FIGURE 6.21. Daily temperature variation at the Martian equator. (After W. M. Sinton and J. Strong.)

is radiated into space at night, without any appreciable replacement from the subsurface or the atmosphere.

The results of an analysis of the data of Sinton and Strong made by G. Leovy in 1965 imply that the conditions on the Martian surface are such that the daily temperature variations are determined mainly by the size of the surface particles, but not by their nature, and by the low atmospheric pressure. The conclusion drawn is that the particles must be quite small, not more than several microns in diameter.

An indication of the manner in which the daily maximum (noon), minimum (sunrise), and mean surface temperatures vary with latitude is provided by the curves in figure 6.22. These were prepared by the meteorologist Y. Mintz, University of California, Los Angeles, partly from experimental data and partly from calculations. The temperatures are those at the time of the winter solstice (midwinter) in the northern hemisphere and the summer solstice (midsummer) in the southern hemisphere. Thus, the curves show approximately the highest and lowest surface temperatures attained during the Martian year. The temperatures given may not be exact, but the general trends are probably reasonably correct.

It should be remembered that the large daily changes in temperature—roughly 100° C or so at the equator, but less at higher latitudes—are those at the surface. By analogy with terrestrial conditions, it is expected that, during the daytime, temperatures in the atmosphere a foot or two above the surface may be lower than the ground temperature by 20° to 30° C (or more). At night, the difference is probably not as great and may become quite small by sunrise. In any event, it is thought that, as on Earth, the daily temperature variation is considerably less in the atmosphere than it is at the actual surface.

3tf 0 30° Latitude S

FIGURE 6.22. Temperature variations with latitude at Martian solstices. (After Y. Mintz.)

3tf 0 30° Latitude S

FIGURE 6.22. Temperature variations with latitude at Martian solstices. (After Y. Mintz.)

A modification of the infrared radiometric method for determining the temperature of a planet is to measure the radiation emitted at longer wavelengths, especially in the microwave (centimeter wavelength) region of the spectrum. Although the energy intensities are fairly weak, they can be determined with fair accuracy. Furthermore, at long wavelengths there is a simple relationship between the energy received and the temperature of the emitting body. For a given long wavelength, the absolute (Kelvin) temperature is directly proportional to the radiated energy.

During the apparitions of 1956 and 1958, C. H. Mayer and his associates of the U.S. Naval Observatory studied the emission from Mars at a wavelength of about 3.15 centimeters. The average blackbody temperature, over the whole disk of the planet, was found to be approximately 210° K (- 63° C). In subsequent years, other radio astronomers have made measurements at wavelengths up to 21.3 centimeters. Although there are some variations in the temperatures derived, they are nearly all within the range of 200° to 220° K, with a rough average of around 210° K.

There is a significant difference between the temperature (210° K) estimated from measurements at the longer (microwave) wavelengths and the mean infrared daytime temperature (about 250° K) over the whole disk. The reason generally accepted for this difference is that the infrared temperature is that of the actual surface of Mars, but the radiations of longer wavelength originate from a short distance below the surface. Because of the poor thermal conductivity of the surface material, the subsurface temperatures do not change significantly from day to night. It is probably more than coincidence that the presumed subsurface temperature of about 210° K is very close to the average temperature (211° K) over the whole planet calculated earlier. Such agreement is to be anticipated.

General Conclusions

The following general conclusions may be drawn from measurements and calculations relating to the surface temperature on Mars.

The maximum temperatures in the Martian equatorial regions, about 25° C (77° F), are not much lower than those reached in the temperate zones of Earth. During the summer, temperatures in the daytime are above the freezing point of water, 0° C (32° F), probably at all latitudes up to about 60° N (in the northern summer) and 70° S (in the southern summer).

Maximum surface temperatures on Mars are attained about an hour after midday and they decrease rapidly, at first, and then more slowly after sunset. During the night, there is no part of the planet where the temperature is above 0° C or even above -20° C (-4° F). The lowest surface temperatures are experienced just before sunrise, after which they increase rapidly toward the maximum. The daily surface temperature range may be more than 100° C (180° F) near the equator, but it is less at higher latitudes. The variation is expected to be smaller in the atmosphere just above the surface than it is at the surface.

As a result of the very low nighttime temperatures, a large proportion of the water vapor in the atmosphere will condense out as hoarfrost before sunrise in the winter months. It will vaporize again, however, and possibly melt temporarily into liquid water in certain locations, during the daytime.

The surface of the dark areas of Mars is about 10° C (18° F) warmer than adjacent bright areas in the daytime. This is because of the lower albedo of the former. The dark areas reflect less of the solar radiation and so they absorb more heat from the Sun than do the bright areas. The surface temperatures are consequently higher in the dark areas in the daytime, but the difference is expected to be small at night.

Temperatures just below the surface are around -60° C ( — 76° F) and so are well below the freezing point of water. In general, the subsurface temperatures are probably lower in the daytime and higher at night than they are at the actual surface. The atmosphere, a foot or so above the ground, may be colder than the surface, especially in the daytime. The manner in which the atmospheric temperature is thought to vary at higher altitudes was described in chapter V.

The surface temperatures during the local summer are higher in the southern than in the northern hemisphere. The winter temperatures are probably lower, as stated in chapter III. The temperatures rise rapidly as summer approaches, and the highest of the year are attained a short time after the local summer solstice. Similarly, the lowest surface temperatures are experienced soon after the winter solstice. The delay between the re spective solstices and the maximum and minimum temperature are expected to be less than on Earth because of the low-density atmosphere and the absence of bodies of water.

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