Sunday, March 17, 2013

Storms on Jupiter

Jupiter is the largest of all the solar system’s planets, more than ten times bigger and three hundred times as massive as Earth. Jupiter is so immense it could swallow all the other planets easily. Its Great Red Spot, a storm that has raged for centuries, is itself wider than Earth. And the Spot is merely one feature visible among the innumerable vortexes and streams of Jupiter’s frenetically racing cloud tops. Yet Jupiter is composed mainly of the lightest elements, hydrogen and helium, more like a star than a planet. All that size and mass, yet Jupiter spins on its axis in less than ten hours, so fast that the planet is clearly not spherical: Its poles are noticeably flattened. Jupiter looks like a big, colourfully striped beach ball that’s squashed down as if some invisible child were sitting on it. Spinning that fast, Jupiter’s deep, deep atmosphere is swirled into bands and ribbons of multihued clouds: pale yellow, saffron orange, white, tawny yellow-brown, dark brown, bluish, pink and red. Titanic winds push the clouds across the face of Jupiter at hundreds of kilometres per hour.
- Ben Bova, Jupiter (2000)

Figure 1: A true colour mosaic of Jupiter constructed from images taken by the narrow angle camera onboard NASA’s Cassini spacecraft on December 29, 2000. Credit: (NASA/JPL/Space Science Institute)

On 4 May 1999, NASA’s Galileo spacecraft was in orbit around Jupiter and it took a series of images of a region within the giant planet’s South Equatorial Belt. A total of three sets of images were taken. The 1st and 2nd sets of images were taken when the region was in daylight while the 3rd set of images was taken when the region was experiencing night. In the images, two storm centres (latitude 14 S, longitude 268 W and latitude 15 S, longitude 263 W) can be clearly seen. The night time images do not show any cloud features, but they show bright spots due to lightning that are situated at where the two storm centres are.

Figure 2: On the left are images of the mapped region, showing the presence of two storm centres. Colour image of the mapped region (top left), night time image showing the lightning storms as black spots over the storm centres (middle left) and map of wind velocity vectors with the longest vector representing 70 m/s (bottom left). The expanded view of the storm (top right) is colour coded where white regions represent high level clouds and red regions represent deep clouds. A sketch of the storm’s cross-sectional profile (bottom right) shows the haze layers as dotted regions and optically thick clouds as solid outlines. The cloud base is likely to be deeper than indicated. Credit: Gierasch, P.J. et al. (2000)

Further analysis was performed for the more western storm (latitude 14 S, longitude 268 W). This active storm system has a length of 4000 kilometres. The bulk of the storm consists of an optically thick cloud which extends from below the 3 bar pressure level up to at least the 0.5 bar pressure level. As a result, the storm cloud has an estimated vertical extent of no less than 50 kilometres. At the cooler upper levels of the storm cloud, condensation is likely to consist of a combination of water, ammonia and ammonium hydrosulphide. Beneath the upper levels, the rest of the storm cloud is expected to consist of only water as a condensable species. This is because temperatures deeper in the atmosphere are too high for other volatile species to condense. Water is also expected to be the primary agent for cloud electrification. Apart from the primary storm cloud, images taken by the Galileo spacecraft also reveal the presence of background clouds in the vicinity. The components of these background clouds are consistant with a sheet of intermittent clouds (possibly ammonia ice) at the 0.9 bar level, a haze layer between the 0.9 bar and 0.2 bar levels and a more rarefied haze layer extending above the 0.2 bar level.

This storm is a form of moist convective updraft. On Jupiter, the temperature excess necessary to power an updraft up to such a vertical extent is estimated to be about 5 degrees Kelvin. This allows the total power output of the storm to be estimated at 5×1015 watts. In October and November of 1997 the Galileo spacecraft detected lightning from 26 storms on the night side of Jupiter. Based on this data, the estimated frequency of lightning storms occurring on Jupiter is 0.66×10-9 per km2. If all lightning storms on Jupiter come from such moist convective storms, then the heat flux curried by these storms when averaged over the entire surface of Jupiter turns out to be 3.3 W/m2.

For over four billion years, Jupiter has been slowly contracting under its own immense gravitational grip. As the stupendous bulk of the giant planet is squeezed, gravitational potential energy is converted into heat. This causes Jupiter to radiate more heat than is receives from the Sun. Jupiter’s internal heat flux averaged over its entire surface, is about 5.4 W/m2. The energy that drives these moist convective storms largely comes from the giant planet’s own internal heat flux. Comparing the internal heat flux with the heat flux carried by moist convective storms, it appears that these storms can effectively carry a large portion of the giant planet’s internal heat flux out into the upper atmosphere. This suggests that moist convection plays an important role in the dynamics of Jupiter’s atmosphere.

1. Gierasch, P.J. et al. 2000, “Observation of moist convection in Jupiter’s atmosphere”, Nature 403, 628-630
2. Little, B. et al. 1999, “Galileo images of lightning on Jupiter”, Icarus 142, 306-323