Winter and summer visit in succession its
northern and southern hemispheres just as occurs on the planet that we
inhabit, and the torrid, temperate, and frigid zones on its surface have
nearly the same angular width as on the earth. In this respect Mars is
the first of the foreign planets we have studied to resemble the earth.
Around each of its poles appears a circular white patch, which visibly
expands when winter prevails upon it, and rapidly contracts, sometimes
almost completely disappearing, under a summer sun. From the time of
Sir William Herschel the almost universal belief among astronomers has
been that these gleaming polar patches on Mars are composed of snow and
ice, like the similar glacial caps of the earth, and no one can look at
them with a telescope and not feel the liveliest interest in the planet
to which they belong, for they impart to it an appearance of likeness to
our globe which at first glance is all but irresistible.
To watch one of them apparently melting, becoming perceptibly smaller
week after week, while the general surface of the corresponding
hemisphere of the planet deepens in color, and displays a constantly
increasing wealth of details as summer advances across it, is an
experience of the most memorable kind, whose effect upon the mind of the
observer is indescribable.
Early in the history of the telescope it became known that, in addition
to the polar caps, Mars presented a number of distinct surface features,
and gradually, as instruments increased in power and observers in
skill, charts of the planet were produced showing a surface diversified
somewhat in the manner that characterizes the face of the earth,
although the permanent forms do not closely resemble those of our
planet.
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