Astronomical Mystery

Soon after Caltech astronomers began aiming long-range radar beams at Venus in 1962, they made an unexpected discovery. They found that the earth's cloud-shrouded neighbor spins not only more slowly than the other planets, but also in the opposite direction.*Long puzzled by Venus' eccentric behavior and dissatisfied with previous attempts to explain it, Geophysicist S. Fred Singer has now come forth with an ingenious theory.

Most scientists have attributed the peculiar spin of Venus to huge tidal bulges created long ago on the surface of the planet by the sun's gravitational field. Such bulges would have acted like brake shoes on the rim of a flywheel; eventually they could have slowed the planet's rotation and perhaps even reversed it. Singer, the Interior Department's deputy assistant secretary for scientific programs, considers this explanation totally inadequate. The solar tidal effect, he says in Science, would have been far too small to account for even Venus' current rate of rotation, only once every 243 earth days. Thus it certainly could not have caused an actual reversal of rotation.

Near Miss. Looking for a better solution, Singer recalled an old suggestion by Nobel Laureate Harold Urey, who argued that in the early days of the solar system the inner planets were accompanied in orbit around the sun by many moonlike bodies. Because only one of these ancient "moons" remains (the earth's), it seems quite likely that most of the others eventually collided with the planets. Singer dismisses the possibility that a direct hit by a moon could have reversed Venus' spin; the moon would have been much too small. But his calculations indicate that a near miss by a moon traveling counter to the direction of Venus' rotation might have turned the trick.

Initially, the momentum of the errant moon would have carried it beyond Venus. Then, as Venusian gravity pulled it back, it would have again sped by the planet--but this time not so far out into space. Eventually, as the tidal forces between the two bodies increased during this strange celestial courtship, the moon would have been drawn into an increasingly smaller orbit around the planet. At the same time, Venus' spin would have been greatly retarded and eventually reversed; the planet's surface would also have become searingly hot from the friction of the tidal movements, and volcanoes would have erupted--giving off the thick clouds of gases that still envelop Venus. Finally, after about 100 years, the moon would have come crashing into Venus, leaving the slow, backward spin as the legacy of a primordial celestial drama.

*The only exception: Uranus, whose backward rotation is an astronomical mystery.

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