7i is the intensity of the light polarized in a plane at right angles to the one containing the Sun (light source), Mars (scatterer), and Earth (observer); /2 is the intensity polarized in a plane that is parallel to the Sun-Mars-Earth plane.

By utilizing polarization measurements on various powders, which simulate the Martian surface, the contribution of the atmosphere was calculated from the observed polarization of the light from Mars. The atmospheric polarization can then be related to the barometric pressure. The first attempt to use polarization measurements to evaluate the atmospheric pressure on Mars was made in 1929 by the French astronomer Bernard Lyot, a pioneer in the study of the Moon and planets by polarimetric methods. On the basis of the assumption that the Martian atmosphere consists mainly of nitrogen, he determined the surface pressure to be not greater than 25 millibars.

Further development of the polarimetric method, utilizing measurements made at several apparitions, led Lyot's associate Adouin Dollfus to conclude in 1950, and again in 1957, that the barometric pressure on the surface of Mars was in the vicinity of 85 to 90 millibars. This result appeared to confirm the values derived from the best photometric measurements, and in a critical review, published in 1954, de Vaucouleurs gave 85 ±4 millibars as "the most probable value of the atmospheric pressure at ground level on Mars."

Recent Determination of the Atmospheric Pressure

For several years, the surface pressure on Mars was accepted as 85 millibars and calculations of the best way of landing an instrument capsule on the planet were based on this atmospheric pressure. In 1963, observations on the infrared spectrum of carbon dioxide on Mars, to be described shortly, indicated that the value of the pressure given above was too high, perhaps by a factor of 4 or more. A reexamination of the older data, with an improved allowance for the polarization of the light scattered by the surface material, gave an atmospheric pressure of about 50 millibars. Then, on the basis of polarization measurements made in France by A. Dollfus and J. Focas during the 1965 opposition period, J. B. Pollack in the United States has estimated the surface pressure to be 15 millibars if the atmosphere consists entirely of carbon dioxide and 19 millbars if it is half nitrogen. If, as some have suggested, allowance should be made for light scattered by very fine solid particles suspended in the Martian atmosphere, then the pressure would be lower still. There are, however, important differences of opinion on this point.

It is of interest that a new approach to the reflectivity method has also led to results for the Martian atmospheric pressure much lower than those given earlier. Both S. Musman (1964) and D. Evans (1965), working independently in the United States, have made use of reflectivity measurements in the ultraviolet region of the spectrum where the light scattered from the surface itself is essentially zero. The reflectivity can then be attributed almost entirely to the atmosphere and from it the surface pressure can be calculated.

By utilizing the Martian albedo at a wavelength of 3300 A, as determined by G. de Vaucouleurs, Musman estimated the pressure to be 19 millibars for an atmosphere consisting entirely of carbon dioxide and 27 millibars for one of nitrogen. Evans, on the other hand, based his calculations on measurements of reflectivity in the ultraviolet spectrum of Mars, in the wavelength range of 2400 to 3500 A, made from a rocket above Earth's tangible atmosphere. The results could be correlated with surface pressure values ranging from 5 to 20 millibars, for possible atmospheres consisting of various mixtures of carbon dioxide, nitrogen, and argon. The most probable pressure value was given as 10 millibars.

A new era in the study of the Martian atmospheric pressure was initiated in 1963 when H. Spinrad, working at the Mount Wilson Observatory, obtained a high-resolution spectrum of Mars in the near-infrared region; that is, at wavelengths slightly longer than visible light. Spinrad was actually looking for the spectrum of water vapor, which was observed, but in addition he noted the presence of weak bands caused by carbon dioxide. The lines in this band, located at wavelengths around 8700 A, are very faint and their effective width is almost independent of the atmospheric pressure. From an analysis of the spectrum, reported early in 1964 by L. D.

Kaplan and G. Miinch in conjunction with H. Spinrad, the abundance of carbon dioxide in the Martian atmosphere was determined to be equivalent to about 55 meter-atmospheres.

In contrast to the carbon dioxide band at 8700 A, there is a much stronger infrared band at a wavelength of about 2 /x (20 000 A), which has been well studied under laboratory conditions. In the latter band, the line widths are known to depend both on the abundance of the carbon dioxide and on the total pressure of the atmosphere. With the abundance derived above from the 8700-A band, which is essentially independent of pressure, the atmospheric pressure at the surface of Mars was calculated to be roughly 25 millibars.

Confirmation for what appeared at the time to be a surprisingly low value of the Martian barometric pressure soon came from studies of the carbon dioxide infrared spectrum of Mars made by T. C. Owen and G. P. Kuiper at the Lunar and Planetary Observatory, University of Arizona, and by V. I. Moroz in the U.S.S.R. The former quoted a mean of 17 millibars for the pressure, whereas the latter gave about 15 millibars, although with a considerable probable error. Subsequently, other observers, using spectroscopic measurements, have reported atmospheric pressures on the surface of Mars in the general range of 10 to 20 millibars. As seen above, this is close to the range of values derived from the newer polarimetric and reflectivity calculations.

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