🌕 Venus may have destroyed and then swallowed its own moon

Venus may not always have been alone: its own moon could have ended up torn apart and then absorbed by the planet.

Earth and Venus have similar sizes and masses, but one difference is immediately obvious. Our planet has a large moon, whereas Venus has none. Venus also rotates extremely slowly on its axis, with a day lasting longer than a year.

Researchers tested a simple idea that could explain these observations: a moon may have formed around Venus and then disappeared.

To understand this scenario, we need to look at tides as something more than the movement of oceans. A moon’s gravity slightly deforms its planet, and this deformation exchanges energy with its orbit. On Earth, this exchange slowly pushes the Moon away. But the result depends strongly on the planet’s rotation speed.

The team simulated moons of varying masses around a young Venus rotating at different speeds. In many cases, the moon initially moves away. Then Venus’s rotation slows and the situation changes. A particular boundary then moves outward until it catches up with the satellite’s orbit.

From that point on, the motion can reverse. The moon stops gaining altitude and begins falling back toward Venus. As it gets closer, the forces exerted between its near side and far side become increasingly different. It may eventually be pulled apart before its debris falls back onto the planet.

The outcome depends greatly on the initial conditions. With an initial Venusian day shorter than about 12 hours, a moon close to the mass of ours can sometimes survive for several billion years. Above about 15 hours, some models instead lead to its destruction. A very massive moon can also accelerate Venus’s slowing and hasten its own fall.

This possibility is interesting because the large impacts common during the Solar System’s youth can produce satellites around rocky planets.

Direct traces would be difficult to find after several billion years. However, a massive disruption could have altered the composition of the surface or atmosphere. Future measurements of Venus may therefore test some indirect effects of this scenario, and astronomers may also investigate whether slowly rotating planets around other stars are less likely to possess large moons.