Small bodies are unable to retain the gases
at their surface, on account of their feebler gravitation. We
find, moreover, that Mercury always presents the same face to the
sun, as it turns on its axis in the same period (eighty-eight
days) in which it makes a revolution round the sun. While,
therefore, one half of the globe is buried in eternal darkness,
the other half is eternally exposed to the direct and blistering
rays of the sun, which is only 86,000,000 miles away. To
Professor Lowell it presents the appearance of a bleached and
sun-cracked desert, or "the bones of a dead world." Its
temperature must be at least 300 degrees C. above that of the
earth. Its features are what we should expect on the nebular
hypothesis. The slowness of its rotation is accounted for by the
heavy tidal influence of the sun. In the same way our moon has
been influenced by the earth, and our earth by the sun, in their
movement of rotation.
Venus, as might be expected in the case of so large a globe
(nearly as large as the earth), has an atmosphere, but it seems,
like Mercury, always to present the same face to the sun. Its
comparative nearness to the sun (67,000,000 miles) probably
explains this advanced effect of tidal action.
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