Why doesn't Venus have a moon like Earth? This is an age-old question in planetary science, because Venus is similar to Earth in size, mass, and composition, yet it orbits without natural satellites. Previous explanations ranged from the possibility that Venus never experienced a large-scale collision that formed a moon, to the possibility that it once had a moon and lost it in a violent event. the planet did not need a subsequent catastrophe to lose its moon, but rather Venus's slow rotation was enough to gradually push a hypothetical moon toward the planet.
Researchers used computer models to test the evolution of the relationship between Venus's rotation and the orbit of a potential moon, starting with multiple scenarios for rotation speed, the moon's mass, and its orbit. However, the study does not prove that Venus actually had a moon; it explores what might have happened if such a moon had formed.
When the tide works in the opposite direction
On Earth, tidal forces transfer some of the planet's rotational energy to the Moon's orbit, causing our Moon to move away from Earth at a rate of about 4 centimeters per year. Venus, on the other hand, rotates once on its axis in about 243 days, in the opposite direction to the rotation of most planets.
In such a system, tidal interaction can work in the opposite direction, causing the moon to lose orbital energy and move inward instead of away.
To put it simply, tides act as a mechanism for energy exchange between a planet and a moon, and can change the distance between them over time. Kane and his colleagues tested models with initial rotational speeds ranging from 5 to 100 hours, and moons with masses ranging from 1% to 10 times that of Earth's moon, with variations in orbits and tidal interaction characteristics.
The researchers began by testing the model on the known Earth-Moon system before applying it to Venus scenarios to ensure the calculations reproduced known astronomical behavior. The results showed that the Moon's fate depends strongly on Venus's rotational speed, mass, and initial orbit.
A moon might fall on its planet
A collision was not inevitable in every scenario. Published results indicate that a moon with the mass of Earth's moon could survive the lifetime of the solar system if Venus rotated in 12 hours or less, while the loss of the moon becomes more likely when the rotation is slower or the moon's mass is large.
On Earth, the Earth rotates faster than the Moon, so tidal forces gradually push it outwards. If the Moon were to become faster than its planet's rotation, the opposite could happen: the Moon would lose orbital energy and slip inwards.
In some cases, the moon reaches the Roche limit, where it is torn apart by tidal forces, over a period of approximately 30 million to 1.7 billion years.
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