Venus's spin changes speed as its dense, fast-flowing atmosphere interacts with mountains on the planet's surface, reports a study published online this week in Nature Geoscience.

Venus rotates slowly, with one revolution taking about 243 Earth days. However, measurements by visiting spacecraft have not agreed on the precise length of a Venusian day. One reason might be the giant bow-shaped atmospheric structure recently spotted appearing and disappearing by the Akatsuki spacecraft. Despite the Venusian atmosphere moving much faster than the planet itself (completing one rotation in four Earth days), the structure remained stationary above a mountainous region. It has been suggested that this structure was an atmospheric wave, caused by the lower atmosphere rising over mountain topography If correct, the atmosphere and solid planet might be more closely linked than originally thought.

To test this hypothesis, Thomas Navarro and colleagues simulate the circulation of the Venusian atmosphere. They find that the bow-shaped structure can indeed be explained by atmospheric waves forming over the mountains, with the waves only forming in the afternoon and vanishing by dusk. The authors also find that the formation of these waves causes atmospheric pressure fluctuations that actually change the rotation rate of the solid planet, depending on the time of day.

The authors find that this effect is small - a change that would alter the length of a Venusian day by only a couple of minutes - but this interplay between the solid planet and its atmosphere may explain at least a part of the discrepancies between past measurements of Venus’s rotation rate.