Venus has no moon today, and a new study proposes a specific reason why: it may once have had one, only to pull it back down and destroy it. The research, led by astrophysicist Stephen Kane at UC Riverside, was published this week in The Astrophysical Journal.
Kane ran computer simulations of hypothetical moons ranging from half to ten times the mass of Earth's own moon, tracking how each would interact with Venus over billions of years. In every scenario, the moon did not drift outward the way Earth's does. Instead, it spiraled inward and ultimately collided with the planet — with the more massive hypothetical moons falling in fastest. If such a moon existed, the simulations suggest it could have survived for roughly two billion years before being destroyed, placing the collision at around 2.5 billion years ago.
A tale of two spins
The difference comes down to rotation. Earth spins once every 24 hours, and that relatively fast spin transfers energy that pushes the Moon outward at about 4 centimeters a year. Venus, by contrast, rotates only once every 243 Earth days — the slowest of any planet in the solar system. That sluggish spin flips the tidal relationship: rather than gaining energy and receding, a Venusian moon would have lost momentum to the planet and been pulled steadily closer until it crashed.
"My study shows Venus didn't require a catastrophe to arrive at what we can see today. It turns out the gravity of the planet itself combined with the rate at which it spins naturally caused the moon to collapse on top of it."
— Stephen Kane, UC Riverside, lead author
The idea offers a tidier explanation than two competing theories long debated by planetary scientists: that Venus never had a moon at all, or that one was destroyed in a sudden ancient impact. Kane's modeling suggests ordinary tidal physics, playing out over billions of years, would be enough on its own.
Direct evidence is hard to come by. Venus underwent a major resurfacing event roughly a billion years ago that erased much of its older geology, so any scars from an ancient lunar collision are likely long buried. Kane has suggested that future seismic studies of Venus's interior, of the kind proposed for upcoming missions, could eventually reveal buried remnants consistent with the impact.
The findings also carry implications beyond Venus: because the same tidal mechanism applies to any slowly rotating planet, the study suggests that exoplanets found orbiting close to their stars — where tidal forces tend to slow rotation over time — may be similarly prone to losing any moons they once had, a factor scientists are increasingly weighing when they assess distant worlds for habitability.
