Identification of Gases by Absorption Spectra

Sunlight passes through the Martian atmosphere, first to the planet, and then after reflection by the surface and the atmosphere. In each passage the molecules and atoms present in the atmosphere of Mars absorb radiations of characteristic wavelength, and it is by the study of these absorption spectra on Earth that information concerning the composition of the Martian atmosphere has been obtained. Of course, if a particular species does not absorb any of the radiation from sunlight or the absorbed radiation is in such a region of wavelength that it does not reach the instruments on Earth, there is no way of determining whether that species is present or not.

The spectroscopic study of Mars was initiated in 1862 by the English astronomer William Huggins, only 3 years after the invention of the method of spectral analysis. Shortly thereafter, similar work was done by others in the United States, in Germany, and in Italy, but in all cases the results were negative. The only absorption lines detected were those present in solar radiation, thus proving, at least, that the light from the planet is reflected sunlight.

Because of their great potential interest, spectroscopic investigations of Mars were continued throughout the latter part of the 19th century and into the 20th century. It was not until 1947, however, that the first definitive results were obtained. The difficulties in these studies arose mainly from the spectral lines normally present in sunlight, the absorption of radiation by gases in the terrestrial atmosphere, the weakness of the light from Mars that reaches instruments on Earth's surface, and inadequacies of the instrumentation. It is of interest that, as recently as the early 1960's, some absorption bands, which seemed to be of great significance when discovered in 1956, were shown to have originated in a rare constituent (HDO) of Earth's atmosphere rather than in Mars (p. 220).

Planetary Atmospheres

Before going on to describe what is known and what is surmised about the atmosphere of Mars, some of the general ideas concerning planetary atmospheres will be reviewed. By far the most abundant constituent element of the universe is hydrogen, and this is followed by helium. Next in order, although much less abundant than helium, are carbon, nitrogen, and oxygen.1 It is widely accepted that the primitive atmospheres of the planets some 4.6

billion years ago consisted mainly of hydrogen and helium. It is further surmised that there were smaller amounts of gaseous compounds of hydrogen with carbon, nitrogen, and oxygen: methane (CH4), ammonia (NH3), and water vapor (H20). There may also have been small quantities of other gases, but those mentioned are thought to have been the most important. It may be noted that helium forms no known compounds, and so this gas would be present only in its elemental state.

It is highly probable that the atmospheres of the larger planets—Jupiter, Saturn, Uranus, and Neptune—consist chiefly of hydrogen and helium with some methane and ammonia. For example, the characteristic absorption lines (bands) of molecular hydrogen, methane, and ammonia have been detected in the spectrum of Jupiter, and there are reasons for believing that helium is also present.

The composition of Earth's atmosphere is, however, quite different from that of Jupiter (as is apparently the case, also of the smaller planets, Mercury, Venus, and Mars). The terrestrial atmosphere, for example, consists of 78.08 molecular (or volume) percent of nitrogen, 20.95 percent of oxygen, 0.93 percent of argon, with minor proportions of carbon dioxide (0.033 percent), water vapor (variable), and some other gases. Significant amounts of helium are present and there are traces of hydrogen, methane, and ammonia, but it is doubtful if these are of cosmological origin; that is to say, they probably do not originate from Earth's primitive atmosphere. Why then do the atmospheres of Earth and the smaller planets differ so markedly in composition from the atmospheres of Jupiter and the other large planets?

The situation can be accounted for by the

1 The cosmic abundance of the chemically inert gas neon is probably similar to that of carbon, but, like helium, it is not important for the present discussion.

difference in the masses, and hence in the gravitational forces, of the large (Jovian- or Jupiter-like) and the small (terrestrial) planets. Because of the smaller gravitational attraction of the terrestrial planets, the lighter gases—gases of low molecular weight—such as hydrogen (molecular weight 2), helium (4), and neon (20), have escaped completely. The same is true of part of the methane (16), ammonia (17), and water vapor (18). In addition, ultraviolet radiation from the Sun can cause the decomposition of these three gases, and the resulting hydrogen will have escaped from the atmosphere in the course of time. The larger gravitational force on the Jovian planets and their greater distance from the Sun, on the other hand, have permitted the retention of the light gases of the primitive atmosphere.

The Origin of Earth's Atmosphere

From the foregoing, it may be concluded that the present atmosphere of Earth must be, at least in part, of secondary origin. It is thought at the present time that the nitrogen and some of the carbon dioxide in the atmosphere came from geochemical sources; the gases are formed bychemical action in Earth's outer layers and are then gradually released from the crust into the atmosphere. Some confirmation of this view is found in the observation that volcanic gases contain nitrogen, carbon dioxide, and water vapor, as well as small quantities of several other gases. Thus, the nitrogen gas, which constitutes almost four-fifths of the atmosphere, is regarded as having been released from Earth's crust by vulcanism, i.e., by volcanic or related action.

The next most important constituent, oxygen, is considered to have resulted from photo-synthetic action by green plants. In the presence of sunlight (and moisture), such plants take up carbon dioxide gas and release oxygen.

The overall process, which also involves incorporation of the carbon dioxide into complex compounds, like sugars and starches, is called photosynthesis, meaning synthesis by light (p. 214). Before the evolution of photo-synthetic plants, Earth's atmosphere probably contained only very small proportions of oxygen produced by the decomposition of water vapor by ultraviolet light from the Sun. After green plants began to develop, perhaps a billion or so years ago, the proportion of oxygen in the air increased considerably, so that now it constitutes about one-fifth of the terrestrial atmosphere.

The third important constituent of the atmosphere is argon, present to the extent of almost 1 percent. This gas originates mainly from the radioactive decay of a form (isotope) of the element potassium having a mass number (atomic weight) of 40. All soils on Earth contain potassium compounds and the radioactive potassium-40 present decays slowly and is thereby partly converted into argon gas which enters the atmosphere.

The water vapor now in Earth's atmosphere is produced by evaporation from the bodies of liquid water which cover a large part of the globe. Some of this water may be of primitive cosmological origin. If there were low temperatures in the upper atmosphere, such as exist at present, they would act as a trap by freezing the water vapor and would thus inhibit its escape. Furthermore, in a moderate depth of liquid water, the ultraviolet radiation would be absorbed in the upper layers and the water below would be protected from decomposition. Much of the water now on Earth's surface, however, has probably resulted from the condensation of steam released from the interior by volcanic or related action. This steam may have been produced by the decomposition of hydrated, that is, water-containing, minerals by heat.

Most of the carbon dioxide in the present atmosphere of Earth apparently came from the interior. An important factor in controlling the abundance of this gas is considered to be the chemical reactions which can occur between carbon dioxide and magnesium and calcium silicates in various minerals. These reactions, such as

MgSiO., + C02 = MgCOs + Si02, lead to the formation of the respective carbonates plus silica. They occur fairly rapidly in liquid water, but only slowly in the presence of water vapor. In the complete absence of water in any form, the reactions of silicates with carbon dioxide take place extremely slowly, if at all. Other factors influencing the proportion of carbon dioxide in Earth's atmosphere are photosynthesis, combustion of coal, oil, and related fuels, and dissolution of the gas in the oceans.

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