FTS observations of Uranus’ near-infrared spectrum

Icarus 220 (2012) 369–382

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The application of new methane line absorption data to Gemini-N/NIFS and KPNO/FTS observations of Uranus’ near-infrared spectrum P.G.J. Irwin a,⇑, C. de Bergh b, R. Courtin b, B. Bézard b, N.A. Teanby c, G.R. Davis d, L.N. Fletcher a, G.S. Orton e, S.B. Calcutt a, D. Tice a, J. Hurley a a

Atmospheric, Oceanic, and Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), UMR 8109 CNRS, Observatoire de Paris, Université Pierre et Marie Curie, Université Denis Diderot, 5 place Jules Janssen, 92195 Meudon Cedex, France c School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, United Kingdom d Joint Astronomy Centre, 660 N. A’ohoku Place, Hilo, Hawaii, HI 96720, USA e Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA b

a r t i c l e

i n f o

Article history: Received 1 February 2012 Revised 11 May 2012 Accepted 11 May 2012 Available online 18 May 2012 Keywords: Uranus, Atmosphere Radiative transfer Atmospheres, Composition

a b s t r a c t New line data describing the absorption of CH4 and CH3D from 1.26 to 1.71 lm (Campargue, A., Wang, L., Mondelain, D., Kassi, S., Bézard, B., Lellouch, E., Coustenis, A., de Bergh, C., Hirtzig, M., Drossart, P. [2012]. Icarus 219, 110–128), building upon previous papers by Campargue et al. (Campargue, A., Wang, L., Kassi, S., Masat, M., Votava, O. [2010]. J. Quant. Spectrosc. Radiat. Transfer 111, 1141–1151; Wang, L., Kassi, S., Campargue, A. [2010]. J. Quant. Spectrosc. Radiat. Transfer 111, 1130–1140; Wang, L., Kassi, S., Liu, A.W., Hu, S.M., Campargue, A. [2011]. J. Quant. Spectrosc. Radiat. Transfer 112, 937–951)) have been applied to the analysis of Gemini-N/NIFS observations of Uranus made in 2010 and compared with earlier disc-averaged observations made by KPNO/FTS in 1982. The new line data are found to improve greatly the fit to the observed spectra and present a huge advance over previous methane absorption tables by allowing us to determine the CH3D/CH4 ratio and also start to break the degeneracy between methane abundance and cloud top height. The best fits are obtained if the cloud particles in the main cloud deck at the 2–3 bar level become less scattering with wavelength across the 1.4–1.6 lm region and we have modelled this variation here by varying the extinction cross-section and single-scattering albedo of the particles. Applying the new line data to the NIFS spectra of Uranus, we determine a new estimate of the CH3D/ 4 CH4 ratio of 2:9þ0:9 0:5  10 , which is consistent with the estimate of de Bergh et al. (de Bergh, C., Lutz, B.L., 4 Owen, T., Brault, J., Chauville, J. [1986]. Astrophys. J. 311, 501–510) of 3:6þ3:6 2:8  10 , made by fitting a disc-averaged KPNO/FTS spectrum measured in 1982, but much better constrained. The NIFS observations made in 2010 have been disc-averaged and compared with the 1982 KPNO/FTS spectrum and found to be in excellent agreement. Using k-tables fitted to the new line data, the central meridian observations of Uranus’ H-band spectrum (1.49–1.64 lm) made by Gemini-N/NIFS in 2010 have been reanalyzed. The use of the new methane absorption coefficients and the modified scattering properties of the cloud particles in the main cloud deck appears to break the degeneracy between cloud height and methane abundance immediately above it in this spectral region and we find that both vary with latitude across Uranus’ disc. Overall, we find that the main cloud deck becomes higher, but thinner from equator to poles, with a local maximum in cloud top height in the circumpolar zones at 45°N and 45°S. At the same time, using the ‘D’ temperature pressure profile of Lindal et al. (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P. [1987]. J. Geophys. Res. 92, 14987–15001) and a deep methane abundance of 1.6% (Baines, K.H., Mickelson, M.E., Larson, L.E., Ferguson, D.W. [1995]. Icarus 144, 328–340) we find that the relative humidity of methane is high near the equator (60%) and decreases sharply towards the poles, except near the circumpolar zone at 45°N, which has brightened steadily since 2007, and where there is a local maximum in methane relative humidity. In tests conducted with the warmer ‘F1’ profile of Sromovsky et al. (2011) we find a similar variation of methane abundance above the main cloud, although for this warmer temperature profile this abundance is dependent mostly on the fitted deep methane mole fraction. Ó 2012 Elsevier Inc. All rights reserved.

⇑ Corresponding author. Fax: +44 1865 272923. E-mail address: [email protected] (P.G.J. Irwin). 0019-1035/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2012.05.017

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1. Introduction The near-infrared spectrum of Uranus contains numerous absorption features of methane and offers one of the best opportunities to probe the vertical structure of Uranus’ condensation cloud decks and hazes (e.g. Irwin et al., 2011; Sromovsky and Fry, 2008). However, the absorption spectrum of methane becomes increasingly complicated at near-IR wavelengths and for many years the available methane line data from sources such as HITRAN (Rothman et al., 2009) and GEISA (Jacquinet-Husson et al., 2010) have been unreliable at wavelengths less than about 2 lm, especially with respect to the weaker absorption lines. For the case of the Earth’s atmosphere these missing weak lines are not important, but for Uranus’ atmosphere, with its very long paths of cold hydrogen–helium–methane atmosphere the missing absorption lines lead to poorly-modelled near-infrared spectra. Hence, until now the interpretation of sub-2-lm spectra has been achieved through the use of band models and their k-distribution derivatives (Irwin et al., 2006; Karkoschka and Tomasko, 2010) fitted to laboratory measured paths of methane at pressures and temperatures as close as possible to those found in Uranus’ atmosphere. Although these studies have provided reasonably good fits to the observations, there have still been significant systematic differences between the observed and modelled spectra. These differences have been attributed to the fact that such band models, for Uranus and Neptune, are forced to extrapolate the observed path spectra significantly beyond the measurement conditions to longer paths and colder temperatures. An important coordinated effort has been made by several groups (‘‘CH4@Titan’’ project: http://www.icb.cnrs.fr/titan) to improve the line lists of methane for the sub-2-lm region, and the new line-list, known as the WKC – 80K line database (Campargue et al., 2010; Wang et al., 2010, 2011) has been applied in its 2011 version to the modelling of Titan’s near-infrared absorption spectrum (de Bergh et al., 2012), which like Uranus is also dominated by long paths of very cold methane. In this paper we apply a slightly updated version of the WKC – 80K line database (Campargue et al., 2012) to the interpretation of Uranus’ near-infrared Hband spectrum, observed by Gemini/NIFS in 2010 and previously reported by Irwin et al. (2011). We then compare these observations with a disc-averaged spectrum of Uranus observed in 1982 with the Kitt Peak National Observatory (KPNO) Fourier Transform Spectrometer (FTS) and previously reported by de Bergh et al. (1986). While preparing this paper, we became aware of a similar study comparing available band and line data for methane as applied to all the giant planet atmospheres, and in particular Uranus (Sromovsky et al., 2012) albeit at a lower spectral resolution than that considered here. Where appropriate, we compare our findings with this study.

2. Measurement and modelling of Uranus’ near-infrared spectrum Observations of Uranus’ near-infrared spectrum at high spectral resolution have been made by a number of authors. de Bergh et al. (1986) analysed observations made in 1982 with the Kitt Peak National Observatory (KPNO) Fourier Transform Spectrometer (FTS). These were disc-averaged observations covering the spectral range 6205–6678 cm1 (1.497–1.611 lm) at a resolution of 1.2 cm1, giving a mean spectral resolving power in this window of R = 5370. These spectra, which were not radiometrically calibrated, were compared with laboratory-measured spectra of CH4 and 4 CH3D to determine a CH3D/CH4 ratio of 3:6þ3:6 2:8  10 . More recently, Irwin et al. (2011, 2012) used the Gemini-N/NIFS instrument to record, with adaptive optics, Uranus’ H-band spectrum

in 2009 and 2010 across Uranus’ disc from 1.49 to 1.8 lm at a similar spectral resolution of R = 5290, but spatially resolved. In the Irwin et al. (2011, 2012) analyses these observed spectra had to be spectrally smoothed to the resolution of the best available band models of Karkoschka and Tomasko (2010) (i.e. 10 cm1) since the quality of line data in this spectral region was not sufficiently reliable. Significant discrepancies remained between the modelled and measured spectra, which were attributed to deficiencies in the available methane absorption coefficients. To analyse the NIFS observations at their native resolution required reliable methane line data, which were not then available. However, the new WKC – 80K database allows us to use the full spectral resolving power of Gemini/NIFS. The WKC – 80K line data parameters have been assigned to line absorption spectra measured at a temperature of 80 K and list 12CH4 and CH3D lines. In some cases, the lower state energy of the transitions could not be determined and so these line data cannot be extrapolated to conditions much different from 80 K. However, as shown in Fig. 13 of Campargue et al. (2012), these transitions, which, in the H-band, are located essentially around 1.53 and 1.61 lm, contribute at most to 20% of the absorption at 80 K. Furthermore, as for Uranus (and Titan) the temperatures are close to 80 K, temperature extrapolation errors should be very small. The WKC – 80K line parameters do not include line broadening coefficients, for which we had to turn to other sources. We accounted for the H2–He broadening conditions encountered in Uranus’ atmosphere in the following way: (1) the foreign-broadened line widths of CH4 were set to 0.06 cm1 (after Margolis, 1993; and Pine, 1992) with a temperature dependence coefficient of 0.44 (after Margolis, 1993); (2) the foreign-broadened line widths of CH3D were set to 0.07 cm1 (Boussin et al., 1999) with a temperature dependence coefficient of 0.6 (Linda Brown, personal communication). To model the NIFS spectra at their native resolution, we made the same assumptions as in our previous papers concerning the temperature profile and bulk composition. Thus, the temperature profile was set to the ‘D’ profile determined from radio-occultation observations by Lindal et al. (1987). The He/H2 ratio was set to 18% (Conrath et al., 1987), while the deep mole fraction of methane was set to 1.6% (Baines et al., 1995), which we have found in our previous papers to provide a good fit to the observations. The methane relative humidity was limited to 30% at all altitudes below the tropopause and the mole fraction set to the resulting cold trap tropopause value in the stratosphere. We note here that Sromovsky et al. (2011) have made a reanalysis of Voyager 2 radio occultation observations (Lindal et al., 1987) and find that by varying the He/ H2 ratio a range of deep CH4 mole fractions and temperature profiles match the observations. Their preferred profile (F1) has a deep methane mole fraction of 4%, a He/H2 ratio of 0.1306, and a temperature profile that would lead to condensation of methane at 1.2 bar, close to the level of a thin cloud layer inferred from the Voyager 2 occultation observations. This profile led to cloud solutions that improved the fit to the HST/STIS observations made in 2002 and reported by Karkoschka and Tomasko (2009). Where appropriate we tested the effect that adopting this profile instead of our reference profile would have on our conclusions. We tested a number of line shape combinations and line wing cut-offs and found we could model the Uranus spectra acceptably well with a Voigt line shape with wings set to zero more than 35 cm1 from the line centres. However, the best agreement was found using the sub-Lorentzian correction recommended by Hartmann et al. (2002) for giant planet atmospheres, in which we included the contribution of lines up to 350 cm1 away from the calculation wavelength. The sub-Lorentzian line shape correction deduced by de Bergh et al. (2012) to best fit the Titan observations (for which the methane lines are nitrogen-broadened rather then hydrogen–helium-broadened) was found to provide a poor fit to

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the observed Uranus spectra as we shall see later. We also tested a further new estimate of the sub-Lorentzian line shape, suggested by Sromovsky et al. (2012). We determined from the calibration ARC spectra that the NIFS observations have a Gaussian instrument function with a FullWidth-Half-Maximum (FWHM) of 0.0003 lm. Radiative transfer calculations with a line-by-line (LBL) model showed that the difference in spectra calculated using square and Gaussian lineshapes with FWHM = 0.0003 lm was negligible. Our retrieval model, NEMESIS (Irwin et al., 2008) uses a correlated-k radiative transfer model, which requires pre-tabulated k-tables of each gas in the atmosphere in question. Hence, k-tables were constructed from the modified WKC – 80K line data, which assumed a square bin of FWHM = 0.0003 lm. The k-tables were generated for the 12CH4 and CH3D isotopes separately and also for a mixture of these isotopes. The WKC – 80K line data assume a CH3D/CH4 ratio of 5  104, which is not suitable for Uranus. Hence, the CH3D lines were scaled such that the combined k-tables were suitable for an atmosphere where CH3D/CH4 = 3.6  104, which is the value determined for Uranus by de Bergh et al. (1986). k-Tables were also generated from these line data at a resolution of 0.004 lm, the UKIRT–UIST resolution, in order to compare with calculations from our previous lower resolution analyses (Irwin et al., 2010, 2011, 2012). Uranus was observed in 2009 and 2010 with the Gemini-N Near-infrared Integral Field Spectrometer (NIFS) in Hawaii (Irwin et al., 2011, 2012). The observations were made using adaptive optics and have a spatial resolution of 0.100 . NIFS is an imaging spectrometer and records complete images of Uranus at 2048 wavelengths from 1.476 to 1.803 lm, with a spectral resolution of R = 5290, and has a pixel size of 0.043  0.10300 . To analyse these spectra at their highest resolution, care must be taken to ensure the spectra are properly corrected for telluric lines and solar lines. To correct for telluric absorption, observations were also made of a standard star, HIP115119 (A0V type, R.A.: 23:19:02.148, Dec.: 12:10:13.53), which was reasonably close to the position of Uranus in 2009 and 2010. The observations were reduced mostly using

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the Gemini-IRAF software (Tody et al., 1993) and correction for stellar absorption features was made using the Spextool (Cushing et al., 2004) package xtellcor_general, using the method of Vacca et al. (2003). We corrected for the airmass difference between the star and planet by modelling the star spectrum to infer the terrestrial atmospheric transmission. We then calculated the atmospheric transmission at the airmass of the planet observation and corrected for it. Photometric correction was achieved by integrating the NIFS observations of the standard star across the entire FOV, using the quoted 2MASS (Cutri et al., 2003) H-magnitude of HIP115119 of 7.156 and the 2MASS H-filter profile. To correct for solar absorption lines in the radiance reflected from Uranus’ clouds, the solar spectrum model of Fiorenza and Formisano (2005) was used, where we took care to account for the Doppler shift caused by Uranus’ calculated speed towards or away from Earth at the time of the observations. We have shown in our previous papers that the Uranus H-band spectra can be modelled with a number of different vertical cloud distributions, ranging from a continuous distribution of particles to discrete clouds. With appropriate settings, all these distributions can yield equally good fits to the observations. In this paper, we use mostly a discrete cloud model (which has relatively few free variables) and limit ourselves to the 2010 NIFS observations. The centre-of-disc spectrum measured in 2010 was modelled with a discrete 2-cloud model described by Irwin et al. (2012), with a lower vertically thin cloud at 2–3 bar (with cloud fractional scale height (i.e. the ratio of the cloud particle density scale height to the pressure scale height) set to 0.1) and an extended haze based at 0.5 bar (with cloud fractional scale height set to 0.5). Other authors (Sromovsky et al., 2012) suggest a slightly more complicated cloud scheme and a modified temperature and methane abundance profile, but we find this discrete cloud model gives an acceptably good fit to the observed spectra with fewest free parameters and fewest assumptions. The particles in both the lower cloud and upper haze were initially assumed to have a singlescattering albedo of 0.75 at all wavelengths (Irwin et al., 2010) and a simple variation of optical depth with wavelength

Fig. 1. Optical depth spectrum (solid line) and single-scattering albedo spectrum (dotted) of the lower 2–3 bar cloud required to achieve a close fit to the observed Uranus spectra using the WKC – 80K line data. The optical depth spectrum has been normalized at 1.6 lm.

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determined by Mie theory for particles with a refractive index of 1.4 + 0i (although this is not technically consistent with a singlescattering albedo less than 1.0). The optical depths and single scattering albedos of the particles were subsequently adjusted as is described below. For the Mie scattering calculation a modified Gamma-distribution of sizes was used with a mean radius of 1 lm and variance 0.05. Instead of using the phase function calculated by Mie scattering, the phase function was modelled with a simpler and more adjustable Henyey–Greenstein function with asymmetry g = 0.7, as in Irwin et al. (2010). The opacity of the upper haze was first adjusted to get a reasonable fit to the spectrum from 1.62 to 1.7 lm, which is a wavelength region insensitive to the lower cloud, and then fixed. Synthetic spectra were then calculated for a range of different lower cloud parameterizations with the cloud particles’ single-scattering albedo, -, varying from 0.65 to 0.99 in steps of 0.01, and the cloud optical thickness at 1.6 lm, s, varying from 0.5 to 5.4 in steps of 0.1. Initial tests showed that in small wavelength ranges there is almost complete degeneracy between the optical depth of the lower cloud and the single-scattering albedo of its particles. However, it was also clear that a single value of each at all wavelengths did not provide a good match to the spectra across the entire 1.62–1.7 lm range. Hence, we constructed a simple retrieval model to combine the spectra precalculated for the grid of opacities and single-scattering albedos with the assumption that the spectral properties of the lower cloud actually vary as smooth polynomial functions of the wavelength. In other words at each wavelength, the precomputed spectra were interpolated to the optical depth and single-scattering albedo calculated from the assumed polynomial coefficients. We then found solutions to these polynomials that provided a good match to the observed spectra with as smooth a variation of spectral properties

as possible. Even then, we found considerable degeneracy, but there is good evidence from limb darkening that the single-scattering albedo is 0.75 in the 1.5–1.6 lm region (Irwin et al., 2011; Sromovsky and Fry, 2007). Hence, we constrained the solution to agree with this and the fitted variation of opacity and single-scattering albedo with wavelength is shown in Fig. 1. To test whether these variations depended on the assumed methane profile we repeated this procedure using the F1 profile of Sromovsky et al. (2012), discussed earlier, and found a very similar variation of both quantities with wavelength. Using these updated opacity and single-scattering albedo spectra, the spectrum at the centre of Uranus’ disc measured by Gemini/NIFS in 2010 was fitted with our discrete cloud model, where the observed cloud opacity profile and methane abundance profile were represented with eight parameters: optical depth of the lower cloud (s1), fractional scale height of the lower cloud (f1), base altitude of lower cloud (z1) relative to the reference level at a pressure of 1 bar, optical depth of the haze (s2), fractional scale height of the haze (f2), base altitude of haze (z2) relative to the 1 bar pressure level, deep methane mole fraction (qCH4) and methane relative humidity (RH) above the condensation level. In our fit we allowed the lower cloud altitude and fractional scale height to vary, but kept the upper haze fractional scale height fixed at 0.5. The Gemini/NIFS field of view is just smaller than the diameter of Uranus’ disc and thus individual frames were dithered over Uranus’ disc. These observations were then offset appropriately and then co-added. Hence, the spectrum shown in Fig. 2 at the pixel nearest Uranus’ centre (latitude 9.3°N) is the average of 5–6 individual 2-min integrations. Our fit to this spectrum, when using the new WKC – 80K line data and our modified opacity and single-scattering albedo spectra are also shown in Fig. 2 and the retrieved atmospheric parameters are listed in Table 1. Fig. 2

Fig. 2. Top panel: comparison of the Gemini-N/NIFS I/F spectrum at the disc’s centre (9.3°N) measured in 2010 compared with that calculated using our correlated-k radiative transfer model using a k-table constructed from the WKC – 80K line data with the Hartmann et al. (2002) line shape. The bottom panel shows the residual between the fitted and measured spectrum. In the top panel, the measured spectrum and error limits are shown by the grey region, while the fit is the red (or black in monochrome) line. In the bottom panel, the errors are shown as the grey region and the residual as the red (or black in monochrome) line. Note that the residuals are larger than the estimated errors, which suggests that the dominant source of discrepancy is forward modelling error rather than random measurement error. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

P.G.J. Irwin et al. / Icarus 220 (2012) 369–382 Table 1 Retrieved atmospheric parameters to achieve the fit to the 2010 centre-of-disc spectrum shown in Fig. 2.

s1 f1 z1 (km) p1 (bar)

s2 f2 z2 (km) p2 (bar) qCH4 RH

Value

Error

1.530 0.299 36.58 2.82 0.0034 0.5 20.0 0.47 1.6% 83%

0.035 0.044 0.82 0.06 0.022 Fixed Fixed Fixed Fixed 20%

N.B. The cloud pressures p1 and p2 are derived from the fitted cloud heights z1 and z2 (which are themselves relative to the 1-bar pressure level). The opacities quoted (s1 and s2) are optical depths at 1.6 lm. The f1 and f2 parameters are the cloud fractional scale heights. The errors on the fitted parameters have been estimated by adding forward modelling error until the retrieved v2/N  1.

demonstrates clearly that by using a combination of the WKC – 80K line data and spectrally varying scattering parameters for the lower 2–3 bar cloud, a very good fit to the NIFS spectra can now be made at NIFS’ native resolving power of 5290. However, the residuals in Fig. 2 are still significantly larger than the estimated measurement error of the Gemini spectra and so we must attribute remaining differences to other forward modelling errors, such as within the spectroscopic parameters themselves (e.g. line broadening coefficients, far-wing line shape, line mixing effects), in our representation of the instrument function, or other sources. The assumed spectral line shape is an important parameter and

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Fig. 3 shows the fit to the spectrum when the methane line shape recommended by de Bergh et al. (2012) for their analysis of Titan observations is used instead of the Hartmann et al. (2002) line shape. It is clear that the de Bergh et al. (2012) line shape, which is found to match well the N2-broadening conditions in Titan’s atmosphere is not appropriate to the H2–He-broadening conditions prevalent in Uranus’ atmosphere. We also tested the line shape recommended by Sromovsky et al. (2012), but found that although it improved the fit compared with the de Bergh et al. (2012) line shape, the best fit was still achieved with the Hartmann et al. (2002) line shape (Fig. 3). It is possible that remaining discrepancies could be reduced by further empirical adjustment of the line wing shape, combined with empirical adjustments of the assumed variation of single scattering albedo and cloud extinction opacity with wavelength. Another part of the discrepancy between the measured and calculated spectrum is due to trying to fit the whole wavelength range simultaneously, which leads to any errors in the estimate of the far-wing line shape needed to fit the 1.6–1.62 lm region, having an adverse effect on the 1.52–1.57 lm region, where the absorption features of CH3D are most easily detectable. This can be seen in Fig. 4, where instead we have fitted just the 1.525–1.565 lm region alone, using the Hartmann et al. (2002) line shape and achieve a much better fit. We note that Sromovsky et al. (2012) also require a spectral variation of scattering properties to obtain a good fit to the observed Uranus spectra using the WKC – 80K data set. In their case, they chose to accomplish this by varying the forward scattering asymmetry parameter, g. It is probably not possible with current observations to ascertain which approach is more likely as the problem is very much underconstrained.

Fig. 3. Same as Fig. 2 except that the line wing shape recommended by de Bergh et al. (2012) from their analysis of KPNO/FTS Titan observations, and the line shape recommended by Sromovsky et al. (2012) have both been used, instead of that recommended by Hartmann et al. (2002) line shape. The spectrum calculated with the de Bergh et al. (2012) is shown in red (or black in monochrome), while that calculated with the Sromovsky et al. (2012) is shown in blue (or dotted black in monochrome). It is clear that the line shape suitable for N2-broadening in Titan’s atmosphere (de Bergh et al. (2012)) is not suitable for H2–He broadening of methane in Uranus’ atmosphere. However, while the calculated spectrum using the Sromovsky et al. (2012) line shape is an improvement over the de Bergh et al. (2012) line shape for this case, the agreement is still not as good as for the Hartmann et al. (2002) line shape.

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Fig. 4. As Fig. 2, but fitted to the 1.525–1.565 lm region only, showing how the effect of not trying to fit the part of the spectrum from 1.6 to 1.62 lm, which is heavily dependent on the assumed methane line shape, allows the model more freedom to fit the individual features in this spectral range much better.

3. CH3D/CH4 ratio Once suitable optical depth, s, and single-scattering albedo, -, spectra for the particles in the lower cloud at 2–3 bar had been determined, the correlated-k NEMESIS model was used to fit the discrete cloud model to the observed 2010 NIFS centre-of-disc spectrum in the 1.54 to 1.57 lm range using the WKC – 80K k-table with different CH3D mole fractions: 3, 4, 5, 6, 7, 8, 9, 10  106. Given that our reference atmospheric profile contains a deep mole fraction of methane of 1.6% (Baines et al., 1995), this corresponds to a range of CH3D/CH4 ratios of (1.88–6.25)  104. The retrieval model was allowed to vary the lower cloud opacity and base height, but all other parameters were fixed with the lower cloud fractional scale height set to 0.1, the base pressure of the extended haze layer set to 0.5 bar and fractional scale height of the haze set to 0.5 as before. The correlated-k model was used for reasons of speed, but once the cloud parameters had been fitted the spectra were recalculated with the line-by-line model to give as accurate a representation of the spectrum as possible, although differences between the correlated-k and line-by-line models were found to be P very small. The variation of v2 (defined here as v2 ¼ N ððyi  fi Þ= 2 ri Þ , where yi are the N measured radiances with estimated errors ri, and fi are the fitted radiances) with the assumed CH3D/CH4 ratio is shown in Fig. 5. We find that the best-fit CH3D/CH4 ratio on this relatively coarse grid to be 3.1  104, which is smaller, but consistent with the previous estimation from disc-averaged observations 4 by de Bergh et al. (1986) of 3:6þ3:6 2:8  10 . It is also consistent with measurements of the D/H ratio in H2 from ISO-SWS observations (Feuchtgruber et al., 1999). Feuchtgruber et al. obtained 5 5:5þ3:5 for the D/H in hydrogen. Using the fractionation fac1:5  10 tor of 1.68 ± 0.23 estimated by Lecluse et al. (1996) for Uranus, this 5 leads to a D/H ratio in methane of 9:2þ8:0 3:4  10 , and a CH3D/CH4 4 ratio (CH3D/CH4 = 4  D/H in methane) of 3:7þ3:2 1:4  10 . New

measurements of the D/H ratio in hydrogen with Herschel-PACS have been made (Lellouch et al., 2011), but are not yet published. It should be noted here that although we model the fitting process in terms of the CH3D mole fraction, what we are actually fitting here is the relative strength of the CH3D and 12CH4 lines in the wavelength region of interest and our determination does not thus vary with the assumed deep methane abundance. As stated earlier, for this analysis we have assumed a 12CH4 mole fraction of 1.6% (Baines et al., 1995), but there is considerable disagreement as to what this value should be and indeed whether it might actually vary with latitude (e.g. Karkoschka and Tomasko, 2010; Irwin et al., 2010; Sromovsky et al., 2011). To check this we repeated the analysis using the ‘F1’ profile of Sromovsky et al. (2011), which has a deep CH4 mole fraction of 4%, and found that we derived an equivalent estimate of the CH3D/CH4 ratio. To confirm the validity of our CH3D/CH4 estimate, the retrieval was then repeated using a line-by-line retrieval model, which we have recently added to NEMESIS, but in this case the CH3D mole fraction was also fitted together with the cloud parameters. Using this model, the best fit was achieved, shown in Fig. 6, with a CH3D/ CH4 ratio of 2.94  104. From Fig. 5 we can see that the quality of fit to the observations varies rather smoothly with the CH3D/CH4 ratio. We noted in Figs. 2 and 4 that we are unable to fit the observations to within the uncertainty expected from random errors in the observations themselves and also from telluric corrections. Hence, we have to assume that the remaining differences are due to errors in the line data themselves or some additional deficiency in our cloud model scheme. We thus added a small amount of forward modelling ‘noise’ to the measurements to reduce the fitted v2/N to be of the order of 1.0. Using this adjusted noise, we find that the range of CH3D mole fractions for which Dv2  1 (i.e. the change in the v2 value as defined above) is (4.9–5.2)  106, which gives a CH3D/CH4 ratio of (3.06–3.28)  104. Taking a more con-

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Fig. 5. Variation of the goodness of fit, v2 , between the measured and modelled 2010 centre-of-disc NIFS spectra in the 1.54–1.57 lm range as a function of the assumed CH3D/CH4 ratio. The range of CH3D/CH4 ratios for which Dv2 6 1 is (3.06–3.28)  104. The range of CH3D/CH4 ratios for which Dv2 6 9 (i.e. 3r certainty) is (2.49– 3.85)  104.

Fig. 6. Line-by-line fit to the 2010 centre-of-disc NIFS observations using the fitted CH3D/CH4 ratio of 2.94  104. The observed spectrum and error limits are shown in grey. Calculations for the extreme ranges of the estimated CH3D/CH4 ratio, 2.49 and 3.85  104, are shown with the blue (or light grey) and red (or dark grey) lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 7. Top panel: Comparison of the disc-averaged KPNO/FTS spectrum made in 1982 (solid line), with the Gemini-N/NIFS observations made in 2010 (red or dotted line) and manually disc-averaged. The residual is shown in the bottom panel. The agreement between the two spectra is rather good and differences can probably be attributed to slightly different instrument functions, wavelength offsets and reduction pipelines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. Close-up of 1.54–1.58 lm region of the KPNO/FTS spectrum (solid line) and the NIFS disc-averaged data (red or dotted line with crosses), showing very good agreement between the two data sets. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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servative limit of Dv2  9 (i.e. the 3r error), we estimate a CH3D mole fraction of (4.0–6.2)  106, which gives a CH3D/CH4 ratio of (2.49–3.85)  104. Fits to the observed spectrum with CH3D/ CH4 ratios of 2.49  104 and 3.85  104 are also shown in Fig. 6. Combining these determinations with our line-by-line fit, our final estimate of the CH3D/CH4 ratio (where we have rounded 4 to 1 decimal place) is 2:9þ0:9 0:5  10 , which is consistent with, but better constrained than the value determined by de Bergh et al. (1986). The estimate of the CH3D/CH4 ratio in this spectral region by de Bergh et al. (1986) comes from a disc-averaged observation of Uranus’ spectrum made by the KPNO/FTS in 1982. To compare the KPNO/FTS observations with the 2010 NIFS observations, the NIFS spectra, which are spatially resolved and cover the whole of Uranus’ disc, were numerically averaged over the disc to give a discaveraged spectrum. The KPNO/FTS spectrum itself was not absolutely calibrated, and so we simply scaled the spectrum to match the disc-integrated NIFS spectrum. The two spectra are compared in Fig. 7 and in closer detail in Fig. 8. It can be seen that the two spectra correspond very well indeed, in spite of the fact that the sub-Earth latitude in 1982 was 73°S compared with 12°N in 2010. Hence, the only difference between the CH3D/CH4 ratio estimated by de Bergh et al. (1986) and the current estimate is through the use of better-constrained line data. 4. Effect of new line data on vertical cloud profile determinations We have seen that the combination of the WKC – 80K line data and lower cloud particles whose spectral properties vary smoothly with wavelength provides a very good fit to the measured GeminiN/NIFS and KPNO/FTS observations. Our previous analysis of the Gemini-N/NIFS observations had to be done at significantly lower

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spectral resolution in order to use the best available methane absorption database then available of Karkoschka and Tomasko (2010), which we shall hereafter refer to as KT10. Using the KT10 absorption data, we found that we were unable to fit the spectrum precisely and that systematic differences existed. When the retrieval of clouds was unconstrained this led to the model concluding the presence of a second deeper cloud at 8 bar, which we believed to be spurious (Irwin et al., 2012). Using the WKC – 80K line data, a low-resolution k-table was constructed for a methane isotopic mix with the CH3D/CH4 ratio set to the original de Bergh et al. (1986) estimate of 3.6  104, with a FWHM = 0.004 lm and step 0.001 lm, to match the resolution of the spectra considered by Irwin et al. (2011, 2012). The centre-of-disc NIFS spectrum of Uranus measured in 2010 was smoothed to this resolution and then fitted with a continuous cloud model using (1) the KT10 k-table and original unmodified spectral properties; (2) the KT10 k-table and the modified spectral properties of the lower cloud; and (3) the new WKC – 80K low resolution k-table and the modified spectral properties of the lower cloud. Fig. 9 shows the fit to the spectrum achieved in all three cases. We can see that simply using the modified spectral properties of the cloud (model 2) greatly improves the quality of the fit, but it is only with the use of the WKC – 80K k-table as well (model 3) that the fit to the observations becomes very good. Fig. 10 shows a comparison of the continuous cloud profile retrieved using the WKC – 80K k-table and modified cloud scattering properties (model 3) with that retrieved using the KT10 k-table and unmodified cloud properties (model 1) and demonstrates that the new model has no need of requiring a deep cloud layer at 8–10 bar. Thus this feature can rightly be attributed to the systematic differences seen between the modelled and measured spectra using the KT10 k-table and original cloud spectral properties (model 1) as suggested by Irwin et al. (2012). It should be noted that this conclusion

Fig. 9. Comparison of the fit to the centre-of-disc 2010 NIFS spectrum and errors (grey region) using discrete clouds and different cross section files and k-tables. Top panel: Karkoschka and Tomasko (2010) k-table and unmodified optical depth and single-scattering albedo spectra. Middle panel: Karkoschka and Tomasko (2010) k-table (KT10) and lower cloud spectral properties modified as shown in Fig. 1. Lower panel: WKC – 80K k-table and modified lower cloud spectral properties. As can be seen a significant part of the improved fit is due to the modified cross-section file, but only the use of the WKC – 80K k-table can provide a very close fit to the observations.

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Fig. 10. Comparison of the continuous cloud structures (in units of optical depth per bar at 1.6 lm) retrieved from an unconstrained fit to the centre-of-disc 2010 NIFS spectrum using the Karkoschka and Tomasko (2010) k-table and unmodified cloud spectral properties (red or solid line) and the WKC – 80K k-table and modified spectral properties (blue or dashed line). Using the new WKC – 80K k-table obviates the need for a second deeper cloud at 8–10 bar. In both cases the grey regions indicate the error range of the estimated opacity profiles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 2 Correlation matrix calculated from the fit to low-resolution observations with the Karkoschka and Tomasko (2010) (KT10) k-table and unmodified lower cloud scattering properties.

s1 f1 z1

s2 f2 z2 qCH4 RH

s1

f1

z1

s2

f2

z2

qCH4

RH

1.00 0.64 0.89 0.36 0.00 0.00 0.00 0.11

0.64 1.00 0.88 0.74 0.00 0.00 0.00 0.28

0.89 0.88 1.00 0.49 0.00 0.00 0.00 0.14

0.36 0.74 0.49 1.00 0.00 0.00 0.00 0.71

0.00 0.00 0.00 0.00 1.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 1.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.00 1.00 0.00

0.11 0.28 0.14 0.71 0.00 0.00 0.00 1.00

underlines the need for precise molecular line lists in order to interpret remotely sensed data correctly since incomplete and inaccurate lists can easily lead to erroneous conclusions. 5. Degeneracy between methane relative humidity and cloud height Recently it has become apparent that the effect of changing the pressure level of the main cloud at 2–3 bar on the near-IR spectrum of Uranus from 1 to 2 lm is almost indistinguishable from that of changing the abundance of methane above its condensation level. Karkoschka and Tomasko (2009) analysed HST observations of Uranus in 2002 at 825 nm, where there is an H2–H2 collision-inducedabsorption band, and compared the variation of reflectivity with latitude with that seen at nearby wavelengths where the atmospheric absorption was dominated by methane absorption. They found that much of the variation in reflection can be attributed to the latitudinal variation of methane abundance, rather than a latitudinal variation of cloud base and cloud height as was concluded by Irwin et al. (2010, 2011, 2012) and Sromovsky and Fry (2007, 2008). In the model used by Karkoschka and Tomasko

Table 3 Correlation matrix calculated from the fit to low-resolution observations with the Karkoschka and Tomasko (2010) (KT10) k-table and modified lower cloud scattering properties, where both the opacity and single-scattering albedo decrease with wavelength.

s1 f1 z1

s2 f2 z2 qCH4 RH

s1

f1

z1

s2

f2

z2

qCH4

RH

1.00 0.84 0.89 0.70 0.00 0.00 0.00 0.49

0.84 1.00 0.99 0.83 0.00 0.00 0.00 0.47

0.89 0.99 1.00 0.80 0.00 0.00 0.00 0.52

0.70 0.83 0.80 1.00 0.00 0.00 0.00 0.75

0.00 0.00 0.00 0.00 1.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 1.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.00 1.00 0.00

0.49 0.47 0.52 0.75 0.00 0.00 0.00 1.00

(2009) the data could be matched by varying the deep abundance of methane from 4% at equatorial latitudes, reducing to 1–2% polewards of 45°N and 45°S. Irwin et al. (2010), using the Lindal et al. (1987) ‘D’ temperature-abundance profile, found this explanation to be inconsistent with the predicted levels of the clouds needed to match the observed spectra from 1.4 to 1.7 lm, since most of the resulting change of methane abundance took place at altitudes below the cloud level, where they cannot be detected. However, Irwin et al. (2012) noted that varying the relative humidity of methane above the condensation level can account for the latitudinal changes seen in the 1.4–1.7 lm spectrum. Since the WKC – 80K data allow us to improve greatly the fit to the NIFS observations, it was of interest to see if they could also help us to break this degeneracy between methane abundance and cloud top height. Using our discrete cloud retrieval model described earlier, we found there was still some degeneracy with respect to the haze base altitude and fractional scale height and thus these were fixed to 20 km relative to the 1 bar reference pressure level (corresponding to a pressure level of 0.5 bar in our reference profile) and 0.5 respectively, which is found to provide a good fit to the strong methane absorption spectral regions.

P.G.J. Irwin et al. / Icarus 220 (2012) 369–382 Table 4 Correlation matrix calculated from the fit to low-resolution observations with the WKC – 80K k-table and modified lower cloud scattering properties, where both the opacity and single-scattering albedo decrease with wavelength.

s1 f1 z1

s2 f2 z2 qCH4 RH

s1

f1

z1

s2

f2

z2

qCH4

RH

1.00 0.73 0.77 0.39 0.00 0.00 0.00 0.21

0.73 1.00 0.96 0.69 0.00 0.00 0.00 0.06

0.77 0.96 1.00 0.63 0.00 0.00 0.00 0.14

0.39 0.69 0.63 1.00 0.00 0.00 0.00 0.26

0.00 0.00 0.00 0.00 1.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 1.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.00 1.00 0.00

0.21 0.06 0.14 0.26 0.00 0.00 0.00 1.00

Table 5 Correlation matrix calculated from the fit to high-resolution observations with the WKC – 80K k-table and modified lower cloud scattering properties, where both the opacity and single-scattering albedo decrease with wavelength.

s1 f1 z1

s2 f2 z2 qCH4 RH

s1

f1

z1

s2

f2

z2

qCH4

RH

1.00 0.68 0.86 0.40 0.00 0.00 0.00 0.08

0.68 1.00 0.87 0.62 0.00 0.00 0.00 0.27

0.86 0.87 1.00 0.57 0.00 0.00 0.00 0.05

0.40 0.62 0.57 1.00 0.00 0.00 0.00 0.33

0.00 0.00 0.00 0.00 1.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 1.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.00 1.00 0.00

0.08 0.27 0.05 0.33 0.00 0.00 0.00 1.00

The centre-of-disc 2010 NIFS Uranus spectrum was fitted with four models and the correlation matrix of the fitted parameters assessed. The four models were: (1) Smoothed 0.004-lm-resolution NIFS spectrum, low resolution KT10 k-table and unmodified lower cloud spectral properties (i.e. our original model); (2) Smoothed 0.004-lm-resolution NIFS spectrum, low resolution KT10 k-table and modified lower cloud spectral properties; (3) Smoothed 0.004-lm-resolution NIFS spectrum, low resolution WKC – 80K k-table and modified lower cloud spectral properties; and

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(4) Unsmoothed 0.0003-lm-resolution NIFS spectrum, high resolution WKC – 80K k-table and modified lower cloud spectral properties. The correlation matrices, which are derived from the computed covariance matrix of the solution, are shown in Tables 2–5 and also in Fig. 11. In the original case (Table 2, Fig. 11a) we see large cross-correlations between methane relative humidity and cloud properties, highlighting the degeneracy previously reported. If only the cloud particle properties are modified, these cross-correlations actually increase slightly (Table 3, Fig. 11b). However, the use of the WKC – 80K k-table and modified lower cloud spectral properties greatly reduces the cross-correlation between methane relative humidity and cloud properties and similar cross-correlations are seen in both the smoothed and the native NIFS resolution simulations. Tables 4 and 5 (Fig. 11c and d) suggest that the new WKC – 80K data should allow us to differentiate between methane relative humidity and lower cloud properties and thus the retrieval model was applied to all the native NIFS resolution (0.0003 lm) spectra measured along Uranus’ central meridian in 2010. The variation of retrieved parameters with latitude is shown in Fig. 12. Here we can see that we find a clear variation with latitude of both cloud base altitude, cloud opacity and methane relative humidity, with enhanced abundances of methane seen at equatorial latitudes. Interestingly, we also see higher abundances under the newly forming polar zone at 45°N, but not at the latitude of the fading zone at 45°S, which might suggest that the zone at 45°N is a region of active convection as this feature forms. We find that the lower cloud generally becomes higher and less optically thick towards the poles, with a local maximum in cloud height at the circumpolar zones at 45°N and 45°S, although the fractional scale height keeps to the range 0.1–0.3. In addition to the methane relative humidity varying with latitude, we also find that the haze opacity is thickest near the equator and thins towards the poles, again with a local maximum at the newly forming zone at 45°N (Irwin et al., 2012). This feature is also apparent in Gemini/NIRI observations of Uranus made in the 1.69-lm CH4-long filter (Irwin et al., 2011). It should be remembered however, that while Tables 2–5 and Fig. 11

Fig. 11. Calculated solution correlation matrices shown in Tables 2–5. Case 1 (top left) low-resolution observations with the KT10 k-table and unmodified lower cloud scattering properties. Case 2 (top right) low-resolution observations with KT10 k-table and modified lower cloud scattering properties, where both the opacity and singlescattering albedo decrease with wavelength. Case 3 (bottom left) low-resolution observations with the WKC – 80K k-table and modified lower cloud scattering properties. Case 4 (bottom right) high-resolution observations with the high resolution WKC – 80K k-table and modified lower cloud scattering properties.

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(a)

(b)

(c)

(d)

(e)

(f)

Fig. 12. Retrieved variation with latitude of the following properties fitted to the central meridian 2010 NIFS spectra: (a) lower cloud opacity at 1.6 lm, (b) lower cloud base altitude (in km with respect to the 1 bar reference level), (c) lower cloud fractional scale height, (d) upper haze opacity at 1.6 lm, (e) methane relative humidity. In these panels the solid lines are the fitted values, while the error ranges are indicated by the dotted lines. The bottom panel (f) shows the measured (solid line) and modelled (dotted line) central meridian H-continuum reflectivity (averaged from 1.561 to 1.583 lm) variation with latitude. For the second panel (b), which shows the variation of lower cloud altitude with latitude, the equivalent pressure scale (in bars) is shown on the right hand side.

indicate that there is lower correlation between methane relative humidity and cloud properties, its correlation with the haze opacity is 0.33, and its correlation with the fractional scale height of the lower cloud is 0.27 and so it may never be possible to disentangle entirely these three parameters in this wavelength range.

As noted earlier, Sromovsky et al. (2011) have reanalyzed Voyager 2 radio occultation observations to determine a revised temperature and abundance profile that is consistent with the radio occultation data and leads to condensation of methane at 1.2 bar, where the presence of a thin cloud was inferred. The preferred

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‘F1’ profile of Sromovsky et al. (2011) has a deep methane abundance of 4% and since condensation does not occur until the 1.2 bar level, varying the deep abundance does vary the column amount of methane above a cloud at the 2 bar level. Hence, we repeated our retrieval of the Gemini/NIFS central meridian spectra using this F1 temperature profile and assuming either the twocloud model discussed previously or the three-cloud model favoured by Sromovsky et al. (2011), which has the main cloud at 2–3 bar, a thin haze at the methane condensation level and an upper level haze. In both cases the methane relative humidity above the condensation level was fixed at 30%, with the deep methane mole fraction allowed to vary instead. This profile is not exactly the same as the Sromovsky et al. (2011) F1 profile, for which in addition to having a fixed deep methane mole fraction of 4%, the relative humidity of methane varied from approximately 80% at the methane condensation level to 20% at the tropopause. It did, however, allow us to explore how well we can fit the observations with a warmer temperature profile for which the methane column abundance above the 2–3 bar level is governed predominantly by the deep methane mole fraction rather than the methane relative humidity above the condensation level. We found we were able to get good fits with these models at equatorial latitudes, but that the fits worsened towards the poles, probably due to retrieval instabilities, which we could not identify. However, in both cases, there was a clear variation of the methane mole fraction with latitude, with a mole fraction of 3–4% seen at equatorial latitudes, decreasing sharply to less than 2% polewards of 40–50°N and S, with a local maximum at 40–50°N. Sromovsky and Fry (2008) note that the relatively flat NICMOS F097N image (Fig. 1 of their paper), which senses down to about the 1-bar level, has much less global structure than the F095N image, which senses down to around the 2-bar level (Fig. 2 of their paper), indicating that methane variability in the 1–2 bar region is responsible for much of the latitudinal variation in the observed spectra. This point was also made by Karkoschka and Tomasko (2009), who state that at high levels, methane may be uniformly mixed with latitude. Whatever the parameterization scheme, what really matters here is the column abundance of methane above the main cloud deck based at 2–3 bars, which is going to be dominated by the methane mole fraction immediately above the cloud, i.e. in the 1–2 bar region. What we have done here is model that abundance in terms of the relative humidity of the cooler Lindal et al. (1987) ‘D’ profile, rather than in terms of the deep methane mole fraction of the warmer Sromovsky et al. (2011) ‘F1’ profile. Whichever scheme is used, however, our conclusion is that the use of the new WKC – 80K line data allows us to discriminate between latitudinal variations in methane abundance above the main cloud deck and latitudinal variations of the base altitude or pressure of the cloud deck.

6. Conclusion The new methane line data in the H-band spectral region of (Campargue et al., 2010, 2012; Wang et al., 2010, 2011) (WKC – 80K) provide very much improved fits to the observed spectrum of Uranus from 1.4 to 1.8 lm and present a huge improvement over previous methane absorption tables. To obtain a really good fit, however, requires that the cloud particles in the lower cloud deck at the 2–3 bar level become less scattering with wavelength across the 1.4–1.6 lm region. In this study we have modelled this variation by varying the extinction cross-section and single-scattering albedo of the particles, but other approaches, such as varying the scattering particle asymmetry parameter (e.g. Sromovsky et al., 2012) may be equally valid. The WKC – 80K line data enable us to fit the native resolution NIFS spectra of Uranus and determine a new estimate of the

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4 CH3D/CH4 ratio 2:9þ0:9 0:5  10 , which is consistent with the esti4 mate of de Bergh et al. (1986) of 3:6þ3:6 2:8  10 , made by fitting a disc averaged KPNO/FTS spectrum measured in 1982, but much better constrained. The NIFS observations made in 2010 have been disc-averaged and compared with the 1982 KPNO/FTS spectrum and found to be in excellent agreement. Although the Gemini/NIFS spectra have an excellent resolving power of 5290, determination of the CH3D/CH4 ratio would be further strengthened by measuring Uranus’ near-infrared H-band spectrum at even higher spectral resolution. In addition, there is a significant need for laboratory measurements of the far wing line shape of CH4 lines broadened by H2 at low temperatures. Using k-tables fitted to the WKC – 80K line data, the central meridian observations of Uranus H-band spectrum made by Gemini-N/NIFS in 2010 have been reanalyzed. The use of the new k-table and modified scattering properties appears to break the degeneracy between cloud height and methane abundance above the cloud (parameterized either through relative humidity or deep mole fraction, depending on the assumed temperature–pressure profile) and we find that both vary with latitude across Uranus’ disc. Overall, we find that the main cloud deck becomes higher, but thinner from equator to poles, with a local maximum in cloud top height in the circumpolar zones at 45°N and 45°S. At the same time we find that the abundance of methane, expressed here in terms of relative humidity, is high near the equator (60%) and decreases sharply towards the poles, except near the circumpolar zone at 45°N, which has brightened steadily since 2007, where there is a local maximum. Whether this is caused by enhanced convection at this latitude or whether we are seeing residual correlation with the other cloud parameters is not clear.

Acknowledgments We are grateful to the United Kingdom Science and Technology Facilities Council for funding this research and also to our support astronomers: Richard McDermid (2009, 2010), Chad Trujillo (2009, 2010), Andy Adamson (2007, 2008), Watson Varricattu (2006), and also to Ilona Soechting and Andrew Gosling in the UK Gemini Office. Nicholas Teanby acknowledges the support of the Leverhulme Trust. Leigh Fletcher was supported by a Glasstone fellowship at the University of Oxford. Glenn Orton was supported by a grant from NASA to the Jet Propulsion Laboratory, California Institute of Technology. Catherine de Bergh, Régis Courtin and Bruno Bézard acknowledge the financial support from the French ‘‘Agence Nationale de la Recherche’’ (ANR project: CH4@Titan). The Gemini Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência e Tecnologia (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina). The United Kingdom Infrared Telescope is operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the UK. References Baines, K.H., Mickelson, M.E., Larson, L.E., Ferguson, D.W., 1995. The abundances of methane and ortho/para hydrogen on Uranus and Neptune: Implications of new laboratory 4–0 H2 quadrupole line parameters. Icarus 144, 328–340. Boussin, C., Lutz, B.L., Hamdounia, A., de Bergh, C., 1999. Pressure broadening and shift coefficients for H2, He and N2 in the 3m2 band of 12CH3D retrieved by a multispectrum fitting technique. J. Quant. Spectrosc. Radiat. Transfer 63, 49–84. Campargue, A., Wang, L., Kassi, S., Masat, M., Votava, O., 2010. Temperature dependence of the absorption spectrum of CH4 by high resolution spectroscopy

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P.G.J. Irwin et al. / Icarus 220 (2012) 369–382

at 81 K: (II) The icosad region (1.49–1.30 lm). J. Quant. Spectrosc. Radiat. Transfer 111, 1141–1151. Campargue, A., Wang, L., Mondelain, D., Kassi, S., Bézard, B., Lellouch, E., Coustenis, A., de Bergh, C., Hirtzig, M., Drossart, P., 2012. An empirical line list for methane in the 1.26–1.71 lm region for planetary investigations (T = 80–300 K). Application to Titan. Icarus 219, 110–128. Conrath, B.J., Gautier, D., Hanel, R., Lindal, G., Marten, A., 1987. The helium abundance of Uranus from Voyager measurements. J. Geophys. Res. 92, 15003– 15010. Cushing, M.C., Vacca, W.D., Rayner, J.T., 2004. Spextool: A spectral extraction package for SpeX, a 0.8–5.5 micron cross-dispersed spectrograph. Publ. Astron. Soc. Pacific 116, 362–376. Cutri, R.M. et al. 2003. 2MASS All Sky Catalog of Point Sources. The IRSA 2MASS AllSky Point Source Catalog, NASA/IPAC Infrared Science Archive. de Bergh, C., Lutz, B.L., Owen, T., Brault, J., Chauville, J., 1986. Monodeuterated methane in the outer Solar System. II. Its detection on Uranus at 1.6 microns. Astrophys. J. 311, 501–510. de Bergh, C. et al., 2012. Applications of a new set of methane line parameters to the modeling of Titan’s spectrum in the 1.58 lm window. Planet. Space Sci. 61, 85– 98. Feuchtgruber, H., Lellouch, E., Bézard, B., Encrenaz, Th., de Graauw, Th., Davis, G.R., 1999. Detection of HD in the atmospheres of Uranus and Neptune: A new determination of the D/H ratio. Astron. Astrophys. 341, L17–L21. Fiorenza, C., Formisano, V., 2005. A solar spectrum for PFS data analysis. Planet. Space Sci. 53, 1009–1016. Hartmann, J.-M., Boulet, C., Brodbeck, C., van Thanh, N., Fouchet, T., Drossart, P., 2002. A far wing lineshape for H2 broadened CH4 infrared transitions. J. Quant. Spectrosc. Radiat. Transfer 72, 117–122. Irwin, P.G.J., Sromovsky, L.A., Strong, E.K., Sihra, K., Teanby, N.A., Bowles, N., Calcutt, S.B., Remedios, J.J., 2006. Improved near-infrared methane band models and kdistribution parameters from 2000 to 9500 cm1 and implications for interpretation of outer planet spectra. Icarus 181, 309–319. Irwin, P.G.J. et al., 2008. The NEMESIS planetary atmosphere radiative transfer and retrieval tool. J. Quant. Spectrosc. Radiat. Transfer 109, 1136–1150. Irwin, P.G.J., Teanby, N.A., Davis, G.R., 2010. Revised vertical cloud structure of Uranus from UKIRT/UIST observations and changes seen during Uranus’ Northern Spring Equinox from 2006 to 2008: Application of new methane absorption data and comparison with Neptune. Icarus 208, 913–926. Irwin, P.G.J., Teanby, N.A., Davis, G.R., Fletcher, L.N., Orton, G.S., Tice, D., Kyffin, A., 2011. Uranus’ cloud structure and seasonal variability from Gemini-North and UKIRT observations. Icarus 212, 339–350. Irwin, P.G.J., Teanby, N.A., Davis, G.R., Fletcher, L.N., Orton, G.S., Calcutt, S.B., Tice, D., Hurley, J., 2012. Further seasonal changes in Uranus’ cloud structure observed by Gemini-North and UKIRT. Icarus 218, 47–55. Jacquinet-Husson, N. et al., 2010. The GEISA spectroscopic database: Current and future archive for Earth and planetary atmosphere studies. J. Quant. Spectrosc. Radiat. Transfer 109, 1043–1059.

Karkoschka, E., Tomasko, M., 2009. The haze and methane distributions on Uranus from HST–STIS spectroscopy. Icarus 202, 287–302. Karkoschka, E., Tomasko, M., 2010. Methane absorption coefficients for the jovian planets from laboratory, Huygens, and HST data. Icarus 205, 674–694. Lecluse, C., Robert, F., Gautier, D., Guiraud, M., 1996. Deuterium enrichment in giant planets. Planet. Space Sci. 44, 1579–1592. Lellouch, E. et al., 2011. Observations of the giant planets with Herschel. In: Proceedings of the EPSC-DPS Joint Meeting, 2–7 October, 2011, Nantes, France, p. 875. Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P., 1987. The atmosphere of Uranus – Results of radio occultation measurements with Voyager 2. J. Geophys. Res. 92, 14987–15001. Margolis, J.S., 1993. Measurement of hydrogen-broadened methane lines in the m4 band at 296 and 200 K. J. Quant. Spectrosc. Radiat. Transfer 50, 431–441. Pine, A.S., 1992. Self-, N2, O2, H2, Ar, and He broadening in the m3 band Q branch of CH4. J. Chem. Phys. 97, 773–785. Rothman, L.S. et al., 2009. The HITRAN 2008 molecular spectroscopic database. J. Quant. Spectrosc. Radiat. Transfer 110, 533–572. Sromovsky, L.A., Fry, P.M., 2007. Spatially resolved cloud structure on Uranus: Implications of near-IR adaptive optics imaging. Icarus 192, 527–557. Sromovsky, L.A., Fry, P.M., 2008. The methane abundance and structure of Uranus’ cloud bands inferred from spatially resolved 2006 Keck grism spectra. Icarus 193, 252–266. Sromovsky, L.A., Fry, P.M., Kim, J.H., 2011. Methane on Uranus: The case for a compact CH4 cloud layer at low latitudes and a severe CH4 depletion at highlatitudes based on re-analysis of Voyager occultation measurements and STIS spectroscopy. Icarus 215, 292–312. Sromovsky, L.A., Fry, P.M., Boudon, V., Campargue, A., Nikitin, A., 2012. Comparison of line-by-line and band models of near-IR methane absorption applied to outer planet atmospheres. Icarus 218, 1–23. Tody, D., 1993. IRAF in the nineties. In: Hanisch, R.J., Brissenden, R.J.V., Barnes, J. (Eds.), Astronomical Data Analysis Software and Systems II, A.S.P. Conference Ser., vol. 52, p. 173. Vacca, W.D., Cushing, M.C., Rayner, J.T., 2003. A method of correcting near-infrared spectra for telluric absorption. Publ. Astron. Soc. Pacific 115, 389–409. Wang, L., Kassi, S., Campargue, A., 2010. Temperature dependence of the absorption spectrum of CH4 by high resolution spectroscopy at 81 K: (I) The region of the 2m3 band at 1.66 lm. J. Quant. Spectrosc. Radiat. Transfer 111, 1130–1140. Wang, L., Kassi, S., Liu, A.W., Hu, S.M., Campargue, A., 2011. The 1.58 lm transparency window of methane (6165–6750 cm1): Empirical line list and temperature dependence between 80 and 296 K. J. Quant. Spectrosc. Radiat. Transfer 112, 937–951.