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0.0001 0.001 0.01 0.1 1 ' 10 Atmospheric pressure, relative to Earth sea-level pressure

Figure 20. Atmospheric pressure as a function of altitude for large and small habitable planets compared with Earth (illustrative).

Earth mass, and mass = 2.35 Earth masses) where atmospheric molecular weight and temperature are the same as for the Earth, a would be 2.82 x 10-5 per foot and 6.23 x 10 5 per foot, respectively. The significance of this is seen more clearly in Figure 20; that is, small planets have "soft" atmospheres in which the atmospheric density changes slowly with altitude, and large planets have "hard" atmospheres in which the change in density with altitude is far more rapid. One effect of this is that at high altitudes (above, say, about 30,000 to 40,000 feet), the smaller planets should have denser atmospheres than the larger planets. This would have a distinct influence on such factors as the altitude ceilings of similar kinds of aircraft flying in the atmospheres of planets with different masses.

In the present study, then, it will be postulated that the mass of habitable planets may vary over the range 0.4 to 2.35 Earth masses; the radius may vary from 0.78 to 1.25 Earth radii; while surface gravity may range from 0.68 to 1.5 g. This size range is illustrated in Figure 21.

Within the given mass range, given proper temperatures in the atmosphere, the atmospheric composition and pressure would depend very much on the past history of volcanic activity and the exosphere temperature. Generally speaking, planets near the lower end of the permissible mass range might be expected to have developed lower internal temperatures during their period of formation and from subsequent radioactivity, to have cooled more rapidly and to have thicker crusts. They might also be expected to show less volcanic activity and, consequently, to have less atmospheric gas and lower atmospheric pressures at their surfaces. Those near the upper end of the permissible mass range might exhibit more volcanism and have higher atmospheric pressures.

Within the above mass range, the planets of larger mass possibly would tend to have developed more internal classification or stratification of minerals and elementary materials; that is, there might be a greater concentration of dense materials at their centers, leaving their crusts relatively less rich in certain heavy metals and heavy minerals.

Rate of Rotation. As discussed in Chapter 3, the rate of rotation of a planet, together with its mean density and degree of concentration of mass toward its center, uniquely determine its oblateness. However, at high rotation rates the internal mass distribution may be affected by the rotation, so the two are not completely independent. Other characteristics also depend in part on rotation rate—surface gravity as a function of latitude, the daily temperature cycles, atmospheric circulation patterns and wind velocities, and, possibly, the magnetic field.

In general, the more rapid the rotation rate, the smaller the day-to-night temperature differences would be. The meteorological factors (wind velocities, cyclone patterns, et cetera) are extremely difficult to estimate in a quantitative manner because not enough is known at present about general planetary meteorology or the prediction of climates from astronomical parameters.

From the standpoint of human habitation, there are two limits related to rotation rates. For slow rotation rates, a limit would be reached when daytime temperatures became excessively high in the low latitudes below some critical latitude and when nighttime temperatures became excessively low poleward from this same latitude, or when the light-darkness cycle became too slow to enable plants to live through the long hot days and long cold nights. If rotation rate were increased steadily, a limiting point would be reached when surface gravity at the equator fell

Mass Radius

Surface gravity h X

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