Hazy Blue Worlds: A Holistic Aerosol Model for Uranus and Neptune, Including Dark Spots

Abstract

We present a reanalysis (using the Minnaert limb‐darkening approximation) of visible/near‐infrared (0.3–2.5 μm) observations of Uranus and Neptune made by several instruments. We find a common model of the vertical aerosol distribution i.e., consistent with the observed reflectivity spectra of both planets, consisting of: (a) a deep aerosol layer with a base pressure >5–7 bar, assumed to be composed of a mixture of H2S ice and photochemical haze; (b) a layer of photochemical haze/ice, coincident with a layer of high static stability at the methane condensation level at 1–2 bar; and (c) an extended layer of photochemical haze, likely mostly of the same composition as the 1–2‐bar layer, extending from this level up through to the stratosphere, where the photochemical haze particles are thought to be produced. For Neptune, we find that we also need to add a thin layer of micron‐sized methane ice particles at ∼0.2 bar to explain the enhanced reflection at longer methane‐absorbing wavelengths. We suggest that methane condensing onto the haze particles at the base of the 1–2‐bar aerosol layer forms ice/haze particles that grow very quickly to large size and immediately “snow out” (as predicted by Carlson et al. (1988), https://doi.org/10.1175/1520-0469(1988)045%3C2066:CMOTGP%3E2.0.CO;2), re‐evaporating at deeper levels to release their core haze particles to act as condensation nuclei for H2S ice formation. In addition, we find that the spectral characteristics of “dark spots”, such as the Voyager‐2/ISS Great Dark Spot and the HST/WFC3 NDS‐2018, are well modelled by a darkening or possibly clearing of the deep aerosol layer only.

Thermodynamics of diamond formation from hydrocarbon mixtures in planets

“…In Neptune and Uranus, methane is a major constituent with a measured carbon concentration from around 2% in the atmosphere 4 and a concentration up to (assumed) 8% in the interior 5 . The methane in the atmosphere absorbs red light and reflects blue light, giving the ice giants their blue hues 6 . Moreover, numerous recently discovered extrasolar planets, some orbiting carbon-rich stars, have spurred a renewed interest in the high-pressure and high-temperature behaviors of hydrocarbons 7 .…”

Latitudinal Variations in Methane Abundance, Aerosol Opacity and Aerosol Scattering Efficiency in Neptune's Atmosphere Determined From VLT/MUSE

“…We fitted to these two angles simultaneously in order to fit both the mean reflectance and limb-darkening spectra, and these two particular zenith angles were chosen to coincide with two of the zenith Cloud opacity profiles retrieved in this analysis from disc-averaged observations, and assumed example methane abundance profiles. Left: Cloud scheme used by IRW22 (Irwin et al, 2022c) in their combined analysis of Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS), IRTF/SpeX and Gemini/Near-Infrared Integral-Field Spectrograph (NIFS). Middle: Revised cloud scheme used in this analysis of Very Large Telescope (VLT)/Multi Unit Spectroscopic Explorer (MUSE) observations.…”

The Runaway Greenhouse Effect on Hycean Worlds

“…If the concentration of the condensible is high enough, compositional gradients stabilize the atmosphere to convection (Guillot 1995;Li & Ingersoll 2015;Leconte et al 2017) and double-diffusive instabilities (Leconte et al 2017) even if the lapse rate is superadiabatic. Molecular gradient-induced convective inhibition has been invoked to explain periodic storms in Saturn's atmosphere (Li & Ingersoll 2015), stable wave ducts for gravity waves in Jupiter's atmosphere (Ingersoll & Kanamori 1995), and the step-like behavior of methane's abundance in Uranus's condensation layer (Irwin et al 2022). Neglecting this effect results in an underestimation of the deep atmospheric temperature in gas giant planets (Leconte et al 2017).…”