Electrons per cu. cm.

FIGURE 5.10. Electron densities in Earth's ionosphere. (After W. B. Hanson.)

(90 to 150 miles), ultraviolet radiation from the Sun causes ionization of some of the heavier species present in the atmosphere in this region; namely, molecules of oxygen and nitric oxide. Finally, above some 250 kilometers (150 miles) is the F2 region. Here the electron (and ion) density, resulting from the ionization of oxygen atoms, which predominate at these higher altitudes, reaches a maximum and subsequently decreases steadily.

The electron density maximum in the F2 region is attributed to the operation of two opposing factors. First, there is a tendency for the electron density to increase with increasing altitude, as indicated by the increase in density between the Fx and F2 regions. This is attributed to the fact that fewer electrons are being lost as a result of recombination with ions to re-form neutral atoms (or molecules). Second, at higher altitudes the mixture of electrons and relatively heavy ions, called a "plasma," diffuses downward under the influence of gravity. This plasma diffusion effect causes the electron (and ion) density to decrease, and it becomes dominant above the F2 maximum.

Another factor which contributes to the occurrence of electron-density maxima in the ionosphere is that at higher altitudes the density of atoms and molecules in the atmosphere is so low that only a small proportion of the ionizing radiations is absorbed. This has the effect of causing the electron density to decrease with increasing altitude. At very low levels, on the other hand, very little of the radiation penetrates, and so the extent of ionization, and hence the electron density, is again low. At intermediate altitudes, however, the atmospheric density is sufficiently high to permit significant absorption of the ionizing radiation, and the electron density will tend to have a maximum value.

The decrease in the electron densities in figure 5.10 from day to night is quite marked. This is not unexpected if solar radiations are responsible for the presence of the electrons and ions in the first place. At night, electrons are lost by recombination with ions but they are not replaced by further ionization, as they would be in the daytime. The distinction between Fi and F„ regions also disappears at night, inasmuch as the ionization processes responsible for these regions are no longer operative.

Because Mars has an atmosphere, it was assumed that it would also have an ionosphere and scientists had speculated about its possible structure. The occultation experiment of Mariner IV provided the first definite information on the subject and, incidentally, proved most of the speculations to be incorrect. Interpretation of the ionosphere section of the phase path change curve in figure 5.8 has led to the plot given in figure 5.11 for the variation of the electron-number density with altitude above the surface of Mars. These results, obtained just prior to the occultation of Mariner IV, apply to the daytime ionosphere at 50° S latitude. They are comparable with the day curve in figure 5.10. It should be noted that the electron density scale in figure 5.10 is logarithmic, whereas in figure 5.11 it is linear.

After occultation, the radio waves from Mariner IV passed through the nighttime atmosphere of Mars and no significant electron density could be detected. This does not necessarily mean that the Martian ionosphere had disappeared completely during the night, but only that the electron density was less than 5 X 103 electrons per cubic centimeter, compared with a maximum of almost 105 electrons per cubic centimeter in the daytime. Consequently, a large proportion of the electrons and ions formed in the ionosphere of Mars in the daytime recombine to form neutral atoms or molecules at night, just as they do in the terrestrial ionosphere.

A possible explanation of the minor maximum at the bottom of the curve in figure 5.11 will be considered later, but first mention must be made of the discussion and differences of opinion concerning the origin of the larger maximum in the electron density. One surprising fact is that the maximum, at an alti-

FIGURE 5.11. Electron densities in the atmosphere of Mars, derived from Mariner IV data.

tude of about 120 kilometers (75 miles), is much lower than had been expected. This indicates that the temperature of the Martian atmosphere is considerably less than anticipated. Further support for this view is provided by the slope of the electron-density curve at altitudes above the main maximum. This matter will be considered in the next section.

Attempts have been made to explain the larger electron-density maximum by associating it with either an E, an Ft, or an F2 region. Fortunately, there are no other possibilities. The majority—but not unanimous—view seems to be that it is not an E region, but it may be either an F± or an F2 region. The Fx model of the Martian ionosphere would imply that the maximum electron density occurs at an altitude where molecules, rather than the lighter atoms, are ionized by solar ultraviolet radiation. The majority of the positive ions present would then be C02+, produced by ionization of the carbon dioxide molecules which are known to be an important constituent of the atmosphere.

If the upper maximum in figure 5.11 represents an F2 region, the predominant ion would presumably be 0+ formed by ionization of oxygen atoms by ultraviolet radiation. The oxygen atoms would themselves be produced by these radiations decomposing the carbon dioxide at lower altitudes, as described on page 81. As a result of the action of atomic and molecular diffusion, the lightest species, the oxygen atoms, would predominate over carbon dioxide and carbon monoxide at altitudes above about 80 kilometers (50 miles).

The consequences of the F-¡ and F2 models, especially with regard to the temperatures in the upper atmosphere of Mars, are quite different, as will be seen shortly. There appears to be no experimental evidence or convincing theoretical argument that can permit a choice to be made at present between the two alternative points of view. A decision must await further information that can be obtained only by the use of spacecraft.

In conclusion, some remarks may be made concerning the smaller maximum in the electron density which, as seen in figure 5.11, occurs at an altitude of about 95 kilometers. According to the calculations made in 1967 by C. Sagan and J. Veverka, this may be attributed to ionization caused by protons from the solar wind, positively charged nuclei of hydrogen with moderately high energies that are emitted continuously by the Sun. Because of Earth's magnetic field, solar-wind protons are deflected before they enter the terrestrial atmosphere so they have no effect on the ionosphere. The magnetic field of Mars, however, is very weak or nonexistent, and the solar protons would then be expected to produce a region of ionization in the Martian atmosphere, with a maximum in the electron density similar to that observed at 95 kilometers. If this maximum is indeed caused by protons, then the larger maximum at the higher altitude is expected to be an F2 layer.

Temperatures in the Atmosphere of Mars

Numerous suggestions have been made concerning the manner in which the temperature of the Martian atmosphere varies with increasing altitude. These predictions are so very different, and based on such little reliable information, that it is not possible to summarize them here. There are, nevertheless, certain general ideas which can be described. Some are based on analogies with the known way in which the temperatures in Earth's atmosphere change with altitude, whereas others involve results derived from limited experimental measurements.

The manner in which the terrestrial atmospheric temperature varies with altitude depends on many different circumstances, such as the latitude, the season of the year, and the

FIGURE 5.12. Temperature variations and regions in Earth's atmosphere.

activity of the Sun. In general, however, five temperature regions may be distinguished, as indicated in figure 5.12. In the lowest region, called the troposphere, the temperature decreases steadily with altitude to a height of roughly 12 kilometers (7.5 miles). The rate of decrease, referred to as the lapse rate, is almost constant at about 6.5° K per kilometer (10.5° K per mile).

The main reason the temperature decreases in the troposphere is that, as the air rises, it expands because of the decrease in the barometric pressure. The expansion is then accompanied by a lowering of the temperature. If the air is not significantly heated by radiation, the theoretical (adiabatic) lapse rate should be equal to g/cp, where g is the gravitational acceleration and cp is the specific heat (heat capacity per gram) at constant pressure of the air in appropriate units. For dry air, the ideal lapse rate should be 9.6° K per kilometer. The observed value is lower, partly because of the presence of water vapor, which releases its latent heat when it is condensed at higher altitudes, and partly because some heat is supplied by radiation.

Above the troposphere lies the stratosphere where the temperature remains almost constant for several kilometers and then increases with altitude to form the mesosphere, or intermediate region. The higher temperatures are caused by the absorption of solar radiation by ozone which attains its maximum concentration in this region. Above the altitude where the temperature is a maximum in the mesosphere, the amount of ozone decreases and the temperature falls correspondingly. Next, in the thermosphere, the temperature of the air is controlled by the heat taken up from the Sun and conducted along the temperature gradient. There is then a sharp increase in temperature toward a constant value which may be higher than 1000° K (727° C) in the exosphere.

In one respect, at least, the temperature variations in the atmosphere of Mars should differ from those in Earth's atmosphere. The proportion of oxygen (02 and O) in the Martian atmosphere is probably too small to yield any significant quantity of ozone (03). Consequently, the temperature will not attain a maximum in the intermediate (mesosphere) region. The general form of the temperature-altitude curve for Mars might then be like that shown in figure 5.13.

FIGURE 5.13. Approximate temperature variations and regions in the atmosphere of Mars.

This curve must not be taken too seriously, but, as far as can be determined at present, it provides a representation of the situation in broad terms. There is probably a troposphere, in which the temperature drops fairly rapidly with increasing altitude, followed by a region in which the temperature decreases more slowly. Then there is a thermosphere in which heating by solar radiation plays the dominant role, followed by an exosphere in which the atmospheric temperature attains an approximately constant value.

If the Martian troposphere consists entirely of carbon dioxide, the theoretical lapse rate {g/cp) would be 4.7° K per kilometer (7.6° K per mile). For an atmosphere consisting of 80 percent carbon dioxide and 20 percent nitrogen, the lapse rate should be about 4.3° K per kilometer (6.9° K per mile). A rough estimate, which may be incorrect, gives the height of the Martian troposphere as about 20 kilometers. The temperature there would be, respectively, 20X4.7 = 94° K or 20X4.3 = 86° K, or roughly 90° K, below that at the surface of the planet. If the average temperature at the surface is taken to be about 250° K (ch. VI), the temperature at the tropopause, the top of the troposphere, would be approximately 160° K, -113° G.

Two methods have been used to estimate the average temperatures of the Martian atmosphere. One is based on the occultation measurements described earlier. According to the table on p. 90, the value was found to be about 180° K from the data before occultation and 235° K after occultation. It would appear, however, that the latter is somewhat high. The other method depends on the wavelength separations of the lines in the band spectrum of carbon dioxide in the atmosphere of Mars. These lines owe their origin to a set of specific values of the energy of rotation of the carbon dioxide molecule about one of its axes. The separa tions can be related to the temperature of the gas.

From the lines in the band at a wavelength close to 1.05 /x, M. J. S. Belton and D. M. Hunten of the Kitt Peak National Observatory, Ariz., in 1966 calculated a temperature of 194° K. Furthermore, they estimated that this average temperature would be equal to the actual temperature at an altitude of 1.3 times the scale height evaluated at the average temperature. The scale height, RT/Mg, with T equal to 194° K and M to 44 (for carbon dioxide), was found to be 9.3 kilometers, which is close to the Mariner IV value obtained before occultation. The average temperature of 194° K would then correspond to that at an actual altitude of 1.3X9.3=12.1 kilometers. If the surface temperature is 250° K, then the atmospheric temperature on Mars will have dropped 250-194=56° K in 12.1 kilometers. The lapse rate would thus be 56/12.1=4.6° K per kilometer, in good agreement with the theoretical value for carbon dioxide. Whether this result is fortuitous or not, it is not possible to say.

Another approach to the evaluation of temperatures in the upper Martian atmosphere is to utilize the electron-number density variation with altitude in figure 5.11. From the slope of the curve representing electron density in the ionosphere at levels higher than that of the maximum—above about 120 kilometers—an electron-density scale height can be calculated. This is the distance within which the electron density changes by a factor of 2.72, and it is equal numerically to 2RT/Mg, where T is now an average temperature of the electrons and ions present and M is the molecular weight of the ions.

From the Mariner IV data, the electron-density scale height in the ionosphere at altitudes from about 120 to 250 kilometers (75 to 154 miles) was found to have a constant value of 29 kilometers. Provided the same ions are the dominant ones throughout this region, as is probable, then the temperature would appear to be constant. Before this temperature can be calculated, the molecular weight of the ions must be known, and this is where a difficulty arises. If the electron-density maximum corresponds to that of an F2 region, the ions should be 0+, with an atomic weight of 16. The constant temperature in the exosphere would then be about 90° K, which is extremely low. On the other hand, for the Ft model, the ions would be C02+ with a molecular weight of 44. The temperature would then be close to 250° K between 120 and 250 kilometers, at least.

In the terrestrial ionosphere the scale height data indicate that the temperature is also constant above the level of maximum electron density in the F2 region. This level is, however, at about 400 kilometers (250 miles), and the temperature in the exosphere is over 1000° K in the daytime. If the Mariner IV scale height of 29 kilometers for the upper levels of the Martian ionosphere is correct, then the exosphere temperature would have to be very much lower, apparently not above 250° K.

Some attempts have been made to develop other models of the upper atmosphere of Mars which lead to higher temperatures in the exosphere than those derived above; for example, in the vicinity of 400° or 500° K. The basic postulates, however, appear to be incompatible with both the F± and F2 models as well as with the Mariner IV value for the electron scale height. The problem of the temperatures in the Martian atmosphere must be added to the list of questions about Mars that still remain unanswered.

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