Retrieving Neptune's aerosol properties from Keck OSIRIS observations. I. Dark regions

• Published in: arXiv.org 2017-06
• Main Authors: Luszcz-Cook, S H, de Kleer, K, de Pater, I, Adamkovics, M, Hammel, H B
• Format: Article
• Language: English
• Subjects: Aerosols; Albedo; Algorithms; Clouds; Columns (structural); Computer simulation; Cubes; Depletion; Forward scattering; Haze; Humidity; Infrared analysis; Markov analysis; Markov chains; Methane; Monte Carlo simulation; Noise spectra; Relative humidity; Spatial data; Spatial resolution; Spectral resolution; Three dimensional analysis; Tropopause; Troposphere
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1706.05049

We present and analyze three-dimensional data cubes of Neptune from the OSIRIS integral-field spectrograph on the 10-m Keck telescope, from July 2009. These data have a spatial resolution of 0.035"/pixel and spectral resolution of R~3800 in the H and K broad bands. We focus our analysis on regions of Neptune's atmosphere that are near-infrared dark- that is, free of discrete bright cloud features. We use a forward model coupled to a Markov chain Monte Carlo algorithm to retrieve properties of Neptune's aerosol structure and methane profile above ~4 bar in these near-infrared dark regions. Using a set of high signal-to-noise spectra in a cloud-free band from 2-12N, we find that Neptune's cloud opacity is dominated by a compact, optically thick cloud layer with a base near 3 bar and composed of low albedo, forward scattering particles, with an assumed characteristic size of ~1\(\mu\)m. Above this cloud, we require a vertically extended haze of smaller (~0.1 \(\mu\)m) particles, which reaches from the upper troposphere (~0.6 bar) into the stratosphere. The particles in this haze are brighter and more isotropically scattering than those in the deep cloud. When we extend our analysis to 18 cloud-free locations from 20N to 87S, we observe that the optical depth in aerosols above 0.5 bar decreases by a factor of 2-3 or more at mid- and high-southern latitudes relative to low latitudes. We also consider Neptune's methane (CH\(_4\)) profile, and find that our retrievals indicate a strong preference for a low methane relative humidity at pressures where methane is expected to condense. Our preferred solution at most locations is for a methane relative humidity below 10% near the tropopause in addition to methane depletion down to 2.0-2.5 bar. We tentatively identify a trend of lower CH\(_4\) columns above 2.5 bar at mid- and high-southern latitudes over low latitudes.