The comparison with Earth is inevitable. Venus is about 95 % the diameter of Earth and has about 82 % of Earth’s mass. It is a rocky planet formed in the same region of the Solar System from similar materials. But while Earth ended up with an extraordinarily large moon, Venus is completely alone.
The question is, why?
A study led by Stephen R. Kane of the University of California, Riverside, along with Franck Selsis, Jeremy Leconte, and Sean N. Raymond, decided to approach the problem from a different angle. Instead of asking only how Venus could have lost a moon, the researchers asked a more fundamental question: If Venus had had one, could it have retained it for 4.5 billion years?
The results of their simulations are fascinating: in many scenarios, the answer is no.
The mechanism responsible is the tides. We usually think of them as the movement of the Earth’s oceans, but the phenomenon is much deeper. The gravity of a satellite slightly deforms the planet it orbits. That deformation and the planet’s rotation produce a continuous exchange of energy and angular momentum between the two bodies.
That's happening right now between us and the Moon.
The Earth completes one rotation approximately every 24 hours, much faster than it takes the Moon to complete its orbit. This difference allows angular momentum to be transferred from the Earth’s rotation to the Moon’s orbit. As a result, the Earth’s rotation slows down imperceptibly, and the Moon moves farther away by approximately four centimeters each year.
But that process does not necessarily have to continue in the same direction forever.
The new study simulated hypothetical Venus-moon systems by varying key parameters: an initial rotation period of Venus ranging from approximately 5 to 100 hours, different orbits and eccentricities, and moons ranging from just one-hundredth to ten times the mass of our Moon.
In addition, the authors used two different formulations of tidal dissipation to determine how much the results depended on the chosen model.
And that's when the problem arose.
As the tides caused by the moon slow the planet's rotation, a fundamental boundary known as synchronous radio. It is the orbital distance at which the satellite's orbital period matches the planet's rotational period.
Except under certain conditions, a moon can receive angular momentum from the planet and drift outward. But if the system's evolution causes the synchronous radius to reach and exceed the moon's orbit, the direction of the exchange changes.
The moon begins to lose orbital energy. And then the return journey begins.
It would not fall like a stone. Over millions of years, its orbit would gradually contract, bringing it progressively closer to Venus. The closer it got, the stronger the tidal forces would become, until it finally reached a region where the gravitational difference between the satellite’s near side and far side could exceed its own cohesion. This is the Roche limit.
The moon could then be shattered, temporarily creating a massive debris field around Venus and eventually depositing much of that material onto the planet. But the most surprising result of the research emerges when the moon's mass is increased.
Intuitively, we might think that a larger moon would be harder to destroy. Simulations show that the exact opposite can happen. A massive moon exerts stronger tidal forces on Venus and can more effectively extract angular momentum from its rotation. Venus slows down more quickly, the synchronous radius shifts, and it ends up catching up to the moon sooner. In certain scenarios, A larger moon hastens its own doom.
Calculations show that for initial rotation periods of Venus slightly longer than 15 hours, or for moons with more than about twice the mass of our Moon, destruction can occur on scales of approximately 30 million to 1.7 billion years, depending on the model and the initial conditions.
Thirty million years seem like an eternity from our perspective. For a planet that is about 4.5 billion years old, that’s practically a blip in its youth.
But here an essential distinction arises: The study does not prove that Venus had a moon.
The researchers modeled what would have happened if it existed. They even found scenarios in which it could have survived to the present day. With an early Venus rotating fast enough—approximately a day shorter than twelve hours in certain scenarios—a moon with a mass comparable to Earth’s could have drifted away and survived throughout the Solar System’s history.
That is why the research cannot be summarized simply by saying that any moon of Venus was doomed.
The interesting thing is actually the exact opposite: The current absence of a moon provides insight into how Venus might have rotated immediately after the massive impacts of its early history and what kind of moon might have formed.
There is also another mystery.
Venus currently rotates exceptionally slowly: it takes about 243 Earth days to complete one rotation, and it does so in a retrograde direction compared to most planets. Its year lasts about 225 days. In other words, a Venusian sidereal day lasts longer than a Venusian year.
The new study seeks to identify scenarios capable of simultaneously explaining two things: the disappearance of an ancient moon and Venus's evolution toward an extremely slow rotation.
And that dual requirement significantly limits the possibilities. That makes a hypothetical missing moon something much more important than just an astronomical curiosity. If it really existed and was eventually destroyed, it could have profoundly altered the evolution of Venus itself.
A moon falling toward a planet would carry enormous amounts of energy and angular momentum. Its destruction and subsequent accretion could affect the planet’s rotation, interior, and geological activity. Researchers even suggest that such an event could be relevant to reconstructing the climatic evolution of Venus, a world that may have had much milder conditions in its distant past. Finding evidence of this will be extraordinarily difficult.
Approximately 80 % of Venus's surface has relatively similar geological ages, a phenomenon related to a major episode of surface renewal that occurred hundreds of millions of years ago. Much of the earlier scar tissue may have been erased by volcanic activity and tectonic processes.
That's why some of the best clues may not be on Venus, but rather inside Venus.
On Earth, seismology allows us to study deep structures that may contain information about the violent events that occurred during the formation of our planet. Similar measurements on Venus could reveal internal anomalies consistent with major impacts or with the incorporation of enormous amounts of material from a former moon. And here, the problem is no longer limited to Venus alone.
We are discovering thousands of planets orbiting other stars and searching among them for Earth-like worlds. One of the recurring questions is whether they have moons, because moons can influence tides, rotation, orbital stability, and potentially the evolution of habitable conditions. The new study offers a word of caution: Creating a moon doesn't mean you can keep it.
A rocky planet may form with a moon and end up alone. Its own rotation, the moon's mass, tidal forces, and the gravity of its star can slowly transform a stable system over a period of hundreds of millions of years.
Venus may have been one of those worlds. We don’t yet know if it had a moon. Nor do we know if there are any recognizable remnants of it beneath its surface. But the new simulations reveal something we hadn’t previously needed to consider to explain its absence: Venus wouldn’t have needed another world to come along and steal its moon.
His own body may have caused him to disappear.
