Scientists have puzzled for years over why Venus, despite being similar to Earth in size, mass, and overall structure, has no moon. New research from UC Riverside suggests a striking possibility: Venus may once have had a moon, only to eventually pull it back into the planet.
Previous explanations generally focused on two possibilities. One suggested that Venus once had a moon that was later destroyed by a massive impact. Another proposed that Venus simply never experienced the kind of collision that could have created a moon in the first place.
A new study published in The Astrophysical Journal offers a different explanation. According to the research, Venus may not have needed a dramatic external event to lose a moon.
“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.”
Why Earth’s Moon Is Moving Away
Scientists know that Earth’s moon is gradually drifting farther from us because the Apollo 11 mission left mirrors on the lunar surface that allow researchers to measure the Earth-Moon distance with extreme precision.
“We know that the moon is slowly moving away from the planet, at a rate of around four centimeters per year,” Kane said.
That movement is closely tied to Earth’s relatively rapid rotation. Our planet completes one spin in about 24 hours. Some of the energy associated with that rotation is transferred to the moon, helping push it slowly outward over time.
Venus behaves very differently.
A single rotation on Venus takes about 243 Earth days. Because the planet spins so slowly, a moon around Venus would not necessarily migrate outward in the way Earth’s moon does. Instead, the combination of Venus’ gravity and its sluggish rotation could cause the moon to gradually spiral inward until it collided with the planet.
Simulations Send the Moon Into Venus
To investigate that possibility, Kane developed computer models that simulate how planets and moons interact through gravity.
He first tested the model using Earth and the Moon, reproducing the known evolution of the system to confirm that the simulation behaved realistically.
Kane then applied the model to Venus. He changed the planet’s rotation rate and tested hypothetical moons with masses ranging from half the mass of Earth’s moon to ten times its mass.
Across most of those simulations, the outcome was remarkably consistent. The moon eventually crashed into Venus.
Larger moons met that fate even faster.
“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 simulations do not prove that Venus ever actually had a moon. Kane thinks it is possible, but whether one formed remains unknown.
What the study does suggest is that if Venus once possessed a moon, that moon may not have been able to survive indefinitely.
Could Evidence Be Hidden Inside Venus?
Finding direct evidence of an ancient moon collision would be difficult because Venus has undergone major geological changes.
Roughly 80% of the planet’s surface is similar in age, which scientists interpret as evidence of a large resurfacing event about a billion years ago. That event may have erased much of Venus’ older geological record.
Clues could instead be hidden deep inside the planet.
Scientists think Earth’s moon formed after a massive collision early in Earth’s history. Seismic studies have identified unusual structures far below Earth’s surface that could contain remnants linked to that ancient impact.
Comparable measurements of Venus might someday reveal whether the planet absorbed a moon of its own.
A Moon Crash Could Have Changed Venus
Such a collision could also provide clues about another major question: whether Venus was ever capable of supporting life.
If a moon crashed into Venus, it would have delivered enormous amounts of energy and angular momentum to the planet. That could have affected Venus’ rotation, geology, and climate.
If the planet once had oceans or other environments that were more favorable to life, a major moon impact could have significantly changed its subsequent evolution.
The possibility is especially intriguing because Venus is Earth’s closest planetary neighbor and may once have been far more Earth-like than the extremely hot world seen today.
What Venus Could Tell Us About Other Earth-Like Worlds
The findings may also matter far beyond our Solar System.
When scientists search for potentially habitable planets around other stars, they often focus on worlds that resemble Earth. Whether those planets have moons is one factor that researchers sometimes consider when thinking about their long-term environments.
Earth’s moon creates tides, may have contributed to continued geological activity, and has played an important role in the planet’s evolution. Still, scientists do not know whether a large moon is actually necessary for life to develop or persist.
“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.”
Kane’s results suggest that forming a moon may be only part of the story. A planet also has to be able to keep it.
Worlds that rotate too slowly could cause their moons to spiral inward, ending in collisions capable of dramatically reshaping the planets themselves.
“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.”
