Giant Planets

1.23 Giant Planets J.I. Lunine University of Arizona,Tucson, AZ, USA 1.23.1 THE GIANT PLANETS IN RELATION TO THE SOLAR SYSTEM 1.23.1.1 Basic Physical and Orbital Parameters 1.23.1.2 Discovery and Historical Investigation of the Giant Planets 1.23.2 ESSENTIAL DETERMINANTS OF THE PHYSICAL PROPERTIES OF THE GIANT PLANETS 1.23.2.1 How We Know the Giant Planets Contain Hydrogen and Helium 1.23.2.2 The Equation of State of Hydrogen and Helium as a Determinant of the Structure 1.23.2.3 The Thermal Infrared Emission of the Giant Planets and Implications for Evolution 1.23.2.4 The Interior Structure of the Giant Planets 1.23.2.5 Elemental and Isotopic Abundances 1.23.2.6 Atmospheric Dynamics and Magnetic Fields 1.23.3 ORIGIN AND EVOLUTION OF THE GIANT PLANETS 1.23.3.1 Basic Model for the Formation of the Planets from a Disk of Gas and Dust 1.23.3.2 Constraints from the Composition of the Giant Planets 1.23.4 EXTRASOLAR GIANT PLANETS 1.23.5 MAJOR UNSOLVED PROBLEMS AND FUTURE PROGRESS REFERENCES

1.23.1

1.23.1.1

1 1 3 4 4 5 7 8 9 10 11 11 12 12 13 14

planets, and the range of magnetic field strengths is larger in the outer solar system as well. It is the giant planets that sport rings, ranging from the magnificent ones around Saturn to the variable ring arcs of Neptune. Were it not for the fact that only Earth supports abundant life (with life possibly existing, but not proved to exist, in the martian crust and liquid water regions underneath the ice of Jupiter’s moon Europa), the terrestrial planets would pale in interest next to the giant planets for any extraterrestrial visitor. Modern telescopic and spacecraft study of Jupiter, Saturn, Uranus, and Neptune, their properties, and their systems of rings, moons, and magnetospheres, has been the purview of the planetary scientist with little connection to the universe beyond until 1995, when the first extrasolar giant planet was discovered. Now the solar system’s giants are the best-studied example of a class of some 100 objects which—while only one has been measured for size and hence density— may be present B10% of Sun-like stars.

THE GIANT PLANETS IN RELATION TO THE SOLAR SYSTEM Basic Physical and Orbital Parameters

Beyond the inner solar system’s terrestrial planets, with their compact orbits and rock– metal compositions, lies the realm of the outer solar system and the giant planets. Here the distance between planets jumps by an order of magnitude relative to the spacing of the terrestrial planets, and the masses of the giants are one to two orders of magnitude greater than Venus and Earth—the largest terrestrial bodies. Composition changes as well, since the giant planets are largely gaseous, with inferred admixtures of ice, rock, and metal, while the terrestrial planets are essentially pure rock and metal. The giant planets have many more moons than do the terrestrial planets, at last count over 150 (plus countless amounts of lesser debris) versus three for the terrestrial 1

Giant Planets

2

Table 1 Basic physical parameters of measured giant planets. Object

Orbital radiusa

Period (years)

Eccentricity of orbit

Inclination (degree)b

Planetary radiusc

Planetary massd

Magn. field e

Jupiter Saturn Uranus Neptune HD209458bf

5.20 9.54 19.2 30.1 0.045

11.9 29.4 83.7 164 0.0096

0.048 0.054 0.047 0.009 0.0

1.3 2.5 0.77 1.77 ?

11.2 9.45 4.01 3.88 1671

318 95.2 14.4 17.1 21976

20,000 590 50 28 ?

Sources: Cox (2000) and Cody and Sasselov (2002). a Semimajor axis, in AU, where 1 AU ¼ Earth–Sun mean distance ¼ 1.496
1013 cm. b Inclination of orbit relative to an invariable plane. c In units of Earth radii, where 1 Earth radius ¼ 6.378
108 cm at the equator. d In Earth masses, where 1 Earth mass ¼ 5.974
1027 g. e In units of the Earth’s dipole field, where the centered magnetic dipole field of the Earth is 3
104 nT. f For orbital parameters refer to Charbonneau et al. (2000).

The basic physical and orbital parameters of the solar system’s giant planets (Cox, 2000) are summarized in Table 1. Included as well in the table are data on the one giant planet beyond our solar system, the companion to the star HD209458, for which size and physical mass information exist in addition to orbital data (Cody and Sasselov, 2002). Notable about this object is its proximity to its parent star, which is much like the Sun, relative to the giant planets of our own solar system. This proximity is responsible for another interesting feature about the extrasolar giant planet, its low density compared to Jupiter and Saturn. Despite the low density, the basic properties of this object are fully consistent with its being primarily a hydrogen–helium planet, like Jupiter and Saturn (and, to a lesser extent, Uranus and Neptune). Hence, we must consider it very much of a class with our own system’s giant planets—affording the first indication that such objects are potentially a common phenomenon in the cosmos. In the context of Jovian mass bodies being detected around other stars, the question often arises as to what might be considered the definition of a planet, and, in particular, where is the crossover between planets and ‘‘failed stars,’’ or brown dwarfs, that do not undergo significant fusion reactions. There is a convenient, if approximate, hierarchy that can be assigned to self-gravitating objects from the most massive stars to Earth-sized planets, corresponding roughly to a progression of powers of 10 in mass. The mass of the Sun, 1.99
1033 g, and that of Jupiter, a bit more than three orders of magnitude smaller, are useful units of currency. Stars 10 times more massive than the Sun can be considered highmass stars, with geologically and astronomically short lifetimes, o107 years, defined by the ‘‘main-sequence’’ phase of stable hydrogen fusion (Kippenhahn and Weigert, 1991). Below 0.1 (strictly, 0.08) solar masses, the interiors of stars are not sufficiently hot to undergo

self-sustained hydrogen fusion, and hence do not possess a stable main sequence phase of hydrogen burning. These so-called brown dwarfs evolve downward in luminosity and surface (photospheric) temperature over time. Below B0.01 (strictly 0.013) solar masses, interior temperatures are insufficient even for the fusion of deuterium to occur. This has been proposed as a convenient boundary between planets and brown dwarfs, with the virtue that it is unambiguously based on the cessation of a particular physical process (Hubbard, 1989). For those who prefer a distinction based on formation processes, motivated by the view that brown dwarfs form from direct gaseous collapse like stars, whereas planets form through accretion of solids and gas in a disk, the threshold may not be too different. There is some suggestion from theory that bodies larger than 10 Jupiter masses (0.01 solar masses) may form preferentially by direct collapse, though the threshold is likely to be uncertain by several factors, and significant overlap in masses generated by the two processes is likely (Bodenheimer et al., 2000). There may even be a weak preference for making bodies of the order of a Jupiter mass (0.001 solar masses) based on the current planet detection statistics (Mayor et al., 1998). At 0.1 Jupiter masses, if Uranus and Neptune are any guide, giant planets are so noncosmic in composition (with a strong enrichment in elements heavier than hydrogen and helium) that a separate class of ‘‘ice giants’’ has been proposed. Whether 0.1 Jupiter mass objects that are more cosmic in composition exist around other stars is not known, but such objects could be difficult to generate without adding large amounts of rocky and icy elements. Finally, at 0.01 Jupiter masses (a few Earth masses), we enter the realm of the terrestrial planets—in essence, the rocky component of the giant planets bereft of icy and gaseous materials. Addition of volatiles to these bodies to form hydrospheres and organic

The Giant Planets in Relation to the Solar System carbon reservoirs is likely to vary significantly from one system to another, so that the possibility of an Earth-sized planet with the volatile complement of the Moon cannot be ruled out (Lunine, 2001).

1.23.1.2

Discovery and Historical Investigation of the Giant Planets

Jupiter and Saturn are naked-eye objects and were known to the ancients; Uranus is barely detectable by the unaided eye but has undoubtedly been seen by numerous keen-eyed individuals in the times before modern lighting systems dimmed the night sky over much of the settled Earth. The classical Greeks, one among many civilizations, that noticed the regular motions of a handful of points of light in the sky, described these moving objects as planets, meaning wandering, and hence their modern name planet. The association of specific planets with particular deities was a practice established by non-Greeks and non-Romans, according to Plato. For example, it was in Babylon that one of the brightest planets was associated with Marduk, king of the gods, and by this was the planet eventually associated with Jupiter, the equivalent in the Roman pantheon (Krupp, 1983). The discovery of Uranus (so named to continue the classical tradition) is credited to W. Herschel, who first detected it with a telescope in 1781 and thought it a new comet. Further observations of its brightness and motion determined that it is a planet with an orbit almost twice the semimajor axis of that of Saturn. Tracking of Uranus over several decades suggested that the planet’s orbital motion deviated from that prescribed by Kepler’s laws, so that the position of a yet more distant planet was predicted independently by J. C. Adams in England and U. J. J. Le Verrier in France. G. Galle and H. L. D’Arrest found the planet, named Neptune, the evening they received the prediction, in 1846. (In fact, Galileo was the first to see Neptune, recording it as a star shifting in the field of view of his 1612 observations of Jupiter, but he did not recognize it as a previously unknown planet (Kowal and Drake, 1980).) Apparent deviations from Keplerian motion in Neptune’s orbit, now regarded largely as spurious, motivated the search for another giant, a search which came up empty handed but led to the twentiethcentury discovery of Pluto and then the Kuiper Belt just beyond the realm of the giant planets. As described more thoroughly in Chapter 1.17, B100 extrasolar giant planets have been indirectly detected between 1995 and 2002 via the

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gravitational pull on their parent stars. Only one of these, HD209458b, has been directly seen transiting in front of its parent star, an observation first made in 1999 (Charbonneau et al., 2000). Unlike Uranus, for which the evocative Olympian name soon replaced Herschel’s initial proposal of ‘‘[King] George’s star,’’ no plans exist to do likewise for the only directly known extrasolar giant. The discovery of the rings and moons of the giant planets constitutes a much more complex history beginning with Galileo’s 1610 discovery of the four large moons of Jupiter and his glimpse of the rings of Saturn. Galileo himself did not understand the annular nature of the rings, thinking Jupiter might be oblate; C. Huygens first discerned this in 1659, and J. D. Cassini discovered the first major division, or gap, in the rings in 1676. As telescopes progressively increased in aperture and became optically more precise, more moons were progressively discovered, a process that continues today. The Pioneer and Voyager flyby spacecraft discovered a handful of the Jovian and Saturnian satellites, and the majority of the known satellites around Uranus and Neptune. The structure of Saturnian ring system was gradually revealed as well, while those of Uranus and Neptune discovered by stellar occultations observed from Earth-based aircraft and ground telescopes. The detailed structures of the three ring systems, and the discovery of the tenuous Jovian ring, were not discerned until the Voyager flybys of the four giant planets in the decade from 1979 to 1989, supplemented by studies in the 1990s using the Hubble Space Telescope (HST) (see Chapter 1.24 for a detailed treatment of the satellites of the outer planets). The history of atmospheric and interior studies of the giant planets is likewise complex. The first atmospheric compounds to be detected in Jupiter were methane and ammonia, detected by telescopic spectroscopy (Wildt, 1932). Measurement of the overall density of Jupiter and Saturn essentially required that the bulk constituent of the interior be hydrogen, but this was not detected in Jupiter until 1957 (Kiess et al., 1960), and later in the other giant planets. The first reasonably accurate quantitative modeling of the composition of Jupiter based on the behavior of hydrogen and helium at high pressure was that of Demarcus (1958). This work established that Jupiter and Saturn are roughly of solar composition, an intriguing fact that was left unexplained by the starformation models of the day (and, in fact, elicited little scientific interest). The discovery of an excess luminosity in Jupiter and Saturn through pioneering thermal infrared work (Low, 1966) indicated that the two objects probably formed

4

Giant Planets

through some sort of collapse process. It would not be until the mid-1970s that serious work on the formation of these objects proceeded—along with ever more detailed compositional measurements and interior models. Much of what was learned about Jupiter and Saturn from groundbased studies required, for Uranus and Neptune, spacecraft observation by Voyager and then the European Space Agency Infrared Space Observatory (Atreya et al., 1999a). In 1995, Jupiter’s atmosphere was studied in situ by the mass spectrometer aboard the Galileo Probe, which transmitted down to a pressure level exceeding 20 bar (Niemann et al., 1996). The Galileo Orbiter, after relaying data from the Probe, made a series of observations of Jupiter from orbit until 2003. The Cassini Orbiter, carrying the Huygens Probe, made relatively distant observations of Jupiter during a flyby in December 2000 and then attained orbit around Saturn in July 2004; it continues its 4-year prime mission of observing Saturn, its rings, moons and magnetosphere, at the time of this writing. Information regarding the interiors of the giant planets from the presence of magnetic fields became available beginning in 1950 with the detection of radio-emissions from Jupiter at frequencies ranging from 10 to 1,000 MHz. The highest-frequency radiation was interpreted, correctly, as synchrotron emission from electrons forced to gyrate along magnetic field lines generated by Jupiter. From the spatial pattern of the emission, observed from the Earth, the strength and dipolar nature of Jupiter’s magnetic field were revealed (Dessler, 1983). Further information on the nature of the field and the ‘‘magnetosphere’’ of charged particles bound to it were made by Pioneer, Voyagers 1 and 2, Ulysses, Galileo, and Cassini. Because of their greater distance from the Earth and smaller magnitudes, the magnetic fields of the three other giant planets awaited discovery by planetary spacecraft, specifically Pioneer 11 at Saturn in 1979, and Voyager 2 at Uranus and Neptune in 1986 and 1989, respectively. Cassini is currently mapping the Saturnian magnetic field and magnetosphere in great detail (e.g., Mitchell et al., 2005). More directly tied to the modeling of the interior of the giant planets are data on the gravitational moments of the giant planets, derived from measuring the path of flyby spacecraft and optical tracking of the natural satellites. Also germane to interior models is the determination of the value of the internal heat emitted from the giant planets over and above that gained by the absorption of incident sunlight. While Earth-based studies were able to determine approximate values, it required the Voyager flybys to refine both

the infrared emission and the angular dependence of the scattering of sunlight by the clouds to most accurately determine the energy balance on each of the giant planets (Pearl and Conrath, 1991).

1.23.2

1.23.2.1

ESSENTIAL DETERMINANTS OF THE PHYSICAL PROPERTIES OF THE GIANT PLANETS How We Know the Giant Planets Contain Hydrogen and Helium

Hydrogen, the most abundant element in the cosmos, is difficult to measure spectroscopically in its molecular form, because it is a symmetric molecule with permitted features in the rotational infrared part of the spectrum only. Pressure-induced, also referred to as collisioninduced, lines occur in the visible and nearinfrared part of the spectrum for dense gases, in which frequent collisions among hydrogen molecules distort the symmetric electronic structures leading to transient electric moments. The fundamental rotation–vibration pressure-induced band for H2 is at 2.4 mm, with overtones extending into the visible. Sharp quadrupole transitions of molecular hydrogen seen at 0.85 and 0.65 mm wavelengths as well as the more diffuse dipolar features establish that hydrogen is present and abundant in the atmospheres of all four of the giant planets of our solar system (Spinrad, 1963). While other molecules, such as methane, dominate the spectra of the giant planets, this is the result of the differing intrinsic strengths of the spectroscopic features; the contribution of hydrogen to their spectra is sufficient to suggest it as the dominant atmospheric constituent. Helium, as the lightest noble gas, is so spectroscopically inactive that its presence can be detected only through the effect this atom has on the shape of the pressure-induced absorption features of hydrogen. The determination is difficult as the effect is subtle, and is most pronounced in the thermal infrared part of the spectrum (10–30 mm) where terrestrial atmospheric water vapor impedes the accuracy of ground-based studies. The Voyager infrared interferometric spectrometer (IRIS) made determinations for Jupiter and Saturn (Conrath and Gautier, 2000), but the atmospheres of Uranus and Neptune are so cold that insufficient sensitivity was available from IRIS. For all four planets, a second determination was obtained by deriving the temperature– pressure profile of the atmosphere through radio-occultation and infrared measurements. The refraction of a radio beacon sent through

Essential Determinants of the Physical Properties of the Giant Planets the atmosphere by Voyager, as it passed behind each of the giant planets, could be converted to a number density profile with altitude under the assumption of a hydrogen-rich atmosphere. This profile, to which is added the constraints of hydrostatic equilibrium and the equation of state of an ideal gas, yields the temperature divided by the atmospheric molecular weight. Infrared brightness temperatures at various altitudes obtained by IRIS can then be matched to the spectrum to extract the molecular weight of the atmosphere. To the accuracy of the measurement only hydrogen and helium contribute to the molecular weight with any significance except perhaps in Neptune where N2 may have a measurable effect (Gautier et al., 1995). Even for Jupiter and Saturn, the collision-induced determination is difficult, and subsequent in situ measurements of the helium abundance with the Galileo mass spectrometer and the helium abundance detector (Von Zahn et al., 1998) have led to reanalysis of the Voyager IRIS determination. The best-fit helium determination for each of the giant planets, expressed as a number (mole) fraction relative to molecular hydrogen, is 0.135970.0027 for Jupiter, 0.13570.025 for Saturn, 0.15270.033 for Uranus, and 0.19070.032 for Neptune (Von Zahn et al., 1998; Conrath and Gautier, 2000; Fegley et al., 1991; Gautier et al., 1995). The solar abundance—that found in the atmosphere of the Sun—is virtually identical to the number for Jupiter, but the value obtained at the time the planets formed and derived by applying corrections associated with sedimentation of helium in the Sun is B15% higher (Guillot et al., 2003). Therefore, helium in the upper layers of both Jupiter and Saturn is depleted relative to the solar primordial value, and hence perhaps relative to the total bulk abundance of this element in the interiors of the two planets.

1.23.2.2

The Equation of State of Hydrogen and Helium as a Determinant of the Structure

While both main sequence stars and giant planets are self-gravitating bodies composed mostly of hydrogen and helium, there is a fundamental difference between the two classes in the nature of the equation of state of these materials. Unlike stars, giant planets do not behave as ideal gases in their interiors. The difference is a result of the densities in the interiors of the two classes of bodies; stars are thermally expanded by virtue of hydrogen

5

fusion, and hence thermal pressure is much larger than electronic or degeneracy pressure. The temperature–pressure profiles in giant planets pass through a complex region of the phase diagram of hydrogen and helium where the atoms and molecules are separated by distances comparable to the dimensions of the particles themselves. Hence, pressure dissociation of the molecules into atoms, and ionization compete with or are more important than ionization and dissociation due to thermal effects. The standard Saha relations for determining the fractional ionization and dissociation as a function of temperature, pressure, and composition do not work in this realm of giant planet interiors. For this reason, much experimental and theoretical effort has been expended to quantify the behavior of hydrogen–helium mixtures under conditions relevant to the giant planets. This behavior of hydrogen and helium, coupled to its ideal gas behavior under much lower pressures, has a fundamental implication for the size of giant planets, as was shown in the classic paper by Zapolsky and Salpeter (1969). Imagine an object that is made of pure hydrogen, or an admixture by mass of 75% hydrogen, 25% helium, and is cold, so that thermal energy plays no role. At sufficiently low mass of such a self-gravitating object, the behavior of the material is that of an incompressible gas for which the addition of more mass leads primarily to an increase in the radius or size of the body. In the limit of high mass, the hydrogen–helium mixture behaves as a degenerate gas, in which electrons may occupy only the lowest energy states available and Fermi statistics apply. Addition of more mass, in this regime (in which relativistic effects that apply at irrelevantly high masses are neglected), leads to a reduction in the radius of the body. Extension of these two regimes toward each other leads to a maximum radius at a critical mass close to, but somewhat larger than, the mass of Jupiter (Zapolsky and Salpeter, 1969). The maximum radius is essentially that of Jupiter for a solar composition of hydrogen and helium; it is less for a superabundance of nonhydrogen elements, and is B15% higher than that of Jupiter for a pure hydrogen body. The observational implication of this analysis, which holds today despite the equation of state of hydrogen and helium having been refined many times since, is that giant planets are not much larger in surface area, hence in reflected brightness, than Jupiter. One cannot make a larger giant planet by simply adding mass; one must add thermal energy. Stars, defined as bodies 75–85 times the mass of Jupiter

Giant Planets

6 Molecular hydrogen Metallic hydrogen “Ice” Rock

~500,000 K ~170 Gbar GI229b (brown dwarf)

~17,000 K ~70 Mbar Jupiter

~22,000 K ~30 Mbar HD209458b (extrasolar giant planet)

~13,000 K ~18 Mbar Saturn

~5,000 K ~8 Mbar Uranus or Neptune

Figure 1 Some known giant planets and brown dwarfs, illustrated with a limited azimuthal slice (pie slice) to correct scale. The interiors are color coded according to the principal materials in each zone. Ice and rock refer to elements common in materials that are icy or rocky at normal pressures. Metallic hydrogen indicates ionization primarily through pressure effects. Modeled central temperature in Kelvin, and pressure in 109 bar, is shown. The radii of all but Gl229b are known directly; for Gl229b, modeling of the brightness versus wavelength must be used to derive the radius. From left to right, the masses (expressed relative to the mass of Jupiter) are 45, 1, 0.7, 0.3, and 0.05. Courtesy of W. B. Hubbard and T. Guillot.

(a mass sufficient to ignite hydrogen fusion and hence sustain huge internal temperatures), are much larger because of the thermal pressure contribution to their structure. Young giant planets, or those so close to their parent stars that substantial energy is gained from stellar photons, may be larger, but except for the earliest million years or so of a giant planet’s existence, radii more than a factor of 50%— surface areas more than a factor of 2—that of Jupiter are not possible. This limits the detectability of such objects in astronomical searches. Brown dwarfs are transitional, in that those close to the hydrogen-burning limit may remain hot and hence thermally expanded for astrophysically interesting timescales. However, others may not be, for example, the brown dwarf Gliese 229 B, while perhaps 40–50 times or so more massive than Jupiter, is in fact smaller in volume (Burrows et al., 2001). Conversely, Uranus and Neptune are substantially smaller than Jupiter both because of their lesser mass and their larger fractional complement of elements heavier than hydrogen and helium; the latter compositional effect yields a factor of 2 smaller radius over and above the mass effect (Zapolsky and Salpeter, 1969). The menagerie of giant planets and brown dwarfs

can be shown conveniently on a single scale, at least as far as physical size is concerned (Figure 1). Detailed modeling of the sizes of the giant planets, as indicators of their composition and thermal history, require equations of state that are more detailed and better tied to experimental measurements than that of Zapolsky and Salpeter (1969). Figure 2 illustrates a modern phase diagram for pure hydrogen along with some of the experimental constraints from which it is obtained, in the temperature– pressure regime appropriate for the interiors of Jupiter and Saturn. Most interesting about the diagram is the prediction in one heavily used equation-of-state model (Saumon et al., 1995) of a so-called plasma phase transition (PPT) by which the relative abundances of molecular and atomic hydrogen change in a discontinuous fashion. This is based on a particular model of the interactions among protons and electrons in dense liquid hydrogen, wherein the free energy is minimized to identify the preferred phases. An alternative model that assumes that the PPT is replaced by a continuous transition from molecular to metallic hydrogen can better fit much of the shock data (Ross, 1998). However, this theory is

Essential Determinants of the Physical Properties of the Giant Planets

7

4.5 iter Jup Saturn

log T (K)

PP

La se rs ho ck

4.0

T?

50

Liquid H+

%

3.5

Y = 0.2

48 Y = 0.2 He im miscib 1 ility?

ock

sh rb. eve

R 3.0

uid

Liq

+

Solid H

H2 id H 2

Sol 2.5 −1.0

−0.5

0.0

0.5 log P (Mbar)

1.0

1.5

2.0

Figure 2 Phase diagram of pure hydrogen displayed as the logarithmic temperature versus pressure. Theoretical temperature–pressure profiles for Jupiter and Saturn are shown. Laser shocking of molecular hydrogen covers the temperature–pressure region indicated by the upper dashed-dot line, while the reverberation shock experiments, at cooler temperatures, measured the electrical conductivity of hydrogen at the points indicated. A possible discrete transition between molecular and atomic (metallic) hydrogen is marked as PPT, while the 50% line below it indicates where half the hydrogen is metallic in a competing model for which the transition is continuous (Ross, 1998). A possible boundary in T–P space below which helium is immiscible in hydrogen, for mass fractions of helium indicated by ‘‘Y,’’ is shown with a hatched double line. Source: Hubbard et al. (2002).

ad hoc and some of the data it does fit have been called into question (Hubbard et al., 1999). The debate is not an academic one, because the different equations of state make different predictions for the solubility of helium in metallic hydrogen. The depleted mixing ratio of helium relative to hydrogen in the Jovian and Saturnian atmospheres suggests that helium is being extracted from the upper envelope of these planets and sequestered in the deep interior, perhaps because of immiscibility. This process should be revealed through the resulting conversion of gravitational potential energy into heat (Stevenson and Salpeter, 1977), and this appears to be the case at least for Saturn (Hubbard, 1989). Regardless of which equation-of-state model is correct, the existence of electrically conducting metallic hydrogen in the interiors of Jupiter and Saturn, combined with their rotation, provide a natural dynamo mechanism for their magnetic fields (Stevenson, 1998). These, in turn, have a pervasive impact on the nature and evolution of the surfaces and atmospheres of the natural satellites. The equation of state is also crucial in determining the radius of the extrasolar giant HD209458b under the extreme optical irradiation by its nearby parent star, as is discussed below.

Finally, Uranus and Neptune represent another challenge in high-pressure equations of state, since their deep interiors are largely rocky and icy elements with no metallic hydrogen. The source of electrically conducting fluid in these bodies may instead be water ionized at high pressure (Podolak et al., 1991).

1.23.2.3

The Thermal Infrared Emission of the Giant Planets and Implications for Evolution

Sources of energy leading to emission of heat, and hence thermal radiation, from planets include sunlight (or the equivalent from the parent star for extrasolar planets), virialized heat of collapse, gravitational differentiation, and radioisotope decay. The second of these sources refers to the conversion of gravitational potential energy into heat as distributed material accretes to form self-gravitating bodies. The third source, referred to as core formation in the evolution of the terrestrial planets, is due to the sinking of heavier material toward the center of the planet with release of gravitational energy. All four of these sources play important roles in the rocky planets during all or some

8

Giant Planets

part of their evolution. Except for the last source, the same holds for the giant planets. Measurement of the net heat flow associated with formation of and internal processes within the giant planets requires measurement of both the thermal infrared emission and absorption of sunlight. The former is a difficult measurement to perform from Earth for Uranus and Neptune. For all four giant planets, measurement of the absorption of sunlight is also difficult, because it requires accounting for scattered sunlight off of clouds, at angles difficult or impossible to measure from the Earth. For these reasons, the energy balance within the four giant planets of our own solar system has required the Voyager spacecraft for definitive measurement. For the well-studied extrasolar giant planet HD209458b, no direct measurement is possible, but the inflation of its radius (see below) provides an indirect indicator of this process. Define the energy balance of a planet as the ratio of the total thermal emission at present relative to the amount of sunlight re-emitted as thermal radiation, so that a value of unity obtains for an object whose sole source is sunlight. One finds a value of 1.6770.09 for Jupiter, 1.7870.09 for Saturn, 1.0670.08 for Uranus, and 2.6170.28 for Neptune (Hubbard et al., 1995). That Uranus and Neptune are so different in regard to the energy balance can be seen also in the entirely coincidental correspondence in the effective radiating temperature of the two planets, which is 59 K despite Neptune being more than 50% further from the Sun. Interpretation of the lack of a signature of internal heat from Uranus is difficult, and some models have invoked different formation conditions while others the extreme tilt of Uranus on its axis, such that during the Voyager flyby one pole of Uranus was pointed toward the Sun (Lunine, 1993). The quiescence of the Uranian atmosphere relative to the Neptunian atmosphere was striking in Voyager images of the two bodies, and the subsequent apparent increase in convective activity on Uranus as imaged by HST suggests that indeed a seasonal effect plays a role in a possibly variable Uranian heat balance (Hammel et al., 2001). For Saturn versus Jupiter the story is a bit clearer. Models of the interior and evolution of Jupiter (described further below) produce the currently observed effective temperature with only sunlight and the original virialized energy of collapse; no additional differentiation is required at present. However, in the case of Saturn, additional energy is required to obtain a body of its effective temperature and mass at an age of 4.56 Gyr, implying that differentiation is taking place (Hubbard et al.,

2002). The main heavy constituent contributing to this additional source of energy as it sinks is most plausibly helium, on the basis of its observed abundance and the phase diagram discussed above, which implies the potential for immiscibility in the current epoch (Stevenson and Salpeter, 1977). The absence of evidence for the additional energy source on Jupiter may indicate that immiscibility began later on Jupiter, and has contributed proportionately less to the total energy balance than in the case of Saturn, or has not begun at all (Fortney and Hubbard, 2002).

1.23.2.4

The Interior Structure of the Giant Planets

Construction of a model of the interior structure of hydrogen–helium objects requires the imposition of an equation of state along with a thermal model, which together prescribes the temperature–pressure–density relationship at any given time in the planet’s history. Integration of the equation of hydrostatic equilibrium then yields the distribution with radius of the mass (strictly mass density) in the interior of the planet. Since the composition and the internal distribution of the chemical elements are unknown, they must be constrained by an additional set of observations, namely, the gravitational field of the planet and its rotation rate (which, when coupled with the planetary oblateness, depends on the mass distribution). Ground-based studies can provide the required data, through the orbital precession of rings and satellites, measured shape and rotation rate, but with significant ambiguities. Satellite measurements, such as from Voyager, provide higher accuracy and—through observation of the magnetic field rotation rate—an indication of the spin rate of the bulk of the planet. The temperature profiles within Jupiter and Saturn are thought to be essentially adiabatic, reflecting the high central temperatures and the dominant role of convection below the observable atmosphere where radiative processes become important. There may be deeper layers restricted in radial extent where the temperature profile becomes subadiabatic, due to a decrease in the total opacity, or by virtue of the behavior of the equation of state of hydrogen and helium. The same may hold for Uranus and Neptune, although with less certainty, because of the possibility that stable compositional gradients could exist and dominate the heat-flow regime. In particular, Uranus’ small heat flow, if primordial and not a function of seasonal insolation, could be the result of a

Essential Determinants of the Physical Properties of the Giant Planets stable compositional stratification and hence subadiabatic temperature profile in the interior (Podolak et al., 1991). The results of the modeling of the interior are summarized in rough pictorial fashion in Figure 1. It is clear that Uranus and Neptune possess multiple layers, and are similar despite the different heat flows. Neptune has a hydrogen-rich outer layer down to a radius 0.8 that of the planet and a mixed layer of ice, rock, and hydrogen and helium below that. Only two Earth masses, or less than 15%, of the planet is hydrogen and helium. Some of the rock might be separated out in the form of a core. Uranus may be more centrally condensed than Neptune, but the relative proportions of rocky, icy, and gaseous elements overall throughout the two bodies are the same. Jupiter and Saturn, alternatively, are dominated by hydrogen and helium, but the abundance of heavy elements—equivalent to some 20 Earth masses of material (Guillot et al., 2003)—is enhanced over solar abundance. It is possible, but not required, that Jupiter and Saturn have small rocky cores. Most of the elements heavier than hydrogen and helium are distributed throughout the deep interior, rather than sequestered as a discrete layer. Unfortunately, the distribution of elements in the interiors of Jupiter and Saturn does not provide a tight constraint on the formation mechanisms described below, as such a distribution could be achieved either by initial seeding of the formation with heavy elements, or by later addition of this material, or both.

1.23.2.5

Elemental and Isotopic Abundances

Measurement of atmospheric abundances in the giant planets is another indicator of

9

their bulk interior composition, although an indirect one, because the interiors are decidedly nonhomogeneous. However, the atmospheric abundances provide strong evidence for an overall enrichment of the elements heavier than hydrogen and helium relative to solar abundance, an enrichment that increases from Jupiter, to Saturn, and then to Uranus and Neptune. Because of the dominance of hydrogen, the elements are present in reduced molecular form—carbon as CH4, nitrogen as NH3, oxygen as H2O, sulfur as H2S, phosphorous as PH3, germanium as GeH4, etc. However, more oxidized molecular forms are not completely absent; for example, carbon monoxide has been detected on Jupiter, although whether its source is from deeper, hotter, and hence more oxidizing regions, or from photochemistry in the stratosphere, has remained an unsolved problem (Bezard et al., 1999). The troposphere of Neptune appears, indirectly, to have nitrogen primarily in the form of N2 rather than NH3 (Gautier et al., 1995). Photochemically produced species such as acetylene (C2H2) and ethane (C2H6) derived from methane are observed in the stratospheres of the giant planets. Table 2 attempts to summarize the abundances of the major elements in the atmospheres of the giant planets, expressed relative to solar abundance. This is a difficult undertaking, and many abundances, particularly for Uranus and Neptune, are not well determined or are controversial. In addition, it is difficult to assign error bars that capture the diverse range of experimental sensitivities and observational wavelengths; hence, the number of significant figures or the range must serve to indicate the uncertainties. However, the general trend of increasing overabundances of the

Table 2 Abundances of major element species in the atmospheres of the giant planets (expressed relative to solar abundance).

Helium Methane (CH4) Ammonia, NH3 Water (H2O) Phosphine, PH3 Hydrogen sulfide (H2S) Arsine (AsH3) Germane (GeH4) Neon Argon Krypton Xenon

Jupiter

Saturn

Uranus

Neptune

0.8 2.5–3.5 3–4 (48 bar) 42 40.2 2.2–2.9 0.6–3 0.1 0.2 2.0–3.0 2.2–3.2 1.8–3.0

0.8 7 0.5–3 ? 6 ? 2–8 6 ? ? ? ?

1 30–60 41? ? ? ? ? ? ? ? ? ?

1.3 30–60 20–40 as N2? ? ? ? ? ? ? ? ? ?

Notes: Units given in solar abundances as known at the time the cited works were published. Readers should consult those works for the abundance values in absolute units. Updated solar abundances are given in Chapter 1.03. Jupiter and Saturn data are from Atreya et al. (1999b), Gautier et al. (2001), Noll et al. (1989), Fink et al. (1978), Flasar et al. (2005), and Roos-Serote et al. (2004); Uranus and Neptune from Gautier et al. (1995).

Giant Planets

10

D/H

10−3

10−4

Semarkona and Bishunpur meteorites Proto-Uranian Hyakutake ices Proto-Neptunain ices Halley Hale-Bopp Earth CH D 3 x HD ISO CH3D ISO HD ISO HD ISO Protosolar HD Galileo

10−5

LL3

Jupiter

HD ISO

Saturn

Uranus

Neptune

Comets

10−6

Figure 3 Map of the deuterium abundance in solar system objects, plotted as D/H mole fraction. The carrier molecules for which deuterium has been measured in each object are labeled. The protosolar value, derived from measurements of 3He products of deuterium fusion in the Sun, and deuterium in the local region of the galaxy, is given, as are the values for a carbonaceous chondrite meteorite, the Earth’s oceans, and comets. Reproduced with permission from Hersant et al. (2001).

heavy elements relative to solar is well illustrated. For Jupiter, in situ measurement of noble gases and molecular species provides the most complete abundance determinations, but even here the condensation of ammonia and water as clouds leads to uncertainties in the abundances of these important species (Atreya et al., 1999b). An outstanding uncertainty in the water abundance was measured by the Galileo probe to be below solar abundance down to 20 bar, but the atmospheric conditions indicated a zone that had been dried out by atmospheric circulation. Modeling derived from near-infrared observations by the Galileo orbiter indicate that the water abundance below the clouds is at least twice solar (Roos-Serote et al., 2004). Isotopic abundance determinations are spotty for all but deuterium, which is seen in HD and CH3D in the giant planets, as shown in Figure 3. The marginally consistent results from remote sensing and Galileo probe measurements for Jupiter illustrate the difficulty in making accurate chemical and isotopic measurements in giant planet atmospheres, but a trend of rising deuterium abundance seems to be present. Because comets, which are likely to be remnants of the planetesimals from which bodies in the outer solar system were built, contain elevated deuterium enrichments relative to protosolar, the trend in the giant planets may well reflect the

increasing relative importance of the icy and rocky phases compared to the gas. The deuterium abundance in Jupiter, added to the abundance of 3He, the light isotope of helium, measured by Galileo, is consistent with the same sum constructed for the local interstellar medium (ISM). However, relative to the local ISM, deuterium in Jupiter is elevated and 3He is depleted. The ISM represents 4.5 Gyr of stellar hydrogen fusion since the birth of Jupiter— consuming deuterium, generating light helium, and expelling these into the interstellar medium. Hence, the trend of these isotopes is fully consistent with the gaseous (hydrogen–helium) component of Jupiter being a sample of protosolar gas that has been bottled up and hence isolated from hydrogen fusion for 4.5 Gyr (Lunine et al., 2000).

1.23.2.6

Atmospheric Dynamics and Magnetic Fields

The wind systems on Jupiter and Saturn are strongly westward at the equator, with eastward jets at latitudes above and below the equator. Equatorial wind speeds on Jupiter approach 150 m s1 in the direction of rotation, and are 3–4 times higher still on Saturn. Circumpolar jet streams are seen as well

Origin and Evolution of the Giant Planets (Simon-Miller et al., 2006). Measurements by the Galileo probe indicate that the winds persist down to the deepest level, B20 bar, measured by the probe. Uranus shows prograde (in the direction of rotation) winds of 150 m s1 at midlatitudes, which decline in speed toward the equator. Because of the low contrast of the Uranian clouds at the time of the Voyager flyby, optical tracking of the winds was difficult. A radio-occultation experiment done as the Voyager spacecraft passed behind Uranus as seen from the Earth indicated retrograde winds at the equator of 100 m s1. On Neptune, where cloud features at high contrast aided tracking by Voyager cameras, a narrow jet at 701 latitude was clocked at 300 m s1. Other than this feature, wind speeds seem comparable to that at Uranus. Cyclonic and anticyclonic systems are abundant on Jupiter, appear on Saturn, and at least one such feature (the Great Dark Spot) appears on Neptune (Lunine, 1993). The winds on Jupiter are consistent with, but do not require, the presence of deep cylindrical flows, consistent with an efficiently convecting planet in which differential rotation is forced to be constant on cylinders (Hubbard et al., 2002). However, other explanations, including shallow meteorological patterns, are possible. The rough increase in wind speed from Jupiter to Neptune may reflect the greater importance of convection impeding rotational momentum on the two more massive giant planets, the effects of declining insolation with distance, or more stochastic effects associated with formation. The magnetic fields of the giant planets provide fiduciary spin rates of the interiors, create magnetospheres of charged and neutral particles that alter the appearance and composition of satellite surfaces and rings, and affect the energy and mass input into the ionospheres of the giant planet atmospheres. Direct measurement of the magnetic fields of the giant planets has revealed diverse geometries. The Jovian magnetic field has large quadrupole and octupole moments, while the Saturnian magnetic field is largely dipolar (Dessler, 1983). Both fields are opposite in polarity to that of the Earth, although this is probably not a significant difference since the Earth’s field has flipped in polarity many times in its history. Uranus and Neptune both possess complex fields with surface intensities comparable to that of the Earth, but quite different geometries. The fields can be described by dipoles that are tilted with respect to the rotational axis and displaced relative to the center of the planet. Large quadrupolar and octupolar components are present as well (Ness et al., 1995). The implication for the field generation within

11

these planets is that, in contrast to Jupiter and Saturn, the dynamo generation is occurring at shallow levels in the interiors of Uranus and Neptune. This, in turn, suggests that an electrically conducting fluid, possibly high-pressure ionized water, must be invoked. The chemical implications of the magnetic fields and associated magnetospheres for the giant planets are too numerous to even outline here (Dessler, 1983). One interesting example of the role of the magnetic field is the aurora, which is particularly observable on Jupiter with HST. The precipitation of charged particles into the Jovian atmosphere, guided along the field lines of the magnetic field, has a number of effects. They directly excite molecules, inducing photon emission at specific wavelengths, they heat the atmosphere and produce thermal emission, and they induce upper atmospheric chemistry. A high-altitude haze seen on Jupiter at latitudes exceeding 601 might be hydrocarbons generated at high altitudes by charged particle chemistry associated with the auroral precipitation.

1.23.3

ORIGIN AND EVOLUTION OF THE GIANT PLANETS

1.23.3.1 Basic Model for the Formation of the Planets from a Disk of Gas and Dust Astronomical observations of disks around other stars, combined with theoretical modeling and meteorite studies, establish with little doubt that planetary systems are the result of the formation of a disk of gas and solids around a growing ‘‘proto-star’’—the central mass that will become a star (Mannings et al., 2000). The disk is a natural consequence of the angular momentum possessed by molecular clouds, the conservation of which during collapse of a clump leads to the development of a disk. Dissipation in the disk caused by turbulent motions, gravitational, and magnetic torques leads to much of the disk mass being transported inward, to the star, while the angular momentum is moved outward (see Chapter 1.04 on the solar nebula). These processes, modeled in detail, lead to the observed fundamental properties of our own solar system: orbits of the major planets confined to a plane, most of the mass in the Sun, and most of the angular momentum in the orbits of the planets. The formation of the regular systems of moons around the giant planets may replicate some of these processes, but with different timescales and some important variations (Mosqueira and Estrada, 2006).

12

Giant Planets

The decrease in accretion rate of material to the disk, in the terminal stages of star and planet formation, leads to a cooling of the disk, so that water ice becomes stable at distances of 5 AU or perhaps inward. The presence of large amounts of solids—silicates plus water ice—encourages the formation of large bodies by accretion (Lissauer, 1987; Stevenson and Lunine, 1988). Giant planets must form in no more than a several million-year timescale, which is the lifetime of the gaseous disk based on astronomical observations (Calvet et al., 2000), although some gas may remain for longer periods of time. Uranus and Neptune may be giant planets, whose accretion was truncated by the loss of gas, or interrupted by dynamical perturbations from Jupiter (Thommes et al., 2002). Terrestrial planets inward of the region of the giant planets will take longer to form, because of the smaller amount of solid material inward of the ice condensation zone, and the presence of Jupiter plays a key role in stirring the orbits of the growing terrestrial planets and encouraging high-velocity collisions (Morbidelli et al., 2000; see also Chapter 1.17 on planet formation). Delivery of water and organics to the Earth from the asteroid belt and from comets was thus strongly affected, if not determined, by the presence of Jupiter prior to the time of significant terrestrial planet growth (Lunine, 2001). Interaction of giant planets with the gaseous and even particulate disk may result in migration inward, disrupting terrestrial planet formation and leading to giant planets at small orbital distances (Lin et al., 1996; Ward and Hahn, 2000). Such migration did not occur in our own solar system, or occurred early, and the migrating giant planet was destroyed through falling into the Sun.

1.23.3.2 Constraints from the Composition of the Giant Planets The specific mechanism for giant planet formation remains undetermined. Direct formation by collapse of gas within a relatively massive gas disk is one possibility (Boss, 2000); accretion of solids first to trigger the accelerated accretion or hydrodynamic collapse of gas is the other (Wuchterl et al., 2000). The substantial overabundance of heavy elements in Jupiter and Saturn, and the existence of Uranus and Neptune as heavy element ‘‘cores’’ surrounded by a much smaller amount of hydrogen–helium, would seem to support the latter model. One might conceive of direct collapse of gas in the disk followed by later

accretion of solids, but this has not been examined quantitatively. It is conceivable that both processes work, leading to giant planets with differing abundances of heavy elements relative to the parent star, but in our own solar system accretion of solids first seems to best explain the compositions of the giant planets. The noble gas, carbon, nitrogen, and sulfur abundances in Jupiter can be compared to the predicted compositions of icy planetesimals to provide details on when and how material was accreted during the formation of Jupiter. Unfortunately the oxygen abundance in Jupiter is poorly constrained, and since water, as the primary oxygen carrier, was the dominant ice in planetesimals as well (based on observations of comets), one requires this abundance to decide among models. In its absence, the current heavy element inventory can be explained by a model in which Jupiter’s ices were derived from very cold, possibly molecular cloud material (Owen et al., 1999), or by a model in which the ices were condensed directly from the nebula in the region of Jupiter formation (Gautier et al., 2001). Future microwave measurements from a planned Jupiter orbiter called Juno, and eventually a deep atmospheric entry probe capable of reaching 2–5 times deeper in pressure than Galileo, are required to determine the water and hence deep oxygen abundance (Lunine et al., 2004).

1.23.4

EXTRASOLAR GIANT PLANETS

Of the 170 or so bodies in Jupiter to 10-Jupiter mass range discovered indirectly by Doppler spectroscopy, only a handful can be directly studied, because they transit across the face of its parent star; the best observed is the planet around the star HD209458 (Charbonneau et al., 2000). The fact that transits occur in this system tightly constrains the orbital inclination of this planet relative to the line of sight to the Earth, removing the inclination ambiguity of the planet’s mass that is inherent in the Doppler spectroscopic technique. The decrease in starlight as the planet passes across its parent star in its close orbit (less than a tenth the semimajor axis of Mercury’s orbit around the Sun) provides a radius for the planet of 1.3–1.4 times the radius of Jupiter (Cody and Sasselov, 2002). With a mass of 0.69 times that of Jupiter, this giant planet is substantially less dense than Saturn. The origin of this extended radius lies in the close proximity of the planet to its parent star. Although some controversy exists as to how stellar photons are transported to the interior

Major Unsolved Problems and Future Progress of the planet (Showman and Guillot, 2002), the net effect is to slow the decline in the radius of the planet as it evolves over billions of years. Thus, the bulk properties of HD209458b are quite consistent with those of a primarily hydrogen–helium giant planet subjected to a degree of heating enormously larger than those experienced by Jupiter and Saturn. More precise measurements of the transits of HD209458b across the face of its parent star with HST have permitted the change of radius with wavelength to be determined (Charbonneau et al., 2002). The radius variation is directly related to the opacity of the atmosphere as a function of wavelength, and hence the abundances of various absorbing species in the atmosphere (Hubbard et al., 2001). In particular, the signature of the D lines of sodium in the optical should be especially strong (Seager and Sasselov, 2000), and this is indeed what is seen. The strength of the sodium feature from the HST data is weaker than expected were the system to possess a heavy element abundance similar to the solar value, but the sodium signature may be reduced through the presence of clouds and chargedparticle chemistry. The transit studies of HD209458b and other extrasolar transiting planets have opened up the study of the atmospheric composition and dynamical processes in extrasolar giant planet atmospheres. However, only a small fraction, B1%, of giant planets around other stars will exhibit transits. Future studies of giant planets around other stars will require direct detection and spectroscopy, and this in turn will demand huge (30 m mirror diameter) ground-based telescopes employing state-of-the-art adaptive optics, or space-borne telescopes with interferometers or coronagraphs to pull planets out from under the light of their parent stars. Such powerful systems will benefit study of our solar system’s own giant planets as well, but additional visits by spacecraft will continue to be required for certain kinds of atmospheric, interior, and magnetospheric investigations.

1.23.5

MAJOR UNSOLVED PROBLEMS AND FUTURE PROGRESS

An exhaustive list of all unsolved problems regarding the giant planets would require much more space than this chapter itself occupies. Briefly, some of the key questions include the following. *

How do giant planets form? Two different models, disk instability versus core accretion

*

*

*

*

13

followed by gas collapse, are viable. They require very different timescales, have very different implications for satellite formation and internal composition, and may have implications for the ubiquity of giant planets and terrestrial planets around other stars. The formation of Uranus and Neptune is even less well understood, and no agreement exists as to whether these are stillborn ‘‘Jupiters’’ or the product of a distinct kind of formation process. What are the detailed internal structures of the giant planets and how are the magnetic fields generated? While the separation of helium from hydrogen seems to be assured for Saturn, it is unclear to what extent this occurs in Jupiter. Further, the distribution of elements heavier than hydrogen and helium remains unclear in Jupiter and Saturn, in part because of equation-of-state uncertainties. The interiors of Uranus and Neptune are even less certain. For these reasons, and because of uncertainties in dynamo theory, the specific details of the magnetic field generation in the giant planets remain uncertain. What is the relationship between the atmospheric circulation patterns and the deep circulation? Fluid planets are in many ways more complex than the solid planets, in that the atmospheric circulation patterns have some relationship—greater or lesser—to deep circulations. How the coupling occurs in the giant planets of our solar system, and the strength of the coupling, remain unresolved. What are the deep abundances of key elements in the giant planets? In spite of the Galileo mission, we do not know the deep oxygen abundance in Jupiter, which can help constrain formation models for the giant planets. Deep abundances in the other giant planets are even more poorly constrained. How do moons and rings form? The solid bodies around the giant planets formed as a consequence of the assembly of the giant planets, but stochastic events such as large collisions may have played crucial roles. For example, we do not know whether the massive Saturnian rings are as old as Saturn itself.

Future prospects for the study of the giant planets in our solar system from space probes center on the continuation of the Cassini Orbiter mission around Saturn, and planned or envisioned future Jupiter orbiters and probes. No missions to Uranus and Neptune are planned. The dramatic advances in large

14

Giant Planets

ground-based telescopes, detectors, adaptive optics systems, and the planned next generation of space-borne telescopes such as NASA’s James Webb Space Telescope will benefit studies both of the giant planets of our own solar system and detection and characterization of giant planets around other stars.

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Treatise On Geochemistry ISBN (set): 0-08-043751-6

pp. 1–15