The realm of terrestrial planets ends at the asteroid belt. Outside this, the relative abundance of volatile elements is higher and the original composition of the solar nebula is still preserved in the giant planets. The first and largest is Jupiter. Its mass is 2.5 times the total mass of all other planets, almost 1/1000 of the solar mass. The bulk of Jupiter is mainly hydrogen and helium. The relative abundance of these elements are approximately the same as in the Sun, and the density is of the same order of magnitude, namely 1330 kg m-3.

During oppositions, the angular diameter of Jupiter is as large as 50". The dark belts and lighter zones are visible even with a small telescope. These are cloud formations, parallel to the equator (Fig. 7.38). The most famous detail is the Great Red Spot, a huge cyclone, rotating counterclockwise once every six days. The spot was discovered by Giovanni Cassini in 1655; it has survived for centuries, but its true age is unknown (Fig. 7.39).

The rotation of Jupiter is rapid; one revolution takes 9h 55 min 29.7 s. This is the period determined from the variation of the magnetic field, and it reflects the speed of Jupiter's interiors where the magnetic field is born. As might be expected, Jupiter does not behave like a rigid body. The rotation period of the clouds is about five minutes longer in the polar region than at the equator. Due to its rapid rotation, Jupiter is nonspherical; flattening is as large as 1/15.

There is possibly an iron-nickel core in the centre of Jupiter. The mass of the core is probably equal to a few tens of Earth masses. The core is surrounded by a layer of metallic liquid hydrogen, where the temperature is over 10,000 K and the pressure, three million atm. Owing to this huge pressure, the hydrogen is dissociated into single atoms, a state unknown in ordinary laboratory environments. In this exotic state, hydrogen has many features typical of metals. This layer is electrically conductive, giving rise to a strong magnetic field. Closer to the surface where the pressure

7.14 Jupiter


Fig. 7.38. A composed image of Jupiter taken by the 114 km/pixel. The dark dot is the shadow of the moon Europa. Cassini spacecraft in December 2000. The resolution is (NASA/JPL/University of Arizona)

is lower, the hydrogen is present as normal molecular hydrogen, H2. At the top there is a 1000 km thick atmosphere.

The atmospheric state and composition of Jupiter has been accurately measured by the spacecraft. In situ observations were obtained in 1995, when the probe of the Galileo spacecraft was dropped into Jupiter's atmosphere. It survived nearly an hour before crushing under the pressure, collecting the first direct measurements of Jupiter's atmosphere.

Belts and zones are stable cloud formations (Fig. 7.38). Their width and colour may vary with time, but the semi-regular pattern can be seen up to the latitude 50°. The colour of the polar areas is close to that of the belts. The belts are reddish or brownish, and the motion of the gas inside a belt is downward. The gas flows upward in the white zones. The clouds in the zones are slightly higher and have a lower temperature than those in the belts. Strong winds or jet streams blow along the zones and belts. The speed of the wind reaches 150 m/s at some places in the upper atmosphere. According to the measurements of the Galileo probe, the wind speeds in the lower cloud layers can reach up to 500 m/s. This indicates that the winds in deeper atmospheric layers are driven by the outflowing flux of the internal heat, not the solar heating.

The colour of the Great Red Spot (GRS) resembles the colour of the belts (Fig. 7.39). Sometimes it is almost colourless, but shows no signs of decrepitude. The GRS is 14,000 km wide and 30,000-40,000 km long. Some smaller red and white spots can also be observed on Jupiter, but their lifetime is generally much less than a few years.

The ratio of helium to hydrogen in the deep atmosphere is about the same as in the Sun. The results of the Galileo spacecraft gave considerably higher abundance than previous estimates. It means that there are no significant differentiation of helium, i. e. helium is not sinking to the interior of the planet as was expected according to the earlier results. Other compounds found in the atmosphere include methane, ethane and ammonia. The temperature in the cloud tops is about 130 K.

Jupiter radiates twice the amount of heat that it receives from the Sun. This heat is a remnant of the

Fig. 7.39. Jupiter's Great Red Spot and its surroundings with several smaller ovals as seen by Voyager 1 in 1979. Cloud details of 160 kilometres are visible. (NASA)

energy released in the gravitational contraction during the formation of the planet. Thus Jupiter is still gradually cooling. The internal heat is transferred outward by convection; this gives rise to flows in the metallic hydrogen, causing the strong magnetic field (Fig. 7.41).

The ring of Jupiter (Fig. 7.40) was discovered in 1979. The innermost toroid-shaped halo is between 92,000-122,500 km from Jupiter's centre. It consists of dust falling from the main ring toward the planet. The main ring extends from the halo boundary out to about 128,940 km, just inside the orbit of the moon Adrastea. The ring particles are small, a few microns only, and they scatter light forward much more effectively than backward. Therefore, they were not discovered prior the Voyager flyby. A ring consisting of such small particles cannot be stable, and new material must en-

Fig. 7.40. Mosaic of Jupiter's ring system taken by the Galileo spacecraft when the spacecraft was in Jupiter's shadow looking back toward the Sun. Jupiter's ring system is composed of three parts: a thin outermost ring, a flat main ring, and

an innermost doughnut-shaped halo. These rings are made up of dust-sized particles that originate from Io, or are blasted off from the nearby inner satellites by small impacts. (NASA/University of Arizona)

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Fig. 7.41. Left: NASA Hubble Space Telescope close-up view of an aurora on Jupiter. The image shows the main oval of the aurora, centred over the magnetic north pole, and diffuse emissions inside the polar cap. (NASA, John Clarke/University of Michigan) Right: The image taken on January 2001 by

NASA's Cassini spacecraft shows the bubble of charged particles trapped in the magnetosphere. The magnetic field and the torus of the ionised material from the volcanoes of Io are drawn over the image. (NASA/JPL/Johns Hopkins University)

ter the ring continuously. The most probable source is Io.

The two faint outermost rings are fairly uniform in nature. The inner of them extends from the orbit of Adrastea out to the orbit of Amalthea at 181,000 km. The fainter outermost ring extends out to Thebe's orbit at 221,000 km.

Jupiter's rings and moons exist within an intense radiation belt of Jupiter's magnetic field. The magnetosphere extends 3-7 million kilometres toward the Sun, depending on the strength of the solar wind. In the opposite direction it stretches to a distance of at least 750 million kilometres, behind Saturn's orbit.

Jupiter is an intense radio source. Its radio emission can be divided into three components, namely thermal millimetre and centimetre radiation, nonthermal deci-metric radiation and burstal-decametric radiation. The nonthermal emission is most interesting; it is partly synchrotron radiation, generated by relativistic electrons in the Jovian magnetosphere. Its intensity varies in phase with Jupiter's rotation; thus the radio emission can be used for determining the exact rotation rate. The deca-metric bursts are related to the position of the innermost large moon, Io, and are possibly generated by the million Ampere electric current observed between Jupiter and the plasma torus at the orbit of Io.

In the beginning of year 2006 there were 63 known moons of Jupiter. The four largest, Io, Europa, Ganymede and Callisto are called the Galilean satellites

(Fig. 7.42), in honour of Galileo Galilei, who discovered them in 1610. The Galilean satellites can already be seen with ordinary binoculars. They are the size of the Moon or even planet Mercury. The other moons are small, most of them only a few kilometres in diameter.

Owing to tidal forces, the orbits of Io, Europa and Ganymede have been locked into a resonance, so that their longitudes k strictly satisfy the equation

Hence the moons can never be in the same direction when seen from Jupiter.

Io is the innermost Galilean satellite. It is a little larger than the Moon. Its surface is spotted by numerous calderas, volcanoes without a mountain. The molten material is ejected up to a height of 250 km, and a part of the gas gets into Io's orbit. The volcanic activity on Io is much stronger than on the Earth. There is a 100 m bulk of the permanent tide raised by Jupiter. Due to the orbital perturbations caused by Europa and Ganymede the orbit of Io is slightly elliptical and therefore the orbital speed varies. The tidal bulk is forced to move with respect to the surface. This generates friction, which is transformed to heat. This heat keeps the sulphur compounds molten beneath the colourful surface of Io. No traces of impact craters are visible. The whole surface is new, being renewed continuously by eruptions. There is no water on Io.

Fig. 7.42. (Top) The Galilean satellites of Jupiter. From left page bottom) Surface details of Ganymede and Callisto to right-. Io, Europa, Ganymede, and Callisto (NASA/DLR). (NASA/Brown University, NASA/JPL) (Right page top) Surface details of Io and Europa. (Right

Europa is the smallest of the Galilean satellites, a little smaller than the Moon. The surface is ice-covered and the geometric albedo is as high as 0.6. The surface is smooth with only a few features more than a hundred metres high. Most of the markings seem to be albedo features with very low relief. Only a few impact craters have been found indicating that the surface is young. The surface is renewed by fresh water, trickling from the internal ocean. Galileo spacecraft has found a very weak magnetic field. The field varies periodically as it passes through Jupiter's magnetic field. This shows that there is a conducting material beneath Europa's sur-

face, most likely a salty ocean that could even be 100 km deep. At the centre, there is a solid silicate core.

Ganymede is the largest moon in the solar system. Its diameter is 5300 km; it is larger than the planet Mercury. The density of craters on the surface varies, indicating that there are areas of different ages. Ganymede's surface is partly very old, highly cratered dark regions, and somewhat younger but still ancient lighter regions marked with an extensive array of grooves and ridges. They have a tectonic origin, but the details of the formations are unknown. About 50% of the mass of the moon is water or ice, the other half being silicates (rocks). Contrary to Callisto, Ganymede is differentiated: a small iron or iron/sulphur core surrounded by a rocky silicate mantle with an icy (or liquid water) shell on top. Ganymede has a weak magnetic field.

Callisto is the outermost of the large moons. It is dark; its geometric albedo is less than 0.2. Callisto seems to be undifferentiated, with only a slight increase of rock toward the centre. About 40% of Callisto is ice and 60% rock/iron. The ancient surface is peppered by meteorite craters; no signs of tectonic activity are visible. However, there have been some later processes, because small craters have mostly been obliterated and ancient craters have collapsed.

The currently known moons can be divided into two wide groups: regular moons containing the small moons inside the orbits of the Galilean satellites, and the Galilean satellites, and irregular moons outside the orbit of the Galilean satellites. The orbits of the inner group are inclined less than one degree to the equator of Jupiter. Most of the outermost moons are in eccentric and/or retrograde orbits. It is possible that many of these are small asteroids captured by Jupiter.

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