Chapter 3 Giant planets

  These are the bodies that dominate the Solar System – provided you think it is size that matters, and are willing to overlook the Sun itself. The four giant planets are illustrated to scale in the lower half of Figure 3, showing how comprehensively their size overshadows the terrestrial planets. The view of Uranus is from the Hubble Space Telescope in orbit about the Earth, whereas the other giant planets are as seen by visiting spacecraft. Their domination by mass is not quite so overwhelming, because they are less dense than the terrestrial planets. Jupiter’s density is only24% of the Earth’s, and Saturn is even less dense and would fl oat if dropped into a suffi ciently large (and purely hypothetical)bucket of water. All of them have rings in their equatorial plane,though only those of Saturn and Uranus are suffi ciently prominent to be visible in Figure 3. Although the rings look solid, they are made of myriads of orbiting particles and are extremely insubstantial. They are discussed, along with the giant planets’satellites, in the next chapter.

  

  By convention, the size of a giant planet is measured from the top of its clouds. These occur in the planet’s troposphere, above which are largely transparent and progressively less dense layers classifi able in the same way as for the Earth’s atmosphere. The base of a giant planet’s troposphere is hard to defi ne and has never been explored even in the case of Jupiter, where in 1995 an entry probe released by the Galileo spacecraft reached a depth of 160kilometres below the cloud-tops before pressure (22 atmospheres)and temperature (153 °C) put paid to it. Probably, the troposphere of each giant planet merges seamlessly into a fl uid interior at temperatures and pressures so high that there is no distinction between gas and liquid. Certainly, there is no solid surface that a human could ever stand upon.

  Basic data for the giant planets are given in Table 5 . The polar diameters quoted there are less than equatorial diameters,because rapid rate of rotation (see Table 2) fl attens their shapes.Jupiter’s polar diameter is 6.5% less, and Saturn’s 10% less, than its polar diameter. The difference is only about 2% for the less gassy and more slowly rotating Uranus and Neptune (and is less than 1% for each terrestrial planet).

  Interiors

  There is no simple way to study the interior of a giant planet, but we can use atmospheric composition (99% hydrogen and helium)and our general knowledge of what the Solar System as a whole is made of to construct a model that is consistent with its measured density, and with the interior pressures that we can infer from this. Below the atmosphere, each giant planet must have a zone consisting mostly of hydrogen molecules (H 2 ) and helium atoms(He), in a state that it is better to call 『fl uid』 rather than either『liquid』 or 『gaseous』. At the very centre, there is probably a rocky inner core, of about three Earth-masses inside Jupiter and Saturn,and about one Earth-mass inside Uranus and Neptune.Surrounding the inner core, there ought to be an outer core of 『ice』composed of unknown proportions of water, ammonia, and methane, amounting to about twice the mass of the Earth inside Jupiter, maybe six Earth-masses inside Saturn, twelve inside Uranus, and fi fteen inside Neptune. We do not know whether these outer and inner cores are molten or solid, because although we can estimate the pressure (a staggering 50 million atmospheres in the centre of Jupiter), we do not know the composition and have only a vague idea of the likely temperature(ranging from in excess of 15,000 °C in the centre of Jupiter to about 2,200 °C at the outer edge of Neptune’s core). Our understanding of how materials behave under such extreme conditions is sketchy, including whether metallic iron could differentiate from the rock and sink towards the centre to form an inner-inner core. The cores of Uranus and Neptune might even be undifferentiated mixtures of ice and rock.

  Table 5　Basic data for the giant planets. Note that the mass units are a thousand times bigger than for theterrestrial planets in Table 3

  Accounting for the cores leaves little more than one Earth-mass for the hydrogen and helium exteriors of Uranus and Neptune,comprising shells about 6,000 kilometres thick. However, the『gas giants』 Jupiter and Saturn have much deeper envelopes of hydrogen and helium surrounding their cores, in excess of 300and 80 Earth-masses, respectively. Hydrogen is easier to model than ice or rock, and scientists are pretty confi dent that at pressures greater than about 2 million atmospheres, hydrogen atoms are squeezed so tightly together that electrons are no longer confi ned about specifi c atoms. Instead, they are able to wander through a sea of hydrogen that behaves like a molten metal. This freedom of electron movement makes 『metallic hydrogen』 an excellent conductor of electricity. A shell of metallic hydrogen (with some helium dissolved in it) surrounding Jupiter’s core probably accounts for about 260 Earth-masses(80% of Jupiter’s total mass), whereas around Saturn’s core it is thought to comprise a more modest 41 Earth-masses (just over40% of Saturn’s total mass). Figure 17 illustrates the full internal structure of Jupiter.

  The internal structure of the giant planets may still be evolving because, with the possible exception of Uranus, they all radiate more heat to space than they receive from the Sun. Jupiter is so massive that it could still be leaking out a signifi cant amount of primordial heat trapped within since its formation, but for Saturn and Neptune this heat excess shows that heat must actually be being generated within. The discrepancy is too large to be radiogenic heat, so internal differentiation may still be occurring.The settling of denser than average material inwards (allowing an inner shell to grow while the surrounding shell becomes thinner but purer) would convert gravitational potential energy into heat.Such heat could come from continuing growth of cores (or inner cores) or, for Saturn only, from inward settling of helium droplets inside its metallic hydrogen layer.

  17.Cut-away diagram showing the proposed internal layers withinJupiter. The principal tropospheric cloud-top zones (bright) and belts(dark) are labelled

  Atmospheres

  Composition

  In contrast to the reasoned speculation about giant planet interiors, understanding of their atmospheres can draw more on observation and measurement. The composition of clouds and the overlying layers can be measured by optical spectroscopy, which is the study of how sunlight of different wavelengths is absorbed at various depths within the atmosphere. In addition, the average molecular mass at each depth can be determined by the amount of refraction experienced by radio signals transmitted by a spacecraft while it disappears from view behind the planet. Also, the Galileo entry probe made various measurements inside Jupiter’s atmosphere during its descent. Table 6 compares the chemical composition of the four giant planets』 atmospheres. In addition to the species listed there, each contains smaller traces of ethyne(C 2 H 2 ), Jupiter has ethene (C 2 H 4 ), and Jupiter and Saturn both have phosphine (PH 3 ), carbon monoxide (CO), and germane(GeH 4 ).

  The topmost layer of continuous clouds on Uranus and Neptune is of methane-ice particles. It is too warm for methane condensation at Jupiter and Saturn, where instead ammonia-ice particles condense to form the topmost clouds. These top cloud layers are about 10 kilometres thick, below which the 『air』 probably becomes clear again. Calculations suggest that in the case of Jupiter, there should be a second layer of clouds made of ammonium hydrosulfi de (NH 4 HS) about 30 kilometres below, and a third cloud layer, this time of water (ice at the top, liquid water droplets below) about 20 kilometres lower still. The Galileo entry probe found probable ammonium hydrosulfi de clouds at about the right depth but did not fi nd any water-ice clouds. Some say the models are wrong; others say that the probe penetrated into a gap between discontinuous water-ice clouds. The same cloud layers are expected at Saturn, but spaced about three times further apart because of Saturn’s lower gravity. Ammonia-bearing clouds are expected below the methane clouds of Uranus and Neptune.

  Table 6　Gases detected in the atmospheres of the giantplanets, showing the measured proportion made up by each

  The atmospheric pressure at the top of Jupiter’s ammonia clouds is a factor of two or three less than the sea-level atmospheric pressure on Earth, whereas on the other giant planets the cloud-top pressure is close to Earth’s sea-level pressure.

  Circulation

  A global pattern of cloud bands running parallel to the equator is visible on Jupiter even through a small telescope. A similar pattern is repeated less dramatically on the other giant planets.Solar heating must play some role in the circulation of this, visible,part of their atmospheres, but it appears to be powered mostly by internal heat and to be controlled by their rapid rotation.

  Traditionally, the dark bands are referred to as 『belts』 and the intervening bright bands as 『zones』. The names given to the main belts and zones on Jupiter are indicated on Figure 17 . Because there is no solid surface to act as a frame of reference, wind speeds on giant planets are measured relative to the planet’s average rate of rotation. On Jupiter, the cloud-top wind blows to the east at up to 130 metres per second across most of the Equatorial Zone. The adjacent edges of the North and South Equatorial Belts share this motion, but the wind speed decreases and ultimately reverses with distance away from the equator across each belt until the Tropical Zones are reached, where the wind direction reverses again, and so on with repeated reversals across each belt and zone until the polar regions.

  In Jupiter’s zones, the atmosphere is mostly rising, leading to condensation of ammonia clouds high up, where they naturally appear bright. Conversely, in the belts the atmosphere is mostly sinking, drawing the cloud-tops lower, so to a depth where they look darker. Local exceptions to this pattern have been identifi ed on Jupiter, and the general rule of rising zones and sinking belts scarcely seems to hold at all on the other giant planets, whose atmospheric circulation is harder to fathom. A complicating factor infl uencing the visibility of zones and belts is the poorly understood nature and abundance of whatever trace compounds add colour to the clouds, and which are expected to result from photochemical reactions. Jupiter’s various hues of yellow and red could be caused by sulfur (released photochemically from either hydrogen sulfi de or ammonia hydrosulfi de), phosphorous (from phosphine), or hydrazine (N 2 H 4 , made photochemically from ammonia).

  Colour variations are less pronounced in Saturn’s atmosphere, and the pattern of zones and belts is less prominent. However, wind speeds are higher, with eastward-blowing winds in excess of 400metres per second prevailing for 10° either side of the equator.

  Rotating storm systems are well known on both Jupiter and Saturn. The most famous is Jupiter’s Great Red Spot. This can be seen in Figure 3, as an oval feature straddling the boundary between the South Equatorial Belt and the South Tropical Zone.It extends 26,000 kilometres from east to west, having a spiral structure and taking about six days to rotate anticlockwise. It has been apparent in telescopic observations since at least 1830.Smaller storms can be made out at a variety of scales on both Jupiter (look along the South Temperate Belt in Figure 17 ) and Saturn. About once every 30 years, during summer in its northern hemisphere, Saturn tends to be disfi gured by a giant storm system that begins as a white spot near the equator, but within a month can spread to encircle the globe before gradually fading from view.

  Whereas Jupiter and Saturn have a yellowish cast, Uranus and Neptune appear bluey-green. This is because we see their cloud-tops through a depth of overlying methane gas, which preferentially absorbs the longer (red) wavelengths of light.

  The 82.1 °Caxial tilt of Uranus makes for extreme seasonal variations. For example, when Voyager 2, the only spacecraft yet to have visited Uranus, fl ew past in 1986, the south pole was in full sunlight and most of the northern hemisphere was suffering decades of darkness. Its southern hemisphere looked disappointingly bland on the Voyager images, but as the Uranian year progressed and the Sun began to rise and set over a wider range of latitudes, the globe began more to resemble the other giant planets ( Figure 18 ). In 2007, Uranus passed through its equinox, and the south pole, followed gradually by the rest of the southern hemisphere, began to drift into long-term darkness,which will peak with southern midwinter in 2028.

  When Neptune was revealed in detail during Voyager 2 ’s 1989fly-by, it resembled a blue version of Jupiter. There was even a giant storm system in the form of a dark spot just south of the equator, which was dubbed the 『Great Dark Spot』 in tribute to its famous Jovian cousin. However, it proved to be shorter-lived, and had vanished by 1994. Unlike at Jupiter and Saturn, the equatorial wind stream on Neptune blows west (opposite to the planet’s rotation), as can be seen by the westward drift of the Great Dark Spot relative to the smaller, more southerly spot in Figure 18 .

  Magnetospheres

  Each of the giant planets has a strong magnetic fi eld. The『magnetic dipole moment』 of Neptune, which is the conventional measure of a planetary magnetic fi eld, is 25 times greater than the Earth’s. Uranus’s is 38 times, Saturn’s is 582 times, and Jupiter’s is1,949 times greater. To generate these fi elds, each planet must contain a zone of electrically conducting fl uid undergoing some kind of convective motion. In the two terrestrial planets with magnetic fi elds (Mercury and Earth), the explanation is a fluid shell of their iron cores. The magnetic fi elds of Jupiter and Saturn are probably generated in the metallic hydrogen layer, stirred into motion by the planets』 relatively rapid rotation. Pressures are too low for metallic hydrogen in Uranus and Neptune, so their magnetic fi elds are harder to account for, but are probably caused by motion within electrically conducting 『ice』 of their outer cores.

  18.Top: Uranus seen by the Hubble Space Telescope in August 1998(left) and July 2006 (right). The change in orientation of the planet’saxis relative to the Sun is apparent from the pattern of the atmosphericbanding. The region around the south pole was still in sunlight in1998, but the axis had become nearly edge-on to the Sun by 2006.High, bright clouds are apparent in the far north in the 1998 image,which also shows the rings and several of the inner satellites. The ringswere edge-on and invisible in 2006, but instead we can see one of theregular satellites (Ariel) and its shadow. Bottom: two images ofNeptune seen by Voyager 2 during its approach in 1989, recordedalmost exactly one planetary rotation apart. The Great Dark Spot andassociated wisps of high, bright nitrogen cirrus clouds are prominent.Note also the general banded structure, and a smaller dark spotfurther south

  An important consequence of a planet having a magnetic fi eld(which applies to Mercury and the Earth too) is that it cocoons the planet inside a zone into which magnetic fi eld lines from the Sun cannot usually penetrate. This zone is called the planet’s『magnetosphere』. The paths of charged particles in the solar wind(chiefl y protons and electrons) are controlled by the Sun’s magnetic fi eld, until they hit the 『bow shock』 of a planet’s magnetosphere, which diverts them past the planet.

  Charged particles can get through sometimes, especially by leaking back up the long magnetotail, down-Sun from the planet.Near the poles, these can be channelled along fi eld lines towards the top of the atmosphere, where their arrival causes glows in the sky called aurorae, well known on Earth and observed also on Jupiter and Saturn.