The...wave is thought to form as the lower atmosphere flows over mountains on Venus' surface. ... Just after entering orbit around Venus in 2015, the Akatsuki orbiter observed a bow-shaped feature in the upper atmosphere over several days. Curiously, the bright structure - which stretched for 10,000km - remained stationary at the altitude of Venus' cloud tops. This is difficult to reconcile with what we know about Venus' thick upper atmosphere, in which clouds streak by at 100 metres per second (m/s). The clouds travel much faster than the slowly rotating planet below, where a Venusian day lasts longer than...