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

Abstract

a b s t r a c tWe have developed a new retrieval approach to modelling near-infrared spectra of Uranus that represents a significant improvement over previous modelling methods. We reanalysed IRTF/SpeX observations of Uranus observed in 2009 covering the wavelength range 0.8-1.8 lm and reported by Tice et al. (Tice, D.S., Irwin,

Aerosols and methane in the ice giant atmospheres inferred from spatially resolved, near-infrared spectra: I. Uranus, 2001–2007

“…number density x extinction cross-section, per bar) for a cloud of particles with a single scattering albedo of and normalized phase function of 1.0. For a cloud located at several few bars, reasonable values based on the literature (Tice et al, 2013;Irwin et al, 2015), as discussed in Section 5.2, would suggest single scattering albedo of ~0.7 and a normalized phase function of ~ 0.4, yielding an optical depth ~3x our reported ' s bar -1 . Figure 6.…”

“…Particles much smaller than this produced less scattering at longer wavelengths and yielded marginally larger residuals; however, the errors in the K-band are large and the constraints are weak given that very little of the reflectance in the H-band originates from the same aerosols probed in the K-band. By including shorter wavelength observations, Irwin et al (2015) had found the scattering cross-sections of aerosols at these heights diminished more strongly in wavelength, consistent with mean particle sizes of only 0.1µm. As Fig.16 shows, using a refractive index suggested by Irwin et al (2015) for the tropospheric cloud, the wavelength dependence across the K-band alone is similar in shape for both particle sizes, with a magnitude that may be shifted based on other factors folded into the effective scattering optical depth; with the H-band dominated by scattering from different depths, our solutions are degenerate among different combinations of scattering parameters.…”

“…The shape of the curves in the K-band are similar for different particle sizes and thus are only weakly constraining in the absence of reflection in the H-band, which is dominated by scattering from greater pressures. Calculations are done assuming Mie theory and a refractive index of 1.38 + i0.075, as found appropriate for the lower tropospheric by Irwin et al (2015).…”

“…Though a variety of cloud models have been employed, there is a consensus that two or more vertical layers of aerosols are required to reproduce the observed reflectance-including, at least, a high tenuous haze above a more substantial cloud layer--within an atmosphere of increasing methane with depth (e.g. Neff et al, 1984;Baines and Bergstralh, 1986;Pollack et al, 1986;Sromovsky and Fry, 2007;Sromovsky et al, 2011;Irwin et al, 2009;Irwin et al 2010;Irwin et al, 2012a; Tice et al, 2013;Irwin et al, 2015). With recent improvements in methane absorption coefficients, near-infrared (NIR) radiative transfer studies have been particularly fruitful in characterizing cloud vertical structure, latitudinal variation, and aerosol properties (Irwin et al, 2010(Irwin et al, , 2012(Irwin et al, , 2015.…”

ExoReL : A Bayesian Inverse Retrieval Framework for Exoplanetary Reflected Light Spectra

“…Our cloud model is linked with a physical model that calculates cloud density and particle size. As presented in the literature, this is likely to bring a correlation between the VMR of methane and the cloud position (Irwin et al 2015;Lupu et al 2016). However, there might be cases in which this correlation is not significant as highlighted by Hu (2019), it depends on a combination of S/N, spectral resolution, and particular combinations of cloud position and methane VMR.…”