A huge stationary structure has been spotted in the fast-moving atmosphere of Venus by JAXA’s Akatsuki spacecraft, reports a study published online this week in Nature Geoscience. The finding is difficult to reconcile with the planet’s thick upper atmosphere, in which clouds streak by at 100 metres per second - much faster than the slowly rotating planet below, where a Venusian day lasts longer than it takes for the planet to circle the Sun.

Makoto Taguchi and colleagues report the detection of an enormous bow-shaped bright region stretching for 10,000 km across the upper atmosphere of Venus at the altitude of the cloud tops. After entering orbit around Venus at the end of 2015, Akatsuki observed the bow-shaped structure over several days. Curiously, the bright region did not move with the background atmospheric winds, but remained stationary above a mountainous region on the planet’s surface.

The authors suggest that the bright region, which is hotter in temperature than the surrounding atmosphere, is the result of a gravity wave generated in the lower atmosphere as it flows over mountain topography - a phenomenon similar to when air passes over mountains on Earth. Although it is unclear whether gravity waves induced by mountains can readily propagate upwards to the cloud tops of Venus, the observations suggest that the atmospheric dynamics of Venus are more complex at depth than previously appreciated.