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During the flyby in 2010, the OSIRIS camera on-board Rosetta acquired hundreds of high-resolution images of asteroid Lutetia's surface through a range of narrow-band filters. While Lutetia appears very bland in the visible wavelength range, Magrin et al. (2012) tentatively identified UV color variations in the Baetica cluster, a group of relatively young craters close to the north pole. As Lutetia remains a poorly understood asteroid, such color variations may provide clues to the nature of its surface. We take the color analysis one step further. First we orthorectify the images using a shape model and improved camera pointing, then apply a variety of techniques (photometric correction, principal component analysis) to the resulting color cubes. We characterize variegation in the Baetica crater cluster at high spatial resolution, identifying crater rays and small, fresh impact craters. We argue that at least some of the color variation is due to space weathering, which makes Lutetia's regolith redder and brighter. [ABSTRACT FROM AUTHOR]

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: The descent imager/spectral radiometer aboard the Huygens probe successfully acquired images and spectra of the surface of Titan. To counter the effects of haze and atmospheric methane absorption it carried a surface science lamp to illuminate the surface just before landing. We reconstruct the reflectance spectrum of the landing site in the 500–1500nm range from downward looking visual and infrared spectrometers data that show evidence of lamp light. Our reconstruction is a followup to the analysis by Tomasko et al. [2005. Rain, winds and haze during the Huygens probe''s descent to Titan''s surface. Nature 438, 765–778], who scaled their result to the ratio of the up- and down flux measured just before landing. Instead, we use the lamp flux from the calibration experiment, and find a significantly higher overall reflectance. We attribute this to a phase angle dependance, possibly representing the opposition surge commonly encountered on solar system bodies. The reconstruction in the visible wavelength range is greatly improved. Here, the reflectance spectrum features a red slope, consistent with the presence of organic material. We confirm the blue slope in the near-IR, featureless apart from a single shallow absorption feature at 1500nm. We agree with Tomasko et al. that the evidence for water ice is inconclusive. By modeling of absorption bands we find a methane mixing ratio of just above the surface. There is no evidence for the presence of liquid methane, but the data do not rule out a wet soil at a depth of several centimeters. [Copyright &y& Elsevier]

Abstract: The descent imager/spectral radiometer (DISR) onboard the Huygens probe investigated the radiation balance inside Titan''s atmosphere and took hundreds of images and spectra of the ground during the descent. The scattering of the aerosols in the atmosphere and the absorption by methane strongly influence the irradiation reaching the surface and the signals received by the various instruments. The physical properties of the surface can only be assessed after the influence of the atmosphere has been taken into account and properly removed. In the broadband visible images (660 to 1000nm) the contrast of surface features is strongly reduced by the aerosol scattering. Calculations show that for an image taken from an altitude of 14.5km, the corrected contrast is about three times higher than in the raw image. Spectral information of the surface by the imaging spectrometers in the visible and near infrared range can only be retrieved in the methane absorption windows. Intensity ratios from the methane windows can be used to make false color maps. The elevated bright ‘land’ terrain is redder than the flat dark ‘lake bed’ terrain. The reflectance spectra of the land and lake bed area in the IR are derived, as well as the reflectance phase function in the limited range from to phase angle. An absorption feature at 1.55 m which may be attributed tentatively to water ice is found in the lake bed, but not in the land area. Otherwise the surface exhibits a featureless blue slope in the near-IR region (0.9–). Brightness profiles perpendicular to the coast line show that the bottoms of the channels of the large scale flow pattern become darker the further they are away from the land area. This could be interpreted as sedimentation of the bright land material transported by the rivers into the lake bed area. The river beds in the deeply incised valleys need not to be covered by dark material. Their roughly 10% brightness decrease could be caused by the illumination as illustrated by a model calculation. The size distribution of cobbles seen in the images after landing is in agreement with a single major flooding of the area with a flow speed of about . [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]

Abstract: Surface morphology and related issues for nuclei of three comets: Halley, Borrelly and Wild 2, are considered in the paper. Joint consideration of publications and results of our analysis of the comets’ images led to conclusions, partly new, partly repeating conclusions published by other researchers. It was found that typical for all three nuclei is the presence of rather flat areas: floors of craters and other depressions, mesas and terraces. This implies that flattening surfaces or planation is a process typical for the comet nuclei. Planation seems to work through the sublimation-driven slope collapse and retreat. This requires effective sublimation so this process should work only when a comet is close to the Sun and if on the nucleus there are starting slopes, steep and high enough to support the “long-distance” avalanching of the collapsing material. If the surface had no starting slopes, then instead of planation, the formation of pitted-and-hilly surfaces should occur. An example of this could be the mottled terrain of the Borelly nucleus. Both ways of the sublimational evolution on the nucleus surface should lead to accumulation of cometary regolith. The thickness of the degassed regolith is not known, but it is obvious that in surface depressions, including the flat-floor ones, it should be larger compared with nondepression areas. This may have implications for the in situ study of comets by the Deep Impact and Rosetta missions. Our morphological analysis puts constraints on the applicability of the popular “rubble-pile comet nucleus” hypothesis (Weissman, 1986. Are cometery nuclei primordial rubble piles? Nature 320, 242–244.). We believe that the rubble pile hypothesis can be applicable to the blocky Halley nucleus. The Borelly and Wild 2 nuclei also could be rubble piles. But in these cases the “rubbles” have to be either smaller than 30–50m (a requirement to keep lineament geometry close to ideal), or larger than 1–2km (a requirement to form the rather extended smooth, flat surfaces of mesa tops and crater floors). Another option is that the Borelly and Wild 2 nuclei are not rubble piles. In relation to surface morphology we suggest that three end-member types of the comet nuclei may exist: (1) impact cratered “pristine” bodies, (2) non-cratered fragments of catastrophic disruption, and (3) highly Sun-ablated bodies. In this threefold classification, the Wild 2 nucleus is partially ablated primarily cratered body. Borrelly is significantly ablated and could be either primarily cratered or not-cratered fragment. Halley is certainly partially ablated but with the available images it is difficult to say if remnants of impact craters do exist on it. Recently published observations and early results of analysis of the Tempel 1 nucleus images taken by Deep Impact mission are in agreement with our conclusions on the processes responsible for the Halley, Borrelly and Wild 2 nuclei morphologies. In particular, we have now more grounds to suggest that decrease in crater numbers and increase of the role of smooth flat surfaces in the sequence Wild 2⩾Tempel 1⩾Borelli reflects a progress in the sublimational degradation of the nucleus surface during comet passages close to the Sun. [Copyright &y& Elsevier]

Abstract: The European SMART-1 mission to the Moon, primarily a testbed for innovative technologies, was launched in September 2003 and will reach the Moon in 2005. On board are several scientific instruments, including the point-spectrometer SMART-1 Infrared Spectrometer (SIR). Taking into account the capabilities of the SMART-1 mission and the SIR instrument in particular, as well as the open questions in lunar science, a selection of targets for SIR observations has been compiled. SIR can address at least five topics: (1) Surface/regolith processes; (2) Lunar volcanism; (3) Lunar crust structure; (4) Search for spectral signatures of ices at the lunar poles; and (5) Ground truth and study of geometric effects on the spectral shape. For each topic we will discuss specific observation modes, necessary to achieve our scientific goals. The majority of SIR targets will be observed in the nadir-tracking mode. More than 100 targets, which require off-nadir pointing and off-nadir tracking, are planned. It is expected that results of SIR observations will significantly increase our understanding of the Moon. Since the exact arrival date and the orbital parameters of the SMART-1 spacecraft are not known yet, a more detailed planning of the scientific observations will follow in the near future. [Copyright &y& Elsevier]

This paper is based on geologic analysis of the surface morphology of nucleus of the Jupiter family comet 67P. This comet was visited by the ESA mission Rosetta, which escorted the comet since May 2014 till the end of September 2016 and studied it by 11 instruments of the mission orbiter and 10 instruments of the lander. The nucleus is ~4 km in diameter, has a bilobate shape with the smaller (Head) and larger (Body) lobes, and the narrow neck between them. For the analysis, primarily images taken by the Rosetta Navigation camera (NavCam) were used and then complemented by selected images from the ROLIS and OSIRIS cameras. Two major types of the nucleus material are distinguished by us and other researchers: 1) the consolidated nucleus material and 2) the loose material, a kind of cometary regolith, covering the nucleus consolidated material. On the surface of the consolidated material rather long (up to hundreds meters) straight lineaments are distinguishable. They probably correspond to fractures and in some cases to strata. Their presence suggests that the consolidated material is rather compact and lacks voids larger than tens of meters across. Surfaces of consolidated nucleus material typically show knobby appearance at the scales from tens of meters and meters to centimeters and millimeters. This suggests that this material is grainy, consisting of more and less resistant (to surface weathering) “particles” on the scale of the visible knobs. The geometric analysis of steep slopes based on the nucleus shape model allowed us to estimate a tensile, shear and compressive strength of the consolidated material. It was shown that the 67P consolidated nucleus material is very fragile, and taking into account the scale effect one can conclude that it is as fragile as fresh fallen snow and maybe even more fragile. In addition, estimates of the compressive strength of the surface material were considered at the sites of the first and the last contacts of the Philae lander with the surface. Observations also showed evidence of various downslope and lateral movements of rather large material masses (landslide? avalanche?) as well as boulders and “fines”, which are driven primarily by gravity and then by the acquired inertia, but in some cases a material transport by dust-gas jets/outbursts could play a role. The latter could also be responsible for formation of the eolian-type ripples. [ABSTRACT FROM AUTHOR]

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]

Abstract: The surface of (21) Lutetia is highly complex with significant interactions between ancient and more recent structures. This work attempts to summarize the surface geomorphology observed using the high resolution images from OSIRIS, the imaging system onboard the European Space Agency''s Rosetta spacecraft. A wide range of surface morphologies are seen including heavily cratered terrain, extensive sets of lineaments, young impact craters, and a ridge, the height of which is more than 1/5th of the mean radius of the body. Very young and very old terrains (as inferred from crater densities) are seen in close proximity. The longest continuous lineament is over 80km long. The lineaments show regional-dependent organization and structure. Several categories of lineament can be described. Lineaments radial to impact craters as seen on other asteroidal bodies are mostly absent. Although the lineaments may be of seismic origin (and possibly the result of several impact-induced events), impacts producing recent large craters place constraints on seismic phenomena. In particular, stronger attenuation of shocks than seen on other asteroidal bodies seems to be required. Inhomogeneous energy transport, possibly matching observed inhomogeneous ejecta deposition may offer explanations for some of the observed phenomena. Some impact craters show unusual forms, which are probably the result of impact into a surface with relief comparable to the resultant crater diameter and/or oblique impact. There is evidence that re-surfacing through landslides has occurred at several places on the object. [Copyright &y& Elsevier]

Abstract: A general circulation model of the Martian Atmosphere is coupled with a 3-dimensional polythermal ice-sheet model of the polar ice caps. With this combination a series of experiments is carried out to investigate the impact of long-term obliquity change on the Martian north polar ice cap (NPC). The behaviour of the NPC is tested under obliquities of , and . With increasing obliquity the area covered by the NPC gets smaller but does not vanish. However, when started from an ice-free condition the models develop an ice cap only for low obliquities. The ‘critical’ obliquity at which a build-up of a new polar cap is possible is . [Copyright &y& Elsevier]

Abstract: A climate model of intermediate complexity, named the Mars Climate Simulator, has been developed based on the Portable University Model of the Atmosphere (PUMA). The main goal of this new development is to simulate the climate variations on Mars resulting from the changes in orbital parameters and their impact on the layered polar terrains (also known as permanent polar ice caps). As a first step towards transient simulations over several obliquity cycles, the model is applied to simulate the dynamical and thermodynamical response of the Martian climate system to different but fixed obliquity angles. The model is forced by the annual and daily cycle of solar insolation. Experiments have been performed for obliquities of (minimum), (present), and (maximum). The resulting changes in solar insolation mainly in the polar regions impact strongly on the cross-equatorial circulation which is driven by the meridional temperature gradient and steered by the Martian topography. At high obliquity, the cross-equatorial near surface flow from the winter to the summer hemisphere is strongly enhanced compared to low obliquity periods. The summer ground temperature ranges from 200K to 250K at in northern summer, and from 220K to 270K at in southern summer. In the atmosphere at 1km above ground, the respective range is 195–225K in northern summer, and 210–250K in southern summer. [Copyright &y& Elsevier]

Context Pinnacles, local promontories of varied shapes including spires with pointed tops, are observed on the surface of the 67P's cometary nucleus in the OSIRIS and NavCam images. Their presence and characteristics allow to infer the magnitude of the surface erosion and the degree of heterogeneity of the consolidated nucleus material. Aims To identify these pinnacles and consider implications, which follow from their presence and characteristics. Methods Identification of pinnacles in the available OSIRIS and NavCam images, as well as in the model images of the nucleus built from the digital terrain model, and an investigation of their morphology. Results We identified and measured 49 pinnacles in 13 regions of the studied part of the nucleus. The pinnacles are typically asymmetric with somewhat different slope angles at different slopes whose maximum values range from 40 to 90°, sometimes with small overhangs. Their heights vary from 10 to 20 to 100–200 m and the foot diameters from 30 to 300 m. Our analysis of the OSIRIS images of the pinnacles showed that they have surface texture similar to that of consolidated nucleus material and are probably composed of it. Conclusions The observed characteristics of pinnacles found on 67P agree with the suggestion that they are erosion remnants formed due to a slowdown of erosion in places where nucleus material is more resistant to sublimational erosion than the material around it. In this case the maximum heights of pinnacles (100–200 m) gives a lower boundary for the amount of surface material lost and their diameters (typically tens of meters) are a measure of the size of the erosion-resistant parts. [ABSTRACT FROM AUTHOR]

Abstract: The cratering history of main belt asteroid (2867) Steins has been investigated using OSIRIS imagery acquired during the Rosetta flyby that took place on the 5th of September 2008. For this purpose, we applied current models describing the formation and evolution of main belt asteroids, that provide the rate and velocity distributions of impactors. These models coupled with appropriate crater scaling laws, allow the cratering history to be estimated. Hence, we derive Steins’ cratering retention age, namely the time lapsed since its formation or global surface reset. We also investigate the influence of various factors—like bulk structure and crater erasing—on the estimated age, which spans from a few hundred Myrs to more than 1Gyr, depending on the adopted scaling law and asteroid physical parameters. Moreover, a marked lack of craters smaller than about 0.6km has been found and interpreted as a result of a peculiar evolution of Steins cratering record, possibly related either to the formation of the 2.1km wide impact crater near the south pole or to YORP reshaping. [Copyright &y& Elsevier]

Abstract: The Venus Express mission will focus on a global investigation of the Venus atmosphere and plasma environment, while additionally measuring some surface properties from orbit. The instruments PFS and SPICAV inherited from the Mars Express mission and VIRTIS from Rosetta form a powerful spectrometric and spectro-imaging payload suite. Venus Monitoring Camera (VMC)—a miniature wide-angle camera with 17.5° field of view—was specifically designed and built to complement these experiments and provide imaging context for the whole mission. VMC will take images of Venus in four narrow band filters (365, 513, 965, and 1000nm) all sharing one CCD. Spatial resolution on the cloud tops will range from 0.2km/px at pericentre to 45km/px at apocentre when the full Venus disc will be in the field of view. VMC will fulfill the following science goals: (1) study of the distribution and nature of the unknown UV absorber; (2) determination of the wind field at the cloud tops (70km) by tracking the UV features; (3) thermal mapping of the surface in the 1μm transparency “window” on the night side; (4) determination of the global wind field in the main cloud deck (50km) by tracking near-IR features; (5) study of the lapse rate and H2O content in the lower 6–10km; (6) mapping O2 night-glow and its variability. [Copyright &y& Elsevier]

The microscope for the Beagle 2 lander, which was launched as part of the European Space Agency''s Mars Express mission on 2 June 2003, will provide images of the Martian surface at around 6 μm resolution. It will provide optical images of the surface of Mars at a resolution 5 times higher than any other experiment currently planned. The device has a working distance of 12 mm and uses a set of 12 light-emitting diodes which surround the aperture to illuminate the sample in four colours. The target is brought into focus using a stepper motor. This article describes the scientific objectives and the design of the microscope. It also discusses initial results from ground calibration exercises which were designed to validate the system and describes aspects of its operation. [Copyright &y& Elsevier]

By successively orbiting both 4 Vesta and 1 Ceres the Dawn mission directly addresses the long-standing goals of understanding the origin and evolution of the solar system. Ceres and Vesta are two complementary terrestrial protoplanets (one apparently “wet” and the other “dry”), whose accretion was probably terminated by the formation of Jupiter. They provide a bridge in our understanding between the rocky bodies of the inner solar system and the icy bodies of the outer solar system. Ceres appears to be undifferentiated while Vesta has experienced significant heating and likely differentiation. Both formed very early in the history of the solar system and while suffering many impacts have remained intact, thereby retaining a record of events and processes from the time of planet formation. Detailed study of the geophysics and geochemistry of these two bodies provides critical benchmarks for early solar system conditions and processes that shaped its subsequent evolution. Dawn provides the missing context for both primitive and evolved meteoritic data, thus playing a central role in understanding terrestrial planet formation and the evolution of the asteroid belt. Dawn is to be launched in May 2006 arriving at Vesta in 2010 and Ceres in 2014, stopping at each to make 11 months of orbital measurements. The spacecraft uses solar electric propulsion, both in cruise and in orbit, to make most efficient use of its xenon propellant. The spacecraft carries a framing camera, visible and infrared mapping spectrometer, gamma ray/neutron spectrometer, magnetometer, and radio science. [Copyright &y& Elsevier]

By successively orbiting both 4 Vesta and 1 Ceres the Dawn mission directly addresses the long-standing goals of understanding the origin and evolution of the solar system. Ceres and Vesta are two complementary terrestrial protoplanets (one apparently “wet” and the other “dry”), whose accretion was probably terminated by the formation of Jupiter. They provide a bridge in our understanding between the rocky bodies of the inner solar system and the icy bodies of the outer solar system. Ceres appears to be undifferentiated while Vesta has experienced significant heating and likely differentiation. Both formed very early in the history of the solar system and while suffering many impacts have remained intact, thereby retaining a record of events and processes from the time of planet formation. Detailed study of the geophysics and geochemistry of these two bodies provides critical benchmarks for early solar system conditions and processes that shaped its subsequent evolution. Dawn provides the missing context for both primitive and evolved meteoritic data, thus playing a central role in understanding terrestrial planet formation and the evolution of the asteroid belt. Dawn is to be launched in May 2006 arriving at Vesta in 2010 and Ceres in 2014, stopping at each to make 11 months of orbital measurements. The spacecraft uses solar electric propulsion, both in cruise and in orbit, to make most efficient use of its xenon propellant. The spacecraft carries a framing camera, visible and infrared mapping spectrometer, gamma ray/neutron spectrometer, magnetometer, and radio science. [Copyright &y& Elsevier]

In January 2005, the Huygens probe will descent through Titan''s atmosphere and the Descent Imager/Spectral Radiometer (DISR) will perform upward and downward looking observations at various spectral ranges and spatial resolutions. One of the subinstruments, the Upward Looking Visible Spectrometer (ULVS), measures the total downward radiation flux including the direct solar beam and also, with a shadow bar over the Sun, the diffuse downward flux. The intensity of the direct solar beam and thus the optical depth can be calculated from the difference of these two measurements. But 10° wide shadow bar also obscures the Solar Aureole Imager (SA) and therefore removes a considerable fraction of the diffuse downward radiation. This fraction can be estimated taking into account the brightness distribution of the SA which is estimated with the Titan Inverse Radiation Model (TIRM). Input to the model are a first guess of the optical depth in dependence on the altitude calculated directly from ULVS measurements and data from another DISR subinstrument, the Solar Aureole Imager SA imager. By assimilating the sparse SA data, TIRM yields a consistent estimate of the scattering phase function and the complete radiance field in dependence on the altitude. By iteratively correcting the initial optical depth estimation using the resultant radiance field and passing it again to TIRM, the model is used to simultaneously solve for optical depths and scattering phase functions. [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]