance from London to Cape Town. A 'tail' emerging from its southern edge extends further eastward.
Credit: Carolyn Porco and CICLOPS/ NASA/JPL-Caltech/SSI
LONDON: A huge storm in Saturn's atmosphere, over 10,000 km wide and visible from Earth as a 'Great White Spot', has been observed in unprecedented detail by two independent groups of astronomers.
Only five thunderstorms of a similar magnitude have been documented in the past 130 years. Normally occurring at 30-year intervals when Saturn's northern hemisphere tilts closest towards the Sun, this latest event surprised astronomers when it came nearly a decade earlier than anticipated.
The nearby positioning of NASA's Cassini spacecraft coupled with the powerful intensity of the weather system meant astronomers were able to record the most detailed observations to date of such a dramatic event, and gather vital evidence to understand how such titanic storms are formed.
"This giant thunderstorm is enormous in its size and energy," said Georg Fischer from the Austrian Academy of Sciences, lead author of one of two papers published this week in Nature. "It is remarkable that it occurred just 20 years after the last one, because usually they occur in intervals of one Saturn year. We don't yet know why this one is so early."
Sighting the storm
Convective storms around 2,000 km wide occur frequently in Saturn's atmosphere but mega storms, up to ten times larger and nicknamed Great White Spots (GWS) due to their appearance from Earth, are much rarer.
On 5 December 2010, two independent groups of astronomers detected early signs of a giant thunderstorm developing on Saturn. Instruments on the Cassini orbiter that has been monitoring the planet since 2004 began to recorded significant lightning activity in the atmosphere.
Almost simultaneously, ground-based telescopes sighted a barely visible white spot of clouds developing in the planet's northern hemisphere.
Combining these observations allowed astronomers to track the subsequent evolution of the storm. The rate of expansion was tremendous, growing from 3,000km to 8,000km wide in just one week until by February 2011 the storm was encircling almost the entire planet.
Over 10 lightening bolts per second
Fischer and his international team of researchers examined the Cassini recordings to map the lightening discharges of the storm by analysing the radio waves they emit, called Saturn electrostatic discharges.
They measured flashes around 10,000 times stronger than Earth lightning, occurring at exceptionally fast frequencies. "The flash rates of this storm are about an order of magnitude higher than previous ones, with peak rates larger than ten per second," explained the researchers.
The second group of astronomers, led by Agustín Sánchez-Lavega from the Higher Technical School of Engineering of Bilbao in Spain, combined observations with numerical simulations to track the formation and movement of clouds during the storm.
This provided insight into the dynamics of Saturn's 'weather layer', a 250-kilometre-thick layer where the main clouds reside. Sánchez-Lavega explained that the cloud patterns point to deep winds that "extend without change to a depth of at least 250km beneath the visible clouds" - providing significant evidence to support current models that "the outbreak of a GWS is due to moist convection induced by water."
