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In the presence of an excess of free oxygen, H2, CO, and CH4 would tend to become oxidized to HaO and C02.

The gases H2, H20, Oa, and C02 may be dissociated photolytically to form H, H+, OH~, O, O", 03 (transiently), and CO. Nitrogen (Na) and CO are extremely stable molecules and are not readily dissociated by sunlight.

Water is a special case because of its high freezing point (triple point). It can be protected from photolytic dissociation by overlying gases (O, Oa, and 03, for example) that absorb in the ultraviolet region of the spectrum. Low temperatures in the atmosphere which permit the freezing or condensation and precipitation of water can prevent it from rising to too high an altitude.

The most interesting planets from the human point of view would be those having the right combination of mass, radius, and exosphere temperature so that they could retain atomic oxygen, but not capture helium and thus "snowball." Further requirements would be suitable surface temperatures, tolerable gravitational forces at the surface (less than about 1.5 times the acceleration of gravity at the Earth's surface), a suitable partial pressure of atmospheric oxygen at the planetary surface (which implies photosynthesis), and the presence of both liquid water and permanent land areas.

Under the tentative assumptions made here, the planetary properties suggested by the diagonally shaded area of Figure 12 would be approximately those of habitable planets.

Classification of Planets by Atmospheric Characteristics. In general planets may be classified as follows: Airless bodies (more or less spherical aggregates having no sensible atmospheres)—for example, the Moon and Mercury; planets with light atmospheres—for example, the Earth, Mars, and Venus; planets with massive atmospheres (mainly hydrogen and helium)—for example, Jupiter, Saturn, Uranus, and Neptune.

Airless bodies (those lying below the Xe line in Figure 12) will conform more or less to the radius-density relationship of Equation (1), page 29.

Planets with light atmospheres (those lying between the lines for He and Xe in Figure 12) will also conform to the relationship of Equation (1), since their atmospheres will be small in mass relative to the mass of the rocky body. In general, the closer the body lies to the He line, the more massive its atmosphere will be. The position of any point representing a planet with a light atmosphere in Figure 12 gives an approximate clue as to its atmospheric composition. Gaseous constituents below the point will be retained, if present; those above will be lost. There also may be constituents in dynamic equilibrium, however, that are replaced as rapidly as they are lost, either by escaping to space or by entering into chemical combination.

Planets with massive atmospheres (those lying above the line for H or H2) will have a composite structure consisting of a compressed rocky core, possibly a shell of water-ice; a compressed inner shell of metallic hydrogen (for very massive planets); and an outer gaseous atmosphere of hydrogen and helium, plus relatively minor quantities of other gases. Mean planetary densities will generally be less than 2.5 grams per cubic centimeter and total masses will be less than 103 or 104 Earth masses (the transition region between planets and stars).

Many of the details of the formation of planetary atmospheres are still somewhat conjectural. For the Earth, and possibly for most planets with light atmospheres, volcanism has apparently provided the primary ingredients from which the present atmosphere (and oceans) have developed.

According to Macdonald (1961), water is the chief component of volcanic gases at the Earth's surface, generally constituting more than 75 per cent, by volume, of all gases collected at volcanic vents. Other common constituents include carbon dioxide (COa), carbon monoxide (CO), sulfur dioxide (S02), sulfur vapor (S), sulfur trioxide (S03), hydrogen sulfide (H2S), hydrogen chloride (HC1), ammonium chloride (NH4C1), and, in lesser abundance, hydrogen (H2), hydrogen fluoride (HF), boric acid (H2B03), methane (CH4), nitrogen (N2), and argon (Ar). Not all of these components are always present and their relative abundances vary considerably, but water is nearly always predominant and is often overwhelmingly so. It is probable that throughout the geologic ages, all the water on the Earth's surface has been produced by volcanoes. The principal volcanic gases have accumulated over the several billion years since the Earth formed, and a number of important physical and chemical changes have taken place. In the presence of water, carbon dioxide has been removed from the atmosphere and converted into carbonate rocks; the other water-soluble, chemically active materials have been dissolved and converted into various minerals; carbon monoxide and methane have been oxidized; hydrogen has been oxidized or has escaped; nitrogen and argon have remained in the atmosphere; oxygen has been produced through photosynthesis ; and water has been retained on the surface as liquid and solid. If volcanism is accepted as the primary mechanism for the production of atmospheres of planets that are not massive enough to capture hydrogen and helium in large quantities, then an understanding of the relationship between degree of volcanic activity and planetary mass is essential for an understanding of the general course of atmospheric evolution.

Actually, our knowledge of the natural forces responsible for volcanic activity, earthquakes, and mountain formation on the Earth is still quite incomplete, so it is difficult at this time to specify general relationships between planetary mass and volcanism. One view, described by Bullard (1954), says that earthquakes and volcanic heat result from the mechanical energy associated with the distortion of the crust. Distortions of the crust would be produced by a general increase in the temperature of the interior of the Earth, with expansion, or by a general cooling, with contraction. Bullard comments that the present state of the Earth's surface strongly suggests contraction. The high interior temperatures of the Earth apparently are due to both gravitational compression and the accumulation of heat released by radioactive materials, principally uranium, thorium, and potassium-40, with radioactivity being the major contributor.

A planet smaller than the Earth would tend to accumulate less heat through gravitational compression during the period of formation. Its ratio of surface area to mass would be larger; hence, for a given internal temperature and thermal conductivity of the rocks, it could lose heat more rapidly. Small planets might also tend to have less concentration of metals toward the center, making the thermal conductivity of the outer parts greater. Thus small planets should have lower internal temperatures than large ones.

Planets with dense atmospheres and strong surface winds, particularly those with atmospheric constituents that can undergo a change of state at the prevailing temperatures (for example, water) will tend to have extensive erosion and horizontal transport of exposed crustal rocks. This tends to upset isostatic equilibrium in the crust and may tend to enhance volcanic activity.

Much more must be learned about the causes of volcanism and about the processes involved in the production of volcanic gases before it can be said that we have a complete understanding of the evolution and development of planetary atmospheres. There are still some little-under-stood factors relating to the Earth's atmosphere—its structure, composition, density gradient and temperature-altitude relationship, and circulation. But knowledge is being accumulated rapidly and undoubtedly much of this will be applicable to the more general problems of planetary meteorology.

Oblateness of Rotating Planets. Most of the discussion up to this point has been concerned with the properties of massive bodies that are not rotating strongly (see Figure 9). The rate of rotation, however, is an important intrinsic property of a planetary object, alfecting its shape, surface gravity, and habitability. In a consideration of general planetary properties, the effects of rotation can not be ignored.

The shape of a rotating body isolated in space depends on its rate of rotation, its mean density, and the distribution of mass within the body. As a given body is caused to rotate more and more rapidly, its equatorial dimensions increase, producing a spheroidal or a pseudospheroidal shape.

Mathematical analyses have been made of the relationship between oblateness (degree of polar flattening) and density parameters for certain simple models. For example, rotating bodies of uniform density throughout (incompressible liquids) with one axis of symmetry (through the poles) are known as Maclaurin's spheroids; some of their properties have been derived by Lamb, Darwin, and Thomson and Tait (Jeans, 1929). Rotating bodies with all the mass concentrated at a point at the center of the body (Roche's models)* have been studied by Jeans, Poincare, and others. Profiles of these idealized bodies (sectioned along the axis of rotation) are

* Roche's models have a nucleus of finite mass but infinitesimal volume, surrounded by an atmosphere of infinitesimal mass but finite volume (Roche, 1873).

shown in Figure 13. All real planetary bodies probably will lie between these two limits.

It is interesting to compare data for the planets of the solar system (Table 7) with the theoretical results mentioned above. This comparison

Maclaurin's Roche's

Spheroids Models

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