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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 OSIRIS camera onboard Rosetta successfully acquired images of asteroid 2867 Steins through a variety of color filters during the flyby on 5 September 2008. The best images of this 5km diameter asteroid have a resolution of 78m per pixel. We process the images by deconvolving with the point spread function and enlarging through the Mitchell–Netravali filter. The enhanced set is analyzed by means of various techniques (PCA, band ratios, stereo anaglyphs) to study surface morphology and search for variegation. We identify a landslide, which supports a YORP origin for Steins’ unusual diamond shape. In addition, we find that the interior of one of two large craters on the south pole is bluer than the rest of the body. [Copyright &y& Elsevier]

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]

During its year-long orbital mission, the Dawn spacecraft has mapped the surface of main-belt asteroid Vesta multiple times at different spatial resolutions and illumination and viewing angles. The onboard Framing Camera has acquired thousands of clear filter and narrow band images, which, with the availability of high-resolution global shape models, allows for a photometric characterization of the surface in unprecedented detail. We analyze clear filter images to retrieve the photometric properties of the regolith. In the first part of the paper we evaluate different photometric models for the global average. In the second part we use these results to study variations in albedo and steepness of the phase curve over the surface. Maps of these two photometric parameters show large scale albedo variations, which appear to be associated with compositional differences. They also reveal the location of photometrically extreme terrains, where the phase curve is unusually shallow or steep. We find that shallow phase curves are associated with steep slopes on crater walls and faults, as calculated from a shape model. On the other hand, the phase curve of ejecta associated with young impact craters is steep. We interpret these variations in phase curve slope in terms of physical roughness of the regolith. The lack of rough ejecta around older craters suggests that initially rough ejecta associated with impact craters on Vesta are smoothed over a relatively short time of several tens of Myr. We propose that this process is the result of impact gardening, and as such represents a previously unrecognized aspect of Vesta space weathering ( Pieters et al., 2012 ). If this type of space weathering is common, we may expect to encounter this photometric phenomenon on other main belt asteroids. [ABSTRACT FROM AUTHOR]

Abstract: During its year-long orbital mission, the Dawn spacecraft has mapped the surface of main-belt asteroid Vesta multiple times at different spatial resolutions and illumination and viewing angles. The onboard Framing Camera has acquired thousands of clear filter and narrow band images, which, with the availability of high-resolution global shape models, allows for a photometric characterization of the surface in unprecedented detail. We analyze clear filter images to retrieve the photometric properties of the regolith. In the first part of the paper we evaluate different photometric models for the global average. In the second part we use these results to study variations in albedo and steepness of the phase curve over the surface. Maps of these two photometric parameters show large scale albedo variations, which appear to be associated with compositional differences. They also reveal the location of photometrically extreme terrains, where the phase curve is unusually shallow or steep. We find that shallow phase curves are associated with steep slopes on crater walls and faults, as calculated from a shape model. On the other hand, the phase curve of ejecta associated with young impact craters is steep. We interpret these variations in phase curve slope in terms of physical roughness of the regolith. The lack of rough ejecta around older craters suggests that initially rough ejecta associated with impact craters on Vesta are smoothed over a relatively short time of several tens of Myr. We propose that this process is the result of impact gardening, and as such represents a previously unrecognized aspect of Vesta space weathering (Pieters et al., 2012). If this type of space weathering is common, we may expect to encounter this photometric phenomenon on other main belt asteroids. [Copyright &y& Elsevier]

After coming to rest on the night side of comet 67P/Churyumov-Gerasimenko, the ROLIS camera on-board Rosetta’s Philae lander acquired five images of the surface below the lander, four of which were with the aid of LED illumination of different colors. The images confirm that Philae was perched on a sloped surface. A local horizon is visible in one corner of the image, beyond which we can see the coma. Having spent a full day on the surface Philae was commanded to lift and rotate, after which a final, sixth, LED image was acquired. The change in perspective allowed us to construct a shape model of the surface. The distance to the foreground was about 80 cm, much larger than the nominal 30 cm. This caused stray light, rather than directly reflected LED light, to dominate the image signal, complicating the analysis. The images show a lumpy surface with a roughness of apparently fractal nature. Its appearance is completely different from that of the first landing site, which was characterized by centimeter to meter-sized debris (Mottola et al., 2015). We recognize neither particles nor pores at the image resolution of 0.8 mm per pixel and large color variations are absent. The surface has a bi-modal brightness distribution that can be interpreted in terms of the degree of consolidation, a hypothesis that we support with experimental evidence. We propose the surface below the lander to consist of smooth, cracked plates with unconsolidated edges, similar to terrain seen in CIVA images. [ABSTRACT FROM AUTHOR]

Abstract: We present a new method to perform disk-resolved photometry in order to investigate the intrinsic photometric properties of the surface of small Solar System bodies. We adopt the standard approach where a shape model is combined with a photometric formalism – in practise the Hapke formalism – to remove the effects of topography and recover the photometric (Hapke) parameters of either the global surface or, in its most elaborated form, the spatial variations of these parameters across the surface. Our method operates in the space of the facets representing the three-dimensional shape of the body, whereas all past analysis have always been performed in the space of the image pixels although they are not intrinsic to the surface of the body. This has the advantage of automatically tracking the same local surface element on a series of images. We first apply our method to images of the nucleus of Comet 9P/Tempel 1 obtained by the High-Resolution Imager (HRI) instrument on board the Deep Impact spacecraft and our derived Hapke parameters are in good agreement with those published by Li et al. (Li, J.-Y. et al. [2007]. Icarus 187, 41–55) within their respective uncertainties. We confirm the presence of an extended region of higher roughness in the southern hemisphere of the nucleus and the higher albedo of the ice-rich regions identified by Sunshine et al. (Sunshine, J.M. et al. [2006]. Science 311, 1453–1455) near the equator. The photometric properties of Asteroid (2867) Steins are then studied from multi-spectral images obtained with the OSIRIS Wide Angle Camera (WAC) on board the Rosetta spacecraft during its flyby on 5 September 2008. Our analysis indicates that the surface is highly porous (∼84%) and that it exhibits both a shadow-hiding opposition effect (SHOE) and probably, a coherent-backscatter opposition effect (CBOE). The single scattering albedo is the highest (SSA=0.57) ever observed among small bodies visited by spacecrafts. Our modelled roughness parameter is indicative of a high microscale roughness. The surface of Steins may in fact exhibit a fractal surface with high roughness present on a large range of scales, from micrometers to centimeters. The geometric and Bond albedos are calculated with the Hapke parameters yielding A p =0.39±0.02 and A B =0.24±0.01. This high albedo is consistent with an iron-poor surface composition similar to aubrite meteorites which are suspected to originate from the E-type asteroids. We find no photometric variations on the surface at the (limited) spatial resolution of the WAC images. [Copyright &y& Elsevier]