"You should be able to get data on the chemical composition of the Martian atmosphere by spectral analysis," said Holt. "Do you have anything definite on that subject?"
"So far our efforts along that line have produced reliable results for two components only, namely carbonic acid and water vapor. We're very certain that there is nitrogen, even though the latter is difficult to detect spectroscopically. We also have reason to believe there's a good deal of argon in Mars' atmosphere. There's difference of opinion as to the oxygen content; some observers insisting that they have proved that it exists. But if, indeed, oxygen is actually present, the quantity is considerably less than on Earth, even percentage-wise."
This remark brought a profound question from Holt. "On what do you predicate vegetation and animal life, if the oxygen content is so low that it cannot even be definitely traced?" he asked.
"There isn't the slightest doubt that plant life exists, and the botanists consider it well within the realm of credibility that Martian plants may live within a sort of 'internal oxygen atmosphere.' A plant applies photosynthesis in order to live, and generates new oxygen in the process, although it does require a certain amount of oxygen for recycling. If we assume that such a plant can store oxygen within its system, there's no reason why it cannot do without any free oxygen in the surrounding atmosphere.
"Now as to animal life, the answer doesn't come quite so easily. Animals, in the ordinary sense of the word, cannot live without oxygen. Nature, however, discovers the most extraordinarily manifold methods of providing animals with oxygen, even on Earth. Fish, for example, attract oxygen from the water through their gills. Monocellular organisms absorb oxygen through their exterior membranes, just as they do their food. Why shouldn't a Martian animal get its oxygen by eating plants which have stored oxygen produced by photosynthesis? It would, of course, demand that the lungs be more intimately connected with the digestive organs than are our own…
"Should you think this hypothesis a little far-fetched, there are other plausible explanations. Take, for example, the condition of symbiosis, which is quite familiar in natural history. Here animals and plants are able to survive jointly under conditions which would be fatal to either party alone. Corals, which are fauna rather than flora, are a case in point. We know that the oxygen content of the water within an extensive bank of coral is far too low to sustain life in the coral creatures inhabiting it. So Nature simply grows oxygen-producing algae throughout the coral bank. Thus it's quite reasonable to assume that Martian animals may live with oxygen-generating plants in some analogous symbiosis.
"We do have animals on Earth which require no oxygen at all to remain alive.
Intestinal parasites, such as tapeworms, are typical of this class. Instead of relying on the chemical process of oxidation as do most other animals, they use fermentation to obtain the energy essential for the maintenance of life. Fermentation is the dissociation of sugar into alcohol and carbon dioxide, which is the process that transforms grape juice into wine or champagne. Fermentation, like oxidation, generates heat. Intestinal parasites exist amid a superfluity of sugar. They are beautifully protected against temperature variations by the bodies of their hosts, so they live extremely contentedly by fermentation without any oxygen whatsoever. Lest you think this example somewhat depraved, I employ it only to bring out Nature's inventiveness in finding ways and means for making life possible in the most inhospitable places."
"Well," said Holt reflectively, "it begins to look as though we might indeed be sticking our noses into a very strange world in this Mars business. But tapeworms or no tapeworms, what about the climate?"
"The average temperature throughout the year on Mars is somewhat lower than it is on Earth because of Mars' greater distance from the Sun, naturally. Nonetheless, it is not so low as this distance might lead one to expect. One of the most important reasons for this is the relatively low albedo of Mars; 60 % of the total energy radiated to the planet from the Sun strikes the surface and is absorbed, in this case, not only the light radiation, but also the heat radiation. Then, too, there's little doubt that clouds form soon after sundown, preventing any very strong nocturnal reradiation. This cloud formation is doubtless due to the rapid cooling of the air after nightfall and the low atmospheric pressure. But the clouds quickly disperse when the Sun returns in the morning. Thus the mean yearly temperature on Mars is explained as being some 48° F, versus about 60° on Earth.
"The atmosphere of Mars is so thin that the temperature contrasts between day and ensuing night are very marked. This also applies to variations between seasons and latitudes. Such manifestations are familiar to us on Earth in regions of high mountains.
On Mars, in regions where the Sun's rays fall vertically at noon, the early morning temperatures will be on the order on -20 °C. These temperatures will rise to around +30 °C at noon, and then decline to approximately zero near nightfall. We must anticipate temperatures of more than 100° below zero in the polar regions in Winter, when the Sun remains below the horizon for months, as it does in our own polar regions. During the polar Summer, when the Sun doesn't set at all, the temperature rises considerably above freezing. Otherwise the almost complete melting of the polar snow caps during this season would find no explanation."
Here Holt interrupted once more. "How do you explain that the melting of the polar snowcaps in Summer is so extensive? Our terrestrial snowcaps don't do that, despite the fact that the mean temperatures there are markedly higher than those on Mars."
"That's a mighty good question, Colonel. It is explained both by the nature of the Martian atmosphere and by the length of the year on that planet. Our spectroscopic determinations lead us to estimate that the average water vapor content of the Martian atmosphere is very low, not more than about 5 % of that of the Earth's air. This low percentage is easily understandable in view of there being no oceans or large lakes from which quantities of water may rise into the atmosphere. The extreme thinness of the latter stimulates the evaporation of moisture from vegetation zones. The relatively cool air becomes saturated after absorbing small amounts of moisture, however, and this means naturally that any air reaching the polar regions will carry lesser amounts of water with it.
Here it will be cooled down to extremely low temperatures in Winter, and will precipitate its water down to extremely low moisture contents. That is to say, it will precipitate its low moisture content considerably more completely than the Earth's atmosphere will. If we balance these conditions against one another, we're in all probability correctly concluding that the daily precipitation during the Winter months in the polar regions is noticeably less than that in those regions on Earth.
"Conversely, we shouldn't forget that a Martian Summer is nearly twice as long as ours, on account of the length of the Martian year. There's much more time available for solar rays to melt the snow of the Martian polar regions.
"These two effects explain the fact that the Martian poles have never formed such thick ice and snow caps as those of Earth.
"It is noteworthy that Summer at the South Pole of Mars is generally considerably warmer, although shorter than at the North Pole. This is related to the pronounced eccentricity of the planet's orbit. During Summer at the South Pole, Mars is near perihelion and closer to the Sun by 43 million kilometers than six Martian months later when it is Summer at the North Pole. It is at perihelion that Mars reaches its highest orbital velocity, being closest to the Sun. Thus Summer in the southern hemisphere, is considerably shorter than in the northern, being only 158 days against 183."