15.11-P

Winds of Change on Uranus?

H. B. Hammel (MIT)

When Voyager 2 flew by Uranus in 1986, it detected discrete cloud features whose motions yielded the first reliable determination of atmospheric rotation periods. These, with measurements of the magnetic field's 17.24-hr period, allowed a first estimate of Uranus' zonal wind profile (Smith et al. 1986, Science 233, 43). Features were only measured in the southern hemisphere, and the profile was assumed to be zonally symmetric and temporally stable.

Now, HST imaging of Uranus in 1994 (Zellner et al. 1994, BAAS 26, 1163) has revealed several discrete cloud features, allowing the first measurements of wind velocities since the Voyager Encounter. Because the images were taken to search for faint satellites, the disk of Uranus was over-exposed in most of the images. However, three useful 12-sec images of Uranus were obtained through the F791W filter (7826 Å, FWHM=1205 Å) on 14 August 1994. In the images, Uranus' 3.7-arcsec diameter extended over 80 Planetary Camera pixels.

The velocities for the two HST features (-47 49 m/s at latitude   ; +38 39 m/s at latitude ) do not appear to match velocities of Voyager features near those latitudes (closer to 30 m/s and 140 m/s respectively), suggesting either a change or more complex structure in the zonal wind profile. Current theoretical work suggests that Uranus may have an asymmetric and time-variable zonal wind profile (Dowling et al. 1997, in preparation), unlike the profiles seen on other giant planets which are both zonally symmetric and stable with time. The HST velocities reported here may be the first observational hint of changing winds on Uranus.

Additional HST images of Uranus optimized for zonal wind measurements are planned for Summer 1997. If Uranus' variable winds are confirmed, they may be indicative of unusual atmospheric dynamics created by the tilt of the planet's rotation axis.

This analysis was supported by internal MIT research funds.