Slow rotation would have doomed any moon of Venus simulations show

Any moon that Venus once possessed would have spiraled inward and crashed into the planet, according to computer modeling by an astrophysicist at the University of California, Riverside.

The study, published in The Astrophysical Journal, offers an explanation for why Venus has no moon even though it resembles Earth in size, mass and structure. It attributes the outcome to the planet's own gravity and slow rotation, and not to any outside event.

Earlier explanations fell into two groups. One held that Venus had a moon that was destroyed when something massive struck it. The other held that Venus never went through the kind of collision that produces a moon in the first place. The new work indicates that neither account fits.

"My study shows Venus didn't require a catastrophe to arrive at what we can see today," said UCR astrophysicist and lead author Stephen Kane. "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."

The Earth-moon system shows the opposite process at work. Mirrors left on the lunar surface by NASA's Apollo 11 landing allow the distance between the two bodies to be measured precisely.

"We know that the moon is slowly moving away from the planet, at a rate of around four centimeters per year," Kane said. He attributed that recession to Earth's relatively fast spin of 24 hours per rotation. Energy from the spin is transferred to the moon, which pushes it outward.

Venus needs 243 Earth days to turn once on its axis. With a spin that slow, the planet's gravity would pull a moon steadily inward toward a collision instead of letting it drift away.

Kane tested the idea with computer models built on the physics of how planetary bodies interact through gravity. He began by reproducing the evolution of Earth and its moon, to confirm the model accurately represented a known system.

He then varied the rotation rate of Venus and the size of its hypothetical moons. The moons tested ranged in mass from half to ten times that of Earth's moon. Most simulations ended the same way, with the moon striking Venus, and heavier moons struck sooner.

"When I made this discovery, I was shocked," Kane said. "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."

The result is not proof that Venus ever had a moon. Kane believes it may have had one, but whether a moon formed there is still an open question. What the study does establish is that a moon of Venus could not have survived indefinitely.

Physical traces of such a collision would be hard to find. Roughly 80% of the surface of Venus is similar in age, pointing to a major resurfacing event about a billion years ago that wiped out much of the planet's earlier geological record.

The interior may hold better evidence. Earth's moon is thought to have formed after a massive collision early in the planet's history, and seismic studies have found unusual structures deep inside Earth that may be remnants of that event. Comparable measurements at Venus could indicate whether it once absorbed a moon.

A collision of that kind also bears on another unresolved question, which is whether Earth's nearest planetary neighbor could ever have hosted life. A moon striking Venus would have delivered enormous energy and angular momentum, with the potential to change the planet's rotation, geology and climate. If Venus once had oceans or other conditions favorable to life, the impact could have redirected the planet's evolution.

The findings reach beyond Venus. Scientists searching for potentially habitable worlds around other stars often look for planets that resemble Earth, and they treat the presence of a moon as one factor that could affect habitability. Earth's moon drives the tides, may have helped keep the planet geologically active, and has strongly shaped its evolution. Whether a large moon is necessary for life is not known.

"My feeling is there are benefits to having a moon, but it isn't required for habitability," Kane said. "The moon has definitely changed the way Earth has evolved through time, but we don't fully know how important that role is."

The modeling suggests that planets able to form moons may still be unable to keep them. Slowly rotating worlds could draw their moons down to the surface, and the planets themselves would be greatly changed in the process.

"When people think about Earth twins around other stars, one question they ask is, 'Does it have a moon?'" Kane said. "My study shows a disturbing scenario for many of those cases. If these planets don't rotate fast enough, the moon will crash to the surface, and that would change the course of history for those planets."

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