Planetary atmosphere observation analysis

Abstract:

Thermal infrared spectra of Saturn from 10-1400 cm-1 at 15 cm-1 spectral resolution and a spatial resolution of 1°-2° latitude have been obtained by the Cassini Composite Infrared Spectrometer [Flasar, F.M., and 44 colleagues, 2004. Space Sci. Rev. 115, 169-297]. Many thousands of spectra, acquired over eighteen-months of observations, are analysed using an optimal estimation retrieval code [Irwin, P.G.J., Parrish, P., Fouchet, T., Calcutt, S.B., Taylor, F.W., Simon-Miller, A.A., Nixon, C.A., 2004. Icarus 172, 37-49] to retrieve the temperature structure and para-hydrogen distribution over Saturn's northern (winter) and southern (summer) hemispheres. The vertical temperature structure is analysed in detail to study seasonal asymmetries in the tropopause height (65-90 mbar), the location of the radiative-convective boundary (350-500 mbar), and the variation with latitude of a temperature knee (between 150 and 300 mbar) which was first observed in inversions of Voyager/IRIS spectra [Hanel, R., and 15 colleagues, 1981. Science 212, 192-200; Hanel, R., Conrath, B., Flasar, F.M., Kunde, V., Maguire, W., Pearl, J.C., Pirraglia, J., Samuelson, R., Cruikshank, D.P., Gautier, D., Gierasch, P.J., Horn, L., Ponnamperuma, C., 1982. Science 215, 544-548]. Uncertainties due to both the modelling of spectral absorptions (collision-induced absorption coefficients, tropospheric hazes, helium abundance) and the nature of our retrieval algorithm are quantified. Temperatures in the stratosphere near 1 mbar show a 25-30 K temperature difference between the north pole and south pole. This asymmetry becomes less pronounced with depth as the radiative time constant for the atmospheric response increases at deeper pressure levels. Hemispherically-symmetric small-scale temperature structures associated with zonal winds are superimposed onto the temperature asymmetry for pressures greater than 100 mbar. The para-hydrogen fraction in the 100-400 mbar range is greater than equilibrium predictions for the southern hemisphere and parts of the northern hemisphere, and less than equilibrium predictions polewards of 40° N. The temperature knee between 150-300 mbar is larger in the summer hemisphere than in the winter, smaller and higher at the equator, deeper and larger in the equatorial belts and small at the poles. Solar heating on tropospheric haze is proposed as a possible mechanism for this effect; the increased efficiency of ortho- to para-hydrogen conversion in the southern hemisphere is consistent with the presence of larger aerosols in the summer hemisphere, which we demonstrate to be qualitatively consistent with previous studies of Saturn's tropospheric aerosol distribution. © 2007 Elsevier Inc. All rights reserved.

Abstract:

The Composite InfraRed Spectrometer (CIRS) on-board the Cassini spacecraft has currently returned around three years worth of data from Saturn's largest moon Titan. One of the unique aspects of CIRS is to take high spatial resolution spectra of the limb of Titan, with sub-scale height (20-40 km) resolutions. This is made possible by the small field-of-view (FOV) of the mid-IR detectors. However, many limb spectra have moderate to large sized FOVs, which introduces bias into retrieved profiles of temperature and abundance unless the finite FOV size is taken into account. The bias can be reduced by calculating a FOV-averaged spectrum comprising a weighted sum of a small number of spectra with infinitesimal FOVs across the FOV. Here we introduce a scheme for incorporating FOV averaging into radiative transfer calculations of CIRS spectra and quantify the errors as a function of number of FOV averaging points, FOV size, tangent altitude, and wavenumber. The optimum number of FOV averaging points for a given observation can then be found by matching the calculated FOV averaging error with the measurement error. This allows for accurate analysis of a vast amount of Cassini-CIRS data. © 2007 Springer Science+Business Media B.V.

Abstract:

We have analyzed data recorded by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft during the Titan flybys T0-T10 (July 2004-January 2006). The spectra characterize various regions on Titan from 70° S to 70° N with a variety of emission angles. We study the molecular signatures observed in the mid-infrared CIRS detector arrays (FP3 and FP4, covering roughly the 600-1500 cm-1 spectral range with apodized resolutions of 2.54 or 0.53 cm-1). The composite spectrum shows several molecular signatures: hydrocarbons, nitriles and CO2. A firm detection of benzene (C6H6) is provided by CIRS at levels of about 3.5 × 10-9 around 70° N. We have used temperature profiles retrieved from the inversion of the emission observed in the methane ν4 band at 1304 cm-1 and a line-by-line radiative transfer code to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere. No longitudinal variations were found for these gases. Little or no change is observed generally in their abundances from the south to the equator. On the other hand, meridional variations retrieved for these trace constituents from the equator to the North ranged from almost zero (no or very little meridional variations) for C2H2, C2H6, C3H8, C2H4 and CO2 to a significant enhancement at high northern (early winter) latitudes for HCN, HC3N, C4H2, C3H4 and C6H6. For the more important increases in the northern latitudes, the transition occurs roughly between 30 and 50 degrees north latitude, depending on the molecule. Note however that the very high-northern latitude results from tours TB-T10 bear large uncertainties due to few available data and problems with latitude smearing effects. The observed variations are consistent with some, but not all, of the predictions from dynamical-photochemical models. Constraints are set on the vertical distribution of C2H2, found to be compatible with 2-D equatorial predictions by global circulation models. The D/H ratio in the methane on Titan has been determined from the CH3D band at 1156 cm-1 and found to be 1.17-0.28+0.23 × 10-4. Implications of this deuterium enrichment, with respect to the protosolar abundance on the origin of Titan, are discussed. We compare our results with values retrieved by Voyager IRIS observations taken in 1980, as well as with more recent (1997) disk-averaged Infrared Space Observatory (ISO) results and with the latest Cassini-Huygens inferences from other instruments in an attempt to better comprehend the physical phenomena on Titan. © 2007 Elsevier Inc. All rights reserved.

Abstract:

Against a backdrop of intensive exploration of the Martian surface environment, intehded to lead to human exploration, some aspects of the modern climate and the meteorology of Mars remain relatively unexplored. In particular, there is a need for detailed measurements of the vertical profiles of atmospheric temperature, water vapor, dust, and condensates to understand the intricately related processes upon which the surface conditions, and those encountered during descent by landers, depend. The most important of these missing data are accurate and extensive temperature measurements with high vertical resolution. The Mars Climate Sounder experiment on the 2005 Mars Reconnaissance Orbiter, described here, is the latest attempt to characterize the Martian atmosphere with the sort of coverage and precision achieved by terrestrial weather satellites. If successful, it is expected to lead to corresponding improvements in our understanding of meteorological phenomena and to enable improved general circulation models of the Martian atmosphere for climate studies on a range of timescales. Copyright 2007 by the American Geophysical Union.