Venus and Earth share many planetary features, such as size, mass, and a similar rocky structure. However, Venus stands apart for one glaring difference: it has no moon.
A new study published in The Astrophysical Journal by UC Riverside astrophysicist Stephen Kane suggests that if Venus once had a moon, the planet’s slow rotation could have driven it inward until it eventually collided with the Venusian surface.
Theories About Venus’s Missing Moon
“My study shows Venus didn’t require a catastrophe to arrive at what we can see today,” Kane said. “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.”
One Drifts Away, Another Slips Closer
The Moon is moving away from Earth at a rate of about four centimeters per year. Scientists use mirrors installed on the Moon during the Apollo 11 mission to measure this distance by reflecting laser pulses back to Earth. The relatively rapid rotation of Earth (once every 24 hours) causes the Moon to drift slowly away.
Venus takes much longer to complete one rotation, requiring the equivalent of 243 Earth days. Since its rotation is so slow, Kane proposes that gravitational forces would cause a moon to spiral inward over time and eventually collide with the planet.
Running the Numbers
To explore this theory, Kane created computer models that simulated the gravitational interactions between the planets and their moons. First, he accurately reproduced the Earth-Moon system to check the model’s reliability. He then changed Venus’s rotational rate and examined hypothetical moons with masses ranging from half to ten times that of Earth’s moon. In most of the simulations, the moon eventually crashed into Venus, with more massive moons colliding with the planet even earlier.
“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.”
What the Study Doesn’t Settle
The study does not determine whether Venus ever had a moon. While Kane thinks a Venusian moon might have formed at one time, this conclusion is still uncertain. However, the modeling indicates that under many conditions, tidal evolution would eventually drive a Venusian moon into the planet.
Finding physical evidence of an ancient moon would be difficult. About 80% of the planet’s surface dates to the same period due to a major resurfacing event around one billion years ago that wiped out most of its earlier geological history. However, any evidence of a collision with an ancient moon may still be found deeper underneath the planet’s surface.
A Collision With Bigger Consequences
This kind of event could affect the planet’s rotation, geology, and climate. If Venus once had oceans or other conditions favorable to life, such an impact could have altered the planet’s subsequent evolution.
Astronomers searching for potentially habitable worlds around other stars often consider the presence of a moon when evaluating habitability. Kane isn’t convinced a moon is essential, though. “My feeling is there are benefits to having a moon, but it isn’t required for habitability,” he 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 research focuses more on which planets get to keep the moons they form in the first place. “When people think about Earth twins around other stars, one question they ask is, ‘Does it have a moon?'” he 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.”
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.