The Altitude of an Infrared-bright Cloud Feature on Neptune from Near-Infrared Spectroscopy

Abstract

We present 2.03È2.30 km near-infrared spectroscopy of Neptune taken 1999 June 2 (UT) with the W. M. Keck ObservatoryÏs near-infrared spectrometer (NIRSPEC) during the commissioning of the instrument. The spectrum is dominated by a bright cloud feature, possibly a storm or upwelling, in the southern hemisphere at approximately 50¡ south latitude. The spectrum also includes light from a dimmer northern feature at approximately 30¡ north latitude. We compare our spectra (j/*j D 2000) of these two features with a simple model of NeptuneÏs atmosphere. Given our model assumption that the clouds are Ñat reÑecting layers, we Ðnd that the top of the bright southern cloud feature sat at a pressure level of bars, and thus this cloud did not extend into the stratosphere bars). A similar 0.14~0 .03 0.05 (P [ 0.1 analysis of the dimmer northern feature yields a cloud-top pressure of 0.084^0.026 bars. This suggests that the features we observed efficiently transport methane to the base of the stratosphere but do not directly transport methane to the upper stratosphere (P \ 10~2 to 10~3 bars), where photolysis occurs. Our observations do not constrain how far these clouds penetrate down into the troposphere. We Ðnd that our model Ðts to the data restrict the fraction of in ortho-para thermodynamic equilibrium to H 2 greater than 0.8.

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

“…3 Reduction of the spectroscopic data consisted of sky and bias subtraction, rectification (for the NIRSPEC images, where the spatial and spectral coordinates are distorted with respect to the pixels on the array), and wavelength calibration. Wavelength calibration of the NIRSPEC images was by means of argon line lamps (calibration and rectification of NIRSPEC spectra is described in detail in Roe et al, 2001), while the NIRC2 data were calibrated based on OH lines from the Earth's atmosphere (Rousselot et al, 2000). In order to correct for telluric absorption, we observed A-type stars (shown in Table 2) which were divided by the spectrum of Vega (spectral type A0 V; Colina et al, 1996b) to get an estimate of the atmospheric and instrumental transfer function.…”

“…When, in 2000, the Keck Telescope's AO system was fully commissioned, we observed Neptune with the adaptive optics system coupled to NIRSPEC to acquire spatiallyresolved spectra.With the arrival of the new NIRC2 camera and spectrograph in 2002, we obtained additional 2 µm (K band) spectra at a spectral resolution R = 2000 and a spatial resolution of 0.06 arcseconds. In this paper we present an analysis of our spectral data using the method of Roe et al (2001). This analysis indicates that, while most of the southern hemisphere cloud features are located near the tropopause at pressures of 0.08-0.14 bars, some of the features in the northern hemisphere are located in the stratosphere, at pressure levels 0.05 bars.…”

“…This is in contrast to the planet's much more uniform appearance in visible light. Relatively dark features seen at visible wavelengths are thought to originate in the planet's troposphere (Stratman et al, 2001), whereas the infraredbright clouds are believed to be located near the tropopause and perhaps in the stratosphere (Baines et al, 1995a;Roe et al, 2001). At both visible and near-infrared wavelengths, the light we receive from Neptune is reflected sunlight.…”

“…In 1999-2000 we imaged the planet using adaptive optics (AO) techniques (Max et al, 2000(Max et al, , 2002, and we took conventional (i.e., ∼ 0.5 spatial resolution) spectra (with the near-infrared spectrometer NIRSPEC) during engineering commissioning time. Using a model of gas absorption in Neptune's atmosphere, we estimated the altitude of bright northern and southern cloud features (Roe et al, 2001). The northern feature was found to be at a pressure level of 0.084 bars, and the southern feature at 0.14 bars.…”

Keck Adaptive Optics Images of Uranus and Its Rings

“…To get a rough estimate of the altitude of the northern latitude features, we used a simple model of Uranus's atmosphere, where we calculated the atmospheric transmission (going into and out of the atmosphere) as a function of pressure level for each of our filters (Roe et al 2001). Only CH 4 , H 2 -H 2 collision induced absorption (CIA), and H 2 -He CIA were included as sources of opacity.…”