Neutral Atmospheres of the Giant Planets: An Overview of Composition Measurements

Measurements of the chemical composition of the giant planets provide clues of their formation and evolution processes. According to the currently accepted nucleation model, giant planets formed from the initial accretion of an icy core and the capture of the protosolar gas, mosly composed of hydrogen and helium. In the case of Jupiter and Saturn (the gaseous giants), this gaseous component dominates the composition of the planet, while for Uranus and Neptune (the icy giants) it is only a small fraction of the total mass. The measurement of elemental and isotopic ratios in the giant planets provides key diagnostics of this model, as it implies an enrichment in heavy elements (as well as deuterium) with respect to the cosmic composition. Neutral atmospheric constituents in the giant planets have three possible sources: (1) internal (fromthe bulk composition of the planet), (2) photochemical (fromthe photolysis ofmethane) and(3) external (from meteoritic impacts, of local or interplanetary origin). This paper reviews our present knowledge about the atmospheric composition in the giant planets, and their elemental and istopic composition. Measurements concerning key parameters, like C/H, D/H or rare gases in Jupiter, are analysed in detail. The conclusion addresses open questions and observations to be performed in the future.     

Coupled Clouds and Chemistry of the Giant Planets— A Case for Multiprobes

In seeking to understand the formation of the giant planets and the origin of their atmospheres, the heavy element abundance in well-mixed atmosphere is key. However, clouds come in the way. Thus, composition and condensation are intimately intertwined with the mystery of planetary formation and atmospheric origin. Clouds also provide important clues to dynamical processes in the atmosphere. In this chapter we discuss the thermochemical processes that determine the composition, structure, and characteristics of the Jovian clouds. We also discuss the significance of clouds in the big picture of the formation of giant planets and their atmospheres. We recommend multiprobes at all four giant planets in order to break new ground.     

Infrared Spectroscopy of Solar-System Planets

Most of our knowledge regarding planetary atmospheric composition and structure has been achieved by remote sensing spectroscopy. Planetary spectra strongly differ from one planet to another. CO₂ signatures dominate on Mars, and even more on Venus (where the thermal component is detectable down to 1 μm on the dark side). Spectroscopic monitoring of Venus, Earth and Mars allows us to map temperature fields, wind fields, clouds, aerosols, surface mineralogy (in the case of the Earth and Mars), and to study the planets’ seasonal cycles. Spectra of giant planets are dominated by H₂, CH₄ and other hydrocarbons, NH₃, PH₃ and traces of other minor compounds like CO, H₂O and CO₂. Measurements of the atmospheric composition of giant planets have been used to constrain their formation scenario.     

The Contributions of Comets to Planets, Atmospheres, and Life: Insights from Cassini-Huygens, Galileo, Giotto, and Inner Planet Missions

Comets belong to a group of small bodies generally known as icy planetesimals. Today the most primitive icy planetesimals are the Kuiper Belt objects (KBOs) occupying a roughly planar domain beyond Neptune. KBOs may be scattered inward, allowing them to collide with planets. Others may move outward, some all the way into the Oort cloud. This is a spherical distribution of comet nuclei at a mean distance of ～50,000 AU. These nuclei are occasionally perturbed into orbits that intersect the paths of the planets, again allowing collisions. The composition of the atmosphere of Jupiter—and thus possibly all outer planets—shows the effects of massive early contributions from extremely primitive icy bodies that must have been close relatives of the KBOs. Titan may itself have a composition similar to that of Oort cloud comets. The origin and early evolution of its atmosphere invites comparison with that of the early Earth. Impacts of comets must have brought water and other volatile compounds to the Earth and the other inner planets, contributing to the reservoir of key ingredients for the origin of life. The magnitude of these contributions remains unknown but should be accessible to measurements by instruments on spacecraft.     

Exoplanet Transit Spectroscopy and Photometry

Photometry and spectroscopy of extrasolar planets provides information about their atmospheres and surfaces. From extrasolar planet spectra and photometry we can infer the composition and temperature of the atmospheres as well as the presence of molecular species, including biosignature gases or surface features. So far photometry has been published for three different transiting hot Jupiters (gas giant planets in short-period orbits), opening the era of comparative exoplanetology.     

Are There Chemical Gradients in the Inner Solar System?

The solar system is apparently stratified with regard to the contents of volatile constituents, as judged from the rocky, volatile-poor inner solar system planets and meteorites and the huge volatile-rich outer planets. However, beyond this gross structure there is no evidence for a systematic increase of the volatiles' abundances with distance from the Sun. Although meteorites show comparatively large differences in volatile element contents they also differ in many other respects, such as Mg/Si-ratios, bulk Fe and refractory element contents. These variations reflect variations in the nebular environment from which meteorites formed. The various conditions of meteorite formation cannot, however, be related in a simple way to heliocentric distances. There are also no systematic variations in the chemistry of the inner planets Mercury, Venus, Earth, Moon, Mars, and including the fourth largest asteroid Vesta, that could be interpreted as a relationship between volatility and composition. Although Mars (as judged from the composition of Martian meteorites) is more oxidized and contains more volatile elements than Earth, this trend cannot be extrapolated to the dry volatile poor Vesta (sampled by HED meteorites) in the asteroid belt. If the Earth-Mars trend reflects global inner solar system gradients then Vesta must have formed inside Earth's orbit and moved out later to its present location. The quality of Mercury and Venus composition data is not sufficient to allow reliable extrapolation to distances closer to the Sun. Recent nebula models predict small temperature gradients in the inner solar system supporting the view that no large variations in volatile element contents of inner solar system materials are expected. This revised version was published online in June 2006 with corrections to the Cover Date.     

Neutral Atmospheres

This paper summarizes the understanding of aeronomy of neutral atmospheres in the solar system, discussing most planets as well as Saturn’s moon Titan and comets. The thermal structure and energy balance is compared, highlighting the principal reasons for discrepancies amongst the atmospheres, a combination of atmospheric composition, heliocentric distance and other external energy sources not common to all. The composition of atmospheres is discussed in terms of vertical structure, chemistry and evolution. The final section compares dynamics in the upper atmospheres of most planets and highlights the importance of vertical dynamical coupling as well as magnetospheric forcing in auroral regions, where present. It is shown that a first order understanding of neutral atmospheres has emerged over the past decades, thanks to the combined effects of spacecraft and Earth-based observations as well as advances in theoretical modeling capabilities. Key gaps in our understanding are highlighted which ultimately call for a more comprehensive programme of observation and laboratory measurements.     

Beta Pictoris and Other Solar Systems

Planetary systems come in a bewildering variety of shapes and sizes. In addition to the exoplanetary systems with giant planets, found in surveys of stellar radial velocity variations, an overlapping class of dusty disk-containing solar systems exists. The disks include large quantities of meteoroids and dust, and a varying complement of gas. Their solid material represents `replenished' dust born in the collisions/sublimation of planetesimals perturbed by planets. We present several such systems, including HR 4796A, HD 141569, HD 100546, and the prototypical replenished disk of Beta Pictoris. We discuss the composition, physical processing, and migration of dust in the disks, their evolutionary status, and the evidence of embedded planets. This revised version was published online in June 2006 with corrections to the Cover Date.     

Mercury Dust Monitor (MDM) Onboard the Mio Orbiter of the BepiColombo Mission

Space Science Reviews - In this chapter, we review the contribution of space missions to the determination of the elemental and isotopic composition of Earth, Moon and the terrestrial planets, with...     

Comparative Study of the Dynamics of the Outer Planets

The two classes of outer planets, Gas Giants and Ice Giants, have distinctly different global circulation patterns and internal structure. Ongoing ground-based observations of the Ice Giants provide clues to better understanding and Galileo and Cassini data will generate constraints for Gas Giant modeling. The composition below the cloud levels, the depths to which the winds penetrate and the processes that sustain the zonal winds and weather systems are not understood. Basic questions concerning the structure, composition and atmospheric dynamics that are sustained on the four giants could be answered by a combination of orbiters and probes. Future missions that could answer these questions are not currently under development.     

Dynamics and Composition of Rings

Planetary rings are found around all four giant planets of our solar system. These collisional and highly flattened disks exhibit a whole wealth of physical processes involving dust grains up to meter-sized boulders. These processes, together with ring composition, can help understand better the formation and evolution of proto-satellite and proto-planetary disks in the early solar system. The present chapter reviews some fundamental aspects of ring dynamics and composition. The forthcoming exploration of the Saturn system by the Cassini mission will bring both high resolution and time-dependent information on Saturn’s rings.     

From Gas to Satellitesimals: Disk Formation and Evolution

The subject of satellite formation is strictly linked to the one of planetary formation. Giant planets strongly shape the evolution of the circum-planetary disks during their formation and thus, indirectly, influence the initial conditions for the processes governing satellite formation. In order to fully understand the present features of the satellite systems of the giant planets, we need to take into account their formation environments and histories and the role of the different physical parameters. In particular, the pressure and temperature profiles in the circum-planetary nebulae shaped their chemical gradients by allowing for the condensation of ices and noble gases. These chemical gradients, in turn, set the composition of the satellitesimals, which represent the building blocks of the present regular satellites.     

Imaging of the outer planets and satellites

Imaging is the most widely applicable single means of exploring the outer planets and their satellites and also complements other planet-oriented instruments. Imaging generally is more effectively carried out from a three-axis stabilized spacecraft than from a spinning one.Both specific experimental and broader exploratory goals must be recognized. Photography of Jupiter from terrestrial telescopes has revealed features which were neither predictable or predicted. Close-up imaging from fly-bys and orbiters affords the opportunity for discovery of atmospheric phenomena on the outer planets forever beyond the reach of terrestrial laboratories and intuition. On the other hand, a large number of specific applications of close-up imaging to study the giant planets are suggested by experience in photography from Earth and Mars orbit, and by ground-based telescopic studies of Jupiter and Saturn. Photographic observations of horizontal and vertical cloud structure at both global and finer scale, and motions and other time changes, will be essential for the study of atmospheric circulation. Size and composition of cloud particles also is a credible objective of fly-by and orbiter missions carrying both imaging and photo-polarimeter experiments.The satellites of the outer planets actually constitute three distinct classes: lunar-sized objects, asteroidal-sized objects, and particulate rings. Imaging promises to be the primary observational tool for each category with results that could impact scientific thinking in the late 70's and 80's as significantly as has close-up photography of Mars and the Moon in the last 10 yr.Finally, it should be recognized that photography occupies a unique role in the interaction between science and the popular mind. This popular, educational aspect of imaging constitutes a unique aspect of 20th Century culture. Imaging therefore is not only a primary basis for scientific discovery in the exploration of the outer planets, but an important human endeavor of enduring significance.Contribution No. 2163 of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91109.This is one of the publications by the Science Advisory Group.     

Formation of the Outer Planets

Models of the origins of gas giant planets and ‘ice’ giant planets are discussed and related to formation theories of both smaller objects (terrestrial planets) and larger bodies (stars). The most detailed models of planetary formation are based upon observations of our own Solar System, of young stars and their environments, and of extrasolar planets. Stars form from the collapse, and sometimes fragmentation, of molecular cloud cores. Terrestrial planets are formed within disks around young stars via the accumulation of small dust grains into larger and larger bodies until the planetary orbits become well enough separated that the configuration is stable for the lifetime of the system. Uranus and Neptune almost certainly formed via a bottom-up (terrestrial planet-like) mechanism; such a mechanism is also the most likely origin scenario for Saturn and Jupiter.     

The X-Ray Spectrometer on the MESSENGER Spacecraft

NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission will further the understanding of the formation of the planets by examining the least studied of the terrestrial planets, Mercury. During the one-year orbital phase (beginning in 2011) and three earlier flybys (2008 and 2009), the X-Ray Spectrometer (XRS) onboard the MESSENGER spacecraft will measure the surface elemental composition. XRS will measure the characteristic X-ray emissions induced on the surface of Mercury by the incident solar flux. The Kα lines for the elements Mg, Al, Si, S, Ca, Ti, and Fe will be detected. The 12° field-of-view of the instrument will allow a spatial resolution that ranges from 42 km at periapsis to 3200 km at apoapsis due to the spacecraft’s highly elliptical orbit. XRS will provide elemental composition measurements covering the majority of Mercury’s surface, as well as potential high-spatial-resolution measurements of features of interest. This paper summarizes XRS’s science objectives, technical design, calibration, and mission observation strategy.     

Upstream Ion Cyclotron Waves at Venus and Mars

The occurrence of waves generated by pick-up of planetary neutrals by the solar wind around unmagnetized planets is an important indicator for the composition and evolution of planetary atmospheres. For Venus and Mars, long-term observations of the upstream magnetic field are now available and proton cyclotron waves have been reported by several spacecraft. Observations of these left-hand polarized waves at the local proton cyclotron frequency in the spacecraft frame are reviewed for their specific properties, generation mechanisms and consequences for the planetary exosphere. Comparison of the reported observations leads to a similar general wave occurrence at both planets, at comparable locations with respect to the planet. However, the waves at Mars are observed more frequently and for long durations of several hours; the cyclotron wave properties are more pronounced, with larger amplitudes, stronger left-hand polarization and higher coherence than at Venus. The geometrical configuration of the interplanetary magnetic field with respect to the solar wind velocity and the relative density of upstream pick-up protons to the background plasma are important parameters for wave generation. At Venus, where the relative exospheric pick-up ion density is low, wave generation was found to mainly take place under stable and quasi-parallel conditions of the magnetic field and the solar wind velocity. This is in agreement with theory, which predicts fast wave growth from the ion/ion beam instability under quasi-parallel conditions already for low relative pick-up ion density. At Mars, where the relative exospheric pick-up ion density is higher, upstream wave generation may also take place under stable conditions when the solar wind velocity and magnetic field are quasi-perpendicular. At both planets, the altitudes where upstream proton cyclotron waves were observed (8 Venus and 11 Mars radii) are comparable in terms of the bow shock nose distance of the planet, i.e. in terms of the size of the solar wind-planetary atmosphere interaction region. In summary, the upstream proton cyclotron wave observations demonstrate the strong similarity in the interaction of the outer exosphere of these unmagnetized planets with the solar wind upstream of the planetary bow shock.     

The internal constitutions of the inner planets and the moon

The internal structures of the moon, Mars, Venus, and Mercury are examined in the light of what is known about the constitution of the earth. The gravitational figure of the earth as obtained from orbits of artificial satellites is used to estimate the possible deviations from hydrostatic equilibrium on other planets. Observations of the orbital and rotational motion of the moon are consistent with the hypothesis that the interior of the moon supports density inhomogeneities of the same order as those supported by the earth. The available data on the moon are insufficient to determine whether or not the moon is differentiated. The orbits of Phobos and Deimos yield an adequate value for the moment of inertia of Mars. The moment of inertia and the mass are consistent with a metallic core containing about 10 per cent of the mass of Mars. The observations of the possible magnetic field of Mars would be of importance both to the understanding of planetary magnetic fields and elucidating the internal structure of that planet. Seismic investigations on the earth yield an equation of state for silicates to pressures of about 1 × 10⁶ bars. This equation of state is used in determining density variation within Mars.The surface heat flow for the earth is consistent with the hypothesis that the concentration of radioactive elements is the same as that in chondritic meteorites. The observed ratio of potassium to uranium in surface and near-surface rocks is not consonant with the chondritic hypothesis. The moon can be of chondritic composition only if it is differentiated with the radioactivity concentrated in the upper few hundred kilometers. A chondritic composition for Mars would require a differentiation in excess of that consistent with its mass and moment of inertia. It is concluded that a chondritic composition is not a satisfactory chemical model for the inner planets.     

The Delivery of Water During Terrestrial Planet Formation

The planetary building blocks that formed in the terrestrial planet region were likely very dry, yet water is comparatively abundant on Earth. Here we review the various mechanisms proposed for the origin of water on the terrestrial planets. Various in-situ mechanisms have been suggested, which allow for the incorporation of water into the local planetesimals in the terrestrial planet region or into the planets themselves from local sources, although all of those mechanisms have difficulties. Comets have also been proposed as a source, although there may be problems fitting isotopic constraints, and the delivery efficiency is very low, such that it may be difficult to deliver even a single Earth ocean of water this way. The most promising route for water delivery is the accretion of material from beyond the snow line, similar to carbonaceous chondrites, that is scattered into the terrestrial planet region as the planets are growing. Two main scenarios are discussed in detail. First is the classical scenario in which the giant planets begin roughly in their final locations and the disk of planetesimals and embryos in the terrestrial planet region extends all the way into the outer asteroid belt region. Second is the Grand Tack scenario, where early inward and outward migration of the giant planets implants material from beyond the snow line into the asteroid belt and terrestrial planet region, where it can be accreted by the growing planets. Sufficient water is delivered to the terrestrial planets in both scenarios. While the Grand Tack scenario provides a better fit to most constraints, namely the small mass of Mars, planets may form too fast in the nominal case discussed here. This discrepancy may be reduced as a wider range of initial conditions is explored. Finally, we discuss several more recent models that may have important implications for water delivery to the terrestrial planets.     

The Interior Structure, Composition, and Evolution of Giant Planets

We discuss our current understanding of the interior structure and thermal evolution of giant planets. This includes the gas giants, such as Jupiter and Saturn, that are primarily composed of hydrogen and helium, as well as the “ice giants,” such as Uranus and Neptune, which are primarily composed of elements heavier than H/He. The effect of different hydrogen equations of state (including new first-principles computations) on Jupiter’s core mass and heavy element distribution is detailed. This variety of the hydrogen equations of state translate into an uncertainty in Jupiter’s core mass of 18M _⊕ . For Uranus and Neptune we find deep envelope metallicities up to 0.95, perhaps indicating the existence of an eroded core, as also supported by their low luminosity. We discuss the results of simple cooling models of our solar system’s planets, and show that more complex thermal evolution models may be necessary to understand their cooling history. We review how measurements of the masses and radii of the nearly 50 transiting extrasolar giant planets are changing our understanding of giant planets. In particular a fraction of these planets appear to be larger than can be accommodated by standard models of planetary contraction. We review the proposed explanations for the radii of these planets. We also discuss very young giant planets, which are being directly imaged with ground- and space-based telescopes.