Neptune’s rotational period suggested by the extraordinary stability of two features

The interior rotation and motions in giant planets have generally been probed only at radio wavelengths from spacecraft near the planet, except for Jupiter’s radio emission detectable from Earth. Here I suggest that Neptune’s interior can be indirectly probed at visible wavelength by tracking 10 features that are connected with a stationary latitudinal speed pattern of 7 m/s amplitude. All 10 features remained aligned at the same longitude throughout the Voyager observation period in 1989. Two of them, the South Polar Wave and South Polar Feature, have been observed from Earth for ∼20 years, but their extraordinary rotational stability was never recognized. They probably pinpoint Neptune’s rotational period (15.9663 ± 0.0002 h), one of the largest improvements in 346 years of measuring the giant planets’ rotations. The previous best estimate of Neptune’s rotational period (16.108 ± 0.006 h) was based on Voyager 2 radio data (Lecacheux, A., Zarka, P., Desch, M.D., Evans, D.R. [1993]. Geophys. Res. Lett. 20, 2711–2714). The new result suggests an upward revision of the mass of Neptune’s core. This finding may also question the accepted value of Uranus’ rotational period. The first reliable wind measurements within 15° of Neptune’s South Pole, based on tracking four features in Voyager images, show a 300 m/s eastward jet peaking near 76° South, while the area within 4° of the South Pole seems to be rotationally locked to the interior. These new observations of the stationary features and winds could address the long-standing question about the depth of the atmospheric circulation and may allow some constraints on convection currents in Neptune’s interior.