Reanalysis of Uranus' cloud scattering properties from IRTF/SpeX observations using a self-consistent scattering cloud retrieval scheme

We find that the bulk of the reflected radiance from Uranus arises from a thick cloud at approximately the 2 bar level, composed of particles that are significantly more absorbing at wavelengths λ > 1.0 μm than they are at shorter wavelengths λ < 1.0 μm. This spectral information provides a possible constraint on the identity of the main particle type, although we find that the scattering properties required are not consistent with any of the available laboratory data for pure NH3, NH4SH, or CH4 ice (all suspected of condensing in the upper troposphere). It is possible that the observed clouds are mixtures of tropospheric condensate mixed with photochemical products diffusing down from above, which masks their pure scattering features. Because there is no available laboratory data for pure H2S or PH3 ice (both of which might be present as well), they cannot be excluded as the cloud-forming species. We note, however, that their absorptive properties would have to be two orders of magnitude greater than the other measured ices at wavelengths greater than 1 μm to be consistent with our retrieval, which suggests that mixing with photochemical products may still be important.