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Abstract: Scattering properties of large (N>2000) aggregates of monomers as found in Titan’s atmosphere during the descent of the Cassini/Huygens probe are numerically calculated. We extend our previous quantitative analysis of small aggregates (N≤256) to verify the obtained conclusions. To calculate the light scattering of such big non-spherical aggregates we use the DDSCAT code based on the discrete dipole approximation (DDA) technique introduced by . The orientation averaged single-scattering phase function as well as the degree of linear polarization is calculated. Clusters with up to 2048 monomers are investigated in the optical wavelength range. We conclude that (a) the applied mathematical aggregation mechanism plays only a minor role, and it is sufficient to test particles formed by diffusion limited aggregation (DLA) with different cluster parameters; (b) the aggregate “fluffiness” can be qualitatively derived from polarization data; (c) for a fixed number of monomers the scattering function of intensity is sensitive to the size parameter of the aggregates (e.g. to the effective cluster density); (d) consequently, characterization of aggregates by their number of monomers only is generally insufficient and more detail cluster descriptions are needed; (e) even when an aggregate is described more explicitly significant intensity variations can be observed for “identical” model particles over the whole range of scattering angles, particular for large fluffy aggregates; (j) random generation averaging should be added to the standard orientation averaging procedure. Finally, we discuss our results in respect of the conclusions of the paper. [ABSTRACT FROM AUTHOR]

Abstract: The Descent Imager/Spectral Radiometer onboard Huygens observed Titan''s surface through the atmospheric methane windows [Tomasko, M.G., Doose, L., Engel, S., Dafoe, L.E., West, R., Lemmon, M., Karkoschka, E., See, C., 2008. A model of Titan''s aerosols based on measurements made inside the atmosphere. Planet. Space Sci. 56, 669–707]. Infrared spectra obtained during the last stage of the descent, for which the atmospheric contribution is negligible, show that the reflectance of the surface around the sit increases with decreasing solar phase angle. Combining these with a spectrum reconstructed from reflected lamp light [Schröder, S.E., Keller, H.U., 2008. The reflectance spectrum of Titan''s surface at the Huygens landing site determined by the Descent Imager/Spectral Radiometer. Planet. Space Sci. 56, 753–769] reveals a strong increase in reflectance towards zero phase angle: the opposition surge. Both shadow hiding and coherent backscatter are required to fit the phase curve with the Hapke [2002. Bidirectional Reflectance Spectroscopy 5. The Coherent Backscatter Opposition Effect and Anisotropic Scattering. Icarus 157, 523–534] model. We find the particle phase function below phase angle to be close to isotropic, which is highly unusual for the surfaces of planetary bodies. A terrain with similar scattering properties has been identified on Triton [Lee, P., Helfenstein, P., Veverka, J., McCarthy, D., 1992. Anomalous-scattering region on Triton. Icarus 99, 82–97], and a connection with the tholins thought to be present on both worlds seems plausible. Indeed, tholin laboratory analogs are found to scatter in similar fashion [Lüthi, 2008. Remote sensing of the surface of Titan: Photometric properties, comparison with analogues, and future microscopic observations. Ph.D. Thesis, Philosophisch-naturwissenschaftlichen Fakultät, Universität Bern]. We conclude that Titan''s unusual phase curve is consistent with the presence of tholins on the surface. Our result provides a lower boundary condition for Titan atmosphere models and as such may aid the ongoing analysis of Cassini data. [Copyright &y& Elsevier]

Abstract: In the frame of fractal modeling of tholin aggregates we made a systematic analysis of their optical properties. Ballistic particle–cluster aggregation (BPCA) and diffusion-limited aggregation (DLA) of spherical primary particles (monomers) identical in material composition were considered. Aggregates composed of identical particles (monodisperse cluster), as well as of size-distributed particles (polydisperse cluster), were simulated. To calculate the light-scattering models, the code based on the superposition T-matrix method is used. Orientationally averaged properties of light scattering by model particles were extracted, and the normalized phase function and the degree of linear polarization were calculated as functions of scattering angle. We concluded that: (a) aggregation mechanism as well as specific internal structure of the clusters play only a minor role, and for the future it is not necessary to investigate aggregates of different types; (b) the intensity is very sensitive both to the size parameter of forming particles x and to the size parameter of the aggregates X; (c) characterization of the aggregates by the number of monomers is insufficient to retrieve physical properties of aggregates from optical measurement; and (d) it is very desirable to include into the analysis polarization data calculated for the different clusters. [Copyright &y& Elsevier]

Imager for Mars Pathfinder (IMP) obtained data of sky brightness as a function of the scattering angle, wavelength, time of day and Sol. This data set is fitted with model calculations to extract the size distribution, shape and the refractive index of the aerosols suspended in the atmosphere. The inferred optical parameters are discussed in context of diurnal variations and compared to those derived from Viking Landers cameras and Phobos KRFM radiometer data. The effects of the scattering and absorption of the solar radiation by the atmospheric aerosols are discussed in terms of their influence on the spectrophotometry of the Martian surface. [Copyright &y& Elsevier]

The Portable University Model of the Atmosphere (PUMA) is a general circulation model of intermediate complexity. In the model setup used herein, the dynamics is driven by restoration to a prescribed temperature field. Given an observed three-dimensional field of restoration temperatures, the feedback of aerosol distribution to the radiation scheme and hence to the dynamics is excluded from calculations. PUMA is adapted to Titan conditions and used to carry out a series of experiments with temperature fields based on radio occultation and infrared spectroscopy measurements taken by Voyager 1 in 1980. The resultant winds are prograde with maximum wind speeds of about 14 m/s in the troposphere. This simulation demonstrates capabilities of a restricted complexity model and provides a contribution to the prediction of the descent trajectory of the Huygens lander in January 2005. [Copyright &y& Elsevier]

During the descent of the Huygens probe through Titan''s atmosphere in January 2005, the Descent Imager/Spectral Radiometer (DISR) will perform upward and downward looking measurements at various spectral ranges and spatial resolutions. This internal radiation density could be estimated by radiative transfer calculations for Titan''s atmosphere. However, to do this, the optical properties—i.e. volume extinction coefficient, single scattering albedo and scattering phase function—have to be prescribed at every altitude, and these are apriori not known. Herein, an inverse approach is investigated, which retrieves the single scattering albedo and the phase function of the aerosols from DISR observations. The method uses data from a DISR subinstrument, the Solar Aureole imager (SA), to estimate the optical properties of the atmospheric layer between two successive observation altitudes. A unique solution for one layer can in principle be calculated directly from a linear system of equations, but due to the sparseness of the data and the unavoidable noise in the measurements, the inverse problem is ill-posed. The problem is stabilized by the regularization method requiring smoothness of the resultant solution. A consistent set of solutions for all layers is obtained by iterating several times downward and upward through the layers. The method is tested in a simulated radiation density scenario for Titan, which is based on a microphysical aerosol model for the haze layer. Within this scenario, the expected coverage of SA data allows a reconstruction of the angular dependence of the scattering phase function with an explained variance better than 90%. [Copyright &y& Elsevier]