After the Voyagers, Jupiter has been revisited in the '90s. The primary objective for the Galileo mission was to penetrate the Jovian atmosphere. This spacecraft, launched on Oct. 13, 1989, separated its probe from the Orbiter on Dec. 7, 1995. Its initial velocity on entering was ~106,000 km/hr (65800 mph) but it then greatly slowed from atmospheric friction aided by a deployed parachute. Data were transmitted for 57 minutes as the probe descended to depths in excess of 150 km (930 miles) where pressures exceeded 24 bars at temperatures around 150° C. No thick, dense clouds were encountered. Almost no oxygen or water were detected and helium and neon levels were below what was expected. The data overall suggested the probe may have entered at an anomalous hot spot with abnormal cloud conditions. As it observes Jupiter itself, the Orbiter has found evidence of large thunderstorms, with rainout, and some lightning. The Orbiter also continues to monitor the Galilean satellites, yielding close-ups of their surfaces that surpass those of Voyager.

Our tour begins with Jupiter, shown here full disk images taken by Voyager 1 (top) enhanced to bring out various color tones and (bottom) taken from the Galileo spacecraft that is a closer approximation to the true color of this planet's atmospheric surface.

The most obvious features are the colored bands (from chemical reactions) in which the darker reddish-browns are called belts (low pressure atmospheric motion) and the lighter are zones (high pressure). In Jupiter's northern hemisphere, jet winds at velocities up to 600 km/hr (420 mph) blow westward south of zones and north of belts and eastward north of zones and south of belts; this is reversed in the southern hemisphere. Global cloud decks include an outer layer of ammonia ice crystals, above a layer containing small amounts of ammonium hydrosulphide (NH₄SH), and possibly a water ice crystal layer beneath. Upwelling in the belts may convert the NH₄SH (perhaps including also phosphorus compounds) into the darker reds by photochemical reactions with sunlight.

A hallmark of Jupiter's atmosphere is the prominent Great Red Spot (GRS), an oval feature (25000 x 12000 km [15500 x 750 miles]) that has persisted at least since its discovery by telescope in the 17th century. The GRS is a high pressure disturbance caused by thermal convection that carries gases up to 8 km (5 miles) higher than its surroundings, thus cooling them; its anticyclonic winds move counterclockwise (in the southern hemisphere) as it drifts through the bands. The GRS experiences a complete rotation in about 6 days. The false color view below is a Voyager close-up of the GRS in which images made by the red and blue filters are enhanced relative to green which also helps to emphasize some of the swirls and plumes peripheral to it. Red areas are higher than blue above the level of the Jovian atmosphere that appears as a quasi-surface.

Both bands and convection plumes show differences in temperature that broadly correlate with the patterns these atmospheric features assume. This is evident in the Galileo Orbiter view below of a segment of bands and spots in which the temperatures derived from the photopolarimeter divide into warmer (yellow) and somewhat cooler (orange-red to black) zones.

The Jovian atmosphere is not only dynamic and turbulent but is electrically charged, as evident from this next view that shows lightning-like discharges imaged when the Orbiter is looking at the nightside of Jupiter's outer atmosphere. These are the first visible lightning flashes seen on another planet.

Aurora effects are also observed as rings at both the north and south magnetic poles of Jupiter. The view below shows a Galileo visible image of the planet, on which are two superimposed insets made with the UV camera on the spacecraft.

Ultraviolet auroral light is generated by emissions resulting from excitation of ions and electrons emanating from the innermost satellite, Io, released during volcanic activity (see next page) that become captured and concentrated by the magnetic field in the polar regions.

Jupiter's ring system is more nebulous than Saturn's, as evident in this Voyager image.

The ring's inner edge begins at about 123000 km (76300 miles) from the planet's center. Its fine particles, still effective as sunlight reflectors, form a continuous band about 5200 km (673 miles) wide and 30 km (18 miles) thick that shows no conspicuous gaps.

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