Holt was making careful notes, but at this moment he glanced up. "What are the planes of the Phobos and Deimos orbits?" he asked.
"Both of them are, with great accuracy, in the plane of the equator of Mars," answered Bergmann.
"Thanks, but now what about the atmosphere of Mars, the climate and the general nature of the surface? I'd like to get some idea of conditions which we're apt to meet when we make a landing there."
"Certainly, Colonel. It is well proven that Mars has an atmosphere, although it's considerably less dense than that of the Earth. Once in a while, the formation of clouds has been noted. As to the surface, its formations are very clearly shown, particularly in infrared photography. At times, however, these formations are covered by some sort of diffuse white or yellow layer which registers particularly on ultraviolet photographs.
"Such layers must be considered cloud formations, and we take the white ones for water vapor and the yellow ones for sand clouds in all probability, whirled up by powerful storm conditions.
"Clouds containing water vapor must of necessity be present, in order to explain the regular appearance of snowfalls in the polar regions in Winter, and the occasional snowfalls in the temperate zones. There's no other possible explanation for regions of hundreds of thousands, or even millions, of square miles being concealed, sometimes in a very brief period, by a blinding white layer. The borders of the polar snowcaps also are frequently surrounded by a veil of clouds. We're inclined to interpret this veil as fog banks, developing over regions where snow is beginning to melt, rather than as clouds in the conventional sense. These fog banks develop in the cold Martian nights. Generally
they are gone by noon."
"What is your guess for the atmospheric pressure on Mars?" asked Holt.
"A mercury barometer at sea level on Earth reads 760 millimeters where a corresponding one on Mars would read only 64 millimeters, if we've estimated correctly. That's only one twelfth of the terrestrial atmospheric pressure at sea level."
"Confound it! That means we'll have to wear pressure suits and employ artificial respiration on Mars!"
"There's no doubt of it, Colonel. Surface atmospheric pressure will be equivalent to that at 60,000 feet above the Earth."
"Then I imagine that the Martian atmosphere isn't nearly so lofty as that of Earth?" asked Holt.
"You might be a little careful about that deduction," said Bergmann. "Remember that increase of pressure at the lower altitudes of the terrestrial atmosphere is caused by terrestrial gravitation. The pressure is higher at low altitudes because the lower we descend, the heavier is the column of air above us. Now the acceleration of gravity on Mars is but 38 % of ours on Earth. So any column of Martian air of equivalent mass would press upon the air below it with only 38 % of the weight with which it would do so upon Earth. Consequently increase in density of Martian air with diminishing altitude necessarily takes place more slowly that upon Earth.
"Putting it another way, we can say that decrease of barometric pressure with increasing altitude takes place more slowly than at home. Expressed in figures, the atmospheric pressure of the terrestrial atmosphere decreases by a power of ten for every 18 kilometers of altitude; that is to say that ground level pressure is reduced at 18 kilometers altitude to a tenth, at 36 kilometers to one ten thousandth of an atmosphere.
But on Mars, the pressure decreases by a power of ten only every 47 kilometers by reason of the weaker field of gravity. So, if we have but a twelfth of our terrestrial pressure at the surface, we shall have one hundred and twentieth of that pressure at 47 kilometers, one twelve hundredth at 94 kilometers…"
"Do you mean that at 94 kilometers above the surface of Mars, the atmospheric pressure is higher than it is 72 kilometers above the Earth?" asked Holt incredulously.
"That's what it is, strange as it may sound. It's got to be that way according to the inviolable laws of physics. Mars' weaker gravitational field just isn't able to compress its atmosphere to such a thin layer as the Earth's can do. Besides, we have visible proof that the Martian atmosphere must have just such a gradual pressure stratification, for we did some measurement on the clouds of Mars and found them to be at least 20 miles high."
"What's that?" exclaimed Holt, "do you mean that you can actually measure the cloud ceiling on Mars?"
"Surely," said Bergmann quietly. "It isn't even very difficult. Every now and then we observe luminous spots at the so-called terminator — the border-line between night and day on the planet. These spots remain luminescent when night has already fallen below them at the surface. Those spots are high-altitude clouds, and their height above the surface is calculated quite easily by their distance from the terminator at the time they occur."
"Well, that's most interesting. But what actually leads you to the conclusion that the pressure at the surface of Mars is one twelfth of that of the Earth? I can understand that the potency of the Martian field of gravitation throws light on the ratio of stratification of its atmospheric shell. But where do we get the absolute pressures themselves?"
Bergmann went on. "That's a bit more complicated. But there are several independent methods by which we determine these values, and their results correlate pretty well. Initially, all these methods give us only the entire mass of a column of air pressing upon a unit of area on the surface of Mars. The law of stratification already referred to then gives us the ground pressure.
"One of those methods is spectral analysis. Another comes via the reflective capacity of the surface of Mars, known as the "albedo" to astronomers. Perhaps you'd like to hear a more accurate description of this latter method…
"The albedo is the reflected percentage of the radiant energy of the entire visible region of the sun's spectrum intercepted by a planet. In the case of Mars, this is 27 %. Now, we know from comparisons with similar objects on Earth, that the reradiation of the Martian desert areas, vegetated regions and snow fields accounts for only 10 % thereof.
Hence the remaining 17 % must be accounted for by the reradiation of its practically cloudless atmosphere, which reflects 74 % of what it receives. So we perceive that the albedo of the atmosphere of Mars is to that of a cloudless terrestrial atmosphere as 17 is to 74, or about 2 to 9.
"Let us now assume that the total mass of a cloudless shell of air increases in the same ratio as its power of reradiation. That should be approximately correct, because the power of reradiation is largely determined by particles suspended in the atmosphere, and the ability of the air to keep such particles in suspension increases as its total number of molecules increases. This leads us to the conclusion that Mars has an air mass above each square inch of surface equal to two ninths of that resting upon the same area on Earth. If we substitute this mass of air in the law of pressure stratification to which I referred, we arrive at a pressure at ground level of one-twelfth of an atmosphere."
Holt smiled. "It seems to me that you go all the way around Robin Hood's barn to reach that conclusion; but I suppose you know how much faith to put in it. In view of the importance to our enterprise of the structure of Mar's atmosphere, I almost think that those revelations, as I may well call them, urgently need a much more solid underpinning."
"No doubt, Colonel. Recently we've been trying to find the atmospheric pressure at ground level by another method, which might be called meteorological. We've set up a ratio between the presumptive air temperature and humidity of a certain section of the Martian atmosphere and the pressure. Using this ratio and the laws of meteorology, we have arrived at the atmospheric pressure at which clouds ought to form. We compared the results with the actually measured cloud height and postulated the computed pressure as existing at this height. By applying the law of pressure stratification, we computed the pressure at ground level as 79 mm of mercury. This is only slightly higher than the pressure obtained by the albedo method. Spectroanalytic results are perhaps the least reliable, because of the technical difficulties attending measurements, but they also are not far from this figure."