"What's the size of the polar caps in Winter?" inquired Holt.
"They're largest towards Spring. The south polar cap extends as far as about the 42nd degree of latitude at this time, having a diameter of almost 5,700 kilometers. It shrinks gradually throughout the short but relatively hot Summer, and usually disappears completely before Fall. The cap at the North Pole does not grow quite so large. Due to the Winter being shorter, the amount of snow falling there is obviously somewhat less. The snow at the end of Winter doesn't usually go beyond the 51st parallel of latitude. In late Summer, this snow doesn't, as a rule, melt entirely due to the Northern Summer being cooler, though somewhat longer. It then shows as a small, white spot around the North Pole, reaching down to around the 87th parallel of latitude."
"What is your general concept of the total amount of water on Mars?" asked Holt.
"This must have an important bearing on the vegetative regions and the canals."
"We've attacked that problem from various angles," answered Bergmann. "The most attractive one seems to be related to the probable amount of snow in the polar regions.
"Our approach is as follows: On Earth, the Sun is capable of melting about six meters of snow during the four months of Summer at and around the poles. Now Mars is more distant from the Sun and therefore receives less solar heat. On the other hand, however, its atmosphere reradiates a much smaller amount of this heat, so that a greater proportion reaches the surface. Furthermore, the Martian Summer is about twice as long as ours. It is hence reasonable to assume that the Sun probably melts about six meters of snow depth on Mars also. Since by the end of the Summer the snow on the polar caps is gone, we may conclude that the mean depth of snow at the polar regions is likewise six meters at the end of a Winter.
"Six meters of snow is equivalent to 60 centimeters of water, and at its maximum extension, the southern polar cap covers about 24 million square kilometers. From this we compute about 14 million millions of cubic meters of water. That's about twenty times the volume of Lake Erie, or a one-hundred-thousandth of the water in our terrestrial oceans.
"You might quite properly object that our assumptions concerning the depth of the snow caps are pretty arbitrary and that the quantity of water might well be one half of our estimated value. We won't take exception to that. An additional consideration is the fact that vegetative areas near the northerly latitudes are seen to be flowering and green when the south polar cap is largest. So we're sure that by no means all the water on Mars collects in the form of one polar snow cap or the other during the Winter in either hemisphere. My figures were intended to approximate the order of magnitude of the water on Mars. Just think, it's only one-hundred-thousandth of Earth's water supply, although the surface of the Earth is only about three and a half times greater than that of Mars!
Makes it a pretty dry planet, doesn't it? Well, that figure alone, no matter how much you may question it, shows pretty plainly how arid Mars has become."
"What makes you think that Mars once was wetter?" inquired Holt.
"The huge zones of vegetation, particularly those in the southern hemisphere seem, for many and various reasons, to have been oceanic basins. The earlier astronomers even named them "Mare" because they thought that they were actually lakes. Today we know that open water bodies of any such size are unimaginable in view of the low atmospheric pressure of Mars — why, they'd evaporate in no time!
"What happened to the prehistoric Martian oceans and lakes is in prospect for our own, incidentally. We can follow on Earth the long process of their shrinkage through the various geological ages, and even through the short span of human history. Rome, for example, was a sea-side town when the Republic flourished, but since then the ocean has receded and Rome lies many miles inland. We find fossilized fish in all sorts of places in the mountains and deserts of the southwest states of America. These leave no doubt that large portions of the American continent were under water and only emerged by reason of the sinking of the oceans and inland lakes. The Great Salt Lake of Utah is a tiny remnant of the prehistoric Lake Bonneville, and even now its level is dropping at a rate which can be measured year by year.
"Planetary water loss is irrevocable and pitiless. On one side, the water sinks into the crevices of the solid crust. These crevices continue to gape open as long as the incandescent interior is undergoing a cooling process and shrinking. On the other hand, water evaporates into the air. This process becomes more rapid as the atmosphere is dissipated and its pressure drops."
"What's that about the atmosphere dissipating?" asked Holt curiously. Bergmann went on coolly: "The molecules of the air are in motion, irregular and uncoordinated. Because they continuously collide, some of them may attain velocities sufficient to overcome gravity and thus escape from the atmosphere into the void. In the case of the Earth, this velocity of escape for a molecule must be not less than 6.9 miles per second, but on Mars, where gravity is much less potent, 3.1 miles per second suffices.
"Those two figures clearly show that the Martian atmosphere must have dissipated much more rapidly than that of the Earth. It's pretty safe to assume that there must have been about two fifths as much air over every square inch of Mars as there is over a square inch on Earth. That's plain from a very simple computation. But, as I said, the amount now present is only two ninths, and the difference is presumably that portion which has been dissipated."
"Now tell me, Doctor," said Holt, "if that much of the Martian atmosphere has been dissipated already, where is the moisture, the water vapor, which the air has absorbed from the evaporation of the erstwhile oceans? Did that fly out into space too?"
"Only a very small part of it," answered Bergmann. "Molecules can escape from the upper strata of the atmosphere only and these strata are necessarily very dry. The major portion of the water vapor must have combined its oxygen with the soil. For many millions of years, some moisture was precipitated on every cold Martian night in the form of dew, oxidizing the metals present in the soil. This split off the hydrogen, permitting it to rise into the upper atmosphere, from whence it dissipated gradually. We suspect," said Bergmann, with a gesture towards the photograph, "that this reddish coloration of the Martian deserts stems mainly from ferrous oxides which were thus formed at the expense of the oceans."
"Looking at this photograph," said Holt, "I find the great contrasts in coloration particularly striking. What conclusions do you draw from them?"
"Well," said Bergmann, taking a deep breath, "the reddish brown, ochre yellow, and orange zones are doubtless arid desert. But their coloration must be much affected by the position of the Sun, and they may therefore be somewhat "subjective" in every picture. Of course, we cannot tell what individual types of minerals make up the whole. If you contemplate taking a geologist along on your expedition to Mars, it ought to be a fascinating problem for him to examine the formations.
"Perhaps he'd find remnants of sea animals in the dried-up ocean beds, or maybe extensive salt deposits in the sinks where the last drops of the oceans lay. We're certain that he could do much to complete and supplement the theories we've built up on Mars' development.
"Now, it is our belief that the greenish zones represent vegetation. These tend to appear primarily around the polar caps at the time the melt begins. It is extremely impressive how the canals grow toward the equator out of these green melting regions, with water from the melted snow or ice.