When we look at Jupiter we see only the uppermost cloud layers in its atmosphere. The composition of the atmosphere at this level can be determined spectroscopically . The first re¬ported observations of dark absorption bands in Jupiter’s spectrum were made by Angelo Secchi in 1863. It was not until the early 1930s that the bands were shown, by Rupert Wildt and T.Dunham, to be due to the presence of methane (CH4) and ammonia (NH3). Although these bands are the dominant feature of Jupiter’s optical spectrum it is wrong to deduce that methane and ammonia are the main constituents of the atmosphere. Some spectral features are intrinsically much weaker than others. An important molecule with such intrinsically weak lines is molecular hydrogen (H2). It was not until 1960 that these lines were identified, by C.C.Keiss, C.H.Corliss and H.K.Keiss, in Jupiter’s spectrum. Although the lines are very weak, molecular hydrogen is undoubtedly the major constituent of Jupiter’s atmosphere. This finding agrees with the low mean density, which requires the predominant constituents of Jupiter to be hydrogen and helium, the lightest elements.
Because of Jupiter’s low atmospheric temperature, no lines of helium can be present in the optical spectrum and the element can¬not be detected, if present, by means of Earth-based observations. Detection from an Earth-orbiting satellite is very difficult. It has been traditional to assume that helium is present hi Jupiter’s atmosphere in much the same proportion as in the Sun. Since almost all the hydrogen must be in molecular form this gives a number ratio He (He + H2) of about 0.12. The first direct evidence of the presence of helium on Jupiter was the observation by Pioneer 10 of a spectral line in the ultraviolet at 58.4nm, but it has not been possible to determine the abundance of helium from this observation.

Small amounts of four other molecules have also been detected; these are water (H20), ethane (C2H6), acetylene (C2H2) and phosphine (PH3). There is also a trace of two isotopes of elements: deuterium (heavy hydrogen) and 13C (carbon-13) have been detected in methane as CH3D and 13CH4 respectively. Further molecules are undoubtedly present but have not yet been detected because their lines in the accessible part of the spectrum are too weak. In particular it is expected that sulphur is present as the molecule hydrogen sulphide (H2S).

Although Jupiter is completely cloud covered like Venus, the visual appearance is quite unlike that of the almost featureless venusian clouds. Even in a small telescope much detail is visible and this detail is always changing. The most prominent features are a series of dark belts and light zones which run parallel to the planet’s equator . Since the Earth is never far from the plane of Jupiter’s equator these appear straight when viewed from the Earth. They seem to be permanent features and have been given the names. The belts and zones are found at equatorial and intermediate latitudes but not in the polar regions. Their colours vary somewhat but in general the zones are whitish or yellowish whereas the belts are more often brownish or reddish.

The belts and zones are not the only features visible on Jupiter. The better photographs taken from the Earth and those taken from spacecraft show a wealth of intricate detail due to spots, festoons, plumes and waves. Colors such as grey, blue and black can be seen; although most features are pastel-coloured some show quite intense colors. Apart from the belts and zones, most of Jupiter’s visible features have a limited lifetime and are seen to form and later disappear, typically after a few weeks or months.

The visible clouds, or at least their upper layer, are composed of small particles of frozen ammonia similar to the water-ice cirrus clouds of the Earth. These must be rather tenuous so that, at’least near the centre of the planetary disc, it is possible to see through them to a second, more substantial cloud layer. The .composition of this lower layer is not certain, but particles of ammonium hydrosulphide (NH4SH) seem most likely. Various suggestions have been made for the source of the coloration. One is that the dominant yellowish color is due to ammonium sulphide ((NH4)2S) in the ammonium hydrosulphide cloud layer. Other possibilities are the ammonium sulphide polymers and the element sulphur itself.

Spacecraft measurements have shown that the tops of the zones are about 9K cooler than the tops of the belts and that the zones reach about 20 km higher in the atmosphere than the belts.

Although the observed features of Jupiter’s disc have a limited lifetime they do, in general, last long enough for measurement of their rotation periods. Such measurements show that there are two distinct rotation systems. The equatorial zone, which extends to about latitudes 10° north and south, forms what is known as SYSTEM i and rotates about 360kmhr-1 more rapidly than the rest of the planet, which forms rotation SYSTEM n. Neither system rotates uniformly, but standard rotation periods have been assigned as follows:
System I : 9hr 50 mm 30.003 sec = 9.8416675hr
System II: 9 hr 55 min 40.062 sec = 9.9277950 hr

These rotation periods define two systems for measuring longitude.On the Earth, which has a solid surface, longitude is measured relative to a meridian fixed on the surface; the particular meridian is the one through Greenwich. Since Jupiter does not have a solid surface this procedure cannot be used. Instead a decision was made that at a certain time the central meridian of Jupiter was to be defined as having longitude zero. Using System I the longitude of the central meridian must then increase by 360°/9.8416675 = 36o.579 per hour, or by 0°.00965 per minute. After one complete rotation period the; longitude of the central meridian has increased by 360″ so that the meridian of zero longitude is again the central meridian. In System II the increase in longitude is 36° .262 per hour, or 0″.60436 per minute. Tables are published that show the longitude of the central meridian in both Systems I and II at regular intervals; the longitude at any other time can then be readily calculated. The longitude of an actual feature on Jupiter can be determined by noting the time at which it transits (i.e. crosses the central meridian) and then looking up the longitude in the tables. In the equatorial zone, System I is used; elsewhere on the planet it is System II. A feature that rotates with exactly the standard period for its appropriate system has the same longitude at every transit. If it rotates more slowly, the longitude increases from one transit to the next; rotation faster than the standard period results in a decreasing longitude. In general, features on Jupiter do show a changing longitude because they do not rotate at exactly the standard rate. The actual rotation period can be easily calculated from the change in longitude from one transit to the next; for most features the rotation periods are not constant but change during their lifetime.

The existence of these different rotation periods, and particularly that of the EQUATORIAL JET which constitutes System I, shows conclusively that the visible surface of Jupiter does not form a solid surface. As discussed below it is not even certain that Jupiter has a solid surface at all, but if it does its rotation period is most likely equal to that of the magnetosphere; this is determined from radio measurements described below. In 1962 a SYSTEM in rotation period of 9hr 55min 29.37 sec was adopted for the standard radio period. Unfortunately more recent measurements have shown this value to be incorrect; the best measurement is now 9hr 55min 29.75sec. This means that a feature which rotates with this latter period will, in System III, drift to greater longitudes at a rate of 3°.6 per year. Since the magnetic field of Jupiter is generated in the central regions of the planet, the rotation period of these regions is likely to be equal to the radio period.

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