The planet Venus is covered by thick clouds of sulfuric acid that move westwards because the entire upper atmosphere rotates much faster than the planet itself. At the cloud tops, about 65 km in altitude, small-scale features are predominantly carried by the background wind at speeds of approximately 100 m s−1. In contrast, planetary-scale atmospheric features have been observed to move slightly faster or slower than the background wind, a phenomenon that has been interpreted to reflect the propagation of planetary-scale waves. Here we report the detection of an interhemispheric bow-shaped structure stretching 10,000 km across at the cloud-top level of Venus in middle infrared and ultraviolet images from the Japanese orbiter Akatsuki. Over several days of observation, the bow-shaped structure remained relatively fixed in position above the highland on the slowly rotating surface, despite the background atmospheric super rotation. We suggest that the bow-shaped structure is the result of an atmospheric gravity wave generated in the lower atmosphere by mountain topography that then propagated upwards. Numerical simulations provide preliminary support for this interpretation, but the formation and propagation of a mountain gravity wave remain difficult to reconcile with assumed near-surface conditions on Venus. We suggest that winds in the deep atmosphere may be spatially or temporally more variable than previously thought.
Figures
![Brightness temperature and UV brightness of the Venus disk.]()
Figure 1: Brightness temperature and UV brightness of the Venus disk. a–e, Sequence of brightness temperature distributions obtained by LIR from 7 December to 11 December 2015. The equator and evening terminator are shown by solid and dashed lines, respectively. The colour bar is valid only for a; the temperature ranges for b–e are adjusted so that the mean temperatures in a circle with a radius of 0.1 RV at the disk centre are constant, where RV is the Venus radius. f, UV brightness image obtained by UVI at a wavelength of 283 nm.
![High-pass-filtered brightness temperature and UV brightness at the cloud top of Venus.]()
Figure 2: High-pass-filtered brightness temperature and UV brightness at the cloud top of Venus. a–c, High-pass-filtered brightness temperature distributions derived from three successive shots by LIR with an interval of a few hours. The surface topography is overlaid by contour lines in a with units of km. Altitudes below 0 km are indicated by white dashed lines. d–f, UV brightness distributions acquired by UVI at a wavelength of 283 nm. The four small circles in each frame indicate the displacement of air parcels advected westwards at a zonal speed of −96 m s−1.
![Disturbance temperature field at 65[thinsp]km altitude associated with a stationary gravity wave calculated by a numerical model.]()
Figure 3: Disturbance temperature field at 65 km altitude associated with a stationary gravity wave calculated by a numerical model. The model solves upward propagation of a gravity wave packet excited by a stationary geopotential disturbance at the bottom boundary of 10 km altitude. The forcing is located at the centre of this plot is indicated by the cross, and has a half-width of ~6° in longitude and latitude. The direction of the background zonal wind (super rotation) is from right to left.
![Disturbance temperature field at 65[thinsp]km altitude associated with a stationary gravity wave calculated by a numerical model.](http://www.nature.com/ngeo/journal/vaop/ncurrent/carousel/ngeo2873-f3.jpg)