Why does Venus have no moon? Scientists have put forward a new explanation

Venus may have had a moon which, over time, changed its direction of motion and fell back onto the planet. A new computer model shows that a second major collision was not necessarily required for its disappearance.

The authors of the study consider the most likely scenario to be one in which the moon formed following an ancient impact, initially moving away from the rapidly rotating Venus, and then began to approach it. However, the study has so far only been published as a preprint and does not prove that such a moon actually existed.

Details

Venus is similar to Earth in size and mass, but rotates very slowly and in the opposite direction to most planets in the Solar System. Furthermore, it has no natural satellite. Scientists are discussing three main explanations: Venus never formed a moon, it was destroyed by tidal forces, or it disappeared following a major collision at a later stage.

Stephen R. Kane of the University of California, Riverside, and his colleagues investigated the second possibility. They calculated the joint evolution of Venus’s rotation and the orbit of a hypothetical moon over a period of 4.5 billion years.

In the model, they varied Venus’s initial rotation period—from 5 to 100 hours—the moon’s mass—from one hundredth to ten times that of Earth’s Moon—the distance from the planet, the orbital eccentricity and the intensity of internal tidal friction. The researchers used two models of tidal evolution, which describe the behaviour of the system near a synchronous orbit in different ways.

When the planet rotates rapidly, tidal interactions can transfer energy to the moon and push it outwards, as happens with Earth’s Moon. However, the Sun simultaneously slows down Venus’s rotation. As a result, the boundary of the synchronous orbit gradually shifts outwards; it may catch up with the moon and alter the direction of its migration.

Following such a reversal, the satellite begins to move towards Venus. Upon crossing the Roche limit, tidal forces tear it apart, and the material from the temporary ring may then fall onto the planet’s surface. In some of the simulated scenarios, this occurred between 30 million and 1.7 billion years after the moon’s formation.

The outcome depended heavily on the initial conditions. In some calculations, a moon of approximately lunar mass near a rapidly rotating Venus persisted for the entire 4.5 billion years. The loss of the moon, together with the planet’s deceleration, occurred only within a limited range of parameters—those most consistent with Venus’s moderate rotation rate following the ancient impact and a moon of lunar mass or greater.

Why this is important

The model offers an explanation in which the absence of a moon is a natural outcome of tidal evolution, rather than the result of a separate, later catastrophe. If young Venus did indeed have a moon, its existence could have influenced the axial tilt, tides, internal heating and surface conditions of the planet.

This also helps to explain why Earth and Venus, which are similar in size, took different paths. The preservation of Earth’s Moon and the possible early loss of Venus’s moon could have been one of the turning points in the history of the two planets.

However, the conclusion remains theoretical. The parameters of Venus’s internal tidal friction are poorly understood; the two models used yield different results for certain fast-moving and massive moons; and no direct geological evidence of an ancient moon has yet been found. The calculations show that the proposed scenario is possible, but do not establish the planet’s actual history.

The authors note that the results may also be useful for studying rocky exoplanets close to their stars: slowly rotating worlds in conditions similar to those on Venus may lose their moons faster than their planetary system has existed.

Source

Stephen R. Kane, Franck Selsis, Jérémy Leconte and Sean N. Raymond, ‘Tidal Demise: The Evolution and Fate of a Hypothetical Venus Moon’, arXiv, 2026. DOI: 10.48550/arXiv.2608.25036. This paper is a preprint and has not yet been peer-reviewed by a scientific journal.

The original article was published on Phys.org. Photograph of Venus: NASA/JPL-Caltech, Mariner 10 spacecraft, public domain.