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

S. H. Luszcz-Cook*, K. de Kleer, I. de Pater, M. Adamkovics, H. B. Hammel

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review

18 Citations (Scopus)


We present and analyze three-dimensional data cubes of Neptune from the OSIRIS integral-field spectrograph on the 10-m W.M. Keck II telescope, from 26 July 2009. These data have a spatial resolution of 0.035/pixel and spectral resolution of R ~3800 in the H (1.47-1.80 μm) and K (1.97-2.38 μm) 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.We construct a set of high signal-to-noise spectra spanning a range of viewing geometries to constrain the vertical structure of Neptune's aerosols in a cloud-free latitude band from 2-12°N. We find that Neptune's cloud opacity at these wavelengths is dominated by a compact, optically thick cloud layer with a base near 3 bar. Using the pyDISORT algorithm for the radiative transfer and assuming a Henyey-Greenstein phase function, we observe this cloud to be composed of low albedo (single scattering albedo =0.45-0.01+0.01), forward scattering (asymmetry parameter g=0.50-0.02+0.02) particles, with an assumed characteristic size of ~1μm. Above this cloud, we require an aerosol layer of smaller (~0.1μm) particles forming a vertically extended haze, which reaches from the upper troposphere (. 0.59-0.03+0.04 bar) into the stratosphere. The particles in this haze are brighter (single scattering albedo =0.91-0.05+0.06) and more isotropically scattering (asymmetry parameter g=0.24-0.03+0.02) than those in the deep cloud. When we extend our analysis to 18 cloud-free locations from 20°N to 87°S, 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 (CH4) profile, and find that our retrievals indicate a strong preference for a low methane relative humidity at pressures where methane is expected to condense. When we include in our fits a parameter for methane depletion below the CH4 condensation pressure, 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 CH4 columns above 2.5 bar at mid- and high-southern latitudes over low latitudes, qualitatively consistent with what is found by Karkoschka and Tomasko (2011), and similar to, but weaker than, the trend observed for Uranus.

Original languageEnglish
Pages (from-to)52-87
Number of pages36
Publication statusPublished - 15 Sept 2016


  • Atmosphere
  • Neptune
  • Radiative transfer
  • Spectroscopy


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