The photometric roughness of Ariel is not unusual

Abstract

Contrary to initial reports, the large-scale roughness parameter θ for Ariel is not significantly higher than for the other Uranian satellites. For the whole-disk phase curve a value of θ near 27° is appropriate, in agreement with the recent work of B. J. Buratti et al. (1990, Icarus 84, 203–214). The earlier value of 42° is definitely spurious and resulted from an inaccurate determination of I/ F in Voyager 2 image FDS 26903.29 (clear filter; phase angle = 144.7°). Hapke parameters derived from the corrected whole-disk phase curve agree well with those determined from an analysis which incorporates disk-resolved photometry (brightness distribution across the satellite's disk at different phase angles). The experience underscores the extreme sensitivity of the θ parameter to high phase angle data.