The methane distribution and polar brightening on Uranus based on HST/STIS, Keck/NIRC2, and IRTF/SpeX observations through 2015

Abstract

HST/STIS observations of Uranus in 2015 show that the depletion of upper tropospheric methane has been relatively stable and that the polar region has been brightening over time as a result of increased aerosol scattering. This interpretation is confirmed by near-IR imaging from HST and from the Keck telescope using NIRC2 adaptive optics imaging. Our analysis of the 2015 spectra, as well as prior spectra from 2012, shows that there is a factor of three decrease in the effective upper tropospheric methane mixing ratio between 30°N and 70°N. The absolute value of the deep methane mixing ratio, which probably does not vary with latitude, is lower than our previous estimate, and depends significantly on the style of aerosol model that we assume, ranging from a high of 3.5$\pm$0.5% for conservative non-spherical particles with a simple Henyey-Greenstein phase function to a low of 2.7%$\pm$0.3% for conservative spherical particles. Our previous higher estimate of 4$\pm$0.5% was a result of a forced consistency with occultation results of Lindal et al. (1987, JGR 92, 14987-15001). That requirement was abandoned in our new analysis because new work by Orton et al. (2014, Icarus 243, 494-513) and by Lellouch et al. (2015, Astron. & AstroPhys. 579, A121) called into question the occultation results. For the main cloud layer in our models we found that both large and small particle solutions are possible for spherical particle models. At low latitudes the small-particle solution has a mean particle radius near 0.3 $\mu$m, a real refractive index near 1.65, and a total column mass of 0.03 mg/cm${}^2$, while the large-particle solution has a particle radius near 1.5 $\mu$m, a real index near 1.24, and a total column mass 30 times larger. The pressure boundaries of the main cloud layer are between about 1.1 and 3 bars, within which H${}_2$S is the most plausible condensable.

arXiv commentThere are 55 pages, 36 figures, 13 tables, and supplemental information. Version 2 corrects labeling errors in two figures (25L and 34), corrects an error in the referenced index of refraction of H2S, corrects associated comparisons with fitted values in several sentences, and adds a new conclusion paragraph to better summarize the comparison

References in corpus (8)

• 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
• Methane depletion in both polar regions of Uranus inferred from HST/STIS and Keck/NIRC2 observations
• Mid-Infrared Spectroscopy of Uranus from the Spitzer Infrared Spectrometer: 2. Determination of the Mean Composition of the Upper Troposphere and Stratosphere
• Uranus at equinox: Cloud morphology and dynamics
• Accurate and Approximate Calculations of Raman Scattering in the Atmosphere of Neptune
• Post-equinox dynamics and polar cloud structure on Uranus
• Reanalysis of Uranus' cloud scattering properties from IRTF/SpeX observations using a self-consistent scattering cloud retrieval scheme

Cited by in corpus (18)

• Deep Atmosphere Composition, Structure, Origin, and Exploration, with Particular Focus on Critical in situ Science at the Icy Giants
• Latitudinal variation in the abundance of methane (CH4) above the clouds in Neptune's atmosphere from VLT/MUSE Narrow Field Mode Observations
• Atmospheric chemistry on Uranus and Neptune
• Ice Giant Circulation Patterns: Implications for Atmospheric Probes
• Uranus in Northern Mid-Spring: Persistent Atmospheric Temperatures and Circulations Inferred from Thermal Imaging
• Uranus and Neptune are key to understand planets with hydrogen atmospheres
• Latitudinal variation of methane mole fraction above clouds in Neptune's atmosphere from VLT/MUSE-NFM: Limb-darkening reanalysis
• Microphysics of Water Clouds in the Atmospheres of Y Dwarfs and Temperate Giant Planets
• Dynamics and Clouds in Planetary Atmospheres from Telescopic Observations
• Evidence of a Polar Cyclone on Uranus from VLA Observations
• Deciphering Sub-Neptune Atmospheres: New Insights from Geochemical Models of TOI-270 d
• Mid-Infrared Observations of the Giant Planets
• Radiative-convective models of the atmospheres of Uranus and Neptune: heating sources and seasonal effects
• Storms and convection on Uranus and Neptune: impact of methane abundance revealed by a 3D cloud-resolving model
• Photochemical Haze Formation on Titan and Uranus: A Comparative Review
• Structure of Jupiter's High-Latitude Storms: Folded Filamentary Regions Revealed by Juno
• Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including Dark Spots
• Late gas released in the young Kuiper belt could have significantly contributed to the carbon enrichment of the atmospheres of Neptune and Uranus