The haze and methane distributions on Neptune from HST–STIS spectroscopy
Introduction
Neptune is probably the planet with the least accurate knowledge about the vertical haze and methane structure, which determines where most sunlight is deposited as heat or scattered back to the observer. Based on estimated condensation levels for methane and H2S near 2 and 4 bars pressure, radiative transfer models with haze or clouds at both layers could explain several observations (Hammel et al., 1989, Baines and Smith, 1990, Baines and Hammel, 1994, Baines et al., 1995, Sromovsky et al., 2001b). The opacity just above the 2-bar layer was called methane haze considering an optical depth on the order of 0.1. The opacity at 3–4 bars was called the H2S cloud since it was considered to be optically thick. Because of intrinsic difficulties of constraining the vertical haze and cloud structure, the altitudes of both layers were assumed and their optical depths were fitted to explain the observations.
Part of Neptune is covered by discrete clouds with a strong temporal variability (cf. references in Hammel and Lockwood (2007)). Even the simplest models can easily place them within about a scale height of the tropopause (0.13 bars pressure).
Since almost all observations that constrain the haze and cloud structure involve methane absorptions, Neptune’s structure was determined by assuming a reasonable vertical methane mixing ratio profile and then interpreting observed features as haze or cloud features. Whether these features could be caused by spatial variations of the methane mixing ratio has not been investigated. Recent mid-infrared images by Orton et al. (2007) indicate major horizontal variability of methane near the tropopause.
This is similar to our state of knowledge about the atmosphere of Uranus several years ago. However, recent work on Uranus revealed that there is no significant observable aerosol enhancement at the methane condensation level (Sromovsky and Fry, 2007), nor at the H2S condensation level (Karkoschka and Tomasko, 2009). The latter work, which we call K&T2009, explained the main observed feature primarily as a latitudinal methane feature and not a haze feature. This investigation was based on a data cube of Uranus, providing images at the spatial resolution of the Hubble Space Telescope (HST) in 1750 wavelengths.
We acquired a similar data cube of Neptune, and we present here its calibration, analysis, and modeling using the methods of K&T2009. We do not repeat the methods here, but focus on additional techniques needed because of the characteristics of the Neptune data. In order to give a rough idea about the pressure levels probed by our Neptune data, we show cumulative optical depths for Rayleigh scattering, methane and hydrogen absorption, and aerosol extinction in Fig. 1.
Our observations occurred on August 3, 2003, just before Neptune’s northern winter solstice. Within the 50+ year long photometric record of Neptune in blue and green filters, our observations were taken after Neptune had brightened for some 20 years with some indication of leveling off during the years after our observations (Hammel and Lockwood, 2007).
The next section describes the observations and data calibration. Section 3 discusses discrete clouds, while the other features are discussed in Sections 4 Latitudinal structure, 5 Model setup, 6 Vertical structure, 7 Comparison with Uranus. Section 4 reports observed latitudinal variations. Section 5 explains our radiative transfer modeling technique. Section 6 discusses the distribution of tropospheric hazes and methane. Section 7 focuses on the comparison between Uranus and Neptune. Section 8 summarizes our work.
Section snippets
Data calibration
Our program GO 9330 consisted of 49 exposures taken with the Space Telescope Imaging Spectrograph (STIS). The four-orbit program was executed during seven successive, shortened HST orbits. The first two exposures were acquisition images in 530–1000 nm (F28X52LP) and clear filters (50CCD). Following were 28 exposures with the grating G750L covering the wavelengths 530–1020 nm at 0.49 nm/pixel. The slit was 0.1 arcsec wide and parallel to the central meridian of Neptune, stepping from the evening
Data analysis
The most obvious features on Neptune are discrete features, which we call discrete clouds. We analyzed discrete clouds based on the five images with at least 4 km-am−1 methane absorption coefficient, displayed in panel D of Fig. 2 (1 km-am = 2.687 × 1024 molecules cm−2). Although the same pattern of discrete clouds dominates images in weaker absorptions, detailed analysis indicates a subtle change of pattern for absorptions weaker than 4 km-am−1, most likely caused by other features at lower altitudes.
Center-to-limb fitting
We used the images of Fig. 5 to determine latitudinal and center-to-limb variations with the same technique as in K&T2009. We excluded the 5% of pixels of the strongest discrete features, the data above the top dashed line in Fig. 3, for which the subtraction of discrete features did not work perfectly, as a few spots visible in the images of Fig. 5 indicate.
Our method provided fitted reflectivities at μ = 0.4, 0.6, and 0.8, called I4/F, I6/F, and I8/F, respectively, and the ratio I8/I4 is a
Composition of atmosphere
We used the same radiative transfer code as in K&T2009 for Uranus, but adjusted the parameters in the following way. For Neptune, we used the calculated gravity at each latitude, which varies between 11.0 and 11.4 m s−2 from equator to pole (Lindal, 1992). For the composition of the atmosphere, we used a 19:81 mixing ratio of helium to hydrogen from Conrath et al. (1991). Conrath et al. (1993) considered the possibility of a mole fraction of 0.003 for nitrogen with a helium mole fraction of ∼0.15
General structure
Generally, results for the vertical distribution of aerosols from methane band observations are very dependent on the assumed methane absorption coefficients. Small changes in the coefficients within their uncertainties give very different best-fitting models. Therefore, in K&T2009, we presented a method that is independent of the coefficients. We calculated two parameters, such as reflectivities at two center-to-limb locations, for models with all methane absorption coefficients between zero
Methane
We compare here our Neptune results primarily with results of K&T2009 about Uranus because both data sets and their analysis were so similar. The most unanticipated result was that Uranus and Neptune have a variable methane profile as function of latitude. For both planets, the methane profile is roughly constant at low latitudes and roughly constant at high southern latitudes, where methane is depleted. For Uranus, the transition is centered on −35° latitude, for Neptune, the best fit is −32°
Summary
We analyzed a HST–STIS data cube of Neptune of 0.1 arcsec spatial resolution and 1 nm spectral resolution between 300 and 1000 nm wavelength, calibrated to accuracies of 0.004 arcsec (spatial), 0.05 nm (spectral), and 1% relative and 5% absolute reflectivity. The combination of data about center-to-limb variations over a wide spectral range, methane absorptions with strengths ranging over four orders of magnitude, and hydrogen absorptions, allowed us to test previous results about the vertical and
Acknowledgments
Support for this work was provided by NASA through Grant No. NNX08AE74G. Support for Program No. HST-GO-09330.01-A was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA Contract NAS5-26555. We thank Patrick Fry for valuable suggestions.
References (23)
- et al.
Clouds, hazes, and the stratospheric methane abundance in Neptune
Icarus
(1994) - et al.
The atmospheric structure and dynamical properties of Neptune derived from ground-based and IUE spectrophotometry
Icarus
(1990) - et al.
The abundances of methane and ortho/para hydrogen on Uranus and Neptune: Implications of new laboratory 4-0 H2 quadrupole line parameters
Icarus
(1995) - et al.
Neptune’s far-infrared spectrum from the ISO long-wavelength and short-wavelength spectrometers
Icarus
(2003) - et al.
Constraints on N2 in Neptune’s atmosphere from Voyager measurements
Icarus
(1993) - et al.
The altitude of Neptune’s cloud features from high-spatial-resolution near-infrared spectra
Icarus
(2003) - et al.
Atmospheric structure in 1994, 1995, and 1996: HST imaging at multiple wavelengths
Icarus
(1997) - et al.
Long-term atmospheric variability on Uranus and Neptune
Icarus
(2007) - et al.
Vertical aerosol structure of Neptune: Constraints from center-to-limb profiles
Icarus
(1989) - et al.
The haze and methane distribution on Uranus from HST–STIS spectroscopy
Icarus
(2009)
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