Mars has only a very thin atmosphere while Mercury and the Moon have no atmosphere to speak of. The temperature structure of the Martian atmosphere is
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affected by the direct absorption of sunlight by dust particles swept into the air (Fig. 3.1). The terrestrial planets with gaseous atmospheres are Venus and Earth.
Fig. 3.2 shows a schematic plot of the temperature of the Earth’s atmosphere as a function of height above the ground. The thermal structure of the Earth’s atmosphere has four distinct regions:
(i) The troposphere is the region that adjoins the ground. It is the region with which we associate weather. Vertical heat transfer in the layer consists mostly of the upward diffusion of infrared photons and fluid transport by convection. The temperature is highest near the ground and steadily decreases to the tropopause, the top of the troposphere.
(ii) The stratosphere is heated by the ultraviolet component of sunlight in forming and destroying the ozone O3. This direct absorption leads to an inversion of the temperature above the tropopause, so that the stratosphere is actually warmer than the troposphere. This increase in temperature with height continues until the stratopause is reached.
Figure 3.1: Thermal structure of the atmosphere of Mars is affected by the amount of dust swept into the air by the winds of that planet. The shaded region between the curves demarcates the theoretical range of temperatures observed in calculations using typical daytime conditions for a dust-laden and clear Martian atmosphere. The dashed curves indicate the limits found by aircraft observations of Mars.
(iii) In the mesosphere, there is a decrease in ozone production and an increased rate of infrared cooling by carbon dioxide (CO2). The temperature decreases with increasing height up to the mesopause.
(iv) In the thermosphere, heat input from sunlight occurs by the photo-dissociation and photo-ionization of molecular oxygen (O2). The balance
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of this heat input leads to a rise of the temperature with increasing height in the thermosphere until temperatures of about 1,000 K are reached.
The primary energy transfer in the lower thermosphere is a downward conduction of heat into the mesosphere.
As the atmosphere is transparent to optical photons, sunlight strikes the ground, with little interaction with the troposphere except for scattering by molecules, dust, and reflection from cloud tops. The troposphere can be regarded as a gaseous atmosphere. It is heated from below by the emanation of infrared radiation from the ground. The troposphere is partially opaque to this emergent infrared radiation. In planetary astronomy, this is known as the greenhouse effect, but the basic principle is the same as that which governs the radiative transfer inside a star.
Figure 3.2: The thermal structure of the Earth’s atmosphere. The various regions are presumably duplicated qualitatively in the other planets. The detailed mechanisms of ultraviolet heating in the upper atmospheres of the other planets may, however, differ from those of the Earth due to the different chemical compositions involved.
As the Sun does not shine equally on the Earth’s equator and poles, we expect some variation of this temperature with latitude. Measurements show that the tropopause at the tropics and polar caps have different temperatures. The tropopause is warmer at the poles (about 225 K) than at the tropics (about 195 K).
This situation is opposite to that which prevails at sea level. As an explanation Victor Starr proposed that the refrigeration of the tropopause at the tropics results from the planetary circulation pattern of the Earth’s atmosphere.
The upper levels of the troposphere are convectively stable while the lower levels are not. As the radiative equilibrium solutions require the temperature in the lower troposphere to drop quickly with increasing height, these solutions may become unstable and allow convection to develop. The purely radiative solutions actually require the ground heated by both the trapped infrared radiation and the incident optical sunlights to be warmer than the air just above it (Fig. 3.3).
This situation produces convection currents at least at ground level. The humidity depends, in part, on the weather patterns that develop as a result of the convection and the large-scale circulation pattern. The combination of all the complications on the Earth produces an average surface temperature of about 290 K, close to the freezing point of water.
Let us now enquire what the surface temperature is on Earth’s sister planet, Venus. Venus is enshrouded with clouds, so that its surface cannot be examined by ground-based telescopes in either the optical or the infrared. Radio measurements can reach the surface. Such measurements indicate that the surface temperature of Venus might be in the neighborhood of 750 K, a temperature high enough to melt lead.
Figure 3.3: The greenhouse effect in planetary atmospheres involves the additional warming of the surface layers due to the partial trapping of the remitted infra-red radiation. Once it was assumed that the name for this effect might be a misnomer as greenhouses may not be kept warm by the same mechanism. However, a reanalysis by Silverstein reveals that the greenhouse effect may be nontrivial even for greenhouses. In any case, the effect is the same as that which keeps the interiors of stars warmer in their interiors than at their surfaces.
Venus is 1.38 times closer to the Sun, but it reflects about 72 percent of the incident sunlight while the Earth reflects 39 percent. Thus, the average flux of sunlight absorbed by Venus is (1.38)2(0.28/0.61) = 0.87 of that absorbed by Earth (i.e., the effective temperature of Venus should only be (0.87)1/4 that of Earth,
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or about 238 K). Venus has whopping greenhouse effects and has a whopping thick atmosphere. Surface pressure is about 90 times that at sea level on Earth.
This atmosphere is composed of 96 percent of carbon dioxide (CO2), which is an efficient absorber in the infra-red. Venus has a much thicker atmosphere than the Earth with a composition heavily weighted toward CO2.