Venus, which shares similar mass, size, and structure with Earth, does not have a moon, leading astronomers to explore various hypotheses regarding its absence. A recent study from researchers at the University of California, Riverside, proposes that instead of being destroyed by a collision, a hypothetical moon may have been gradually pulled into Venus by its gravity and slow rotation until it fragmented and was consumed.
Steven Kane, an astrophysicist and lead author of the study, stated, "Venus didn’t require a catastrophe to arrive at what we can see today. 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 rotation of a planet can influence its satellites, causing variations in gravitational strength. For example, Earth's moon is drifting away at a rate of approximately 1.5 inches per year due to Earth's relatively fast rotation of 24 hours. In contrast, Venus takes 243 Earth days to complete one rotation, making it the slowest rotating planet in the solar system. This slow rotation, combined with Venus' gravity, suggests that if a moon existed, it would be drawn inward rather than pushed outward.
To test this hypothesis, Kane and his team developed a computer model to simulate gravitational interactions between celestial bodies. They verified the model's accuracy by replicating the evolution of Earth's moon and then conducted simulations with varying factors, including Venus' rotation speed and the mass of a hypothetical moon. The results indicated that under many conditions, the moon would spiral inward and eventually break apart due to Venus' gravity.
Kane noted, "When I made this discovery, I was shocked. 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." In some scenarios, such as when the planet's rotation was artificially sped up, a moon could survive for up to 4.5 billion years, but this is unlikely given Venus' current slow rotation.
The study does not confirm that Venus once had a moon, and finding physical evidence would be challenging due to the planet's geologically young surface, which has undergone significant volcanic and tectonic activity. Any remnants of a moon may be buried beneath the surface. Seismological surveys on Earth have provided insights into the moon's formation, and similar studies on Venus could potentially reveal traces of a swallowed moon.
The fate of Venus' moon could also offer insights into whether Venus had conditions suitable for life. The absorption of a moon could have transferred substantial energy and altered the planet's rotation, geology, and climate, impacting its evolutionary path.
This research also has implications for the search for habitable exoplanets, as it suggests that planets with slower rotation speeds may be less likely to retain moons, which could influence their potential for supporting life. Kane remarked, "My feeling is there are benefits to having a moon, but it isn’t required for habitability."
The findings indicate that even Earth-like exoplanets may not retain moons if their rotation speeds are insufficient, raising questions about their habitability.