Atmospheres, Planetary

Atmospheres are a natural consequence of the origin and evolution of planets. If planets are of sufficient size, they may have captured some nebular gas while they formed. Accretionary and radioactive heating can also release gases that were brought into the planet in solid carriers. The atmospheres of Venus, Earth, and Mars are composed of the same gases (C02, N2, H20, 02), but in markedly different amounts and proportions, reflecting their different evolutionary histories. For example, the rise of life on Earth 

Noble gas abundances in planetary atmospheres and Cl carbonaceous chondrites, relative to silicon and solar abundances. After Porcelli and Pepin (2004). 

In contrast to the terrestrial planets, the giant planets are massive enough to have captured and retained nebular gases directly. However, concentrations of argon, krypton, and xenon measured in Jupiter s atmosphere by the Galileo spacecraft are 2.5 times solar, which may imply that its atmosphere preferentially lost hydrogen and helium over the age of the solar system. 

In this chapter, we only briefly consider ices, focusing on their relationship to organic matter and noble gases in the ISM and in the solar nebula. Our understanding of ices is rudimentary because, unlike meteorites, ices have been studied only by remote sensing. 

Interstellar grains with ice mantles probably comprised a significant amount of the material that collapsed to form the solar nebula. Heating of this material caused the icy mantles to sublimate, producing a vapor that subsequently condensed as crystalline ices as the nebula cooled. By mass, H20 ice rivals rock in terms of potentially condensable matter from a gas of cosmic composition. The amount of water ice depends, of course, on the extent to which oxygen is otherwise tied up with carbon as CO and/or C02 (Prinn, 

Since particles are ubiquitous in planetary atmospheres [1.34], they play roles in most atmospheric processes. 

Cloud formation and dynamics is a subject which requires considerable information from almost all the areas of aerosol microphysics [1.37,18]. Thermodynamics determines aerosol growth to cloud droplet size, and electrical processes in clouds may play a role in the onset of precipitation. Clouds may scavenge the atmosphere of aerosols through capture of particles by cloud droplets, with the rates and mechanisms described by subtleties of their interaction forces and aspects of kinetic theory. 

A problem of overriding importance for planetary atmospheres as a whole is that of the albedo, the ratio of the energy radiated to space by the planet to that incident upon it [1.39]. Particles and cloud droplets are responsible for reflection back to space of a part of the incident energy, while they play roles in the capture and thermalization of the thermal emissions of the solid earth. In this latter regard, particles may be particularly important in the question of stratospheric heating [1.40,41]. Besides particle interaction with light, microphysics and kinetic theory may also be important in understanding aspects of the thermal transfer problem. 

In Earth s upper atmosphere (and, in planetary probe missions, the atmospheres of other planets), atmospheric composition can be measured in situ with mass spectrometers and related instrumentation. However, UV measurements provide a capability for remote sensing of atmospheric composition and its variation with altitude, geographic location, and time, which supplements and extends in situ measurements where available, and can be applied to many objects not yet visited by spacecraft (e.g., in observations of other planets by spacecraft in near-Earth orbit). In addition to atmospheric composition, UV measurements can remotely sense the atmospheric temperature structure and its spatial and temporal variations. Also, information on the fluxes, energies, and spatial distributions of incoming energetic particles (such as those that produce Earth s polar auroras) can be obtained. 

PRC98-04 ST Scl OPO January 7, 1998 J. Clarke (University of Michigan) and NASA 

Hydrogen Lyman a and the OH molecular band emission near 310 nm are the two most prominent spectral features in comets. These indicate that water is indeed the dominant volatile constituent of comets other materials (which are responsible for the ground-accessible features, such as CH, CN, C2, and NH band emissions) are only minor constituents. Because of the small mass of 

Ultraviolet observations of several comets, including West (1975) and Halley (1986) (Fig. 13), revealed that CO is also a prominent cometary constituent. Ultraviolet observations have also resulted in the detection of minor species not previously observed in comets, including atomic and diatomic sulfur, ionized carbon, and the SH radical. Future, more sensitive observations and ones extending to shorter wavelengths are expected to reveal other constituents, such as H2, N2, N, N+, and 0+. 

Current extrasolar planets are all much larger than the Earth. The total count at present (9 September 2005) is 168 found in 144 planetary systems, of which 18 contain multiple planets. The first to be discovered was 51-Pegasi in the constellation of Pegasus by the radial velocity method. It is about 0.45 Mjupiter and has an orbital period around the star of about 4.5 days. Of the 168 planets found so far only nine are present within a habitable zone around their star. The survey of the star catalogue for planets has only just started and we have found a large number of planets very quickly - solar systems, at least, are not special. 

The preceding calculation of the thermal energy balance of a planet neglected any absorption of radiation by molecules within the atmosphere. Radiation trapping in the infrared by molecules such as CO2 and H20 provides an additional mechanism for raising the surface temperature - the greenhouse effect. The local temperature of a planet can then be enhanced over its black body temperature by the atmosphere. 

The noble gas elements act as a record of the deposited material because they are essentially chemically inert and are also trapped within the ice of comets and meteorites. The late-heavy bombardment era must have affected both the Earth and the Moon similarly so an estimate of the collision frequency may be obtained by using the record of impacts on the Moon s surface. The collision rate calculated 

The delivery of volatiles to Earth and Mars must have been similar but where has the early Martian atmosphere gone The atmosphere of the inner planets can be seen in Table 7.3. Cometary and meteorite impacts can deliver material to a planet but are also responsible for a process called impact erosion where the atmosphere could be lost due to an impact such as the Earth-Moon capture event. Current estimates suggest that impact erosion may be responsible for the loss of 100 times the current mass of the Martian atmosphere. 

As the planet acquires a volatile molecule inventory it begins to develop an atmosphere, and in the case of the Earth this also includes the extensive circulation of water in the hydrosphere. The weight of the volatiles trapped in the atmosphere of a planet leads to a mean surface pressure, po, given by  

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The astrochemistty of ions may be divided into topics of interstellar clouds, stellar atmospheres, planetary atmospheres and comets. There are many areas of astrophysics (stars, planetary nebulae, novae, supemovae) where highly ionized species are important, but beyond the scope of ion chemistry . (Still, molecules, including H2O, are observed in solar spectra [155] and a surprise in the study of Supernova 1987A was the identification of molecular species, CO, SiO and possibly ITf[156. 157]. ) In the early universe, after expansion had cooled matter to the point that molecules could fonn, the small fraction of positive and negative ions that remained was crucial to the fomiation of molecules, for example [156]... 

For remote sensing, spectroscopy at THz frequencies holds the key to our ability to remotely sense enviromnents as diverse as primaeval galaxies, star and planet-fonuing molecular cloud cores, comets and planetary atmospheres. 

Through spectroscopic observations and sometimes tenuous deductions there has accumulated a significant picture of the makeup of the planetary atmospheres. Doubt pervades much of this picture, yet it represents our starting place in knowledge as we venture outside our own atmosphere for the first time. Table 25-V summarizes a part of this information—the maximum surface temperatures and the chemical compositions. Naturally, these compositions are incomplete ... 

The composition of the planetary atmospheres is fairly constant. This is indeed surprising in view of the fact that molecules such as methane, ammonia, and carbon dioxide are easily decomposed by the ultraviolet radiation from the sun. Presumably other reactions regenerate those substances that are light sensitive. 

The solar spectrum is, of course, as well studied as our planetary atmosphere will permit. More information will be forthcoming as spectra from man-made satellites are recorded above the atmosphere. At this time, the spectra of many diatomic molecules have been detected. These are not the familiar, chemically stable molecules we find on the stockroom shelf. These are the molecules that are stable on a solar stockroom shelf. Figure 25-3 shows some of these and the location in the periodic table of the elements represented. 

Volume 22 Joseph W. Chamberlain. Theory of Volume 38 Planetary Atmospheres An Introduction to Their Physics and Volume 39 Chemistry. 1978 ... 

The CO2 content of the planetary atmosphere plays a vital role. A relatively high CO2 partial pressure was certainly an important precondition for solving the problem of the faint, young sun . It is assumed that the sun was much cooler four billion... 

Composition of Planetary Atmosphere (Today) - Molar% in Volume... 

Of the alkenes (Figure 5.5) only ethene has been detected and of the aromatics only benzene has been seen unambiguously surprisingly propene has not been seen despite its well-understood microwave spectrum. Of interest to the origins of life is the onset of polymerisation in HCN to produce cyanopolyynes. These molecules could provide a backbone for the formation of information-propagating molecules required for self-replication. The survival of these species in a planetary atmosphere depends on the planet oxidation would be rapid in the atmosphere of today s Earth but what of the early Earth or somewhere altogether more alkane-based such as Titan ... 

This pathway sets the direction for the remainder of this chapter but has the same basic strategy for the understanding of meteorite chemistry, comet chemistry, planetary atmospheric chemistry, prebiotic chemistry and ultimately the chemistry of an organism. This pathway is the molecule-up view of the Universe. 

In principle, it is now possible to construct a complete network of interconnecting chemical reactions for a planetary atmosphere, a hot molecular core or the tail of a comet. Once the important reactions have been identified the rate constants can be looked up on the database and a kinetic model of the atmosphere or ISM molecular cloud can be constructed. Or can it Most of the time the important reactions are hard to identify and if you are sure you have the right mechanisms then the rate constants will certainly not be known and sensible approximations will have to be made. However, estimates of ISM chemistry have been made with some success, as we shall see below. 

Meteoroid - the name given to a meteorite or a meteor before it enters the Earth s (or any other planetary) atmosphere. 

The temperature profile of a planetary atmosphere depends both on the composition and some simple thermodynamics. The temperature decreases with altitude at a rate called the lapse rate. As a parcel of air rises, the pressure falls as we have seen, which means that the volume will increase as a result of an adiabatic expansion. The change in enthalpy H coupled with the definition of the specific heat capacity... 

The rate of photolysis, J, depends on the absorption cross-section, a, the number density, the scale height and the angle, all of which are unique properties of a planetary atmosphere. For the Earth and the Chapman mechanism for ozone the O3 concentration maximum is 5 x 1012 molecules cm-3 and this occurs at 25 km, shown in Figure 7.12, and forms the Chapman layer structure. 

Kobayashi K. et al. (2001). Formation of Bioorganic Compounds in Simulated Planetary Atmospheres by High-Energy Particles or Photons. Adv. Space Res. 27(2) 207-215. 

The similarities in products and pathways between interstellar molecules and terrestrial laboratory experiments imply a unity of physical and chemical laws in the universe. Given certain conditions and appropriate energy sources, the same chemical pathways will be followed to create certain products from the elements. That is not to say that life, even in primitive form, could be supported in interstellar space. The significant precursor molecules found in interstellar space are at extremely low concentrations, but if they were transported to planetary atmospheres, perhaps by comets, they might then react in the proper environment and evolve into self-replicating systems. 

The decompositions of C302, CO, C02, CS2, COS, CSe2 and COSe are dealt with in this section. Apart from carbon suboxide, this is a group of stable, un-reactive compounds. Considerable emphasis has been placed on the investigation of the photolytic decompositions of some of these compounds which are thought to provide useful sources of atoms (C, O, S and Se) and free radicals (C20). The photochemistry of carbon dioxide has particular relevance to the chemistry of planetary atmospheres, although to date the mechanism of C02 photolysis remains obscure. 

Thiemens MH (1988) Heterogeneity in the nebula evidence from stable isotopes. In Meteorites and die Early Solar System. Kerridge JF and Matthews MS (eds) University of Arizona Press, Tucson, p 899-923 Thiemens MH (1999) Mass independent isotope effects in planetary atmospheres and die early solar system. Science 283 341-345... 

Thiemens MH (1999) Mass-independent isotope effects in planetary atmospheres and the early solar system. Science 283 341-345... 

It might seem at first glance that arriving at the dipole moment p of an ellipsoidal particle via the asymptotic form of the potential < p is a needlessly complicated procedure and that p is simply t>P, where v is the particle volume. However, this correspondence breaks down for a void, in which P, = 0, but which nonetheless has a nonzero dipole moment. Because the medium is, in general, polarizable, uP, is not equal to p even for a material particle except when it is in free space. In many applications of light scattering and absorption by small particles—in planetary atmospheres and interstellar space, for example—this condition is indeed satisfied. Laboratory experiments, however, are frequently carried out with particles suspended in some kind of medium such as water. It is for this reason that we have taken some care to ensure that the expressions for the polarizability of an ellipsoidal particle are completely general. 

Noble gases are most abundant in planetary atmospheres, although even there they are only minor components. They have been measured in the gas envelopes of Venus, Earth (of course), Mars, and Jupiter. We will consider their utility in understanding planetary differentiation and atmospheric evolution shortly, but first we will focus on their rather miniscule abundances in meteorites and other extraterrestrial materials. 

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