Uranus’ cloud particle properties and latitudinal methane variation from IRTF SpeX observations

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

“…This is necessary to lower the single-scattering albedo sufficiently to reduce the reflectivity of the peaks at k > 1 lm relative to those at k < 1 lm. This adjustment to the single-scattering albedo is very similar to that empirically arrived at by Irwin et al (2010) and later by Tice et al (2013). However, in this case we have achieved this result with a (1) self-consistent scattering property retrieval (green), (2) original T2013 model with empirically determined scattering properties (blue) and (3) original T2013 model with empirically determined scattering properties, but using the Spitzer temperature-abundance profile (red).…”

“…The a priori real refractive index spectra are shown by the dot-dash lines and the inferred Kramers-Kronig spectrum by the solid line. The cross symbols show the imaginary refractive indices necessary to reproduce the single-scattering albedo spectra inferred for the TC by Tice et al (2013). Comparing the size of the error bars we can see that the imaginary refractive index of the TC particles is well constrained in the absorption peaks at 1.3 and 1.6 lm, but is less well constrained elsewhere.…”

“…Most recently, Tice et al (2013) analysed IRTF/SpeX observations of Uranus made in 2009 and attempted to determine cloud parameters that could be applied over the entire SpeX range of 0.8-1.8 lm using both compact and vertically extended clouds/hazes. Tice et al (2013) found a variation of methane abundance with latitude that was consistent with Karkoschka and Tomasko (2009), but also found that the particles in the main cloud deck at 2-3 bars must be significantly more absorbing at longer wavelengths with the single-scattering albedo varying from 1.0 for k < 1.0 lm to 0.7 for wavelengths longer than 1.4 lm, a conclusion previously reached from UKIRT/UIST observations by Irwin et al (2010). In this paper we attempt to reconcile the conclusions of Tice et al (2013) with that of a reanalysis of HST/STIS observations by Sromovsky et al (2011), who approached the problem from the perspective of first determining a vertical profile of methane (and H 2 , He and Ne) that was consistent with both the Voyager 2 radio-occultation observations (Lindal et al, 1987) and also cloud and haze vertical distributions inferred from observations in the visible and near-IR.…”

“…Reanalysis of IRTF spectra with a simple two-cloud model Tice et al (2013) (henceforth T2013) analysed SpeX long-slit spectra of Uranus, recorded in 2009 at NASA's Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. The slit was aligned with Uranus' central meridian and spectra recorded from 0.8 to 1.8 lm with a spectral resolution of 1200 and with an average 'seeing' that varied from 0.5 00 in the H-band (1.4-1.8 lm) to 0.6 00 in the I-band (0.8-0.9 lm).…”

“…Hence, particles with higher values of n r seem less able to reproduce the peak radiances at lower wavelengths and thus our analysis favours solutions with lower values of n r . Table 1 Derived scattering properties of the Tropospheric Cloud and Tropospheric Haze using the 2-cloud Tice et al (2013) model, with the a priori imaginary refractive index, n i , set to 0.001 ± 50% at all wavelengths. The imaginary part of the refractive index spectrum (n i ) is retrieved by the model and a Kramers-Kronig analysis used to construct the real part (n r ).…”