Equatorial jet in the lower to middle cloud layer of Venus revealed by Akatsuki

摘要

The Venusian atmosphere is in a state of superrotation where prevailingwestward winds move much faster than the planet's rotation. Venus is coveredwith thick clouds that extend from about 45 to 70 km altitude, but thermalradiation emitted from the lower atmosphere and the surface on the planet'snightside escapes to space at narrow spectral windows of the near-infrared. Theradiation can be used to estimate winds by tracking the silhouettes of cloudsin the lower and middle cloud regions below about 57 km in altitude. Estimatesof wind speeds have ranged from 50 to 70 m/s at low to mid-latitudes, eithernearly constant across latitudes or with winds peaking at mid-latitudes. Herewe report the detection of winds at low latitude exceeding 80 m/s using IR2camera images from the Akatsuki orbiter taken during July and August 2016. Theangular speed around the planetary rotation axis peaks near the equator, whichwe suggest is consistent with an equatorial jet, a feature that has not beenobserved previously in the Venusian atmosphere. The mechanism producing the jetremains unclear. Our observations reveal variability in the zonal flow in thelower and middle cloud region that may provide clues to the dynamics of Venus'satmospheric superrotation.