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

摘要

We present and analyze three-dimensional data cubes of Neptune from theOSIRIS 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 ofR~3800 in the H and K broad bands. We focus our analysis on regions ofNeptune's atmosphere that are near-infrared dark- that is, free of discretebright cloud features. We use a forward model coupled to a Markov chain MonteCarlo algorithm to retrieve properties of Neptune's aerosol structure andmethane 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 thickcloud layer with a base near 3 bar and composed of low albedo, forwardscattering particles, with an assumed characteristic size of ~1$\mu$m. Abovethis cloud, we require a vertically extended haze of smaller (~0.1 $\mu$m)particles, which reaches from the upper troposphere (~0.6 bar) into thestratosphere. The particles in this haze are brighter and more isotropicallyscattering than those in the deep cloud. When we extend our analysis to 18cloud-free locations from 20N to 87S, we observe that the optical depth inaerosols above 0.5 bar decreases by a factor of 2-3 or more at mid- andhigh-southern latitudes relative to low latitudes. We also consider Neptune's methane (CH$_4$) profile, and find that ourretrievals indicate a strong preference for a low methane relative humidity atpressures where methane is expected to condense. Our preferred solution at mostlocations is for a methane relative humidity below 10% near the tropopause inaddition to methane depletion down to 2.0-2.5 bar. We tentatively identify atrend of lower CH$_4$ columns above 2.5 bar at mid- and high-southern latitudesover low latitudes.