Formation and Composition of Planetesimals

The composition of planetesimals depends upon the epoch and the location of their formation in the solar nebula. Meteorites produced in the hot inner nebula contain refractory compounds. Volatiles were present in icy planetesimals and cometesimals produced in the cold outer nebula. However, the mechanism responsible for their trapping is still controversial. We argue for a general scenario valid in all regions of the turbulent nebula where water condensed as a crystalline ice (Hersant et al., 2004). Volatiles were trapped in the form of clathrate hydrates in the continuously cooling nebula. The epoch of clathration of a given species depends upon the temperature and the pressure required for the stability of the clathrate hydrate. The efficiency of the mechanism depends upon the local amount of ice available. This scenario is the only one so far which proposes a quantitative interpretation of the non detection of N₂ in several comets of the Oort cloud (Iro et al., 2003). It may explain the large variation of the CO abundance observed in comets and predicts an Ar/O ratio much less than the upper limit of 0.1 times the solar ratio estimated on C/2001 A2 (Weaver et al., 2002). Under the assumption that the amount of water ice present at 5 AU was higher than the value corresponding to the solar O/H ratio by a factor 2.2 at least, the clathration scenario reproduces the quasi uniform enrichment with respect to solar of the Ar, Kr, Xe, C, N and S elements measured in Jupiter by the Galileo probe. The interpretation of the non-uniform enrichment in C, N and S in Saturn requires that ice was less abundant at 10 AU than at 5 AU so that CO and N₂ were not clathrated in the feeding zone of the planet while CH₄, NH₃ and H₂S were. As a result, the 14N/15N ratio in Saturn should be intermediate between that in Jupiter and the terrestrial ratio. Ar and Kr should be solar while Xe should be enriched by a factor 17. The enrichments in C, N and S in Uranus and Neptune suggest that available ice was able to form clathrates of CH₄, CO and the NH₃ hydrate, but not the clathrate of N₂. The enrichment of oxygen by a factor 440 in Neptune inferred by Lodders and Fegley (1994) from the detection of CO in the troposphere of the planet is higher by at least a factor 2.5 than the lower limit of O/H required for the clathration of CO and CH4 and for the hydration of NH₃. If CO detected by Encrenaz et al. (2004) in Uranus originates from the interior of the planet, the O/H ratio in the envelope must be around of order of 260 times the solar ratio, then also consistent with the trapping of detected volatiles by clathration. It is predicted that Ar and Kr are solar in the two planets while Xe would be enriched by a factor 30 to 70. Observational tests of the validity of the clathration scenario are proposed.     

Evolution of Interstellar Ices

Infrared observations, combined with realistic laboratory simulations, have revolutionized our understanding of interstellar ice and dust, the building blocks of comets. Ices in molecular clouds are dominated by the very simple molecules H₂O, CH₃OH, NH₃, CO, CO₂, and probably H₂CO and H₂. More complex species including nitriles, ketones, and esters are also present, but at lower concentrations. The evidence for these, as well as the abundant, carbon-rich, interstellar, polycyclic aromatic hydrocarbons (PAHs) is reviewed. Other possible contributors to the interstellar/pre-cometary ice composition include accretion of gas-phase molecules and in situ photochemical processing. By virtue of their low abundance, accretion of simple gas-phase species is shown to be the least important of the processes considered in determining ice composition. On the other hand, photochemical processing does play an important role in driving dust evolution and the composition of minor species. Ultraviolet photolysis of realistic laboratory analogs readily produces H₂, H₂CO, CO₂, CO, CH₄, HCO, and the moderately complex organic molecules: CH₃CH₂OH (ethanol), HC(=O)NH₂ (formamide), CH₃C(=O)NH₂ (acetamide), R-CN (nitriles), and hexamethylenetetramine (HMT, C₆H₁₂N₄), as well as more complex species including amides, ketones, and polyoxymethylenes (POMs). Inclusion of PAHs in the ices produces many species similar to those found in meteorites including aromatic alcohols, quinones and ethers. Photon assisted PAH-ice deuterium exchange also occurs. All of these species are readily formed and are therefore likely cometary constituents. This revised version was published online in June 2006 with corrections to the Cover Date.     

Rheological and Thermal Properties of Icy Materials

Laboratory measurements of physical properties of planetary ices generate information for dynamical models of tectonically active icy bodies in the outer solar system. We review the methods for measuring both flow properties and thermal properties of icy planetary materials in the laboratory, and describe physical theories that are essential for intelligent extrapolation of data from laboratory to planetary conditions. This review is structured with a separate and independent section for each of the two sets of physical properties, rheological and thermal. The rheological behaviors of planetary ices are as diverse as the icy moons themselves. High-pressure water ice phases show respective viscosities that vary over four orders of magnitude. Ices of CO₂, NH₃, as well as clathrate hydrates of CH₄ and other gases vary in viscosity by nearly ten orders of magnitude. Heat capacity and thermal conductivity of detected/inferred compositions in outer solar system bodies have been revised. Some low-temperature phases of minerals and condensates have a deviant thermal behavior related to paramount water ice. Hydrated salts have low values of thermal conductivity and an inverse dependence of conductivity on temperature, similar to clathrate hydrates or glassy solids. This striking behavior may suit the dynamics of icy satellites.     

Vertical distribution of disequilibrium species in Jupiter's troposphere

Sources of organic matter and inorganic tracers on Jupiter, including solar UV photolysis, lightning discharges, and convective quenching of hot gases from the lower atmosphere, are reviewed in light of Earth-based and Voyager data with the purpose of predicting the tropospheric steady-state abundances and vertical distributions of HCN, CH₂O, and other species.It is concluded that a steady-state mole fraction of HCN in the Jovian troposphere of only 10^-12 could be maintained by vertical transport of hot gases from the deep atmosphere. The observed HCN abundance (roughly X_HCN = 10^-9) appears to be due to photochemical reactions.After HCN, the most abundant organic disequilibrium species in the troposphere is probably C₂H₆, derived from direct photolysis of CH₄ at high altitudes, with a mole fracton of 10^-10 at the H₂O cloud level. Inorganic tracers of disequilibrium processes are also briefly summarized.     

Aeronomy of Extra-Solar Giant Planets

The intense stellar UV radiation field incident upon extra-solar giant planets causes profound changes to their upper atmospheres. Upper atmospheric temperatures can be tens of thousands of kelvins, causing thermal dissociation of H₂ to H. The stellar ionizing flux converts H to H⁺. The high temperatures also drive large escape rates of H, but for all but the planets with the smallest orbits, this flux is not large enough to affect planet evolution. The escape rate is large enough to drag off heavier atoms such as C and O. For very small orbits, when the hill sphere is inside the atmosphere, escape is unfettered and can affect planet evolution.     

New Horizons: Anticipated Scientific Investigations at the Pluto System

The New Horizons spacecraft will achieve a wide range of measurement objectives at the Pluto system, including color and panchromatic maps, 1.25–2.50 micron spectral images for studying surface compositions, and measurements of Pluto’s atmosphere (temperatures, composition, hazes, and the escape rate). Additional measurement objectives include topography, surface temperatures, and the solar wind interaction. The fulfillment of these measurement objectives will broaden our understanding of the Pluto system, such as the origin of the Pluto system, the processes operating on the surface, the volatile transport cycle, and the energetics and chemistry of the atmosphere. The mission, payload, and strawman observing sequences have been designed to achieve the NASA-specified measurement objectives and maximize the science return. The planned observations at the Pluto system will extend our knowledge of other objects formed by giant impact (such as the Earth–moon), other objects formed in the outer solar system (such as comets and other icy dwarf planets), other bodies with surfaces in vapor-pressure equilibrium (such as Triton and Mars), and other bodies with N₂:CH₄ atmospheres (such as Titan, Triton, and the early Earth).     

On the Flux of Water and Minor Volatiles from the Surface of Comet Nuclei

Surface temperature and the available effective energy strongly influence the mass flux of H₂O and minor volatiles from the nucleus. We perform computer simulations to model the gas flux from volatile, icy components in porous ice-dust surfaces, in order to better understand results from observations of comets. Our model assumes a porous body containing dust, one major ice component (H₂O) and up to eight minor components of higher volatility (e.g. CO, CH₄, CH₃OH, HCN, C₂H₂, H₂S), The body's porous structure is modeled as a bundle of tubes with a given tortuosity and an initially constant pore diameter. Heat is conducted by the matrix and carried by the vapors. The model includes radially inward and outward flowing vapor within the body, escape of outward flowing gas from the body, complete depletion of less volatile ices in outer layers, and recondensation of vapor in deeper, cooler layers. From the calculations we obtain temperature profiles and changes in relative chemical abundances, porosity and pore size distribution as a function of depth, and the gas flux into the interior and into the atmosphere for each of the volatiles at various positions of the body in its orbit. In this paper we relate the observed relative molecular abundances in the coma of Comet C/1995 O1 (Hale-Bopp) and of Comet 46P/Wirtanen to molecular fluxes at the surface calculated from our model. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Solar System d/h Ratio: Observations and Theories

The measured D/H ratios in interstellar environments and in the solar system are reviewed. The two extreme D/H ratios in solar system water - (720±120)×10⁻⁶ in clay minerals and (88±11)×10⁻⁶ in chondrules, both from LL3 chondritic meteorites - are interpreted as the result of a progressive isotopic exchange in the solar nebula between deuterium-rich interstellar water and protosolar H₂. According to a turbulent model describing the evolution of the nebula (Drouart et al., 1999), water in the solar system cannot be a product of thermal (neutral) reactions occurring in the solar nebula. Taking 720×10⁻⁶ as a face value for the isotopic composition of the interstellar water that predates the formation of the solar nebula, numerical simulations show that the water D/H ratio decreases via an isotopic exchange with H₂. During the course of this process, a D/H gradient was established in the nebula. This gradient was smoothed with time and the isotopic homogenization of the solar nebula was completed in 10⁶ years, reaching a D/H ratio of 88×10⁻⁶. In this model, cometary water should have also suffered a partial isotopic re-equilibration with H₂. The isotopic heterogeneity observed in chondrites result from the turbulent mixing of grains, condensed at different epochs and locations in the solar nebula. Recent isotopic determinations of water ice in cold interstellar clouds are in agreement with these chondritic data and their interpretation (Texeira et al., 1999). This revised version was published online in June 2006 with corrections to the Cover Date.     

The Stratospheric and Mesospheric NOy in the 2002–2004 Polar Winters as measured by MIPAS/ENVISAT

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board ENVISAT, provided global (pole-to-pole, the polar night winter regions) measurements of nearly all constituents of the NO_y family (including NO, NO₂, HNO₃ and H₂O₅) from July 2002 to the end of March 2004 from the upper stratosphere up to the middle mesosphere. The inter-annual variability of the NO₂ and HNO₃ abundances in the Arctic and Antarctic winters from September 2002 through March 2004 was enormous with tremendous hemispheric asymmetry and extraordinary values in two winters. The origin of these variations and of the extreme measured values has been analyzed on the basis of the changing atmospheric dynamics (using the CH₄ tracer) and solar activity, including the extraordinary solar protons events of Oct–Nov 2003.     

Influence of Solar Wind on the Global Electric Circuit,and Inferred Effects on Cloud Microphysics,Temperature, and Dynamics in the Troposphere

There are at least three independent ways in which the solar wind modulates the flow of current density (Jz) in the global electric circuit. These are (A) changes in the galactic cosmic ray energy spectrum, (B) changes in the precipitation of relativistic electrons from the magnetosphere, and (C) changes in the ionospheric potential distribution in the polar caps due to magnetosphere-ionosphere coupling. The current density J _z flows between the ionosphere and the surface, and as it passes through conductivity gradients it generates space charge concentrations dependent on J _z and the conductivity gradient. The gradients are large at the surfaces of clouds and space charge concentrations of order 1000 to 10,000 elementary charges per cm³ can be generated at cloud tops. The charge transfers to droplets, many of which are evaporating at the cloud-clear air interface. The charge remains on the residual evaporation nuclei with a lifetime against leakage of order 1000 sec, and for a longer period the nuclei also retain coatings of sulfate and organic compounds adsorbed by the droplet while in the cloud.The charged evaporation nuclei become well mixed with more droplets in many types of clouds with penetrative mixing. The processes of entrainment and evaporation are also efficient for these clouds. The collection of such nuclei by nearby droplets is greatly increased by the electrical attraction between the charge on the particle and the image charge that it creates on the droplet. This process is called electroscavenging. Because the charge on the evaporation nuclei is derived from the original space charge, it depends on J _z, giving a rate of electroscavenging responsive to the solar wind inputs.There may be a number of ways in which the electroscavenging has consequences for weather and climate. One possibility is enhanced production of ice. The charged evaporation nuclei have been found to be good ice forming nuclei because of their coatings, and so in supercooled clouds droplet freezing can occur by contact ice nucleation, as the evaporation nuclei are electroscavenged. Although quantitative models for the all the cloud microphysical processes that may be involved have not yet been produced, we show that for many clouds, especially those with broad droplet size distributions, relatively high droplet concentrations, and cloud top temperatures just below freezing, this process is likely to dominate over other primary ice nucleation processes. In these cases there are likely to be effects on cloud albedo and rates of sedimentation of ice, and these will depend on J _z.For an increase in ice production in thin clouds such as altocumulus or stratocumulus the main effect is a decrease in albedo to incoming solar radiation, and in opacity to outgoing longwave radiation. At low latitudes the surface and troposphere heat, and at high latitudes in winter they cool. The change in meridional temperature gradient affects the rate of cyclogenesis, and the amplitude of planetary waves. For storm clouds, as in winter cyclones, the effect of increased ice formation is mainly to increase the rate of glaciation of lower level clouds by the seeder-feeder process. The increase in precipitation efficiency increases the rate of transfer of latent heat between the air mass and the surface. In most cyclones this is likely to result in intensification, producing changes in the vorticity area index as observed. Cyclone intensification also increases the amplitude of planetary waves, and shifts storm tracks, as observed.In this paper we first describe the production of space charge and the way in which it may influence the rate of ice nucleation. Then we review theory and observations of the solar wind modulation of J _z, and the correlated changes in atmospheric temperature and dynamics in the troposphere. The correlations are present for each input, (A, B, and C), and the detailed patterns of responses provide support for the inferred electrical effects on the physics of clouds, affecting precipitation, temperature and dynamics.     

A Comparative Study of the Influence of the Active Young Sun on the Early Atmospheres of Earth, Venus, and Mars

Because the solar radiation and particle environment plays a major role in all atmospheric processes such as ionization, dissociation, heating of the upper atmospheres, and thermal and non-thermal atmospheric loss processes, the long-time evolution of planetary atmospheres and their water inventories can only be understood within the context of the evolving Sun. We compare the effect of solar induced X-ray and EUV (XUV) heating on the upper atmospheres of Earth, Venus and Mars since the time when the Sun arrived at the Zero-Age-Main-Sequence (ZAMS) about 4.6 Gyr ago. We apply a diffusive-gravitational equilibrium and thermal balance model for studying heating of the early thermospheres by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by IR-radiating molecules like CO₂, NO, OH, etc. Our model simulations result in extended thermospheres for early Earth, Venus and Mars. The exospheric temperatures obtained for all the three planets during this time period lead to diffusion-limited hydrodynamic escape of atomic hydrogen and high Jeans’ escape rates for heavier species like H₂, He, C, N, O, etc. The duration of this blow-off phase for atomic hydrogen depends essentially on the mixing ratios of CO₂, N₂ and H₂O in the atmospheres and could last from ∼100 to several hundred million years. Furthermore, we study the efficiency of various non-thermal atmospheric loss processes on Venus and Mars and investigate the possible protecting effect of the early martian magnetosphere against solar wind induced ion pick up erosion. We find that the early martian magnetic field could decrease the ion-related non-thermal escape rates by a great amount. It is possible that non-magnetized early Mars could have lost its whole atmosphere due to the combined effect of its extended upper atmosphere and a dense solar wind plasma flow of the young Sun during about 200 Myr after the Sun arrived at the ZAMS. Depending on the solar wind parameters, our model simulations for early Venus show that ion pick up by strong solar wind from a non-magnetized planet could erode up to an equivalent amount of ∼250 bar of O⁺ ions during the first several hundred million years. This accumulated loss corresponds to an equivalent mass of ∼1 terrestrial ocean (TO (1 TO ∼1.39×10²⁴ g or expressed as partial pressure, about 265 bar, which corresponds to ∼2900 m average depth)). Finally, we discuss and compare our findings with the results of preceding studies.     

Long Term Climate Records from Polar Ice

One of the great challenges in climate research is to investigate the principal mechanisms that control global climatic changes and an effective way to learn more about it, is the reconstruction of past climate changes. The most important sources of information about such changes and the associated composition of the atmosphere are the two large ice caps of Greenland and Antarctica. Analysis of ice cores is the most powerful means we have to determine how climate has changed over the last few climatic cycles, and to relate this to changes in atmospheric composition, in particular to concentrations of the principal greenhouse gases – CO₂, CH₄ and N₂O (carbon dioxide, methane, and nitrous oxide).Transitions from cold ice age climates to warmer interstadials have always been accompanied by an increase of the atmospheric concentration of the three principal greenhouse gases. This increase has been, at least for CO₂, vital for the ending of glacial epochs. A highly simplified course of events for the past four transitions would then be as follows: first, changing orbital parameters initiated the end of the glacial epoch; second, an increase in greenhouse gases then amplified the weak orbital signal; third, in the second half of the transition, warming was further amplified by decreasing albedo, caused by melting of the large ice sheets in the Northern Hemisphere going parallel with a change of the ocean circulation.The isotopic records of Greenland ice cores show evidence for fast and drastic climatic changes during the last glacial epoch. Possible causes and mechanisms of such changes and their significance as global climatic events are discussed here. Ice core results also enable the reaction of the environment to past global changes to be investigated.It will also be discussed how reliable stable isotope records are as a local temperature proxy and how representative paleoclimatic results from Greenland and Antarctica are in relation to global climate.     

Radio emissions from the outer heliosphere

For nearly fifteen years the Voyager 1 and 2 spacecraft have been detecting an unusual radio emission in the outer heliosphere in the frequency range from about 2 to 3 kHz, Two major events have been observed, the first in 1983–84 and the second in 1992–93. In both cases the onset of the radio emission occurred about 400 days after a period of intense solar activity, the first in mid-July 1982, and the second in May–June 1991. These two periods of solar activity produced the two deepest cosmic ray Forbush decreases ever observed. Forbush decreases are indicative of a system of strong shocks and associated disturbances propagating outward through the heliosphere. The radio emission is believed to have been produced when this system of shocks and disturbances interacted with one of the outer boundaries of the heliosphere, most likely in the vicinity of the the heliopause. The emission is believed to be generated by the shock-driven Langmuir-wave mode conversion mechanism, which produces radiation at the plasma frequency (f _p ) and at twice the plasma frequency (2f _p ). From the 400-day travel time and the known speed of the shocks, the distance to the interaction region can be computed, and is estimated to be in the range from about 110 to 160 AU.Abbreviations PWS Plasma Wave Subsystem - AU Astronomical Unit - DSN Deep Space Network - NASA National Aeronautics and Space Administration - GMIR Global Merged Interaction Region - MHD Magnetohydrodynamic - CME coronal mass ejection - f _p plasma frequency - R radial distance - AGC automatic gain control     

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.     

Thermal and Chemical Evolution of Comet Nuclei and Kuiper Belt Objects

The structure and composition of comet nuclei are mainly altered during two short phases that are separated by a very long hibernation phase. Early evolution—during and immediately after formation—is the result of heating caused by radioactive decay, the most important source being ²⁶Al. Several studies are reviewed, dealing with evolution throughout this phase, calculated by means of 1-D numerical codes that solve the heat and mass balance equations on a fixed spherically symmetric grid. It is shown that, depending on parameters, the interior may reach temperatures above the melting point of water. The models thus suggest that comets are likely to lose the ices of very volatile species during early evolution; ices of less volatile species are retained in the cold subsurface layer. As the initially amorphous ice is shown to crystallize in the interior, some objects may also lose part of the volatiles trapped in amorphous ice. Generally, the outer layers are far less affected than the inner part, resulting in a stratified composition and altered porosity distribution. The second phase of evolution occurs when comet nuclei are deflected into the inner solar system and is dominated by the effect of solar radiation. Now the outer layers are those mostly affected, undergoing crystallization, loss of volatiles, and significant structural changes. If any part of a comet nucleus should retain its pristine structure and composition, it would be well below the surface and also well above the core.     

Photochemistry in Outer Solar System Atmospheres

The photochemistries of the H₂-He atmospheres of the gas giants Jupiter, Saturn and ice giants Uranus and Neptune and Titan’s mildly reducing N₂ atmosphere are reviewed in terms of general chemical and physical principles. The thermochemical furnace regions in the deep atmospheres and the photochemical regions of the giant planets are coupled by vertical mixing to ensure efficient recyling of photochemical products. On Titan,mass loss of hydrogen ensures photochemical evolution of methane into less saturated hydrocarbons. A summary discussion of major dissociation paths and essential chemical reactions is given. The chapter ends with a overview of vertical transport processes in planetary atmospheres.     

Thermospheric X-Ray and Euv Heating by the Young Sun on Early Venus and Mars

In view of the low H₂O abundance in the present Venusian and Martian atmospheres several observations by spacecraft and studies suggest that both planets should have lost most of their water over the early active period of the young Sun. During the first Gyr after the Sun arrived at the Zero- Age-Main-Sequence high X-ray and EUV fluxes between 10 and 100 times that of the present Sun were responsible for much higher temperatures in the thermosphere-exosphere environments on both planets. By applying a diffusive-gravitational equilibrium and thermal balance model for investigating radiation impact on the early thermospheres by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by CO₂ IR emission in the 15μm band we found expanded thermospheres with exobase levels between about 200 km (present) and 2000 km (4.5 Gyr ago). The higher temperatures in the upper atmospheres of both planets could reach “blow-off” conditions for H atoms even at high CO₂ mixing ratios of 96%. Lower CO₂/N₂ mixing ratio or higher contents of H2O vapor in the early atmospheres could have had a dramatic impact from the loss of atmosphere and water on both planets. The duration of this phase of high thermal loss rates essentially depended on the mixing ratios of CO₂, N₂, and H₂O in the early atmospheres and could have lasted between about 150 and several hundred Myr.     

Chemical Composition of Icy Satellite Surfaces

Much of our knowledge of planetary surface composition is derived from remote sensing over the ultraviolet through infrared wavelength ranges. Telescopic observations and, in the past few decades, spacecraft mission observations have led to the discovery of many surface materials, from rock-forming minerals to water ice to exotic volatiles and organic compounds. Identifying surface materials and mapping their distributions allows us to constrain interior processes such as cryovolcanism and aqueous geochemistry. The recent progress in understanding of icy satellite surface composition has been aided by the evolving capabilities of spacecraft missions, advances in detector technology, and laboratory studies of candidate surface compounds. Pioneers 10 and 11, Voyagers I and II, Galileo, Cassini and the New Horizons mission have all made significant contributions. Dalton (Space Sci. Rev., 2010, this issue) summarizes the major constituents found or inferred to exist on the surfaces of the icy satellites (cf. Table 1 from Dalton, Space Sci. Rev., 2010, this issue), and the spectral coverage and resolution of many of the spacecraft instruments that have revolutionized our understanding (cf. Table 2 from Dalton, Space Sci. Rev., 2010, this issue). While much has been gained from these missions, telescopic observations also continue to provide important constraints on surface compositions, especially for those bodies that have not yet been visited by spacecraft, such as Kuiper Belt Objects (KBOs), trans-Neptunian Objects (TNOs), Centaurs, the classical planet Pluto and its moon, Charon. In this chapter, we will discuss the major satellites of the outer solar system, the materials believed to make up their surfaces, and the history of some of these discoveries. Formation scenarios and subsequent evolution will be described, with particular attention to the processes that drive surface chemistry and exchange with interiors. Major similarities and differences between the satellites are discussed, with an eye toward elucidating processes operating throughout the outer solar system. Finally we discuss the outermost satellites and other bodies, and summarize knowledge of their composition. Much of this review is likely to change in the near future with ongoing and planned outer planet missions, adding to the sense of excitement and discovery associated with our exploration of our planetary neighborhood.     

Molecular Hydrogen

Observations of H₂ line emission in galactic and extragalactic environments obtained with the Infrared Space Observatory (ISO) are reviewed. The diagnostic capability of H₂ observations is illustrated. We discuss what one has learned about such diverse astrophysical sources as photon-dominated regions, shocks, young stellar objects, planetary nebulae and starburst galaxies from ISO observations of H₂ emission. In this context, we emphasise use of measured H₂ line intensities to infer important physical quantities such as the gas temperature, gas density and radiation field and we discuss the different possible excitation mechanisms of H₂. We also briefly consider future prospects for observation of H₂ from space and from the ground. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands, and the United Kingdom), and with the participation of ISAS and NASA.     

The Planets and Titan Observed by ISO

Infrared spectroscopic observations of planets and Saturn's satellite Titan with the Infrared Space Observatory led to many significant discoveries that improved our understanding on the formation, physics and chemistry of these objects. The prime results achieved by ISO are: (1) a new and consistent determination of the D/H ratios on the giant planets and Titan; (2) the first precise measurement of the ¹⁵N/¹⁴N ratio in Jupiter, a valuable indicator of the protosolar nitrogen isotopic ratio; (3) the first detection of an external oxygen flux for all giant planets and Titan; (4) the first detection of some stratospheric hydrocarbons (CH₃, C₂H₄, CH₃C₂H, C₄H₂, C₆H₆); (5) the first detection of tropospheric water in Saturn; (6) the tentative detection of carbonate minerals on Mars; (7) the first thermal lightcurve of Pluto. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands, and the United Kingdom), and with the participation of ISAS and NASA.