Page:The Solar System - Six Lectures - Lowell.djvu/116



equalizing process the Sun, Jupiter, and Saturn now seem to be.

Jupiter, however, has progressed beyond the Sun in that the outer layers of his substance have cooled down enough to condense into cloud, due, possibly, to the planet's smaller mass. On the surface of the Sun things are still kept largely uniform by the terrific heat, and the slower rotation lets us perceive no latitudinal layers. On the contrary, Jupiter's disk is striated with belts of various tone and tint, according almost exactly to the parallels; while the albedo, or relative brightness of the disk, 62 per cent, of absolute whiteness, indicates that most of it is cloud.

These clouds are quite unlike our terrestrial ones. Jupiter's clouds are not Sun-raised, but self-raised condensations. On the one hand, the Sun's action there, only $$\scriptstyle \frac{1}{27}$$ of what it is here, is impotent to produce the effect we see; on the other, the cloud zones show a persistence quite disregardant of the Sun. They are not ephemeral like ours, but long-lived, lasting for weeks, months, and even years. They must, therefore, be Jove-caused.

In another feature Jupiter resembles the Sun. Its disk darkens to the limb. None of the smaller planets do this. The only thing capable