New Measurements of the Winds of Uranus

Hubble Space Telescope imaging of Uranus in 1994, 1997, 1998, and 2000 revealed 13 cloud features, allowing the first measurements of wind velocities at northern latitudes not accessible to the Voyager cameras and new measurements of southern-latitude wind velocities determined during the 1986 Voyager encounter. Images acquired with the Keck 10-meter telescope adaptive optics system in June 2000 also showed some of the same features. Wind speeds inferred from feature motions—along with additional measurements by Karkoschka (1998, Science 280 ) and Voyager measurements (Smith et al . 1986, Science 246 )—indicate a zonal wind profile that is asymmetric with respect to the equator. Small but consistent differences are seen between the newer data and a profile determined from Voyager data: nearly all the newer measurements have slightly slower velocities than expected. We cannot yet determine whether the source of these differences is a slowly changing Uranian wind profile or subtle latitudinal structure in a temporally constant profile. If Uranus' winds vary with time, this may indicate unusual atmospheric dynamics created by the 98° tilt of the planet's rotation axis.