Jupiter

I

Introduction

Jupiter, fifth planet from the Sun, and the largest in the solar system. Named after the ruler of the gods in Roman mythology, Jupiter has 1,400 times the volume of the Earth, but is only 318 times as massive. The mean density of Jupiter is therefore only about one quarter that of the Earth, indicating that the giant planet must consist of gas rather than the metals and rocks of which the Earth and the other inner planets are composed.

Orbiting the Sun at a mean distance 5.2 times as great as that of Earth, Jupiter makes a complete revolution in 11.9 years. It takes only 9.9 hours to rotate once on its axis. This rapid rotation causes an equatorial bulge that is apparent even in telescopic views of the planet. The rotation is not uniform. The banded appearance of Jupiter’s belts is due to the presence of strong atmospheric currents, reflecting the different rotation periods at different latitudes. These belts are made more apparent by the pastel colours of the clouds. These colours are also apparent in the famous brownish-red oval called the Great Red Spot. The colours come from traces of compounds formed by ultraviolet light, lightning discharges, and heat. Some of these compounds may be similar to the organic molecules that formed on the ancient Earth as a prelude to the origin of life (see Exobiology).

II

Composition, Structure, and Magnetic Field

Scientific knowledge of Jupiter and its satellites increased enormously in 1979 with the successful fly-bys of the Voyager 1 and 2 spacecraft launched by the US National Aeronautics and Space Administration (NASA). Spectroscopic observations from Earth had shown that most of Jupiter’s atmosphere is molecular hydrogen, H₂. Infrared studies from the Voyager spacecraft confirmed this, indicating that 87 per cent of the molecules are H₂, with helium, He, constituting most of the remaining 13 per cent. Because the helium molecule has about twice the mass of the hydrogen molecule, these figures indicated that the mass of helium present is about a quarter of the total mass. The interior of the planet must have essentially the same composition as the atmosphere in order to yield the observed low density. Apparently, then, this huge world consists mostly of the two lightest and most abundant elements in the universe, a composition similar to that of the Sun and other stars. Jupiter may therefore represent a direct condensation of a portion of the primordial solar nebula—the great cloud of interstellar gas and dust from which the entire solar system formed about 4.6 billion years ago.

Scientists also collected a large amount of information about Jupiter when fragments of Comet Shoemaker-Levy 9 crashed into the planet in July 1994. The collisions stirred up the planet’s atmosphere, heating interior gases to incandescence and bringing them to the surface. Astronomers captured detailed images of these gases with telescopes on Earth and in space. They used spectroscopes to analyse the gases in order to verify and expand their knowledge of the composition of Jupiter’s atmosphere.

Still more knowledge was gained from the entry into Jupiter’s atmosphere of an entry probe that was part of the Galileo unmanned mission to the planet. In December 1995 Galileo went into orbit around Jupiter after a circuitous six-year flight during which it had flown once past Venus and twice past the Earth, using a gravity assist on each occasion to boost it on its way. Also in December 1995 the entry probe separated from the main craft and entered the atmosphere, deploying parachutes to slow it down. For an hour it transmitted data to the mother craft orbiting above, as it descended to approximately 160 km (100 mi) below the visible cloud-tops before being crushed by the atmospheric pressure. The data were relayed to Earth over a period of months from the orbiter, which continued its investigation of the Jovian system.

Jupiter radiates about twice as much energy as it receives from the Sun. The source of this energy is apparently a very slow gravitational contraction of the entire planet. This is the way in which stars form. However, Jupiter would need to be almost 100 times as massive to produce a temperature at its centre high enough to release nuclear energy in reactions like those that power the Sun and other stars.

In Jupiter’s turbulent, cloud-filled atmosphere, hydrogen-based molecules, such as methane, ammonia, and water, predominate. Periodic temperature fluctuations in the upper atmosphere reveal a pattern of changing winds like that in the equatorial region of the Earth’s stratosphere. Photographs showing sequential changes in Jovian clouds suggest the birth and decay of giant cyclonic storm systems. New understanding has been gained from meteorological data obtained from the Galileo spacecraft.

Planetary physicists had expected Jupiter’s composition to be very similar to that of the primordial gas cloud from which the solar system formed—a composition that survives in today’s Sun. After initial uncertainty, the proportion of helium was confirmed to be about 24 per cent, close to the amount in the Sun. Proportions of heavier elements, such as carbon, nitrogen, and sulphur, are rather greater than in the Sun, probably because they have been increased by billions of years of bombardment by meteoroids and comets.

However, the scientists were surprised to find that significantly less water vapour was detected than they had expected. According to one theory, the “missing” water is locked in Jupiter’s rocky core. According to another, water vapour is not distributed evenly through the outer atmosphere, and the Galileo entry probe happened to penetrate a “desert” region.

The entry probe reported winds of over 650 km/h (400 mph) during its descent. However, it established that anticyclonic and other weather systems appeared to rotate more slowly between jet streams, much like those found in the Earth’s upper atmosphere. In the anticyclonic system studied by the probe, upwelling movements in the centre of the feature are thought to bring ammonia to the top of the atmosphere, where it freezes; it then descends towards the periphery of the system, again entering into the gaseous state. This suggested that the winds seen at the top of the atmosphere are not driven by solar heating of a shallow layer, but rather are part of a system of atmospheric motions that extends deep into the Jupiter’s interior and is driven mainly by the planet’s internal heat.

Ammonia freezes in the low temperature of Jupiter’s upper atmosphere (-125° C/-193° F), forming the white cirrus clouds seen in many photographs of the planet transmitted by the Voyager spacecraft. At lower levels, ammonium hydrosulphide can condense. Coloured by other compounds, clouds of this substance may contribute to the widespread tawny clouds of the planet. The temperature at the tops of these clouds is about -50° C (-58° F), and the atmospheric pressure there is about twice the sea-level atmospheric pressure on Earth. Through holes in this cloud layer, radiation escapes from a region where the temperature reaches 17° C (63° F). Still deeper, warmer layers have been detected by radio telescopes, which are sensitive to cloud-penetrating radiation.

Although only the barest skin of the planet is directly visible, calculations show that the temperature and pressure continue to increase towards the interior. The pressure reaches values at which hydrogen first liquefies and then assumes a metallic, highly electrically conducting state. A solid core, including iron and silicates, may exist at the centre.

Jupiter possesses a magnetic field, which is generated deep within these layers. At the top of the atmosphere, this is 14 times stronger than the Earth’s magnetic field. Its polarity is the opposite to that of the Earth’s field, so a terrestrial compass taken to Jupiter would point south. The Jovian magnetic field is responsible for huge radiation belts of trapped charged particles that encircle the planet out to a distance of 10 million km (6 million mi).

III

Jupiter’s Satellites and Rings

At least 63 significant satellites of Jupiter have been discovered. The four largest were first observed in 1610 by Galileo. They were subsequently named after mythological lovers of the god Jupiter (or Zeus in the Greek pantheon): Io, Europa, Ganymede, and Callisto. This tradition has been followed in the naming of other satellites. Recent measurements have shown that the mean density of the largest satellites follows the trend apparent in the solar system itself. Io and Europa, close to Jupiter, are dense and rocky like the inner planets, while Ganymede and Callisto, at greater distances, are composed largely of water ice and have lower densities. During the formation of both planets and satellites, proximity to the central body (the Sun or Jupiter) evidently prevented the more volatile substances from condensing.

Callisto is almost as big as Mercury, and Ganymede is bigger than Mercury, being the largest satellite in the solar system. If they orbited the Sun as independent bodies, they would be considered planets. The icy crust of these two bodies is marked by numerous impact craters, the record of an early bombardment, probably by comet nuclei, similar to the asteroidal battering that scarred the Earth’s Moon and other inner solar-system bodies. Callisto is the most heavily cratered of the Galilean satellites, suggesting that its surface is the oldest. In 1997 the Galileo spacecraft discovered that it has a tenuous atmosphere of hydrogen and carbon dioxide, and in 1998 the discovery of a variable magnetic field suggested the possible presence of an ocean of salty water beneath its crust, similar to that of Europa (see below).

The Galileo spacecraft also detected a magnetic field associated with Ganymede suggesting that it must have generated enough internal heat to maintain a partially molten interior. It appears to have a metallic core 400 to 1,280 km (250 to 800 mi) in diameter surrounded by a mantle of ice and silicates, and a thick water-ice crust. The surface is a mixture of two terrain types: 40 per cent of the satellite is covered by highly cratered dark regions that appear to be old, while the remaining 60 per cent is covered by younger, light, grooved terrain probably formed by tensional fracturing or the release of water from below. The large craters on Ganymede have no relief, probably due to their gradual adjustment into the soft icy surface; they are called “palimpsests”. Ganymede has evidently had a complex geological history.

Europa is the smallest of the Galilean satellites and has an extremely smooth surface with a reflectance that is five times that of our Moon. It too has two types of terrain: mottled, brown or grey hilly units, and large smooth plains criss-crossed with cracks. The patterns made by the latter are similar to those seen in terrestrial polar sea ice, and Galileo images reveal a huge number of what appear to be ice rafts that have moved relative to one another, evidently over a mobile layer beneath. Data from Galileo’s magnetometer indicate the strong possibility that below the ice there is a water ocean generated by internal warming driven by constant tidal distortions. There is a possibility that life could have developed in this ocean. After the end of the Galileo orbiter’s primary mission in December 1997, NASA decided to make Europa the focus of an extended mission, the Galileo Europa Mission (GEM), a two-year project including eight further fly-bys of the satellite.

The most remarkable of the Galilean satellites is unquestionably Io. Its surface has a bizarre appearance: yellowish, brown, and white areas dotted with black features. Io is racked by volcanism that is driven by the dissipation of tidal energy in the satellite’s interior. Sulphur dioxide (SO₂) issues from the vents and condenses on the surface, forming a local, transient atmosphere. The white regions are solid sulphur dioxide; the other markings are presumably caused by other sulphur compounds. Ten volcanoes were observed to be erupting during the Voyager fly-bys in 1979. Galileo images show that many further eruptions have taken place. In particular, the prominent volcano Prometheus, first imaged by Voyager in 1979, had by 1997 developed an eruption plume emanating from a point 75 to 95 km (45 to 60 mi) west of the original hot spot, while a new dark lava flow had emerged from its vent. Scientists have speculated that the plume is fed by vaporized sulphur-dioxide-rich snow under the lava flow. Comparison of Voyager and Galileo maps of Io reveal several other new plumes and considerable changes to those observed in 1979. Measurements of Loki, the most powerful of Io’s volcanoes, indicated that it was putting out more heat than all the Earth’s active volcanoes combined. Other Galileo images show mountains 16 km (52,000 ft) high—nearly twice the height of Mount Everest.

The remaining moons are very much smaller and not as well studied as the four Galilean satellites. Four lie within the orbit of the Galilean satellites, and their diameters range in size from 20 km/12.5 mi (Adrastea) to 189 km/117 mi (Amalthea). In order of increasing distance from Jupiter they are: Metis, Adrastea, Amalthea, and Thebe. The remaining satellites, beyond the Galilean satellites, are concentrated in several groups and may represent captured bodies. They range in diameter from 2 km/1.2 mi to 186 km/116 mi. The two groups that contain the largest eight comprise, in order of increasing distance from Jupiter: Leda, Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae, and Sinope.

Close to Jupiter, the Voyager spacecraft discovered a faint system of rings, comprising a main ring and an outer, “gossamer” ring, itself made up of inner and outer rings orbiting separately. The material in these rings must be continuously renewed, since the rings are visibly moving in towards the planet. In 1998 scientists studying images from the Galileo spacecraft confirmed that the rings consist of dust produced by collisions of cosmic material with the inner satellites. The gossamer rings are made up of material from Amalthea and Thebe, while the material in the main ring comes from Metis and Adrastea. There is also a halo, made up of dust pulled from the outer rings by Jupiter’s powerful electromagnetic field.