Venus's slow spin dooms moons

A new study led by UC Riverside astrophysicist Stephen Kane found that Venus's sluggish 243-day rotation makes it impossible for any moon to survive. Computer simulations showed that any moon would crash into the planet within 30 million to 1.7 billion years.

Earth and Venus took different paths

Earth and Venus are nearly identical in size and composition, yet Earth has a moon while Venus doesn't. Our moon stabilizes Earth's tilt and controls ocean tides, possibly keeping the planet geologically active. Venus's missing moon may explain why the two worlds evolved so differently.

Rotation speed determines moon fate

Earth spins fast enough every 24 hours to push its moon away at four centimeters per year. Venus rotates once every 243 days, so its gravity pulls any orbiting moon inward instead. This fundamental difference explains why Venus cannot keep a moon.

Heavier moons spiral in faster

Kane tested moons ranging from half Earth's moon's mass to ten times heavier. Every simulation showed the same result: the heavier the moon, the faster it spiraled into Venus. No moon size could escape the planet's gravitational pull.

“When I made this discovery, I was shocked.”

Moon formation creates a catch-22

Moons form when massive collisions throw debris into orbit. But Venus faces a paradox: impacts fast enough to create a moon leave the planet spinning too slowly to keep it. Impacts that spin Venus fast enough scatter debris too close to coalesce into a moon.

No catastrophe needed for Venus

Earlier theories suggested Venus lost its moon to a massive collision or never had one due to bad luck. Kane's research shows neither is necessary. Venus's slow rotation alone explains why it has no moon today.

“Venus didn't require a catastrophe to arrive at what we can see today.”