Selenology: Around the Lunar Edge

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"This is the most spectacular find of the Lunar Orbiter program," said Geologist John McCauley of the U.S. Geological Survey. "If it were fully visible on the side of the moon facing the earth," said University of Arizona Astronomer Gerard Kuiper, "a whole mythology would have been built up about it." Both scientists were referring to the Orientale Basin, which is located on the extreme western edge of the visible face of the moon and affords only a meager side view to earthbound astronomers. Photographed head-on for the first time by NASA's Orbiter 4, it bears a surprising resemblance to a giant bull's-eye, fully 600 miles in diameter.

Clearly visible in the Orbiter picture are the Cordillera Mountains, which are 20,000 ft. high and completely ring the basin. Within them, scientists can pick out as many as six additional concentric rings of smaller mountains, separated by relatively flat plains that are partially filled with darker material. Giant cracks radiate from the edge of the bull's-eye; the lunar surface for another 600 miles beyond is littered by coarse debris that was obviously hurled from the basin with tremendous force.

Crust over Lava. Geologist McCauley believes that Orientale was formed by the impact on the moon of a meteorite or a comet between 30 and 60 miles in diameter. Because more of the collision debris lies to the west of the crater, McCauley speculates that the intruder approached the moon from the east, overtaking it in its orbit around the earth and plowing into the lunar surface in a "trailing impact." He also suggests that Orientale is the youngest of the large lunar basins—only about 500 million years old—because it has relatively few small craters superimposed on it and is only partially filled with the dark volcanic material that covers the bottoms of older basins.

To Astronomer Kuiper, the mountain ranges seem much like the concentric ridges that would be formed if a boulder crashed into a layer of ice over water. The dark material is similar to the water that would ooze into the valleys between the ridges. This similarity reinforces his belief that Orientale was formed when there was a relatively thin lunar crust over a molten-lava interior—possibly 4.5 billion years ago. It was around this time, he believes, that the moon swept its path clear of large chunks of moonlike material that also circled the earth in the lunar orbit. Most of the basins were formed, Kuiper suggests, when these subsatellites crashed into the lunar surface.

The remarkable picture of the Orientale Basin was one of the last shots taken by Orbiter 4, which returned telephoto pictures of 99% of the front face of the moon and increased the portion of the lunar backside photographed by all of the Orbiters to nearly 75%. Although its photographic mission has been completed, Orbiter 4 still has contributions to make. NASA scientists last week fired its velocity-control engine to drop the spacecraft into a lower lunar orbit in which it will be painstakingly tracked in an experiment to learn more about the moon's gravity.