Both Uranus and Neptune are thought to have strong zonal winds with
velocities of several hundred meters per second. These wind velocities,
however, assume solid-body rotation periods based on Voyager 2 measurements of
periodic variations in the planets' radio signals and of fits to the planets'
magnetic fields; 17.24h and 16.11h for Uranus and Neptune, respectively. The
realization that the radio period of Saturn does not represent the planet's
deep interior rotation and the complexity of the magnetic fields of Uranus and
Neptune raise the possibility that the Voyager 2 radio and magnetic periods
might not represent the deep interior rotation periods of the ice giants.
Moreover, if there is deep differential rotation within Uranus and Neptune no
single solid-body rotation period could characterize the bulk rotation of the
planets. We use wind and shape data to investigate the rotation of Uranus and
Neptune. The shapes (flattening) of the ice giants are not measured, but only
inferred from atmospheric wind speeds and radio occultation measurements at a
single latitude. The inferred oblateness values of Uranus and Neptune do not
correspond to bodies rotating with the Voyager rotation periods. Minimization
of wind velocities or dynamic heights of the 1 bar isosurfaces, constrained by
the single occultation radii and gravitational coefficients of the planets,
leads to solid-body rotation periods of ~16.58h for Uranus and ~17.46h for
Neptune. Uranus might be rotating faster and Neptune slower than Voyager
rotation speeds. We derive shapes for the planets based on these rotation
rates. Wind velocities with respect to these rotation periods are essentially
identical on Uranus and Neptune and wind speeds are slower than previously
thought. Alternatively, if we interpret wind measurements in terms of
differential rotation on cylinders there are essentially no residual
atmospheric winds.Comment: Accepted for publication in Icarus, 20 pages, 4 tables, 9 figure
