Jupiter and its moons

1.4 The atmosphere

David A. Rothery Teach Yourself Planets, Chapter 9, pp. 107-39, Hodder Education, 2000, 2003.

Copyright © David Rothery

It is probably the heat from Jupiter's interior that powers Jupiter's winds. This is in contrast to the Earth's atmosphere, where solar heating and condensation of water vapour provide the impetus for winds. Jupiter's prevailing winds follow a pattern that is described as 'zonal', meaning confined into discrete zones of latitude. The fast westerly winds near Jupiter's equator have already been mentioned. This atmospheric stream coincides with the pale equatorial zone visible in Figure 9.1. To both north and south there is a darker belt where the wind blows in the opposite direction at several tens of metres per second. The winds become super fast again in the direction of the planet's rotation in each of the pale zones at higher latitudes. As might be expected, where zones of opposite flowing winds touch, the shearing between air masses creates complex patterns (Figure 9.4).

Figure 9.4: Galileo image, 34,000 km wide, showing the boundary between westerly (eastward-blowing) winds in Jupiter's pale equatorial zone (lower third of the image) and the easterly winds in the darker belt to its north[.] Arrows show wind speed and strength relative to the dark spot, which is a site upon which dry atmosphere is converging and sinking[.]
NASA

Generally, the atmosphere is rising in the pale zones and sinking in the dark belts, and effectively Jupiter has a series of several Hadley cells between the equator and the two poles. Jupiter shows very little latitudinal variation in temperature, so these cells must be very efficient at transferring heat from the equator towards the poles. However, the situation is complicated by the fact that so much heat comes from inside the planet too.

Figure 9.5: Galileo image, 30,000 km across, showing the Great Red Spot in June 1996[.] The spiral pattern within the Great Red Spot is apparent, reflecting its anticlockwise rotation, and smaller spots and eddies can be seen at the edge of the south tropical zone to its south[.] This image was recorded in near infrared light, at which wavelength the Great Red Spot is more reflective than its surroundings[.]
NASA

A particularly notable feature of the interaction between competing zonal winds is the generation of eddies where the wind flows in a circular or spiral pattern. The most famous of these is Jupiter's Great Red Spot (Plate 7, Figure 9.5), which has been apparent in telescopic observations since at least 1830 and sits in the dark belt between the pale equatorial zone and the pale south tropical zone. It takes about a week for the Great Red Spot to rotate, and winds around its periphery can reach over 100 m per second. Jupiter has many other storm systems, usually white rather than red, that can individually persist for several years or even decades (Figure 9.6).

[Click 'view document' to open Plate 7, Voyager view of part of Jupiter and its Great Red Spot. This spiral storm pattern is about twice the width of the Earth and takes about a week to rotate. NASA.]

View document

Figure 9.6: Galileo image, 25,000 km across, showing a remarkable confluence of spiral storm systems on Jupiter in February 1997[.] The two large white ovals at left and right were storm systems first identified telescopically in 1938[.] They were rotating anticlockwise, whereas the pear-shaped mass between them was rotating clockwise[.] These three systems merged into a single feature early in 1998[.] The spiral storm system in the lower right corner was drifting eastward at 500 km per day relative to its more northerly neighbours[.]
NASA

In addition to water and the atmospheric constituents listed in the planetary facts table, spectroscopic studies have shown that Jupiter's atmosphere contains traces of gases such as water vapour, hydrogen deuteride (HD, a molecule consisting of an atom of ordinary hydrogen bonded to an atom of 'heavy' hydrogen), ethane (C2H6), ethyne (C2H2), phosphine (PH3), carbon monoxide (CO) and germane (GeH4).

The issue of what imparts the colour to features such as the Great Red Spot is a controversy that has not been resolved despite the Galileo entry probe's success. Compounds of sulfur, phosphorous and carbon have all been suggested. However, we do at least have a reasonable knowledge of the basic composition of the clouds. Clouds occur where conditions of temperature and pressure are such that a constituent in the atmosphere, which may be only a trace (as, for example, water in the atmosphere of Earth and Mars), is more stable as a liquid or solid than as a gas.

The whiteness of Jupiter's pale zones is caused by condensation of ammonia-ice (NH3) as the atmosphere rises and cools towards the cloud top temperature of about -150°C. In the dark belts, as the atmosphere sinks it becomes hotter, so the ammonia clouds evaporate allowing us to see to deeper and darker levels. About 50 km below the ammonia clouds is a layer of clouds of ammonium hydrosulfide (NH4HS) at about 0°C and below this probably a layer of water-ice clouds at about 50°C. The Galileo entry probe passed through the ammonium hydrosulfide cloud layer, but failed to find the water clouds, probably because it arrived in a dry downwelling zone where the water content of the atmosphere was lower than average.