Selenology: An Earthlike Moon

"One of the greatest scientific achievements of the civilization of man," exulted NASA Planetologist Donald Gault last week. By means of the Surveyor 5 spacecraft, man had reached across a void of some 240,000 miles, studied the surface of the moon, and analyzed its chemical composition. That analysis, scientists reported last week, indicated that part of the lunar surface is strikingly similar to the earth's.

The most plentiful substance identified by Surveyor's alpha particle "jewel box" (TIME, Sept. 22) is virtually the same as the basaltic rock that forms the ocean floors and is found in such widely scattered locations as the Hudson River Palisades, the Brazilian Plateau, the Hawaiian Islands and India. Like its counterpart on earth, the lunar material consists largely of oxygen (58%) and silicon (18.5%). It also contains aluminum (6.5%), iron-nickel (5.5%), magnesium (3%), and smatterings of carbon.

Surveyor 5's overworked TV camera, which has transmitted more pictures (18,006) to earth than Surveyors 1 and 3 together, also confirmed the presence of some iron compounds on the moon. Focusing on a powerful magnet that scientists had attached to one of the spacecraft's footpads, the camera transmitted pictures of a thin layer of dust—kicked up in the landing—on the magnet. By comparing these shots with control pictures made on earth, where dirt with known percentages of iron filings had been scattered on a magnet, the scientists established the presence of iron compounds.

Surveyor's findings are admittedly based on tiny samples. The spacecraft's alpha-particle analyzer covered only two 4-sq.-in. areas and probed them to a depth of only 1/1,000th of an inch. And it is conceivable that the lunar highland regions have a chemical composition different from that in the Sea of Tranquillity, where Surveyor landed. But scientists believe that Surveyor's findings are typical of the makeup of the other lunar seas, or basins.

The basaltic character of Surveyor's samples has strengthened the belief of many scientists that the floor of the Sea of Tranquillity was formed by extensive lava flow. Such flow, they say, could emerge only from a lunar interior that was—and could still be—hot and molten. The earthlike characteristics of the lunar surface also support the theory that the moon and earth were part of the same pre-planetary dust cloud when the solar system was young, but later split to go their separate ways.