Computer simulations in a Sept. 14 Astrophysical Journal study found Venus would usually pull any early moon inward until it struck the planet, rather than keeping a long-lived satellite.
A 243-Earth-day Venus rotation is the key driver: unlike Earth, whose faster spin pushes the Moon outward by about 1.6 inches a year, Venus' slow spin and gravity would sap a moon's orbit.
Most modeled moons—from half the Moon's mass to 10 times larger—ended in impact; a moon at least twice Earth's could last 30 million to 1.7 billion years before crashing.
The team found a smaller moon might have survived only if early Venus spun with a day shorter than about 12 hours, conditions Kane said do not fit leading views of the planet's past.
The idea remains hard to test because Venus' surface has been reworked by volcanism, but researchers say chemical or seismic clues could reveal whether a moon impact helped trigger the planet's runaway greenhouse state.