Methane comets

Alkanes have the general molecular formula C H2 +2 The srmplest one methane (CH4) rs also the most abundant Large amounts are present rn our atmosphere rn the ground and rn the oceans Methane has been found on Juprter Saturn Uranus Neptune and Pluto and even on Halley s Comet... 

The next most likely possibility is cometary delivery of the atmosphere but again there are some problems with the isotope ratios, this time with D/H. The cometary D/H ratios measured in methane from Halley are 31 3 x 10-5 and 29 10 x 10-5 in Hayuatake and 33 8 x 10-5 in Hale-Bopp, whereas methane measurements from Earth of the Titan atmosphere suggest a methane D/H ratio of 10 5 x 10-5, which is considerably smaller than the ratio in the comets. The methane at least in Titan s atmosphere is not exclusively from cometary sources. Degassing of the rocks from which Titan was formed could be a useful source of methane, especially as the subnebula temperature around Saturn (100 K) is somewhat cooler than that around Jupiter. This would allow volatiles to be more easily trapped on Titan and contribute to the formation of a denser atmosphere. This mechanism would, however, apply to all of Saturn s moons equally and this is not the case. 

The volatile-trapping mechanism has a further problem associated with the temperature. Very volatile molecules such as N2, CO and CH4 are not easily trapped in laboratory ice simulation experiments unless the ice temperature is 75 K, which is somewhat lower than the estimated Saturnian subnebula temperature. This has led to the suggestion that the primary source of nitrogen within the Titan surface ices was NH3, which became rapidly photolysed to produce H2 and N2 upon release from the ice. The surface gravity is insufficient to trap the H2 formed and this would be lost to space. However, the origin of methane on Titan is an interesting question. Methane is a minor component of comets, with a CH4/CO ratio of clCT1 compared with the present atmospheric ratio of > 102. The D/H ratio is also intermediate between that of comets and the solar nebula, so there must be an alternative source of methane that maintains the carbon isotope ratio and the D/H isotope ratio and explains the abundance on Titan. 

Hydrate clathrates of organic compounds are thought to be responsible for the behavior of ice in the heads of comets and in wet methane under pressure.22 Unless methane is carefully dried, high-pressure lines may become clogged with the ice-like gas hydrate. There may be large deposits of methane hydrates, the ice that burns. beneath the ocean floor. 

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. 

Probably the best general description of a comet ever made was by the American astronomer and comet authority Fred Whipple (1906-2004). Whipple called comets "dirty snowballs." That term is apt because cometary nuclei consist primarily of ices of water, ammonia, carbon dioxide, and methane mixed with dust particles. A comet that is distant from the Sun is essentially invisible because its nucleus is so small and dark. As the comet approaches the Sun, however, solar radiation vaporizes some of the ices that make up the nucleus. The gases thus released, along with some of the dust in the nucleus, form the familiar and spectacular features of a comet. 

The chemical composition of short- and long-term comets tends to he somewhat different. Probably the most important factor difference is that short-term comets tend to spend more time close to the Sun than long-term comets do. Each time a comet passes around the Sun, it loses some of its mass because of outgassing caused by solar radiation. As a result, short-term comets tend to have lower amounts of "volatile substances (those that sublime or evaporate at relatively low temperatures), such as water ice, methane ice, and ammonia ice. 

In 1941, astronomer Walter S. Adams (1876-1956) discovered another new chemical species in interstellar space. Herzberg generated the same species in the laboratory by burning methane. Comparison of experiment and theory confirmed that it was CH. In 1942, Herzberg investigated a set of emission bands observed earlier in a comet (comet 1940c) and centered... 

Nineteenth-century spectroscopes allowed astronomers to record emission spectra of stars, comets, and planets, and even to discover a new element (helium) on the Sun. In Germany, Hermann Vogel (1842-1907) discovered that the major components of Jupiter s atmosphere are hydrogen and helium, very similar to the Sun. In the earlier years of the 20th century, Vesto M. Slipher (1875-1969), at the Lowell Observatory, recorded spectra from Jupiter, Saturn, Uranus, and Neptune and in the early 1930s German astronomer Rupert Wildt (1905-76) analyzed them and discovered traces of methane (CH4) and ammonia (NH3) in the atmospheres of these frigid outer planets. 

Figure 20. Formation of methane and water due to the water-gas shift and Fischer-Tropsch reactions (A) heating of metallic Co in C02 H2 atmosphere and (B) pulses of CO2 over Comet heated in 20 vol% H2.

The results shown in Figure 20 depict the investigation of the reactions between CO, CO2, H2O and H2 in the presence of the active metallic cobalt under conditions of COD decomposition. The metallic cobalt obtained by the decomposition of COD in hydrogen was heated at a rate of 10 K min in an atmosphere of CO2 and H2 mixed in the ratio 1 2 (Figure 20A). In transient experiments (Figure 20B), the pulses of CO2 were injected into a system in which COmet was heated in an atmosphere of 20 vol% H2, balance He. The first experiment confirmed that, imder die applied conditions, the formation of methane begins at about 190 C. The maximum yield of CH4, calculated from the results of PulseTA experiments shown in Figure 20B, was observed at about 400 C. 

Hydrocarbons are also found in outer space. Asteroids and comets contain a variety of organic compounds. Methane and other hydrocarbons are found in the atmospheres of Jupiter, Saturn, and Uranus. Saturn s moon Titan has a solid form of methane—water ice at its surface and an atmosphere rich in methane. Whether of terrestrial or celestial origin, we need to understand the properties of alkanes. We begin with a consideration of their shapes and how we name them. 

PAHs comprise a large percentage of the carbon found in interstellar space. They have even been observed in interstellar ice (Halley s comet, for example). It has been shown that ultraviolet irradiation of PAHs in ice produces aromatic ketones (Chapter 9), alcohols (Chapter 7), and other compounds, suggesting a role of PAHs in prebiotic chemistry (see A Word about Methane, Marsh Gas, and Miller s Experiment, page 60). 

The interest in the possibility of extraterrestrial clathrate hydrates on comets and the outer planets continues, for example, methane hydrate on Titan and CO2 hydrate on Mars. Moreover, this has sparked an interest in clathrate hydrate phases at high pressure. ... 

Space is not as empty as it once seemed. True, there are no men on the moon or canals on Mars, but there are oceans on Titan (methane) and three other moons (under ice). Water can be detected many places from its intense infrared color. This color is especially rich in comets and can be found in nebulae and dark lunar craters, even on the dark side of Mercury. Much of this water predated even the sun. 

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