The altitude of Neptune cloud features from high-spatial-resolution near-infrared spectra

Abstract

We report on observations of Neptune from the 10-meter W.M. Keck II Telescope on June 17-18 (UT) 2000 and August 2-3 (UT) 2002 using the adaptive optics (AO) system to obtain a spatial resolution of 0.06 arcseconds. With this spatial resolution we can obtain spectra of individual bright features on the disk of Neptune in a filter centered near 2 microns. The use of a gas-only, simple reflecting layer radiative transfer model allows us to estimate the best fit altitudes of 18 bright features seen on these 4 nights and to set a constraint on the fraction of hydrogen in ortho/para equilibrium. On these nights there were three main types of features observed: northern hemisphere features in the range from +30 to −45 degrees; southern hemisphere features in the range from −30 to −50 degrees; and small southern features at −70 degrees. We find that the altitudes of the northern features are in the range from 0.023-0.064 bar, which places them in Neptune's stratosphere. Southern features at −30 to −50 degrees are mainly at altitudes from 0.10 to 0.14 bars. The small features at −70 degrees are somewhat deeper in the upper troposphere, at 0.17 and 0.27 bars. This pattern of features located at higher altitudes in the northern hemisphere and lower altitudes in the south has also been noted by previous observers. The best fits for all the observed spectra give a value of 1.0 for the fraction of hydrogen in ortho/para equilibrium; the value of the helium fraction is less well constrained by the data at 0.24. We suggest that the southern mid-latitude features are methane haze circulated up from below, while the −70 • features may be isolated areas of upwelling in a general area of subsidence. Northern bright features may be due to subsidence of stratospheric haze material rather than upwelling and condensation of methane gas. We suggest that convection efficiently transports methane ice clouds to the tropopause in the Southern mid latitudes and thus plays a key role in the stratospheric haze production cycle.

Dispersion in Neptune’s zonal wind velocities from NIR Keck AO observations in July 2009

“…Baines & Smith 1990;Gibbard et al 2002;Irwin et al 2011;Karkoschka & Tomasko 2011;Luszcz-Cook 2012). Our finding that equatorial features are deepest, while the SPFs in the south are found above them (∼0.3 bar), is different from the results of Gibbard et al (2003), which suggest a trend of increasing altitude with latitude from south to north. However, clouds at these equatorial latitudes were not observed at the earlier epoch (see Gibbard et al (2003)).…”

“…For a range in altitudes of 2 scale heights, and assuming that the altitudes of observed cloud features are similar to those for which we derive the cloud-top pressures, vertical wind shear should not contribute more than ∼60 m/s to zonal wind dispersion. If we extend this range of depths up to the 2.4 bar bottom cloud we assume in our models and down to 0.02 bar (lower limit for northern features found by Gibbard et al (2003)), then feature altitudes span no more than ∼5 scale heights. For this range of depths vertical wind shear should not contribute more than ∼150 m/s to zonal wind dispersion.…”

“…The precise values of the derived northern cloud pressures do not, however, agree with these works, which find them in the stratosphere at 0.023-0.064 bar. We expect these differences to be related to the limitations of our data (we only have broadband measurements, not spectra), differences in the sensitivity of our measurements to different altitudes (for example, Gibbard et al (2003) measures mostly in K'-band, which is not sensitive to altitudes as deep as those we probe in H-band), and the simplicity of our model -previous studies have favored models with a more complicated haze structure and which vary other model parameters (e.g. Baines & Smith 1990;Gibbard et al 2002;Irwin et al 2011;Karkoschka & Tomasko 2011;Luszcz-Cook 2012).…”

“…Sromovsky et al 2001b;Gibbard et al 2003;Luszcz-Cook 2012). The precise values of the derived northern cloud pressures do not, however, agree with these works, which find them in the stratosphere at 0.023-0.064 bar.…”

Neptune’s zonal winds from near-IR Keck adaptive optics imaging in August 2001

“…2.2, the concentration of clouds in circumferential bands at 30-50 degrees South is typical of AO images from Keck Gibbard et al 2003;Luszcz-Cook et al 2010) and CFHT (Roddier et al 1998), and of 850 and 890 nm images from HST (Hammel and Lockwood 1997;Sromovsky et al 2001b). These are likely similar to the bright cloud features seen in Voyager 2 images in methane absorption bands, and to those that appear as streaky white clouds at visible wavelengths (Smith et al 1989;Limaye and Sromovsky 1991).…”