http://www.archive.org/stream/throughspaceandt031585mbp/throughspaceandt031585mbp_djvu.txt wrote:
THROUGH SPACE & TIME (1934)
BY SIR JAMES JEANS
<<[Astronomers] find no direct evidence of water vapour in the [Martian] atmosphere, although it has often been thought that there is a certain amount of circumstantial evidence that water vapour is present.
It is noticed that dark patches appear regularly in the Martian spring, and fade away again in the autumn mainly in the tropical regions and southern hemisphere. It was at first thought that these were real seas of water, but this is now considered improbable. For one thing, they vary too much and too rapidly in colour; one, for instance, was observed to change from blue-green to chocolate-brown and back again within a very few months. They also resemble the supposed seas on the moon in never reflecting the sunlight, as sheets of real water would do. At one time astronomers thought they might be forests, or masses of vegetation. Since then the surface of Mars has been examined in the same way as the surface of the moon, and appears to be of somewhat similar composition possibly volcanic lava or some such substance. Thus the dark patches may be produced by showers of rain wetting a dead dry surface like that of the moon.
Mars has days and seasons very like our own. It takes 24 hours and 37 minutes to turn on its axis, so that its day is slightly longer than ours. And as this axis is tilted at an angle of 25 10', as against the earth's angle of 23 27', we must expect to find the Martian seasons rather more pronounced than ours on earth; there will be a greater difference between summer and winter. On top of this, however, there is a further cause of variation in the climate on Mars.
The earth's path round the sun is very nearly circular not quite, since the earth's distance from the sun is 3 per cent, less in December than in June. We inhabitants of the northern hemi- sphere are closest to the sun at our mid-winter, while people in the southern hemisphere are closest at their mid-summer. Thus the small variations in our distance from the sun go to lessen the difference between summer and winter in the northern hemisphere, but accentuate it in the southern hemisphere. As a consequence, we must go to the South Pole rather than to the North for extremes of climate.
Nevertheless, the earth's distance from the sun does not vary enough to produce any great effect on our climate. It is different with Mars, whose path is nothing like so circular as that of the earth. Our distance from the sun varies by less than 3 million miles, but that of Mars varies by more than 26 million miles. Thus, when Mars approaches the sun, the climate of the whole planet becomes appreciably warmer; as it recedes, the whole planet gets colder. These alternations of general coldness and general warmth are of course superposed on top of the ordinary Martian seasons.
The maximum of general warmth, the time when Mars is nearest the sun, occurs shortly before mid-summer in the southern hemisphere, so that on Mars, as on earth, we must go to the southern hemisphere for extremes of climate. Furthermore, the extremes will be far more marked than with us.
Now if we are planning to land our rocket on Mars, we may as well take advantage of what warmth there is even so, we shall soon find there is little enough. Let us then arrange to arrive when Mars is nearest the sun i.e. at the middle of the period of general warmth and to land slightly south of the equator at mid-day. Here we may find a temperature as high as 60 degrees Fahrenheit.>>
Seasonal Dark Streaks on Mars
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