RADAR IMAGE of Venus, taken by the Magellan spacecraft, reveals the planet's surface in false color as seen from its north pole. Venus rotates in the clockwise direction around its north pole, whereas all the other solar system planets rotate counterclockwise.
COURTESY OF NASA

One of biggest mysteries in the solar system is why Venus has no moon. A new model suggests that our sister planet may have in fact had a moon, but that it was destroyed.

Earth's moon is thought to have formed when a Mars-size body struck the early Earth, hurling material into orbit, where it coalesced. Normally, material launched ballistically would merely fall back down to the surface, but the impact temporarily distorted Earth's shape and therefore its gravitational field; the lopsided gravity allowed material to remain in orbit. Since it formed, the moon has gradually receded from Earth because of gravitational interactions between the two bodies: the moon raises tides on Earth, and these tides back-react onto the moon, accelerating it at the expense of Earth's rotation.

Venus, being nearly identical in size and general composition to Earth, presumably would also have gotten whacked by large bodies. One possibility is that these bodies did not distort Venus's gravity enough to allow the debris to remain in orbit. Another is that a moon did form and drifted outward to the point that it escaped altogether. The problem with the latter hypothesis is that the outward drift should have taken billions or even tens of billions of years.

At the Division for Planetary Sciences conference in Pasadena, Calif., on October 9, Alex Alemi, a sophomore undergraduate at the California Institute of Technology, and Caltech planetary scientist David Stevenson argued that the mystery may be connected to another oddity of Venus: its rotation rate, which is not only extremely slow (once every 243 Earth days) but also backward (clockwise as seen from above its north pole, rather than counterclockwise, as it is for Earth and the other planets). They suggested that Venus underwent not one but two large impacts.

The first hit on the side of Venus that caused the planet to spin counterclockwise. It created a moon that began to drift away, like Earth's. The second slammed into the side of Venus that caused it to spin clockwise--canceling the effect of the first collision. The cancellation need not have been exact; the sun's gravity could have completed the task of slowing and even reversing Venus's rotation. The reversal changed the gravitational interactions between the moon and planet, causing the moon to start moving inward and ultimately collide with the planet. The second impact may or may not have created a moon, too. If it did, this moon would have been swept up by the first one on its inward plunge toward doom.

Stevenson says that this model can eventually be tested by looking at isotopic signatures in Venusian rocks. For now, its main significance may simply be to restart discussion over Venus's lack of a moon, a conundrum that planetary scientists have barely explored.