Rings of Uranus: Not so thick,not so black

The optical thickness of the rings of Uranus has been thoroughly measured by many stellar occultations. However, we show that the optical thickness so obtained is larger by an extinction efficiency factor of 2 than the fractional area physically filled by particles which is commonly used to infer both particle reflectivity and particle size. By neglecting this factor, previous work has overestimated particle packing density and therefore underestimated individual particle reflectances and sizes (as well as overestimated collision frequencies). This has led to concern as to why the ring particles seemed unusually black and small. We present new estimates of particle reflectance which include both this effect and an improved radiative transfer treatment, and show that the ring particles, while still quite dark, are no longer mysteriously so. Particle sizes, while not strongly constrained, could easily lie in the macroscopic size range characteristic of other planetary rings.