IN STARRY SKIES

Evening Post, Volume CVI, Issue 112, 16 November 1928, Page 16

IN STARRY SKIES

SCENERY OF THE MOON THE MYSTERIOUS RAYS (By "Omega Centauri.")' Last week wo proposed for consider, ation the most puzzling of all tho strange problems presented by tho surface features of the moon, and asked our readers to frame some theory to account for tho marvellous system of raya which/diverge from tho crater Tycho. To-day we. show a beautiful photograph of the region of Copernicus which was taken on 15th September, 1919, with tho 100-inch telescope at Mount Wilson. It shows another magnificent system of rays, but instead of simplifying the problem it appears at first sight to intensify tho difficulties of the solution. For whilst there are somo striking resemblances between the

two systems there are also characteristic differences. The rays from Tycho are straight like tho spokes of a wheel. Those from Copernicus and Kepler are feathery and full of delicate tracery. Can wo frame any theory that will account for the differences as well as the general likeness? Before discussing this problem further let us examine thephotograph which is before us. Copernicus stands out prominently in 1 -.o centre. Its mountain ring, 56 miles in diameter, risca 12,000 ft above its inner plain. Eays can be traced from it almost to tho margins of the picture, which shows an area of about 750 by 650 miles. Kepler, over 300 miles to the east, has a similar system of rays. Eratosthenes, a beautiful ring 37 mites in diameter, lies near tho end of the Apennine chain. Two hundred and fifty miles north of it is Timoeharis, with the two small craters named after tho selcnographcr Beer to tho west and Lambert twice as far to the cast. Continuing tho lino through theso craters, but curving a little upwards, tho crater Euler, 19 miles across, and tho smaller one, Brayley, occur at nearly equal intervals. Another line, from Timoeharis to Kepler, contains Pytheas and the group of which Tobias Mayer is the largest. The latter group is amongst the Carpathian Mountains. From Copernicus in a crooked line to the south-cast lio Eeinhold, Landsberg, and the tiny Euclides. The last of these is the centre of a brilliant patch of rays just east of the Riphean Mountains. Near the middle of the upper edge of tho picture aro the flattened rings Parry and Bouplantl, the former encroaching slightly on the latter. Under the. surfaces of Maro Imbrium, Sinus Acstuum,

[and Oceauus Proeellarum, which fill a great part of the north, north-west,' and east of the picture,1 the ruins of many buried rings may still bo discerned. Tho origin of the seas and craters has already been discussed; let us now return to the problem of the rays. We have pictured the moon at ■ a period in which its interior was. still molten and its solid crust comparatively thin. "We saw its surface roughened by nioteoric bombardment and covered with tho fragmentary debris scattered by innumerable oxplosions. Soino largo round areas, however, in tho geologically re. cent past, had been smoothed and darkened by deluges of very fluid lava thrown up by huge meteoritos which pierced the crust and detonated in the molten interior. These areas are now called seas. As the crust thickened it i beeamo moro and more difficult for a moteorito to reach the liquid within. At this stage a particularly large meteorite with a specially high velocity might penetrate the covering of uncouaolidated material and give a shattering blow to the solid crust beneath. This would be far more capablo of producing long radiating cracks than any pressure acting from within. If the explosion of tho meteorite now establishes communication with the liquid interior the whole crust will be subjected 'to a brief wave of intense hydrostatic prcs-

sura, under which super-heated .liquid and Imprisoned gases will be forced as a spray through all the cracks. This will riso through the unconsolidated material and condense and crystallise on the surfaces of the fragments, thus giving them a high reflecting power. The sudden pressures is relieved by the formation of! the crater and disappears as suddenly as it was produced, so no appreciable quantity of lava is ejected. No change of level takes place along the cracks, and the irregularities are not smoothed out. The cracks themselves aro hidden by the fragmentary material above them. An oblique illumination casts no new shadows, but the original roughness remains with its mingled light and shade. But when the high sun shines down into every hollow the crystalline deposit is lighted up, and the rays'appear clear and distinct. _ But, why aro the rays" round Copernicus and Kepler somewhat different in character from those round Tycho? The former craters arc in lunar seas, the latter in a light area. It seems as if the surfaces which have been sprayed and smoothed by very liquid lavas, and over which there is a much thinner deposit of fragmentary material, suffer a more feathery fracture than the original crust. It is probable then that the rays are due to. cracks, though no cracks can be seen; that they are not formed by lava ejected through the cracks, which would be darker instead of lighter than its surroundings, and which, if in sufficient quantity, would cast shadows. They are merely crystal, line deposits formed from vapours ejected suddenly by a'wave of compression whic*h forced super-heated liquids against tho shattered crust.