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

Abstract

We 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 to 1.8 microns and reported by Tice et al. (2013). By retrieving the imaginary refractive index spectra of cloud particles we are able to consistently define the real part of the refractive index spectra, through a Kramers-Kronig analysis, and thus determine self-consistent extinction cross-section, single-scattering and phase-function spectra for the clouds and hazes in Uranus' atmosphere. We tested two different cloud-modelling schemes used in conjunction with the temperature/methane profile of Baines et al. (1995), a reanalysis of the Voyager-2 radio-occultation observations performed by Sromovsky, Fry and Tomasko (2011), and a recent determination from Spitzer (Orton et al., 2014). We find that both cloud-modelling schemes represent the observed centre-of-disc spectrum of Uranus well, and both require similar cloud scattering properties of the main cloud residing at approximately 2 bars. However, a modified version of the Sromovsky, Fry and Tomasko (2011) model, with revised spectral properties of the lowest cloud layer, fits slightly better at shorter wavelengths and is more consistent with the expected vertical position of Uranus' methane cloud. 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 > 1.0 micron than they are at wavelengths < 1.0 micron. This spectral information provides a possible constraint on the identity of the main particle type.

arXiv comment52 Pages, 14 Figures

References in corpus (2)

• Methane on Uranus: The case for a compact CH4 cloud layer at low latitudes and a severe CH4 depletion at high-latitudes based on re-analysis of Voyager occultation measurements and STIS spectroscopy
• Mid-Infrared Spectroscopy of Uranus from the Spitzer Infrared Spectrometer: 1. Determination of the Mean Temperature Structure of the Upper Troposphere and Stratosphere

Cited by in corpus (14)

• Aerosols in Exoplanet Atmospheres
• Probable detection of hydrogen sulphide (H${}_2$S) in Neptune's atmosphere
• Developing Atmospheric Retrieval Methods for Direct Imaging Spectroscopy of Gas Giants in Reflected Light I: Methane Abundances and Basic Cloud Properties
• The methane distribution and polar brightening on Uranus based on HST/STIS, Keck/NIRC2, and IRTF/SpeX observations through 2015
• Latitudinal variation in the abundance of methane (CH4) above the clouds in Neptune's atmosphere from VLT/MUSE Narrow Field Mode Observations
• ExoReL${}^{\Re }$: A Bayesian Inverse Retrieval Framework For Exoplanetary Reflected Light Spectra
• HST/WFC3 Observations of Uranus' 2014 storm clouds and comparison with VLT/SINFONI and IRTF/SpeX observations
• How Does Thermal Scattering Shape the Infrared Spectra of Cloudy Exoplanets? A Theoretical Framework and Consequences for Atmospheric Retrievals in the JWST era
• Spectral analysis of Uranus' 2014 bright storm with VLT/SINFONI
• Analysis of gaseous ammonia (NH${}_3$) absorption in the visible spectrum of Jupiter - Update
• Aerosols and Methane in the Ice Giant Atmospheres Inferred from Spatially Resolved, Near-Infrared Spectra: I. Uranus, 2001-2007
• Cloud Parameterizations and their Effect on Retrievals of Exoplanet Reflection Spectroscopy
• Radiative-convective models of the atmospheres of Uranus and Neptune: heating sources and seasonal effects
• Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including Dark Spots