Mars, the first planet more distant
from the sun than the earth, is the special subject of this chapter,
and will be described and discussed a few lines further on. Jupiter,
Saturn, Uranus, and Neptune, the four giant planets, all more distant
than Mars, and each more distant than the other in the order named,
are all regarded as uninhabitable because none of them appears to
possess any degree of solidity. They may have solid or liquid nuclei,
but exteriorly they seem to be mere balls of cloud. Of course, one can
imagine what he pleases about the existence of creatures suited to the
physical constitution of such planets as these, but they must be
excluded from the category of habitable worlds in the ordinary sense
of the term. We go back, then, to Mars.
It will be best to begin with a description of the planet. Mars is
4230 miles in diameter; its surface is not much more than one-quarter
as extensive as that of the earth (.285). Its mean distance from the
sun is 141,500,000 miles, 48,500,000 miles greater than that of the
earth. Since radiant energy varies inversely as the square of
distance, Mars receives less than half as much solar light and heat as
the earth gets. Mars' year (period of revolution round the sun) is 687
days. Its mean density is 71 per cent of the earth's, and the force of
gravity on its surface is 38 per cent of that on the surface of the
earth; i.
Pages:
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209