Page:The Moon (Pickering).djvu/79

Rh situated in the vicinity of a prominent streak centre, such as Tycho, for instance, are distributed not uniformly, but with a tendency to occur along radial lines. Their positions probably indicate lines of weakness, or cracks in the surface, due to the original contraction of the crust. In the Hawaiian craters to which we have already compared those of the Moon radial cracks are sometimes found extending twenty miles or more directly away from the summits. Along these cracks are found small crater cones a few hundred feet in height from which occasionally issue jets of steam and other volcanic gases.

It is evident that upon the Moon, where the atmosphere is very rare, when those craterlets upon the rim of Tycho, for instance, become active, they would give rise to a wind blowing away from that crater in every direction. As this wind proceeded outward it would be reinforced by the wind from the various active craterlets that it encountered upon the way; there would, therefore, in general, be no opportunity for diverse currents, and if the wind were strong enough the bright streaks would necessarily lie radially as we observe them.

The objection to this explanation is the doubt as to the adequacy of such a wind to produce the results observed. Another and perhaps better explanation is that electrical repulsion, such as we see in our auroral streamers, acting in the rare lunar atmosphere, furnished the radial force which caused the arrangement in question. In this case the little white triangular area proceeding from each craterlet, which helps to form the white band that we observe, merely represents a little cometary tail of vapour, which has been rendered visible to our eyes by deposition upon the lunar surface in the solid form.

We thus see that the reason why these white radial bands are formed upon the Moon, but not upon the Earth, is due not to any difference in the volcanic eruptions of the two planets, but rather to the great difference in the densities of their atmospheres.