Venus is covered by a thick blanket of clouds that prevents direct observation of its surface. Most of the planet's characteristics, including its rotation, therefore remained a mystery until the early twentieth century, until the advent of radar observations and space missions.
Based on the similarities found up to then with the Earth, it was believed that Venus had a rotation period of a few hours, comparable to that of our planet.
Spectroscopic observations conducted in the early twentieth century, however, did not reveal any Doppler shift. It was therefore assumed that Venus rotated much slower than expected.
The confirmation came between the 1950s and 1960s, when radar observations confirmed that the planet's rotation period is 243 Earth days.
It was also discovered that Venus rotates on itself in a retrograde direction, therefore in the opposite direction to that of all the other planets in the Solar System (with the exception of Uranus), which instead have prograde rotation.
There are several hypotheses to explain the retrograde rotation of Venus. None of these have been universally accepted by the scientific community, but they all start from the same assumption, that is, that Venus at the time of its formation had a progressive rotation and with a period comparable to that of the other planets.
Following a series of events, the rotation of Venus then slowed down more and more, until it became retrograde and assumed the current period.
This could be due to the impact with a large asteroid or a protoplanet, which hit Venus in its early stages of formation.
Another theory suggests that the rotation of Venus has been slowed down due to the tidal interactions between the Sun, its dense atmosphere, its core and its mantle.
Credit: NASA, JPL.
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