New research suggests the slow rotation of ‘Earth’s twin’ could have caused its one-time satellite to spiral back into the planet in a horror cosmic event
Venus may have swallowed its own moon in a giant fiery gulp, according to astronomers.
New research suggests the slow rotation of ‘Earth’s twin’ could have caused its one-time satellite to spiral back into the planet. The finding offers a possible explanation for why Venus, despite being similar to Earth in size, mass and structure, has no natural satellite. Scientists have previously suggested that Venus may once have had a moon which was destroyed by a massive collision.
Another possibility was that the planet never experienced the kind of impact needed to form a moon in the first place. A team from the University of California, Riverside, suggest neither explanation fits – and that one explanation is that it swallowed its moon. UCR astrophysicist and lead author Stephen Kane stated: “My study shows Venus didn’t require a catastrophe to arrive at what we can see today.”
He added: “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 study, published in The Astrophysical Journal, examined how Venus’ unusually slow rotation could affect a hypothetical moon. Scientists can precisely measure the distance between Earth and the Moon because mirrors left on the lunar surface by the Apollo 11 mission allow researchers to measure the distance between the two bodies.
Kane stated: “We know that the moon is slowly moving away from the planet, at a rate of around four centimeters per year.”
That gradual separation is linked to Earth’s relatively rapid rotation, which takes about 24 hours. Energy from the planet’s spin is transferred to the Moon, causing it to move progressively farther away.
Venus presents almost the exact opposite scenario. The planet takes about 243 Earth days to complete one rotation. Its slow spin, combined with its gravitational pull, would cause a moon to spiral inwards rather than move away. To investigate the idea, Kane created computer models based on the physics governing gravitational interactions between planetary bodies.
He first reproduced the evolution of the Earth-Moon system to check that the model could accurately recreate a known example. He then varied Venus’ rotation rate and the size of hypothetical moons, testing satellites with masses ranging from half to 10 times that of Earth’s Moon.
In most simulations, the outcome was the same: the moon eventually collided with Venus. The more massive the moon, the more quickly the collision occurred. Kane stated: “When I made this discovery, I was shocked.”
He added: 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 findings do not prove that Venus once had a moon. Kane believes it may have done, but whether one ever formed remains an open question. What the research does indicate is that, if Venus did have a moon, it could not have remained in orbit indefinitely.
Finding direct evidence of such a collision would be challenging. About 80% of Venus’ surface is believed to be similar in age, suggesting a major resurfacing event around a billion years ago wiped away much of the planet’s earlier geological record.
Scientists believe Earth’s Moon formed following a huge collision early in the planet’s history. Seismic studies have since identified unusual structures deep inside Earth which could be remnants of that impact. Similar measurements on Venus could potentially provide clues about whether the planet once absorbed a moon.
Such an event could also shed light on another longstanding question: whether Earth’s nearest planetary neighbour was ever capable of supporting life. A moon colliding with Venus would have transferred enormous amounts of energy and angular momentum to the planet. The impact could have altered its rotation, geology and climate.
If Venus once had oceans or other conditions favourable to life, such a collision could have significantly changed the course of its evolution.
The implications extend beyond our Solar System. Scientists searching for potentially habitable planets around other stars often focus on worlds that resemble Earth. Having a moon is sometimes considered one factor that could affect a planet’s habitability.
Earth’s Moon drives tides, may have helped maintain geological activity and has played a significant role in the planet’s evolution. Scientists do not yet know, however, whether a large moon is essential for life.
“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 findings suggest that forming a moon and retaining one may be two very different things. Slowly rotating planets could cause their moons to spiral towards their surfaces, potentially transforming the planets in the process.
Kane stated: “When people think about Earth twins around other stars, one question they ask is, ‘Does it have a moon?'” He added: “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.”