Simon Calcutt

Abstract:

Ammonia (NH3) has been detected both on Saturn and Jupiter, and although its concentration and distribution has been well-studied on Jupiter, it has proven more difficult to do so on Saturn due to higher sensitivity requirements resulting from Saturn's lower atmospheric temperatures and the dominance of Saturn's phosphine which masks the ammonia signal. Using far-infrared measurements of Saturn taken by Cassini/CIRS between February 2005 and December 2010, the latitudinal variations of upper tropospheric ammonia on Saturn are studied. Sensitivity to NH3 in the far-infrared is explored to provide estimates of temperature, para-H2 and PH3, from 2.5cm−1 spectral resolution measurements alone, 0.5cm−1 spectral-resolution measurements alone, and 0.5cm−1 measurements degraded to 2.5cm−1 spectral resolution. The estimates of NH3 from these three different datasets largely agree, although there are notable differences using the high emission angle 0.5cm−1 data, which are asserted to result from a reduction in sensitivity at higher emission angles. For low emission angles, the 0.5cm−1-retrieved values of NH3 can be used to reproduce the 2.5cm−1 spectra with similar efficacy as those derived directly from the 2.5cm−1 resolution data itself, and vice versa. Using low emission angle data, NH3 is observed to have broad peak abundances at ±25° latitude, attributed to result from condensation and/or photolytic processes. Lack of data coverage at equatorial latitudes precludes analysis of NH3 abundance at less than about 10° latitude. Noise levels are not sufficient to distinguish fine zonal features, although it seems that NH3 cannot trace the zonal belt/zone structure in the upper troposphere of Saturn.

Abstract:

Saturn's moon Titan has a nitrogen atmosphere comparable to Earth's, with a surface pressure of 1.4 bar. Numerical models reproduce the tropospheric conditions very well but have trouble explaining the observed middle-atmosphere temperatures, composition and winds. The top of the middle-atmosphere circulation has been thought to lie at an altitude of 450 to 500 kilometres, where there is a layer of haze that appears to be separated from the main haze deck. This 'detached' haze was previously explained as being due to the co-location of peak haze production and the limit of dynamical transport by the circulation's upper branch. Here we report a build-up of trace gases over the south pole approximately two years after observing the 2009 post-equinox circulation reversal, from which we conclude that middle-atmosphere circulation must extend to an altitude of at least 600 kilometres. The primary drivers of this circulation are summer-hemisphere heating of haze by absorption of solar radiation and winter-hemisphere cooling due to infrared emission by haze and trace gases; our results therefore imply that these effects are important well into the thermosphere (altitudes higher than 500 kilometres). This requires both active upper-atmosphere chemistry, consistent with the detection of high-complexity molecules and ions at altitudes greater than 950 kilometres, and an alternative explanation for the detached haze, such as a transition in haze particle growth from monomers to fractal structures.

Abstract:

New line data describing the absorption of CH 4 and CH 3D from 1.26 to 1.71μm (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 CH 3D/CH 4 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 2-3bar level become less scattering with wavelength across the 1.4-1.6μm 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 CH 3D/CH 4 ratio of 2.9-0.5+0.9×10-4, which is consistent with the estimate of de Bergh et al. (de Bergh, C., Lutz, B.L., Owen, T., Brault, J., Chauville, J. [1986]. Astrophys. J. 311, 501-510) of 3.6-2.8+3.6×10-4, 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 (1.49-1.64μm) 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. © 2012 Elsevier Inc.

Abstract:

A far-infrared emission band attributed to volatile or refractory haze in Titan's stratosphere has been decreasing in intensity since Cassini's arrival in 2004. The 220cm-1 feature, first seen by the Voyager Infrared Interferometer Spectrometer, has only been found in Titan's winter polar region. The emission peaks at about 140km altitude near the winter stratospheric temperature minimum. Observations recorded over the period 2004-2012 by the Composite Infrared Spectrometer on Cassini show a decrease in the intensity of this feature by about a factor of four. Possible seasonal causes of this decline are an increase in photolytic destruction of source chemicals at high altitude, a lessening of condensation as solar heating increased, or a weakening of downwelling of vapors. As of early 2012, the 220cm-1 haze has not yet been detected in the south. The haze composition is unknown, but its decrease is similar to that of HC3N gas in Titan's polar stratosphere, pointing to a nitrile origin. © 2012. The American Astronomical Society. All rights reserved.

Abstract:

This paper describes new measurements and modelling of the absorption of methane gas, one of the most important gases observed in the atmospheres of the outer planets and Titan, between 9000 and 14,000cm -1 (0.7 to 1.1μm) and compares them with current best available spectral models.A series of methane spectra were measured at the UK's Natural Environment Research Council (NERC) Molecular Spectroscopy Facility (based at the Rutherford Appleton Laboratory, Oxfordshire, UK) using a Brüker 125HR Fourier transform spectrometer. To approximate the conditions found in outer planet atmospheres, the spectra were measured over a wide range of pressures (5bar to 38mbar) and temperatures (290-100K) with path lengths of 19.3, 17.6, 16.0 and 14.4m. The spectra were recorded at a moderate resolution of 0.12cm -1 and then averaged to 10cm -1 resolution prior to fitting a series of increasingly complex band-models including temperature dependence. Using the most complex model, a Goody line distribution with a Voigt line shape and two lower energy state levels, the typical rms residual error in the fit is between 0.01 and 0.02 in the wings of the main absorption bands.The new spectral parameters were then compared with the measured spectra and spectra calculated using existing data and shown to be able to accurately reproduce the measured absorption. The improvement in the temperature dependence included in the model is demonstrated by comparison with existing cold methane spectral data for a typical Jovian path. © 2012 Elsevier Ltd.