The 2 spacecraft have produced a lesscontinuous record in the lower clouds because the longer-wavelength infrared instruments capable of observing at this altitude (the infrared channel of VIRTIS on Venus Express and the IR2 camera on Akatsuki) both failed after fulfilling their expected lifetimes. VIRTIS studied the clouds from 2006 to 2008, finding that the wind speeds did not vary with latitude during that time period (although they did vary with time). However, when Akatsuki observed the lower clouds in 2016, Takeshi Horinouchi of Hokkaido University discovered that a jet with much faster wind speeds is sometimes present at the equator. This recurrent jet might explain the billows and vortices observed in the lower clouds. We combined Venus Express and Akatsuki data with low-cloud data from Galileo, NASA’s Pioneer Venus entry probes, Soviet VEGA balloons, and numerous groundbased observations to reconstruct the equatorial wind speeds from 1978 to 2017 and found that their speed has varied by more than 100 kilometers (60 miles) per hour in that time.

To get a more complete picture of the 3-dimensional circulation of Venus’ winds through all levels of the nightside and dayside of Venus, we had help from the worldwide campaign of observations organized for the second Venus flyby of MESSENGER in June 2007. We performed feature tracking on images from Venus Express, MESSENGER, and amateur observations with small telescopes. We obtained Doppler wind speeds from spectra taken at large, ground-based telescopes, and we predicted wind speeds using temperature measurements from orbit and ground. We learned that the atmospheric circulation on the nightside of Venus is surprisingly different from the dayside.

With VIRTIS, we were able to image the upper-level clouds on the nightside with high resolution for the very first time. Our measurements revealed that superrotation on the nightside, rather than being homogeneous like on the dayside, sometimes becomes chaotic, with wind speeds as slow as 70 kilometers (43 miles) per hour in some places. VIRTIS was also able to observe a particular emission from oxygen molecules that occurs at an altitude of about 100 kilometers (60 miles) on the nightside well above the clouds, where superrotation is supposed to vanish. Ricardo Hueso (UPV/EHU, Spain) and Dmitry Gorinov (Space Research Institute, Russia) performed enormously challenging tracking of these fast airglow emissions and confirmed that at this altitude, the east-to-west superrotation is replaced by another mean circulation called “solar to antisolar.” As is hypothesized to happen on hot Jupiter exoplanets tidally locked to their stars, Venus’ highest-altitude winds blow from the area where solar insolation is maximum (the subsolar point) to the antipode at the antisolar point.

What's Next?

In the short term, worldwide campaigns of observations are being prepared to coordinate with the Venus flybys of the ESA/JAXA Mercury mission BepiColombo in October 2020 and August 2021 and probably also NASA’s Parker Solar Probe in December of this year. However, despite all we have learned from more than 60 years of spacecraft study, there are critical gaps in our knowledge of Venus’ atmosphere.

We know almost nothing about the deep atmosphere of Venus below 40 kilometers (25 miles), which contains more than 75 percent of the total mass of the atmosphere and controls the interaction between the surface and the atmosphere. Only in situ measurements from probes and landers can provide information for this region. Descent probes and short-lived (1 hour) and long-lived (1 year) landers have been proposed; more futuristic projects consider low-altitude balloons or surface rovers. New missions capable of making in situ measurements after a gap of more than 30 years will likely change our view of the planet all over again. After all, Venus’ appearances are deceptive.