THE INNER PLANETS

Chemists have long been fascinated by the composition of the planets, asteroids, comets, meteors, and other objects that make up the solar system. Are these bodies made of the same elements and compounds, in the same proportions, as the Earth itself How do their surfaces and atmospheres differ, if at all, from those of Earth If chemical differences among solar system bodies do exist, how can they be explained What does the chemical composition of solar system bodies tell scientists about their possible origins  

Prior to 1960, chemists had few tools with which to answer these questions. In some cases, it was possible to make reasonably good estimates of a planet s density, for example, based on its mass and volume. Both characteristics can be determined relatively easily from the planet s orbital path. Once a planet s density had been estimated, scientists could develop models that would fit the estimated density. 

For example, planets with densities similar to those of Earth (about 5.5 g/cm3) were thought to contain some combination of lightweight minerals such as silicates (densities of about 3.5 g/cm3) and more dense materials such as iron and nickel (densities of about 8.5 g/cm3). Those with lower densities, such as Mars (density = 3.93 g/cm3), were thought to consist primarily of rocky, silicate-like materials with little heavy metals. Those with much lower densities, such as Jupiter (density = 1.36 g/cm3), were assumed to consist almost entirely of gaseous elements and compounds. 

Scientists believe that the planets were born out of a mass of gas and dust rotating around the Sun. (Jon Lomberg/Photo Researchers, Inc.) 

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In about 4 billion years our sun will also develop into a red giant. The diameter will then reach the orbit of Mars, and the inner planets will cease to exist. 

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. 

It is also possible that neither of these mechanisms for providing water to the inner planets is correct. Another hypothesis is that absorption of water onto dust particles in the accretion disk might account for the Earth s oceans (Drake, 2005). As already mentioned, the amount of water required to explain Earth s water is not large on a per-gram basis. Regardless of whether comets, asteroids, or nebular particles were the source of our planet s oceans, the water likely came from more distant regions of the nebular disk. 

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],... 

Of all meteorites, the volatile-rich Cl chondrites are believed to most closely resemble the chemistry of the solar system (Wasson and Kallemeyn, 1988 Palme and Jones, 2004). Chondrites directly condensed from the Solar Nebula (Wasson and Kallemeyn, 1988, 536). Large numbers of chondrites are believed to have agglomerated into the planetesimals, which eventually formed the inner planets of our solar system (Wasson and Kallemeyn, 1988, 536). 

Miss Muxdroozol nods. The sun must have died by now. She pauses, It must have become a red giant, hundreds of times its original size. The inner planets were burned to cinders. Her voice takes on a sad timbre. The land was hot enough to melt lead. ... 

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. 

Detected rotational H2 and ro-vibrational CO, CO2, C2H2, HCN, as well as H2O and OH lines trace hot gas in the inner, planet-forming disk zone with T > 300 K (Brittain et al. 2003 Lahuis et al. 2006 Salyk et al. 2008), see Fig. 4.3. These lines are a good measure of temperature and high-energy radiation fields, and presumably sensitive to disk accretion, which could be a stress-test for advanced chemo-dynamical models. In Table 4.1 the various molecules used to study protoplanetary disks are overviewed. 

Chemical and isotopic compositions of the inner planets and asteroids... 

In our own solar system, nearly all volatiles complementary to the inner planets (3 X 10 Mg) were so lost. Earth and Venus contain only about 10 their complement of C, and even lesser amounts of H O, N, and noble gases. Since the retained C appears to show the imprint of the Fischer-Tropsch reaction, it seems likely that the lost C, too, had been involved in this process. 

Most meteorites are depleted in moderately volatile and highly volatile elements (see Figures 2-4). The terrestrial planets Earth, Moon, Mars, and the asteroid Vesta show similar or even stronger depletions (e.g., Palme et aL, 1988 Palme, 2001). The depletion patterns in meteorites and in the inner planets are qualitatively similar to those in the ISM. It is thus possible that the material in the inner solar system inherited the depletions from the ISM by the preferential accretion of dust grains and the loss of gas during the collapse of the molecular cloud that led to the formation of the solar system. There is, however, little support for this hypothesis ... 

The highly noncircular orbits of embryos and the long accretion timescales allowed considerable radial mixing of material over distances of 0.5-1.0 AU (Wetherill, 1994). It is likely that each of the inner planets accreted material from throughout the inner solar system, although the degree of radial mixing depends sensitively on the mass distribution of the embryos at this time (Chambers, 2001). The relative contributions from each part of the disk would have been different for... 

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