A moon orbiting Venus would not survive. Tides would pull it inward until it hit the planet, according to a new model by astrophysicist Stephen Kane of the University of California, Riverside.
The reason is Venus's rotation. Earth spins quickly and in the same direction the Moon orbits, so the Moon drifts away by about four centimeters a year. Venus takes 243 Earth days to turn once, and it spins backward.
One outcome
Kane simulated moons ranging from half the mass of Earth's Moon to 10 times larger. Nearly every run ended the same way: the moon fell onto Venus, and heavier moons fell faster.
"I thought surely the broad range of scenarios I was exploring would lead to a variety of results. But it all went pretty much in the same direction," Kane says.
The paper does not claim Venus ever had a moon. It shows only that if it did, the moon could not have lasted.
"My study shows Venus didn't require a catastrophe to arrive at what we can see today," Kane says. In his view, the planet's gravity and spin rate alone would have brought any moon down.
Beyond the solar system
The same math applies to slowly rotating planets around other stars that otherwise resemble Earth. They could lose their moons too.
"When people think about Earth twins around other stars, one question they ask is, 'Does it have a moon?' My study shows a disturbing scenario for many of those cases," Kane says. The paper appears in The Astrophysical Journal.
