THE OUTER PLANETS

The solar system is sometimes divided into two parts consisting of the inner planets—Mercury, Venus, Earth, and Mars—and the outer planets—Jupiter, Saturn, Uranus, Neptune, and, until recently, Pluto. One might imagine that understanding the chemical and physical properties of the inner planets would help in understanding the chemical and physical properties of the outer planets. No such luck. The two groups of planets differ from each other in some fundamental and important ways. 

One of the most obvious differences between inner planets and outer planets is size. Earth, the largest of the inner planets, has a diameter of about 7,926 miles (12,750 km). By comparison, Neptune, the smallest of the outer planets, has a diameter of about 30,800 miles (49,500 km), more than four times as far across as Earth. Jupiter, the largest of the outer planets, has a diameter of about 88,740 miles (142,800 km), 12 times that of the Earth. In fact, 1,400 spheres the size of the Earth could be fit inside Jupiter. 

The composition of the outer planets is also very different from that of the inner planets. Mercury, Venus, Earth, and Mars are all made of rocky-like material with a density of about 5.5 g/cm3. By contrast, the outer planets seem to consist largely of gases (which accounts for their sometimes being called the gas giants) with densities of about 0.69 g/cm3 for Saturn to 1.54 g/cm3 for Neptune. These 

As on Mars, the principal contribution of infrared spectroscopy and radiometry to an understanding of the dynamics of the atmospheres of the outer planets has been the provision of information on the temperature fields. Ground-based measurements, data from the infrared radiometers on Pioneers 10 and 11, and measurements of infrared spectra from Voyager 1 and 2 have all provided significant information on atmospheric temperature structure. The largest set of spatially resolved data has been obtained by the Voyager infrared instruments. We will discuss analyses of those data to illustrate their usefulness. 

Sufficiently large quantities of spatially resolved data have been obtained for all four giant planets to permit the meridional, upper tropospheric thermal stmcture of each to be reasonably well defined. The latitudinal dependence of temperature at selected pressure levels are shown in Fig. 9.2.6 for Jupiter. These results have 

Venus provides an example of a slowly rotating body with a deep atmosphere. The surface pressure is approximately 95 bars and the rotational period in inertial space is 243 days. Infrared remote sensing has been applied to the study of the dynamics of the atmosphere of Venus, and an example of such an application is discussed. 

We now relate the measured temperatures to the mean zonal wind. In this case it is not possible to use the geostrophic thermal wind equation (9.2.29) as an examination of Eq. (9.2.21) shows. The ratio of the second term to the first term in the brackets on the left side of the equation is of the order of the ratio of the 243-day planetary rotation period to the four-day atmospheric rotation period or 60 hence, the second term dominates. This suggests a first approximation  

The lower boundary condition, u(zo), is the zonal wind speed at level zo- Elson (1979) has applied this diagnostic relation to the temperatures shown in Fig. 9.2.10. The lower boundary was taken to be at the cloud top height, zo = 65 km, and the wind speed, u(zo), was based on observed cloud motion. The resulting cyclostrophic component of the zonal wind in the northern hemisphere is shown in Fig. 9.2.11 as a function of latitude and z. At levels above the zero contour line pure cyclostrophic balance cannot hold since 94 /90 becomes positive. Other terms in Eq. (9.2.21) then become important in this part of the atmosphere. This result has led to the application of more complex diagnostic models by Taylor et al. (1980). 

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There may, of course, be types of life with a wholly different chemical basis to our own, for example, a low temperature life on the outer planets which is based on reactions in liquid ammonia. 

Effects of condensation are also seen in the bulk compositions of the planets and their satellites. The outer planets, Uranus and Neptune, have overall densities consistent with their formation from icy and stony solids. The satellites of Uranus have typical densities of 1.3g/cm which would tend to indicate a large ice com-... 

Water and carbon play critical roles in many of the Earth s chemical and physical cycles and yet their origin on the Earth is somewhat mysterious. Carbon and water could easily form solid compounds in the outer regions of the solar nebula, and accordingly the outer planets and many of their satellites contain abundant water and carbon. The type I carbonaceous chondrites, meteorites that presumably formed in the asteroid belt between the terrestrial and outer planets, contain up to 5% (m/m) carbon and up to 20% (m/m) water of hydration. Comets may contain up to 50% water ice and 25% carbon. The terrestrial planets are comparatively depleted in carbon and water by orders of magnitude. The concentration of water for the whole Earth is less that 0.1 wt% and carbon is less than 500 ppm. Actually, it is remarkable that the Earth contains any of these compounds at all. As an example of how depleted in carbon and water the Earth could have been, consider the moon, where indigenous carbon and water are undetectable. Looking at Fig. 2-4 it can be seen that no water- or carbon-bearing solids should have condensed by equilibrium processes at the temperatures and pressures that probably were typical in the zone of fhe solar... 

After planetary accretion was complete there remained two groups of surviving planetesimals, the comets and asteroids. These populations still exist and play an important role in the Earth s history. Asteroids from the belt between Mars and Jupiter and comets from reservoirs beyond the outer planets are stochastically perturbed into Earth-crossing orbits and they have collided with Earth throughout its entire history. The impact rate for 1 km diameter bodies is approximately three per million years and impacts of 10 km size bodies occur on a... 

The interiors of planets, moons, and many asteroids either are, or have been in the past, molten. The behavior of molten silicates and metal is important in understanding how a planet or moon evolved from an undifferentiated collection of presolar materials into the differentiated object we see today. Basaltic volcanism is ubiquitous on the terrestrial planets and many asteroids. A knowledge of atomic structure and chemical bonding is necessary to understand how basaltic melts are generated and how they crystallize. Melting and crystallization are also important processes in the formation of chondrules, tiny millimeter-sized spherical obj ects that give chondritic meteorites their name. The melting, crystallization, and sublimation of ices are dominant processes in the histories of the moons of the outer planets, comets, asteroids, and probably of the Earth. 

D. A. Rothery, Satellites of the Outer Planets , 1992, Clarendon Press, Oxford. 

Of a special astronomical interest is the absorption due to pairs of H2 molecules which is an important opacity source in the atmospheres of various types of cool stars, such as late stars, low-mass stars, brown dwarfs, certain white dwarfs, population III stars, etc., and in the atmospheres of the outer planets. In short absorption of infrared or visible radiation by molecular complexes is important in dense, essentially neutral atmospheres composed of non-polar gases such as hydrogen. For a treatment of such atmospheres, the absorption of pairs like H-He, H2-He, H2-H2, etc., must be known. Furthermore, it has been pointed out that for technical applications, for example in gas-core nuclear rockets, a knowledge of induced spectra is required for estimates of heat transfer [307, 308]. The transport properties of gases at high temperatures depend on collisional induction. Collision-induced absorption may be an important loss mechanism in gas lasers. Non-linear interactions of a supermolecular nature become important at high laser powers, especially at high gas densities. 

It is, therefore, noteworthy that almost immediately upon Welsh and associates discovery of collision-induced absorption in hydrogen [128, 129, 420], Herzberg found the first direct evidence of the H2 molecule in the atmospheres of the outer planets [181, 182], He was able to reproduce in the laboratory the unidentified diffuse feature at 827.0 nm observed by Kuiper in the spectra of Uranus and Neptune, using an 80 m path of hydrogen at 100 atmospheres pressure and a temperature of 78 K. The feature is the S3(0) line of the 3 — 0 collision-induced rotovibrational band of the H2 molecule [182]. 

We know today that hydrogen and helium are overwhelmingly the most abundant species in the atmospheres of the outer planets, but direct evidence for their presence was virtually absent prior to the work mentioned [145]. Supermolecular spectroscopy had to be discovered before such evidence could be understood and it comes as no surprise that soon after Welsh s discovery many other uses of collision-induced absorption were pointed out in various astrophysical studies. Supermolecular absorption and emission have become the spectroscopy of the neutral, dense regions, especially where non-polar gases prevail. 

Trafton has shown in 1964 that the opacity in the far infrared of the atmospheres of the outer planets is due to the rototranslational band of H2-H2 and H2-He pairs [393], It is now clear that collision-induced absorption plays a major role in the thermal balance and atmospheric structure of the major planets. The Voyager emission spectra of Jupiter and Saturn show dark fringes in the vicinity of the So(0) and So(l) lines of H2, Fig. 7.3, which are due to collision-induced absorption in the upper,... 

A. Borysow and L. Frommhold. Theoretical collision induced rototranslational absorption spectra for the outer planets H2-CH4 pairs. Astrophys. J., 304 849, 1986. 

L. Trafton. Observational studies of collision induced absorption in the atmospheres of the outer planets. In J. Szudy, ed., Spectral Line Shapes 5, p. 755, Ossolineum, Warsaw, 1989. 

A (gas) clathrate hydrate is a crystalline compound which can be obtained by the formation of a hydrogen-bonded host lattice around one or more species of guest molecules. It s a pleasing thought to a crystallographer that when it snows on the outer planets it might snow gas hydrates. (Jeffrey and McMullan, 1967.)... 

Oxygen is the most abundant element in the Earth s crust and accounts for 23 % of the mass of the atmosphere. In fact, Earth is the only planet in the solar system with an oxidizing atmosphere. On Mars, oxygen provides only 0.15% of the atmospheric mass and in the atmospheres of the outer planets, oxygen is essentially nonexistent. In the hot atmosphere of Venus, the oxygen has reacted and is present mainly as carbon dioxide. In that form, and as certain other gaseous oxides, it contributes to the warming of the planet (Box 15.1). 

The necessary starting point for any study of the chemistry of a planetary atmosphere is the dissociation of molecules, which results from the absorption of solar ultraviolet radiation. This atmospheric chemistry must take into account not only the general characteristics of the atmosphere (constitution), but also its particular chemical constituents (composition). The absorption of solar radiation can be attributed to carbon dioxide (C02) for Mars and Venus, to molecular oxygen (02) for the Earth, and to methane (CH4) and ammonia (NH3) for Jupiter and the outer planets. 

Among places where condensates accreted into significant solid bodies, such as planets, habitable realms have always been rarer than places that were either too cold or too hot for life to exist. Much of our Solar System s mass is still far too hot for life. Most of the deep interiors of the gas giants and rocky planets are too hot, as is, of course, the Sun itself. Most of the surface area of solid bodies in the Solar System are too cold - the icy satellites of the outer planets and the myriad comets and Kuiper Belt Objects on the far outer fringes of the Solar System. In this sense, places like the surfaces of Earth and Mars and Europa s subsurface ocean are indeed very rare places. 

According to the foregoing analysis, conformers such as gauche- and anri-butane or chair and twist-boat cyclohexane would be considered to be diastereomers of each other. However, under most conditions these conformers interconvert so rapidly that butane and cyclohexane are considered to be single species and not mixtures of stereoisomers. When we have to write chemistry books for people living on the outer planets of the solar system, we might have to modify these concepts. 

Figure 2-54 shows Kepler and his planetary model based on the regular solids [84], According to this model the greatest distance of one planet from the sun stands in a fixed ratio to the least distance of the next outer planet from the sun. There are five ratios describing the distances of the six planets which were known to Kepler. A regular solid can be interposed between two adjacent planets so that the inner planet, when at its greatest distance from the sun, lays on the inscribed sphere of the solid, while the outer planet, when at its least distance, lays on the circumscribed sphere. 

Reactions of C with H, H2 are of fundamental importance to the carbon chemistry in interstellar clouds, and some of the reaction paths prevalent in dense interstellar clouds may also be of significance in the reducing atmospheres of the outer planets. These reactions initiate a complex sequence which produce CH, CH and lead eventually to molecules such as CH, These molecules are important pre-... 

On the whole observation of H3 emission has emerged as a useful ground-based observational method for the study of plasma activities in the outer planets. 

Probably this requires timescales of <10 yr (Podosek and Cassen, 1994). In contrast, the most widely accepted dynamic models advocated for the formation of the terrestrial planets (Wetherill, 1986), involve protracted timescales —10 -10 yr. Application of these same models to the outer planets would mean even longer timescales. In fact, some of the outermost planets would not have yet formed. Therefore, the bimodal distribution of planetary density and its striking spatial distribution appear to require different accretion mechanisms in these two portions of the solar system. However, one simply cannot divide the accretion dynamics into two zones. A range of rate-limiting processes probably controlled accretion of both the terrestrial and Jovian planets and the debates about which of these processes may have been common to both is far from resolved. There almost certainly was some level of commonality. 

The mission to Venus, hy the Galileo spacecraft in 1989-90, produced relatively modest new data. The mission s primary objective was the planet Jupiter, and a visit to Venus was included only to provide a "gravity assist —a way to give the spacecraft the impetus it needed to get to the outer planet. During its closest approach of about 10,000 miles (16,000 km) from the planet, however, Galileo was able to carry out additional spectroscopic studies of Venus s clouds, collect photographs of its middle atmosphere clouds, and analyze radioactive sources present in the clouds. 

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