Temporal and Spatial Variation of the Geometric Albedo of Uranus, 1953-1996

G. W. Lockwood (Lowell Observatory)

Using photoelectic broadband B magnitudes of Uranus obtained annually from 1953 to 1966 by Serkowski and Jerzykiewicz, and Strömgren b, y magnitudes obtained from 1972 through 1996 by Thompson and me, I have assembled a lightcurve for Uranus suitable for investigating seasonal and intrinsic variability and the distribution of reflectivity with latitude. The B and b filters lie in the blue continuum at 440 and 472 nm, respectively, while the y filter at 551 nm includes the methane band at 546 nm. The 1953-1966 B observations cover sub-Earth latitudes from 60 N to 0. The 1972-1996 observations bracket the ``pole-on'' 1986 apparition and Voyager encounter, and include sub-Earth latitudes from 82S to 45S.

The 1972-1996 y lightcurve is asymmetrical. The rising branch prior to 1986 is steeper than the falling branch after 1986, and the discrepancy between its brightness in 1975 (sub-Earth latitude 43S) and 1996 (sub-Earth latitude 45S) is 0.024 mag. The corresponding b magnitudes for these two apparitions are identical.

I modeled the lightcurve using the known oblateness of Uranus (0.024) and the observed latitudinal variation of I/F obtained during the 1986 Voyager encounter (Smith et al., Science, 4 July 1986). The b lightcurve is reasonably consistent with the Voyager blue filter I/F distribution, but the y lightcurve indicates a stronger brightening toward the pole than shown by the Voyager green I/F curve.

This work is supported by NASA grant NAG5-3948.