Changes in the Moon.

South Canterbury Times, Issue 6028, 3 October 1889, Page 3

Changes in the Moon.

What,is the matter with the moon, that astronomical obeervors describe as changes in her surface, disappearance of mountains, formation of new craters? Simply that the moon is showing that she is not such a “ dead world ” as she is commonly described to be, and is again experiencing one of those eruptive attacks which have pitted her face with pock-marks and dotted it with pimples till one can hardly find a sound spot about her. The new'eraters are reported to bo, one a mile and three quarters in diameter, another larger, and some smaller ones. Those larger ones would be huge craters on the earth, but they are mere babies to some of the lunar craters, which rim up to 100 miles in diameter. 'The larger ones are like vast plains, surrounded by high and steep mountain walls, the plain being usually lower than the region outside the walls. In the case of one very largo crater, however, the crater plain is level, or nearly level, with the top of the surrounding wall. Several attempts have been made to account for , the formation of such large craters on the moon. One idea deserves mention on account of its own peculiarity : it is suggested that they were formed by the fall of large meteoric masses from space, the larger craters presenting just the appearance produced by dropping a weight into a pasty substance, and no doubt tbe fall, with solar velocities, of meteorites weighing some thousands of tons, would melt both the meteorites and the surfaces they struck into a paste. A more probable view is that lunar volcanoes are attributable to the same cause which produces terrestrial volcanoes, the crushing of the outer cooled portion as it sinks upon a cooling and shrinking inner portion. It is a simple matter of calculation whether the force of gravity on the moon is sufficient to cause such forcible contraction, and in doing so to produce heat enough to melt ordinary rock substances or metals, and tbe calculation being made, shows that the force is sufficient. The crushing pressure of such contraction is equal to the weight of a wall of tbe moon’s matter, of a height equal to one-fourth of her diameter, that is, a wall 576 miles high. Supposing the moon’s crust material to be of the same nature as the earth’s, but only half as dense, (the whole moon having only half tho average density of the whole earth) say one and a-half times 1 as heavy as water, the weight of such a wall on tho moon (where gravity is but *163 what it is on the earth’s surface) would be about 140 tons per square inch of its base. This is sufficient to crush and melt most if not all terrestrial rooks, and sot up , volcanic action, and that pressnre may by 1 not improbable circumstances be doubled or even trebled. Allo wing then that the moon’s interior is still cooling and shrinking, there is force enough in the gravitation of her cooled crust to melt some part of it as a consequence of tho shrinking of tho interior. Other results of crust pressures are seen in chains of mountains—“ wrinkles " —said to be 20,000 feet high. A peculiar thing about the lunar volcanoes is the disappearance of the molten matter after it has been emitted, i and has filled up high walla around the limits of its spread. This disappearance seems to take place so rapidly as to suggest that the moon contains cavernous spaces below the region where the volcanic forces operate, into which tho molten matter presently finds its way before it has had time to cool—save in the case of the large crater above referred to, which remains filled to the brim of its walls. According to this view lunar ovruptiona consist of emissions of molten material which spreads around tho vent and continually fences itself in by moraines of cooled slag ; the molten matter presently eats its way through internal barriers and sinks into tbe cavernous interior, leaving behind it a mountainous circular moraine of chilled slug—the “ crater wall, ” —and taking with it a more or less considerable depth of tho previous surface melted while Ijing upon and flowing off it, thus forming a crater plain at a lower level than the surrouning country.