Atmospheres inner planets

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. 

The Earth s oceans reveal an abundance of water that corresponds to —1/1000 of the planet s mass. Mars, too, once had liquid water that sculpted its surface, and water ice still resides at its poles and in its subsurface at high latitudes. The high D/H ratio in the atmosphere of Venus suggests that it once may have contained water in similar abundance to the Earth. Even Mercury, baking in the Sun s glare, appears to have water ice at its poles. The amounts of water in the terrestrial planets are modest, relative to the amounts of water in gas- and ice-rich planets in the outer solar system, but the importance of water for planetary habitability demands that we discuss how the inner planets got their water. 

Collision-induced absorption has been studied in the laboratory in various dense gases besides hydrogen and mixtures of hydrogen and helium, especially in oxygen, nitrogen, methane, etc., and mixtures of such gases which are of interest in the atmospheres of the inner planets [5, 131, 58],... 

But what was there, in addition to water, on the primitive Earth The four outer planets of the solar system (Jupiter, Saturn, Uranus and Neptune) are still made up mainly of hydrogen, helium, methane, ammonia and water, and it is likely that those same chemicals were abundant everywhere else in the solar system, and therefore even in its four inner planets (Mercury, Venus, Earth and Mars). These were too small to trap light chemicals, such as hydrogen and helium, but the Earth had a large enough mass to keep all the others. It is likely therefore that the Earth s first atmosphere had great amounts of methane (CH4), ammonia (NHJ and water, and was, as a result, heavy and reducing, like Jupiter s. 

Jupiter s general characteristics have been well known for some time. Unlike the inner planets, it has no distinct dividing line he tween an outer atmosphere and an inner core, mantle, and crust. Instead, Jupiter consists of elements, compounds, and other chemical species that are normally gaseous hut that may occur as liquids or solids the closer they are to the planet s center. As the diagram below shows, the outermost region of the planet, the "atmosphere" that is visible from Earth, consists of clouds of ammonia, methane, and water. The pressure within the cloud layer is about one atmosphere, and the temperature, about 165 K (about -100 °C). 

Ip and Fernandes [101] calculated that 6 x 1024 to 6 x 1025 g of cometry material could have been delivered to Earth at the time of the formation of the great Oort Cloud of comets. This amount is equivalent to 4-40 times the present mass of the oceans, assuming about 50% of the cometary mass is ice. Owen and Bar-Nun [102] examined the ability of amorphous ice formed at temperatures below 100K to trap ambient gases. By comparison of the compositions of gases trapped by ice with the compositions of the interstellar medium, comets, and planetary atmospheres, Owen and Bar-Nun [102] concluded that icy comets delivered a considerable fraction of the volatiles to the inner planets. Owen [83] emphasized that the potential supply of cometary materials is more than adequate. 

A number of arguments can be made in favor of cometary carriers for inner planet volatiles. As noted above, reduction of the Ne/Ar ratio relative to the solar ratio, resembling the elemental pattern on Venus, is likely in such ices. Modeling discussed below indicates that a source of this nature could have supplied essentially identical primary atmospheres to both Venus and Earth if an initially Venus-like atmosphere on Earth were later elementally fractionated in hydrodynamic escape powered by a giant Moon-forming impact. 

Xenon isotopes in oceanic basalts provide the most basic evidence on early differentiation of our planet and on formation of the atmosphere. Several Xe isotopes are produced by extinct radioactivity, providing a method to quantify events at the very start of Earth history near 4500 million years ago to a high precision, within several tens of millions of years. The difference between Xe/ °Xe of the mantle and atmosphere is fundamental evidence for their early separation (e.g., Thompson 1980 Staudacher and Allegre 1982). It is thought that formation of the atmosphere involved some early degassing of the mantle. Whether the Earth ever had a primary atmosphere is still an open question, and ultimately depends upon whether the inner planets formed in the presence of a solar nebula gas phase. The timing of these early events, the degree to which Xe isotope differences reflect variations in I/Xe and Pu/Xe of source reservoirs, and the... 

Hydrogen and helium make up over 98% of the Sun s mass and are the most abundant elements in the universe. The small black circle in the upper left is the planet Venus passing between the Sun and the Earth, a rare event that occurred in June of 2012 and will not occur again until 2117. The gases that make up the atmospheres of the inner planets like Venus have a very different composition from that of the Sun. 

By the time our sun had formed, countless stars had already completed their life cycles. The clot of gas that produced the sun was a mixture of primordial hydrogen and heavier elements. These heavier elements were essential to producing the inner planets, satellites, asteroids, and other objects in the solar system which cannot be constructed from hydrogen and helium. The bulk of the heavy elements in the solar system is in the outer portions of the sun itself, which contains more than 99.87 percent of all the mass of the solar system. (The outer portions of the sun do not mix with the core, where nuclear reactions destroy heavy elements.) The abundances of yttrium and the lanthanides in the sun s atmosphere have been determined spectroscopically and are believed known with medium accuracy (Ross and Aller, 1976). Pieces of the Earth, the Moon, and meteorites, all of which condensed from the same batch of material as the sun, have been analyzed chemically to determine their abundances. 

The Sill and Wilkening proposal that clathrates formed in the cold outer parts of the solar system and then transported to the inner solar system (e.g., in comets) might help account for the atmospheres of the terrestrial planets. They contend that infall of 1 ppm of ice-methane clathrate with noble gases dissolved as just described could account for the present inventories of Ar, Kr, and Xe in the terrestrial atmosphere. 

Models of irradiated disks predict four chemically distinct zones (see Fig. 4.1). (I) Zone of ices in the cold mid-plane opaque to incoming radiation. Chemistry in this region is dominated by cold gas-phase and grain-surface reactions. Here Infrared Space Observatory (ISO) and Spitzer observations confirmed the existence of ices, various silicates and PAHs (polycyclic aromatic hydrocarbons e.g. van den Ancker et al. 2000 van Dishoeck 2004 Bouwman et al. 2008). (II) Zone of molecules, a warm molecular layer adjacent to the mid-plane, dominated by ultraviolet/X-ray-driven photochemistry (III) the heavily irradiated zone of radicals, a hot dilute disk atmosphere deficient in molecules and (IV) the inner zone, inside of the ice line where terrestrial planets form. 

Venus. Venus is characterized only by the immensely valuable but still incomplete and relatively imprecise reconnaissance data from the Pioneer Venus and Venera spacecraft missions of the late 1970s. Additional in situ measurements, at precisions within the capabilities of current spacecraft instrumentation, are now necessary to refine atmospheric evolution models. Unfortunately, the possibilities of documenting the volatile inventories of the interior of the planet are more remote. A significant question that must be addressed is whether nonradiogenic xenon on Venus is compositionally closer to SW-Xe (as seen on Mars) or to the U-Xe that is seen on the Earth and so is expected to have been present within the inner solar system. Also, the extent of xenon fractionation will be an important parameter for hydrodynamic escape models if intense solar EUV radiation drove hydrodynamic escape on the Earth, it would also impact Venus, while losses from the Earth driven by a giant impact would not be recorded there. 

Depletion of rare gases in Earth s atmosphere in comparison with cosmic abundances suggests that any primary atmosphere captured at the planet s early accretion could have been lost by an impact with one or more large bodies during the later stages of the accretion [66,74], and by T-tauri solar winds of high-energy particles which could readily blow volatile elements out of the inner Solar System [75]. 

Chemical equilibrium abundances for many C-, N-, and 0-bearing gases for T = 500-2500K and P = 10" -1000 bars for a solar composition gas can be found in 54). Although these calculations were applied to brown dwarfs and giant planet atmospheres, the pressure conditions also include those appropriate for O-rich photospheres and their inner CSEs (10" - 10" bars). 

Table 2.8 Composition of the atmospheres of the inner terrestrial planets (in ppm when ...

The major geochemical reservoirs of the Earth that are currently in existence—inner and outer core, upper and lower mantle, upper and lower continental cmst, oceanic cmst, sedimentary shell, oceans, and atmosphere—were established early in the planet s history. On the other hand, the sizes and compositions of these reservoirs have changed over... 

Our own solar system was formed ca. 4.5 billion years ago out of matter from supernovas and other interstellar substances, and the earth is one of the nine planets in this system. The different zones within the planet are shown in Figure 3.2. The mantle constitutes ca. 83% of the earth s total volume and consists mainly of iron and magnesium silicates. The lithosphere is the sohd portion of the earth as contrasted with the atmosphere and the hydrosphere. It includes the earth crust and the upper mantle. Between 100 and 400 km depth in the upper mantle is a plastic shell, the partially molten asthenosphere. Currents in that medium cause the movements of the continents. The core is believed to consist of an alloy of iron and nickel, and to contain also up to 10% of a lighter element, perhaps sulfur or oxygen. The inner central core is solid, the outer fluid. The temperature in the center is about 4000°C. 

The ability to associate a physical surface temperature to the spectrum relies on the existence and identification of spectral windows probing the surface. For an Earth-like planet there are some atmospheric windows that can be used in most of the cases, especially between 8 and 11 pm as seen in Fig. 5.1. Such identification is not trivial for non Earth-analogue atmospheres. Note that this window would however become opaque at high H2O partial pressure (e.g. the inner part of the Habitable Zone (HZ) where a lot of water is vaporized) and at high CO2 pressure (e.g. a very young Earth or the outer part of the HZ). 

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