Search

Sodium and, in a lesser way, potassium atomic components of surface-bounded exospheres are among the brightest elements that can be observed from the Earth in our Solar System. Both species have been intensively observed around Mercury, the Moon and the Galilean Moons. During the last decade, new observations have been obtained thanks to space missions carrying remote and in situ instrumentation that provide a completely original view of these species in the exospheres of Mercury and the Moon. They challenged our understanding and modelling of these exospheres and opened new directions of research by suggesting the need to better take into account the relationship between the surface-exosphere and the magnetosphere. In this paper, we first review the large set of observations of Mercury and the Moon Sodium and Potassium exospheres. In the second part, we list what it tells us on the sources and sinks of these exospheres focusing in particular on the role of their magnetospheres of these objects and then discuss, in a third section, how these observations help us to understand and identify the key drivers of these exospheres.

The STereo imaging Channel (STC) is a double wide-angle camera developed to be one of the channels of the SIMBIO-SYS instrument onboard of the ESA BepiColombo mission to Mercury. STC main goal is to map in 3D the whole Mercury surface. The geometric and radiometric responses of the STC Proto Flight model have been characterized on-ground during the calibration campaign. The derived responses will be used to calibrate the STC images that will be acquired in flight. The aim is to determine the functions linking the detected signal in digital number to the radiance of the target surface in physical units. The result of the radiometric calibration consists in the determination of well-defined quantities: (1) the dark current as a function of the integration time and of the detector temperature, settled and controlled to be stable at 268 K; (2) the read out noise, which is associated with the noise signal of the read-out electronic; and (3) the fixed pattern noise, which is generated by the different response of each pixel. Once these quantities are known, the photon response and the photo-response non-uniformity, which represents the variation of the photon responsivity of a pixel in an array, can be derived. The final result of the radiometric calibration is the relation between the radiance of an accurately known and uniform source, and the digital numbers measured by the detector.

At the beginning of the Ceres investigation, the Dawn-NASA mission discovered a large bright spot (BS) in the Occator crater floor. Several other smaller bright spots were discovered during the following phases of the mission. In this paper, a complete survey for the detection of BS on the Ceres surface have been made by using the hyperspectral data acquired by Visible and Infrared Mapping Spectrometer (VIR). The hyperspectral images span the spectral range from 0.2 to 5 μm, by using two channel, the VIS channel with a spectral sampling of 1.8 nm and a IR channel with a spectral sampling of 9.8 nm. Finally a catalogue of 92 BS has been compiled and their compositional properties have been examined. In particular, five spectral parameters have been applied to perform the analysis: the photometrically corrected reflectance and four band depths, related to spectral absorptions at 2.7 μm (OH fundamental indicative of phyllosilicates), at 3.05 μm (due to ammoniated clays), at 3.4 and 4.0 μm (carbonate overtones). The 90% of BS are impact-related features (ejecta, crater rim, crater floor, crater wall). The two brightest BS, Cerealia and Vinalia Faculae, are located on the Occator crater floor. Most of BSs show features similar to the average Ceres surface, which has low reflectance and is composed of Mg-phyllosilicates and ammoniated clays, with a reduced abundance of Mgsbnd Ca carbonates. Cerealia and Vinalia Faculae are a peculiar BS family, with a high abundance of Na-carbonates and Al-rich phyllosilicates. Oxo and a companion bright spot represents a third category, depleted in phyllosilicates and with a high to moderate albedo. Carbonate composition ranges from Mg/Ca to Na components. Haulani, Ernutet, Kupalo, and other two BS's represent another group, with intermediate properties between the typical BS and the Oxo family: they are moderately rich in carbonates and slightly depleted in Mg- and ammoniated phyllosilicates. The four families probably explain a single evolutionary path followed by the BS from the formation to their maturity: initially the very fresh bright spots would possess characteristics similar to Cerealia and Vinalia Faculae; with time, salts and OH volatilize and a light mixing with surrounding material would produce Oxo-like BS's; additional strong mixing would form Haulani-like BS, which finally become a typical bright spots.

Introduction Some of the comets visited by spacecraft missions display some circular depressions at their surface: 81P/Wild 2 (Brownlee et al. 2004), 9P/Tempel 1 (Belton et al. 2013), 103P/Hartley 2 (Bruck Syal et al. 2013), 67P/C-G (Vincent et al. 2015). For 67P, they consist of circular holes, half holes or cliffs, with a size range of tens of meters to a few hundreds of meters (Ip et al. 2016). Owing to the high precision of the shape model obtained from the Rosetta/OSIRIS images (Preusker et al. 2015, Sierks et al. 2015), it is possible to investigate the thermal processing of 67P’s surface in relation to the formation and evolution of these features (Mousis et al. 2015, Vincent et al. 2015, Guilbert-Lepoutre et al. 2016). Methods We aim to investigate the formation and evolution of 67P’s circular depressions (or pits, thereafter) by thermally-induced processes (for instance sublimation and amorphous water ice crystallization) on its current orbit. In a departure from the aforementioned studies, we consider a high-resolution shape model of the nucleus, which allows to study several facets for each pit: at the bottom, and on the walls. For each facet, the complete thermal environment is considered, including self-heating and shadowing, either by neighboring facets or due to the complex global morphology of the comet. We compute the illumination, self-heating and shadowing conditions for 125k facets during a full orbit, with a time step of ~8 min, then use these conditions as an input of a 1D thermal evolution model for each facet. The model includes standard features: heat conduction, phase transitions, gas diffusion, erosion, dust mantling (De Sanctis et al. 2005, 2010, Lasue et al. 2008). Various initial setups have been considered, and many tests were conducted to assess the influence of each parameter. The behaviour of 30 circular depressions (pits, half pits and cliffs) was studied in detail (see Figure 1). Results and discussion We find that the following processes do not contribute significantly to the evolution of pits: sublimation of CO and CO2, crystallization of amorphous water ice, and dust mantling. When added to the model, they induce a relatively limited effect, altering the results by less than 10%. Sublimation of water, and therefore erosion, is the main acting process. We find that direct illumination is the main driver for gas production and erosion. Self-heating is not negligible, and in many cases, it allows to sustain some processing for longer periods of time and enhance local erosion. This is especially true for surface features located close to the neck, where facets additionally receive the VIS+IR flux from the small lobe. The total flux received per orbit is crucial, so is the flux received at perihelion. In this regard, we find strong differences between the Northern and Southern hemispheres of the nucleus, observed in other studies (Keller et al. 2015, Tosi et al. 2019). Finally, there is a tendency for facets in the North which are directed towards the equator to sustain more erosion than other facets at similar latitudes. At the scale of a given pit, there is a general tendency for cliffs and walls to receive more energy than the bottoms, and thus erode more. With time, the fate of a circular depression on 67P is thus to become wider and shallower. Nevertheless, in limited instances of small deep pits (such as Seth01), self-heating can be the driver for erosion of both the walls and bottom, since direct illumination is very limited. However, local erosion rates remain relatively low compared to erosion rates sustained by pits with direct illumination by the Sun. In general, we find that the erosion sustained after 10 orbits cannot reach the size extent of pits as they were observed by Rosetta. It is therefore very unlikely that current illumination conditions were able to produce those features. This results joins previous studies (Besse et al. 2015, 2017, Guilbert-Lepoutre et al. 2016). Because we have performed this study with a uniform set of thermo-physical parameters for all facets, we cannot exclude that local heterogeneities, such as the presence of ice patches in the bottom of some pits (Lamy et al. 2018) may help accelerate the erosion at depths in those pits. Fig .1: Erosion sustained after 10 orbits in the current illumination conditions (solar+shadowing+self-heating), for a selection of facets in the 125k resolution shape model. Acknowledgements This study is part of a project that has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 802699). We gratefully acknowledge support from the PSMN (Pôle Scientifique de Modélisation Numérique) of the ENS de Lyon for the computing resources. References Belton et al. (2013) Icarus, 222, 477-486 Besse et al. (2015) EPSC conference, id.EPSC2015-114 Besse et al. (2017) ACM conference Brownlee et al. (2004) Science, 304, 1764-1769 Bruck Syal et al. (2013) Icarus, 222, 610-624 De Sanctis et al. (2005) A&A, 444, 605-614 De Sanctis et al. (2010) Icarus, 207, 341-358 Guilbert et al. (2016) MNRAS, 462, 146-155 Ip et al. (2016) A&A, EDP Sciences, 591, A132 Keller et al. (2015) A&A, 583, A34 Lamy et al. (2018) COSPAR Scientific Assembly, id. B1.1-6-18 Lasue et al. (2008) P&SS, 56, 1977-1991 Mousis et al. (2015) ApJL, 814, L5 Preusker et al. (2015) A&A, 583, A33 Sierks et al. (2015) Science, 347, aaa1044 Tosi et al. (2019) Nat. Astron, 3, 649-658 Vincent et al. (2015) Nature, 523, 63-66

The STereo Imaging Channel (STC) is one of the three channels of the SIMBIO-SYS instrument on board the BepiColombo ESA spacecraft. The design of the camera consists in a double wide-angle camera with two sub-channels looking at ±20° with respect to the nadir direction. Each sub-channel can acquire three quasi-contiguous areas of the Mercury surface in different colours determined by the filters mounted on the detector. The filters are divided in two categories: 4 broad band filters (20 nm of bandwidth and centred at 420, 550, 750 and 920 nm respectively) necessary to the chemical analysis of the Hermean surface; 2 panchromatic (PAN) filters with 200 nm of bandwidth and centred at 600 nm, designed for the stereo acquisition. The nominal Field of View (FoV) of each sub-channel is 5:38°x4:8°. The in-flight stellar calibration will be performed during the nominal mission using stellar fields images. To effectively plan this calibration activity, two analyses have been performed: the first one consists in simulating the observation of stars having different apparent magnitude to derive the best integration time needed to reach a specific Signal to Noise Ratio. Considering the characteristics of the STC camera and of its CMOS detector, the threshold magnitude needed for a star to be detectable will also be determined. The second part consists in selecting the stellar fields from the ESA GAIA archive and Tycho stellar catalogue that contain a pre-defined minimum number of stars required to perform the in-flight geometrical calibration. This selection have been performed taking into account stars brighter than the threshold defined in the first part.

We studied the distribution in the Urvara–Yalode region of Ceres (latitudes 21–66°S, longitudes 180–360°E) of main spectral parameters derived from the VIR imaging spectrometer onboard the NASA/Dawn spacecraft, as an overall study of Ceres mineralogy reported in this special issue. In particular, we analyzed the distribution of reflectance at 1.2 µm, band depth at 2.7 and 3.1 µm, ascribed to magnesium and ammoniated phyllosilicates, respectively. Whereas the average band depths of this region are lower than eastern longitudes, reflecting the E-W dichotomy of abundance of phyllosilicates on Ceres, spectral variations inside this region are observed in the following units: (a) the central peak of the Urvara crater (45.9°S, 249.2°E, 170 km in diameter), showing a deep 3.1 µm band depth, indicating an ammonium enrichment; (b) the cratered terrain westwards of the Yalode basin (42.3°S, 293.6°E, 260 km in diameter), where absorption bands are deeper, probably due to absence of phyllosilicates depletion following the Yalode impact; (c) the hummocky cratered floor of Yalode and Besua (42.4°S, 300.2°E) craters, characterized by lower albedo and band depths, probably due to different roughness; (d) Consus (21°S 200°E) and Tawals (39.1°S, 238°E) craters, whose albedo and band depths decreasing could be associated to different grain size or abundance of dark materials. Twenty-two small scale (i.e., lower than 400 m) bright spots are observed: because their composition is similar to the Ceres average, a strong mixing may have occurred since their formation.

Aims. Comet C/2020 F3 (NEOWISE) is considered to be the brightest comet observed in the northern hemisphere since the passage of comet C/1995 O1 (Hale-Bopp) in 1997. Since the study of comets offers a unique opportunity to investigate the early stages of the formation and evolution of our Sun and the Solar System, we obtained high-resolution optical spectra (R = λ/Δλ = 11 5000) of comet NEOWISE. The unique passage and its brightness yielded spectra with a large number of emission lines, providing information on the coma composition and the physical and chemical processes occurring in the nucleus. The spectra have been used to generate a catalog of emission lines to be used for future studies of comets since there are no catalogs in the literature with such a high spectral resolution. Methods. Two high-resolution spectra of comet NEOWISE were obtained, on 26 July 2020 (geocentric distance of 0.7 AU) and 5 August 2020 (geocentric distance of 0.89 AU), with the High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N) echelle spectrograph installed on the 360 cm Telescopio Nazionale Galileo. The spectra cover the range between 383 and 693 nm, and have been extracted using the HARPS-N Data Reduction Pipeline. To analyze the spectra and compile the high-resolution catalog, we collected several laboratory molecular line lists that cover the same wavelength range as that of our spectra. To validate the final identification, we compared our catalog with other atlases that resulted from the spectral analysis of other comets. Results. We generate a high-spectral-resolution catalog of emission lines observed in comet NEOWISE, providing the identification for 4488 lines. We found cometary lines due to CN, CH, C2, C3, and NH2 and atomic lines due to NaI and [OI].

Thermal inertia is a key information to quantify the physical status of a planetary surface; it can be retrieved by comparison between theoretical and observed temperature diurnal profiles. We have calculated the surface temperature for a set of locations on Ceres' surface with a thermophysical model that provides temperature as a function of thermal conductivity and roughness, and we have determined the values of those parameters for which the best fit with the observed data is obtained. The observed temperatures have been retrieved form spatially-resolved data from the Dawn mission. In our previous work [Rognini et al., 2019], we have found that the average thermal inertia for the overall surface of Ceres is low (from 1 to 15 to 60 ​J ​m−2 ​s−½ K−1), as expected according to the general trend observed in the Solar System for atmosphere-less bodies, while the thermal inertia of the very bright faculae found in the floor of the Occator crater could not be well defined. Using more recently acquired VIR high resolution data we find that the central part of the Cerealia facula displays a thermal anomaly ( ~ 10 K above the average) compatible with a higher thermal inertia with respect to the surrounding regions, while the Vinalia facula does not display any consequently could have a grain size comparable with the Ceres’ surface average.

International audience

2I/Borisov is the first interstellar comet discovered on 2019 August 30, and it soon showed a coma and a dust tail. This study reports the results of images obtained at the Telescopio Nazionale Galileo telescope, on La Palma - Canary Islands, in 2019 November and December. The images have been obtained with the R filter in order to apply our dust tail model. The model has been applied to the comet 67P/Churyumov-Gerasimenko and compared to the Rosetta dust measurements showing a very good agreement. It has been applied to the comet 2I/Borisov, using almost the same parameters, obtaining a dust environment similar to that of 67P/Churyumov-Gerasimenko, suggesting that the activity may be very similar. The dust tail analysis provided a dust-loss rate Qd ≍ 35 kg s-1 in 2019 November and Qd ≍ 30 kg s-1 in 2019 December.

BepiColombo is the fifth cornerstone mission of the European Space Agency (ESA) dedicated to study the Mercury planet. The BepiColombo spacecraft comprises two science modules: the Mercury Planetary Orbiter (MPO) realized by ESA and the Mercury Magnetospheric Orbiter provided by the Japan Aerospace Exploration Agency. The MPO is composed by 11 instruments, including the ‘Spectrometer and Imagers for MPO BepiColombo Integrated Observatory System’ (SIMBIOSYS). The SIMBIOSYS suite includes three optical channels: a Stereoscopic Imaging Channel, a High Resolution Imaging Channel, and a Visible and near Infrared Hyperspectral Imager. SIMBIOSYS will characterize the hermean surface in terms of surface morphology, volcanism, global tectonics, and chemical composition. The aim of this work is to describe a tool for the radiometric response prediction of the three SIMBIOSYS channels. Given the spectral properties of the surface, the instrument characteristics, and the geometrical conditions of the observation, the realized SIMBIOSYS simulator is capable of estimating the expected signal and integration times for the entire mission lifetime. In the simulator the spectral radiance entering the instrument optical apertures has been modelled using a Hapke reflectance model implementing the parameters expected for the hermean surface. The instrument performances are simulated by means of calibrated optical and detectors responses. The simulator employs the SPICE (Spacecraft, Planet, Instrument, C-matrix, Environment) toolkit software, which allows us to know for each epoch the exact position of the MPO with respect to the planet surface and the Sun.

Full list of authors: Cremonese, G.; Capaccioni, F.; Capria, M. T.; Doressoundiram, A.; Palumbo, P.; Vincendon, M.; Massironi, M.; Debei, S.; Zusi, M.; Altieri, F.; Amoroso, M.; Aroldi, G.; Baroni, M.; Barucci, A.; Bellucci, G.; Benkhoff, J.; Besse, S.; Bettanini, C.; Blecka, M.; Borrelli, D.; Brucato, J. R.; Carli, C.; Carlier, V.; Cerroni, P.; Cicchetti, A.; Colangeli, L.; Dami, M.; Da Deppo, V.; Della Corte, V.; De Sanctis, M. C.; Erard, S.; Esposito, F.; Fantinel, D.; Ferranti, L.; Ferri, F.; Ficai Veltroni, I.; Filacchione, G.; Flamini, E.; Forlani, G.; Fornasier, S.; Forni, O.; Fulchignoni, M.; Galluzzi, V.; Gwinner, K.; Ip, W.; Jorda, L.; Langevin, Y.; Lara, L.; Leblanc, F.; Leyrat, C.; Li, Y.; Marchi, S.; Marinangeli, L.; Marzari, F.; Mazzotta Epifani, E.; Mendillo, M.; Mennella, V.; Mugnuolo, R.; Muinonen, K.; Naletto, G.; Noschese, R.; Palomba, E.; Paolinetti, R.; Perna, D.; Piccioni, G.; Politi, R.; Poulet, F.; Ragazzoni, R.; Re, C.; Rossi, M.; Rotundi, A.; Salemi, G.; Sgavetti, M.; Simioni, E.; Thomas, N.; Tommasi, L.; Turella, A.; Van Hoolst, T.; Wilson, L.; Zambon, F.; Aboudan, A.; Barraud, O.; Bott, N.; Borin, P.; Colombatti, G.; El Yazidi, M.; Ferrari, S.; Flahaut, J.; Giacomini, L.; Guzzetta, L.; Lucchetti, A.; Martellato, E.; Pajola, M.; Slemer, A.; Tognon, G.; Turrini, D. -- This is an open access article, The SIMBIO-SYS (Spectrometer and Imaging for MPO BepiColombo Integrated Observatory SYStem) is a complex instrument suite part of the scientific payload of the Mercury Planetary Orbiter for the BepiColombo mission, the last of the cornerstone missions of the European Space Agency (ESA) Horizon + science program. The SIMBIO-SYS instrument will provide all the science imaging capability of the BepiColombo MPO spacecraft. It consists of three channels: the STereo imaging Channel (STC), with a broad spectral band in the 400-950 nm range and medium spatial resolution (at best 58 m/px), that will provide Digital Terrain Model of the entire surface of the planet with an accuracy better than 80 m; the High Resolution Imaging Channel (HRIC), with broad spectral bands in the 400-900 nm range and high spatial resolution (at best 6 m/px), that will provide high-resolution images of about 20% of the surface, and the Visible and near-Infrared Hyperspectral Imaging channel (VIHI), with high spectral resolution (6 nm at finest) in the 400-2000 nm range and spatial resolution reaching 120 m/px, it will provide global coverage at 480 m/px with the spectral information, assuming the first orbit around Mercury with periherm at 480 km from the surface. SIMBIO-SYS will provide high-resolution images, the Digital Terrain Model of the entire surface, and the surface composition using a wide spectral range, as for instance detecting sulphides or material derived by sulphur and carbon oxidation, at resolutions and coverage higher than the MESSENGER mission with a full co-alignment of the three channels. All the data that will be acquired will allow to cover a wide range of scientific objectives, from the surface processes and cartography up to the internal structure, contributing to the libration experiment, and the surface-exosphere interaction. The global 3D and spectral mapping will allow to study the morphology and the composition of any surface feature. In this work, we describe the on-ground calibrations and the results obtained, providing an important overview of the instrument performances. The calibrations have been performed at channel and at system levels, utilizing specific setup in most of the cases realized for SIMBIO-SYS. In the case of the stereo camera (STC), it has been necessary to have a validation of the new stereo concept adopted, based on the push-frame. This work describes also the results of the Near-Earth Commissioning Phase performed few weeks after the Launch (20 October 2018). According to the calibration results and the first commissioning the three channels are working very well. © 2020, The Author(s)., We gratefully acknowledge funding from the Italian Space Agency (ASI) under ASI-INAF agreement 2017-47-H.0. The SIMBIO-SYS instrument has been developed by Leonardo under ASI contract I/054/10/0.

International audience; K nowledge of the surface temperature distribution of a comet's nucleus proves to be of fundamental importance for a number of reasons: the evaluation of the thermophysical properties (thermal inertia and roughness, at several spatial scales), the characterization of the thermal behaviour of peculiar surface units (for example, local ice exposures, or inherently cold material units not to be confused with shaded areas), and for the understanding of the physical processes affecting the surface and the shallow subsur-face layers (for example, sublimation of volatile compounds). The distribution of surface temperatures of a cometary nucleus, closely measured by a spacecraft, can be compared with theoretical models meant to predict the thermophysical properties of the nucleus at various depths and with previous thermal observations carried out by both Earth-based and space-based telescopes, with the goal of validating and/or improving those models 1. Furthermore, the temporal evolution of the surface temperatures, primarily driven by insolation, is critical in triggering the activity of a comet, allowing the migration of volatile compounds from the interior of the nucleus and the production of gas and dust observable with different techniques 2. In the past, thermal surveys of minor bodies were carried out using ground-based and space-based facilities. However, due to the limited spatial resolution of those observations, in most cases the derived thermal properties were only global averages, and the determined temperatures were highly model dependent. Before Rosetta, direct measurements of cometary nuclei surface temperatures were obtained during short periods for a handful of comets, namely 1P/ Halley 3 , 19P/Borrelly 4 , 9P/Tempel 1 1 and 103P/Hartley 2 5 , with a maximum spatial resolution of approximately 30 m per pixel. Here we study the surface temperature distribution of the nucleus of comet 67P/Churyumov-Gerasimenko as derived by the Visible InfraRed and Thermal Imaging Spectrometer, Mapping channel (VIRTIS-M, hereafter VIRTIS) 6 in Rosetta's early global mapping phase after comet encounter. These data cover the pre-perihelion period from 1 August to 23 September 2014, when the heliocentric distance decreased from 3.62 to 3.31 au and the spacecraft was in the altitude range 61-13 km above the surface, resulting in a spatial resolution from approximately 15 to 3 m per pixel (most data showing a resolution of 13 to 15 m per pixel). In this period, the solar phase angle ranged from 17° to 93°, which Knowledge of the surface temperature distribution on a comet's nucleus and its temporal evolution at different timescales is key to constraining its thermophysical properties and understanding the physical processes that take place at and below the surface. Here we report on time-resolved maps of comet 67P/Churyumov-Gerasimenko retrieved on the basis of infra-red data acquired by the Visible InfraRed and Thermal Imaging Spectrometer (VIRTIS) onboard the Rosetta orbiter in 2014, over a roughly two-month period in the pre-perihelion phase at heliocentric distances between 3.62 and 3.31 au from the Sun. We find that at a spatial resolution ≤15 m per pixel, the measured temperatures point out the major effect that self-heating, due to the complex shape of the nucleus, has on the diurnal temperature variation. The bilobate nucleus of comet 67P also induces daytime shadowing effects, which result in large thermal gradients. Over longer periods, VIRTIS-derived temperature values reveal seasonal changes driven by decreasing heliocentric distance combined with an increasing abundance of ice within the uppermost centimetre-thick layer, which implies the possibility of having a largely pristine nucleus interior already in the shallow subsurface.

Rosetta observes sublimating surface icesComets are “dirty snowballs” made of ice and dust, but they are dark because the ice sublimates away, leaving some of the dust behind on the surface. The Rosetta spacecraft has provided a close-up view of the comet 67P/Churyumov-Gerasimenko as it passes through its closest point to the Sun (see the Perspective by Dello Russo). Filacchioneet al.detected the spectral signature of solid CO2(dry ice) in small patches on the surface of the nucleus as they emerged from local winter. By modeling how the CO2sublimates, they constrain the composition of comets and how ices generate the gaseous coma and tail. Fornasieret al.studied images of the comet and discovered bright patches on the surface where ice was exposed, which disappeared as the ice sublimated. They also saw frost emerging from receding shadows. The surface of the comet was noticeably less red just after local dawn, indicating that icy material is removed by sunlight during the local day.Science, this issue p.1563, p.1566; see also p.1536

Spectral parameters of Ceres measured by the Dawn/VIR imaging spectrometer are studied as a function of illumination angles, by applying a semi-empirical method based on a statistical analysis of the VIR dataset acquired up to September 2016. The study also focuses on the photometry of the Occator faculae, i.e. the brightest spots of the Ceres surface, showing an albedo up to eight times the Ceres average. The considered semi-empirical approach takes into account the small extension (and hence small dataset) of this region and lays the groundwork to apply scattering models even on such a limited area. The behavior of Ceres visible and infrared reflectance with phase angle is similar to other asteroids belonging to its same spectral class, i.e. C-type. The depth of the bands at 2.7 µm (phyllosilicates), 3.1 µm (ammonium), 3.4 µm (magnesium carbonates) and the infrared spectral slope linearly increase with phase angle, showing analogies with other asteroids and occurrence of phase reddening. The different behavior of the 3.9 µm band depth (also due to Mg carbonates), independent of illumination angles, could indicate that other carriers contribute to the 3.4 µm band and play a more important role in photometry outside the carbonate deposits. The phase function of the Occator faculae is much steeper than expected from its high albedo. Mixture of bright and dark material and larger roughness can be at the basis of this result. The phyllosilicate bands show a steeper increase with phase angle with respect to the Ceres average, due to the lower presence of dark materials, and/or again larger roughness. The absence of trends with phase angles of the two carbonate bands and of the spectral slope suggests that carbonates do not produce phase reddening.

The Ceres surface is globally composed of Mg-phyllosilicates, ammoniated clays, carbonates and dark components. To obtain a more detailed mineralogical and geological investigation, the dwarf planet surface has been divided into fifteen quadrangles. The aim of this work is to investigate the abundance of phyllosilicates and ammoniated clays in the Kerwan quadrangle, classified as Ac-H-7 and spanning from 22°S to 22°N in latitude and from 72°E to 144°E in longitude. Maps of band depth distribution at 2.7 µm and 3.1 µm have been performed and compared with a map of geometric albedo estimated at 1.2 µm. Phyllosilicates and ammoniated clays generally correlate in the Kerwan quadrangle, even if departure from this behavior is observed in the floor of Kerwan, Inamahari and Homsuk craters. The greatest abundance of ammoniated phyllosilicates is found in Rao and Kerwan ejecta, while Bonsu and Tafakula floors are the most depleted in volatile, as well as Inamahari and Dantu ejecta. Six bright spots are detected in the Kerwan quadrangle, and the one richest in carbonate is related to Dantu ejecta in the southeast region. Some younger features (such as Rao or Kerwan ejecta) show deeper band depths than older terrain, a contrast trend with respect to the entire Ceres surface. Since this correlation is observed in a few other places on other quadrangles but not on the entire Ceres surface, it is possible that recent impact events could have been masked this correlation.

We analyze 2–5μm spectroscopic observations of the dust coma of comet 67P/Churyumov-Gerasimenko obtained with the Visible InfraRed Thermal Imaging Spectrometer (VIRTIS-H) instrument on board Rosetta from 3 June to 29 October 2015 at heliocentric distancesrh= 1.24–1.55 AU. The 2–2.5μm color, bolometric albedo, and color temperature were measured using spectral fitting. Data obtained atα= 90° solar phase angle show an increase in bolometric albedo (0.05–0.14) with increasing altitude (0.5–8 km), accompanied by a possible marginal decrease in color and color temperature. Possible explanations include dark particles on ballistic trajectories in the inner coma and radial changes in particle composition. In the phase angle range 50°–120°, phase reddening is significant (0.031%/100 nm deg−1) for a mean color of 2%/100 nm atα= 90°, which might be related to the roughness of the dust particles. Moreover, a decrease in color temperature with decreasing phase angle is also observed at a rate of ~0.3 K deg−1, consistent with the presence of large porous particles, with low thermal inertia, and showing a significant day-to-night temperature contrast. Comparing data acquired at fixed phase angle (α= 90°), a 20% increase in bolometric albedo is observed near perihelion. Heliocentric variations in dust color are not significant in the time period we analyzed. The measured color temperatures vary from 260 to 320 K, and follow arh−0.6variation in therh= 1.24–1.5 AU range, which is close to the expectedrh−0.5value.

We characterized 67P/Churyumov-Gerasimenko's cometary activity during its inbound arc before perihelion (2014 August-2015 January). We focused on the geomorphological regions of the Northern hemisphere observed by the ESA/Rosetta space probe during this time period. The GIADA dust detector characterized the physical properties of the fluffy and compact particles ejected from the nucleus; the VIRTIS imaging spectrometer detected exposed water ice.

We present an analysis of transient events observed by the Visible InfraRed Thermal Imaging Spectrometer, instrument aboard Rosetta, for the dates of 2015 August 10, September 13 and 14, during the two months surrounding the comet perihelion passage of the Rosetta spacecraft. We detected and characterized events with life-times ranging from 26 min down to 6 min. The temporal evolution of the outburst shows a sudden increase of radiance from quiescent coma to the maximum in a few minutes. This rapid onset is correlated with a change of the visible dust colour from red, 15-18± 3 {{ per cent}}/100 nm, to bluer with values of 7-10± 0.3 {{ per cent}}/100 nm. The dust morphology of these outbursts can be classified into two main types: narrow and collimated plumes (August 10, September 13) and broad blobs (September 14). The observations suggest that there are localized regions on the surface that are more prone to outbursts than the rest of the nucleus. The projected dust velocity during the outburst events ranges between 22.2 ± 2.2 m s-1 and 64.9 ± 10.6 m s-1. The total ejected mass during an outburst event is estimated to be between 10 and 500 tons for a duration of 6-26 min assuming size distribution indices between -2.5 and -3.

The paper 'Comet 67P outbursts and quiescent coma at 1.3 AU from the Sun: dust properties from Rosetta/VIRTIS-H observations' was published in MNRAS 469, S443 (2017). While performing a follow-up investigation, we discovered a numerical error in the algorithms that were developed to model the infrared continuum emission from a population of dust particles. The results of the scattering models for compact or moderately porous grains (Mie theory) and fluffy grains (Rayleigh-Gans-Debye theory, RGD) are both affected. Though the general conclusions of the paper are unchanged, the quantitative constraints obtained on the dust size distribution in the quiescent coma are slightly different.

Earth and orbital-based radar observations of asteroids provide a unique opportunity to characterize surface roughness and the dielectric properties of their surfaces, as well as potentially explore some of their shallow subsurface physical properties. If the dielectric and topographic properties of asteroid’s surfaces are defined, one can constrain their surface textural characteristics as well as potential subsurface volatile enrichment using the observed radar backscatter. To achieve this objective, we establish the first dielectric model of asteroid Vesta for the case of a dry, volatile-poor regolith—employing an analogy to the dielectric properties of lunar soil, and adjusted for the surface densities and temperatures deduced from Dawn’s Visible and InfraRed mapping spectrometer (VIR). Our model suggests that the real part of the dielectric constant at the surface of Vesta is relatively constant, ranging from 2.3 to 2.5 from the night- to day-side of Vesta, while the loss tangent shows slight variation as a function of diurnal temperature, ranging from 6 × 10−3 to 8 × 10−3. We estimate the surface porosity to be ∼55% in the upper meter of the regolith, as derived from VIR observations. This is ∼12% higher than previous estimation of porosity derived from previous Earth-based X- and S-band radar observation. We suggest that the radar backscattering properties of asteroid Vesta will be mainly driven by the changes in surface roughness rather than potential dielectric variations in the upper regolith in the X- and S-band.

Recent observations of water emission around Ceres suggest the presence of an ice layer on or beneath the surface of this asteroid. Several mechanisms have been suggested to explain these plumes, among which cometary-like sublimation seems to be plausible, since there is a correlation between the magnitude of the emission and the change in the heliocentric distance along the orbit. In this work, we applied a comet sublimation model to study the plausible scenarios that match with Herschel observations of the water flux (1026 molecules s-1). Each scenario is characterized by a well-defined set of physical and orbital parameters. Moreover, a study of the dynamic evolution of the H2O plume has been performed, showing that an optically thin transient atmospheric envelope, with a typical timescale of some tens of days, can be maintained by the H2O surface emission. Our simulations could be useful theoretical support for the Dawn NASA mission by giving a better understanding of the physical conditions for water sublimation and ice stability.

The Stereo Channel (STC) is a double wide-angle camera developed to be one of the channels of the SIMBIO-SYS instrument onboard of the ESA BepiColombo mission to Mercury. STC main goal is to map in 3D the whole Mercury surface. The geometric and radiometric responses of the STC Proto Flight model have been characterized on-ground during the calibration campaign. The derived responses will be used to calibrate the STC images that will be acquired in flight. The aim is to derive the functions that link the detected signal in digital number to the radiance of the target surface in physical units. The result of the radiometric calibration consists in the determination of well-defined quantities: i) the dark current as a function of the integration time and of the detector temperature, nominally fixed at 268 K; ii) the Read Out Noise, which is associated with the noise signal of the read-out electronic; iii) the Fixed Pattern Noise, which is generated by the different response of each pixel; iv) once these quantities are known, the photon response and the Photo Response Non-uniformity, which represent the variation of the photon-responsivity of a pixel in an array, can be derived. The final result of the radiometric calibration is the relation between the radiance of an accurately known and uniform source, and the digital numbers measured by the detector.

The VESPA data access system focuses on applying Virtual Observatory (VO) standards and tools to Planetary Science. Building on a previous EC-funded Europlanet program, it has reached maturity during the first year of a new Europlanet 2020 program (started in 2015 for 4 years). The infrastructure has been upgraded to handle many fields of Solar System studies, with a focus both on users and data providers. This paper describes the broad lines of the current VESPA infrastructure as seen by a potential user, and provides examples of real use cases in several thematic areas. These use cases are also intended to identify hints for future developments and adaptations of VO tools to Planetary Science., Comment: Planetary and Space Sciences (in press), Special Issue "Enabling Open and Interoperable Access to Planetary Science and Heliophysics Databases and Tools". 43 pages, 14 figures, 1 table

We investigate the region of crater Haulani on Ceres with an emphasis on mineralogy as inferred from data obtained by Dawn's Visible InfraRed mapping spectrometer (VIR), combined with multispectral image products from the Dawn Framing Camera (FC) so as to enable a clear correlation with specific geologic features. Haulani, which is one of the youngest craters on Ceres, exhibits a peculiar "blue" visible to near-infrared spectral slope, and has distinct color properties as seen in multispectral composite images. In this paper, we investigate a number of spectral indices: reflectance; spectral slopes; abundance of Mg-bearing and NH4-bearing phyllosilicates; nature and abundance of carbonates, which are diagnostic of the overall crater mineralogy; plus a temperature map that highlights the major thermal anomaly found on Ceres. In addition, for the first time we quantify the abundances of several spectral endmembers by using VIR data obtained at the highest pixel resolution ( 0.1 km). The overall picture we get from all these evidences, in particular the abundance of Na- and hydrous Na-carbonates at specific locations, confirms the young age of Haulani from a mineralogical viewpoint, and suggests that the dehydration of Na-carbonates in the anhydrous form Na2CO3 may be still ongoing.

Aims.Using data from the Rosetta mission to comet 67P/Churyumov–Gerasimenko, we evaluate the physical properties of the surface and subsurface of the nucleus and derive estimates for the thermal inertia (TI) and roughness in several regions on the largest lobe of the nucleus.Methods.We have developed a thermal model to compute the temperature on the surface and in the uppermost subsurface layers of the nucleus. The model takes heat conduction, self-heating, and shadowing effects into account. To reproduce the brightness temperatures measured by the MIRO instrument, the thermal model is coupled to a radiative transfer model to derive the TI. To reproduce the spatially resolved infrared measurements of the VIRTIS instrument, the thermal model is coupled to a radiance model to derive the TI and surface roughness. These methods are applied to Rosetta data from September 2014.Results.The resulting TI values from both instruments are broadly consistent with each other. From the millimetre channel on MIRO, we determine the TI in the subsurface to be −1m−2s−0.5for the Seth, Ash, and Aten regions. The submillimetre channel implies similar results but also suggests that higher values could be possible. A low TI is consistent with other MIRO measurements and in situ data from the MUPUS instrument at the final landing site of Philae. The VIRTIS results give a best-fitting value of 80 JK−1m−2s−0.5and values in the range 40–160 JK−1m−2s−0.5in the same areas. These observations also allow the subpixel scale surface roughness to be estimated and compared to images from the OSIRIS camera. The VIRTIS data imply that there is significant roughness on the infrared scale below the resolution of the available shape model and that, counter-intuitively, visually smooth terrain (centimetre scale) can be rough at small (micrometre–millimetre) scales, and visually rough terrain can be smooth at small scales.

We present 2-5 $\mu$m spectroscopic observations of the dust coma of 67P/Churyumov-Gerasimenko obtained with the VIRTIS-H instrument onboard Rosetta during two outbursts that occurred on 2015, 13 September 13.6 h UT and 14 September 18.8 h UT at 1.3 AU from the Sun. Scattering and thermal properties measured before the outburst are in the mean of values measured for moderately active comets. The colour temperature excess (or superheat factor) can be attributed to submicrometre-sized particles composed of absorbing material or to porous fractal-like aggregates such as those collected by the Rosetta in situ dust instruments. The power law index of the dust size distribution is in the range 2-3. The sudden increase of infrared emission associated to the outbursts is correlated with a large increase of the colour temperature (from 300 K to up to 630 K) and a change of the dust colour at 2-2.5 $\mu$m from red to blue colours, revealing the presence of very small grains ($\leq$ 100 nm) in the outburst material. In addition, the measured large bolometric albedos ($\sim$ 0.7) indicate bright grains in the ejecta, which could either be silicatic grains, implying the thermal degradation of the carbonaceous material, or icy grains. The 3-$\mu$m absorption band from water ice is not detected in the spectra acquired during the outbursts, whereas signatures of organic compounds near 3.4 $\mu$m are observed in emission. The H$_2$O 2.7-$\mu$m and CO$_2$ 4.3-$\mu$m vibrational bands do not show any enhancement during the outbursts., Comment: Accepted for publication in the Monthly Notices of the Royal Astronomical Society

JIRAM is an imager/spectrometer on board the Juno spacecraft bound for a polar orbit around Jupiter. JIRAM is composed of IR imager and spectrometer channels. Its scientific goals are to explore the Jovian aurorae and the planet's atmospheric structure, dynamics and composition. This paper explains the characteristics and functionalities of the instrument and reports on the results of ground calibrations. It discusses the main subsystems to the extent needed to understand how the instrument is sequenced and used, the purpose of the calibrations necessary to determine instrument performance, the process for generating the commanding sequences, the main elements of the observational strategy, and the format of the scientific data that JIRAM will produce.

Infrared observations of the coma of 67P/Churyumov-Gerasimenko were carried out from July to September 2015, i.e., around perihelion (13 August 2015), with the high-resolution channel of the VIRTIS instrument onboard Rosetta. We present the analysis of fluorescence emission lines of H$_2$O, CO$_2$, $^{13}$CO$_2$, OCS, and CH$_4$ detected in limb sounding with the field of view at 2.7-5 km from the comet centre. Measurements are sampling outgassing from the illuminated southern hemisphere, as revealed by H$_2$O and CO$_2$ raster maps, which show anisotropic distributions, aligned along the projected rotation axis. An abrupt increase of water production is observed six days after perihelion. In the mean time, CO$_2$, CH$_4$, and OCS abundances relative to water increased by a factor of 2 to reach mean values of 32%, 0.47%, and 0.18%, respectively, averaging post-perihelion data. We interpret these changes as resulting from the erosion of volatile-poor surface layers. Sustained dust ablation due to the sublimation of water ice maintained volatile-rich layers near the surface until at least the end of the considered period, as expected for low thermal inertia surface layers. The large abundance measured for CO$_2$ should be representative of the 67P nucleus original composition, and indicates that 67P is a CO$_2$-rich comet. Comparison with abundance ratios measured in the northern hemisphere shows that seasons play an important role in comet outgassing. The low CO$_2$/H$_2$O values measured above the illuminated northern hemisphere are not original, but the result of the devolatilization of the uppermost layers., Comment: Accepted for publication in the Monthly Notices of the Royal Astronomical Society

Water ice-rich patches have been detected on the surface of comet 67P/Churyumov-Gerasimenko by the VIRTIS hyperspectral imager on-board the Rosetta spacecraft, since the orbital insertion in late August 2014. Among those, three icy patches have been selected, and VIRTIS data are used to analyse their properties and their temporal evolution while the comet was moving towards the Sun. We performed an extensive analysis of the spectral parameters, and we applied the Hapke radiative transfer model to retrieve the abundance and grain size of water ice, as well as the mixing modalities of water ice and dark terrains on the three selected water ice rich areas. Study of the spatial distribution of the spectral parameters within the ice-rich patches has revealed that water ice follows different patterns associated to a bimodal distribution of the grains: ~50 {\mu}m sized and ~2000 {\mu}m sized. In all three cases, after the first detections at about 3.5 AU heliocentric distance, the spatial extension and intensity of the water ice spectral features increased, it reached a maximum after 60-100 days at about 3.0 AU, and was followed by an approximately equally timed decrease and disappearanceat about ~2.2 AU, before perihelion. The behaviour of the analysed patches can be assimilated to a seasonal cycle. In addition we found evidence of short-term variability associated to a diurnal water cycle. The similar lifecycle of the three icy regions indicates that water ice is uniformly distributed in the subsurface layers, and no large water ice reservoirs are present., Comment: submitted to MNRAS

International audience; VIRTIS-M observations of the nucleus of comet 67P/Churyumov-Gerasimenko acquired from 2014 August to 2015 May have been analysed to investigate surface temporal variability at both seasonal and diurnal scales. The measured reflectance spectra are studied by means of comet spectral indicators (CSI) such as slopes in the visible and infrared ranges, and 3.2 mum band area and band centre. CSI maps derived from data acquired at different heliocentric distances (from 3.62 to 1.72 au) along the inbound leg of the comet's orbit are used to infer surface water ice abundance. We measure a global scale enrichment of water ice from 2014 August to 2015 May across the body of the comet, along with variability at small spatial scale, possibly related with the local insolation conditions. Analysis of water ice diurnal variability is performed on 2014 August observations. Water ice appears at the border of receding shadows in the neck of the comet (Hapi), sublimating in less than 1 h, after exposure to sunlight. As similar variability is not observed in other regions of the comet, we interpreted this as the expression of a diurnal cycle of sublimation and re-condensation of water ice, triggered by sudden shadowing produced on the neck by the body and the head of the nucleus.

This work is dedicated to the application to 67P/Churyumov-Gerasimenko of a new quasi-3D approach for non-spherically shaped comet nuclei with the aim to interpret the current activity of the comet in terms of initial characteristics and to predict shape and internal stratification evolution of the nucleus. The model is applied to differently shaped nuclei taking into account the characteristics of Comet 67P/Churyumov-Gerasimenko deduced from observations. We focus our attention on the combined effects that shapes and obliquity have on the comet surface and sub-surface evolution. We discuss the results in terms of activity, local dust mantle formation and disruption, erosion of the surface and internal stratigraphy. The results show that differently shaped nuclei can have different internal structures leading to different activity patterns and behaviors. Our calculations have shown that local variations in the dust and gas fluxes can be induced by the nucleus shape. The distribution of “active” areas on Comet 67P/Churyumov-Gerasimenko is different because of different shapes, reflecting the illumination conditions on the surface. These shapes can influence the structure of the inner coma, but the coma far away from the nucleus is only marginally affected by the nucleus shape. However, different comet behaviors can arise from differently shaped comet nuclei, especially in terms of local activity, surface and sub-surface characteristics and properties. The water flux local distribution is the most influenced by the shape as it is directly linked to the illumination. Irregular shapes have large shadowing effects that can result in activity patterns on the comet surface. The effects of different pole directions are discussed to see the relations with the nucleus activity and internal structure. It is shown that the orientation of the rotation axis plays a strong role on the surface evolution of 67P/Churyumov-Gerasimenko, determining seasonal effects on the fluxes. The activity of the comet changes greatly with the nucleus obliquity leading to pre–post-perihelion differences in the activity and seasonal effects. The effects of the dust deposition and crust formation on the cometary activity have also been simulated and are discussed with respect to 67P/Churyumov-Gerasimenko observations. The dust mantling is also strongly obliquity dependent, with different surface distributions of the dust-covered regions according to the different comet pole orientations. Finally, we show that our model can reproduce the fluxes behavior near perihelion in terms of amplitude and asymmetry, and we estimate 20% of the illuminated surface to be active.

After having observed the planets from his house in Padova using his telescope, in January 1611 Galileo wrote to Giuliano de Medici that Venus is moving around the Sun as Mercury. Forty years ago, Giuseppe Colombo, professor of Celestial Mechanics in Padova, made a decisive step to clarify the rotational period of Mercury. Today, scientists and engineers of the Astronomical Observatory of Padova and of the University of Padova, reunited in the Center for Space Studies and Activities (CISAS) named after Giuseppe Colombo, are busy to realize a stereo camera (STC) that will be on board the European (ESA) and Japanese (JAXA) space mission BepiColombo, devoted to the observation and exploration of the innermost planet. This paper will describe the stereo camera, which is one of the channels of the SIMBIOSYS instrument, aiming to produce the global mapping of the surface with 3D images.

Context. The Centaurs are a dynamical class of minor bodies in the Solar System, moving on chaotic orbits with perihelion lying between Jupiter and Neptune orbits. P/2004 A1 (LONEOS) is a recently discovered object belonging to this class, observed at the TNG telescope in La Palma (Canary Islands) when it was at the heliocentric distance Rh of 5.54 AU, but it already displayed a welldeveloped coma and a long, sharp tail-like structure. Aims. We want to investigate whether it is possible to explain the strong activity of this body in terms of the usual sublimation mechanisms. Methods. We simulated the thermal evolution of LONEOS using a nucleus thermal evolution and differentiation model and took into account that it is being injected for the first time on an inner orbit as a consequence of a close encounter with Saturn experienced in 1992. Results. We show that, considering its peculiar dynamical history, it is possible to explain the activity of this Centaur with the sublimation of very volatile ices.

Aims. We study the distant dust environment of two Long Period Comets: C/2003 O1 (LINEAR) (observed at r h = 7.39 AU) and C/2004 K1 (Catalina) (observed at r h = 3.43 AU). The case of C/2003 O1 is particularly interesting since the comet has a quite large perihelion heliocentric distance (r h = 6.85 AU).Methods. We analysed R -band images taken at the CAHA 2.2 m telescope to characterise the properties of the cometary dust coma. The images of both comets were also used as input into an inverse numerical model, to derive information about the dynamical parameters of coma dust grains until a time ~500 days before the observation.Results. Both the comets appeared to be active, and of similar shape and size despite the great difference in their heliocentric distance of observation. C/2003 O1 exhibited a tail extending to at least 3.7 105 km. It is a very active object, since its Af ρ is 552 ± 36 cm within the inner 5”. C/2004 K1 showed a similar long tail, extending to 2.6 105 km. Its Af ρ is 539 ± 35 cm within the inner 5”. An almost constant dust grain ejection velocity between 0.5 and 0.9 m/s at 1 cm dust size has been derived for C/2003 O1 and an increase from 0.5 to 2 m/s at 1 cm dust size for C/2004 K1, reflecting the different volatile dragging the dust environment (probably CO for C/2003 O1 and water for C/2004 K1). Model results allow some speculations about the comet nucleus size of C/2003 O1 (LINEAR) and its CO content: for a Q CO similar in value to those observed for other distant cometary objects, a comet radius R n from 13 to 17 km can be inferred.

In March 2015, the NASA Dawn spacecraft entered orbit around Ceres, the largest object in the main asteroid belt. The Visible and InfraRed (VIR) mapping spectrometer onboard Dawn unveiled the mineralogy of Ceres at unprecedented spatial resolution. The VIR thermally-corrected average spectrum of Ceres revealed the presence of several absorption bands in the 2.5 to 4 µm region (at 2.7 µm, 3.1 µm, 3.3-3.4 µm, and 4 µm). The 2.7-µm band is diagnostic of hydrous minerals, the 3.1 µm band is associated with NH4-phyllosilicates, and the simultaneous occurrence of both the 3.3-3.4 and 4-µm bands is indicative of carbonates [1]. Although Ceres displays general homogeneity in surface composition, some exceptions can be found at the local scale. For example, Ahuna Mons is a unique isolated elliptical mountain (21x13 km), made up of a summit with sub-radial arcuate structures (ridges and troughs) and steep flanks formed by talus material [2]. Based on FC image data Ahuna Mons was interpreted to be a cryovolcano, formed by ascent of cryomagma and extrusion onto the surface followed by dome development (ibid). VIR spectroscopic analysis of Ahuna Mons highlights a lower abundance of hydrous minerals and NH4-phyllosilicates phases and a higher abundance of Na-carbonates than the surrounding areas observed at the same spatial resolution. Similarly, crater Haulani, characterized by widespread bright ejecta mixed with dark material, shows a reduction of the 2.7 and 3.1-µm bands in VIR data, probably due to lower amounts of hydrous material and NH4-phyllosilicates. Haulani's bright ejecta are characterized by a negative ("blue") spectral slope in the visible to near infrared spectral range. In contrast, the bright material unit in the center of crater Occator was discovered to be the most concentrated extraterrestrial deposit of Na-carbonates [3]. Even though each of these features has a distinct morphology, they may share some spectral characteristics. Here we discuss the mineralogy of these notable geologic features, with the goal of shedding light on their origin.

Context. On 2005 July 4, the NASA spacecraft Deep Impact delivered an impactor on the comet 9P/Tempel 1 to study the material ejected from the nucleus. A worldwide observation campaign accompanied the mission, to characterize the activity of 9P/Tempel 1 before and after the impact. Aims. At La Palma (Canary Islands), the comet was observed from July 2 to July 9 using the echelle spectrograph SARG on the Telescopio Nazionale Galileo (TNG). Fifteen spectra were obtained with a resolving power of R = 29 000 in the spectral range 4620–7920 A. Many interesting emission lines can be found in this range, in particular the [OI] lines at 5577 A (“green line”) and at 6300 and 6364 A (“red doublet ”). From the analysis of these lines it is possible to derive information on the processes that produce these emissions. Methods. The three atomic oxygen lines are clearly visible in most of the spectra. The intensity ratio between the green line and the sum of the red lines, indicative of the parent of these lines, was computed for 9 of the 15 spectra. The value of the intensity ratio for the night of July 5 was compared with the model results obtained from a coupled chemistry transport model. Results. The intensity ratio of green to red oxygen lines obtained from the observed spectra and the one derived from the model suggest water is the main parent of the [OI] emissions on comet 9P/Tempel 1. Conclusions.

A surprisingly large number of short-period comets have been observed with significant activity (a coma and even a well-developed dust tail) at heliocentric distances greater than 3 au, where the water sublimation rate is low and thus the sublimation of other volatiles, such as for example CO, could drive the presence of a coma. As CO is not the main ice in the comet nucleus, the dust release from the nucleus surface as a result of CO drag is expected to be very different from that caused by water. In order to investigate the complexity of the transition between bare nucleus and the presence of a well-developed coma, to compare activity levels and to obtain information concerning evolutionary differences resulting from distinct dynamical histories, we started a long-term programme of CCD imaging of distant short-period comets. In this paper we present the results of the first observing run, performed at the 3.5-m Telescopio Nazionale Galileo on La Palma on 2004 December 17. Five comets were imaged in the R band (three numbered short-period comets and the two fragments of an unnumbered short-period comet): 36P/Whipple, 111P/Helin–Roman–Crockett, 159P/LONEOS, and P/2004 V5 (LINEAR–Hill) A and B. The heliocentric distance of targets was 3.47 ≤Rh≤ 4.43 au. The targets presented several levels of activity, ranging from a stellar appearance to a well-developed coma and tail. The stellar appearance of 111P/allowed a range to be derived for the nuclear radius rnucleus from 0.46 to 1.39 km (assuming a ‘classical’ albedo value of 0.04), depending on the presence of an unresolved coma. For the active comets, we measured dust production levels in terms of the quantity Afρ, which was in the range 11 ≤Afρ≤ 224 cm. The distant activity of the target comets is analysed in relation to the target dynamical history (in terms of the perihelion heliocentric distance). A possible preliminary conclusion can be obtained, namely that the hypothesis of distant activity ‘induced’ by higher temperature owing to perihelion lowering cannot be univocally invoked for short-period comets.

After a cruise phase of more than ten years the Rosetta spacecraft entered orbit around comet 67P/Churyumov-Gerasimenko (CG) on 6 August 2014. From this date the spacecraft began to escort the comet and will follow it till the end of the nominal mission (December 2015). This allowed very accurate nucleus imaging making 67P/CG the sixth comet to be directly observed from a spacecraft after 1P/Halley , 9P/Temple 1, 19P/Borrelly, 81P/Wild 2 and 103P/Hartley 2. The Rosetta orbiter carries eleven instruments and among these, VIRTIS, the Visible and Infrared Thermal Imaging Spectrometer (Coradini et al. 2007). This experiment is composed of two sensor heads, -M (Mapper) and -H (High resolution). The -H sensor is a high resolution punctual spectrometer mainly devoted to the study of the coma properties in the 1.88-5.03 micrometers, while -M produces hyperspectral images of the target in the 0.2-5.1 micrometers range with an angular resolution of 250 micr-rad x 250 micro-rad. Data from VIRTIS-M are crucial to assess nucleus spectrophotometric properties: on global scale with disk-integrated observations and, thanks to its imaging capabilities, at larger spatial resolutions with disk-resolved measurements. The latter case is of paramount importance in order to map compositional variability on the surface and possibly link it to sources of activity. This work focuses on VIRTIS-M data acquired from July 2014 up to February 37 2015. The analysis we performed is divided into two main subjects: full-disk photometry (sec. 2) and disk-resolved photometry (sec. 3). In sec. 2, we discuss rotational curves (sec. 2.1), full-disk phase curves (sec. 2.2), color ratios (sec. 2.3) and the derivation of the geometric albedo (sec. 2.4). In sec. 3, taking advantage of the large dataset acquired by VIRTIS-M we calculate a photometric reduction by means of a Hapke simplified model thus assessing the spectrophotometric properties of the surface. Also a zonal photometric reduction has been derived for four macro-regions, in order to better investigate compositional and morphological differences on the nucleus (sec. 3.5). In sec. 4 the full-disk and the disk-resolved dataset are used together to derive a complete phase curve of the comet. Finally a comparison to photometric properties of other comets is shown in sec. 5 and in sec. 6 a summary of the main findings is reported as well as a discussion of future developments of this work with new observations in the coming months.

Context. Centaurs reside in a region of the Solar System where many volatile species begin to have significant sublimation rates, and several examples of cometary activity have been observed in this group. The source of activity is not well known for most of the active Centaurs. If the sublimation rate of water is low, the sublimation of other volatiles, such as CO, could drive the presence of a coma.Aims. The aim of this paper is to study the dust environment of the active Centaur P/2004 A1 (LONEOS), which was observed to have a significant coma and a very long perspective anti-tail (neck-line) at a heliocentric distance of 5.5 AU.Methods. R -band images taken at the TNG telescope were used as input for an inverse numerical model, meant to derive the physical parameters of dust grains in the coma.Results. The Centaur showed an asymmetric coma and a neck-line extending at least to 1.5 105 km in the solar direction, emitted half an orbit before the observation. The Af ρ value measured in a 5'' aperture is 334 ± 15 cm, indicating a significant dust production rate, comparable to that of several short-period comets at much lower heliocentric distances. The emitted grains are larger than 1 cm, and the dust production rate has been around 100 kg s-1 during the last ten years. For R n = 10 km and a maximun size of the uplifted grain >3 cm, a CO molecular abundance Q > 1030 mol s-1 is required if it is uniform over all the surface.

The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta spacecraft has provided evidence of carbon-bearing compounds on the nucleus of the comet 67P/Churyumov-Gerasimenko. The very low reflectance of the nucleus (normal albedo of 0.060 ± 0.003 at 0.55 micrometers), the spectral slopes in visible and infrared ranges (5 to 25 and 1.5 to 5% kÅ(-1)), and the broad absorption feature in the 2.9-to-3.6-micrometer range present across the entire illuminated surface are compatible with opaque minerals associated with nonvolatile organic macromolecular materials: a complex mixture of various types of carbon-hydrogen and/or oxygen-hydrogen chemical groups, with little contribution of nitrogen-hydrogen groups. In active areas, the changes in spectral slope and absorption feature width may suggest small amounts of water-ice. However, no ice-rich patches are observed, indicating a generally dehydrated nature for the surface currently illuminated by the Sun.

The report of Commission 15 was prepared primarily by the chairpersons of its two working groups: the Minor Planet Working Group and the Comet Working Group. In particular, the Minor Planet section was created by A. Cellino with a little help from E. Tedesco and the Comet section by T. Yamamoto with the assistance of D. Bockelee-Morvan, W. Huebner, A. Bhardwaj, D. Biesecker, L. Jorda, H. Kawakita, H. U. Keller, H. Kimura, A. Kouchi, and D. Prialnik. E. Tedesco was responsible for the Introduction, final editing, and merging of the two reports. Scientific activity in the field has continued to grow in the past three years, as evidenced by publication of 700 papers in the refereed literature, compared with about 400 during the previous triennium. A comprehensive overview of so large a publication list cannot be accomplished in the space at our disposal. We have therefore chosen to highlight a representative subset of these publications to provide a snapshot of the current state of the field, and, as in the last several reports, without including a comprehensive bibliography. Instead, a complete list of the references used in creating this report, assembled by searching the ADS abstract service (http://adsabs.harvard.edu/abstract−service.html) to generate a list of refereed papers published between July 2002 and June 2005, inclusive, is available in the Archive section of the Division III Physical Studies of Comets and Minor Planets web site. This site can be reached (since it does not have a permanent home) via a link from the IAU home page.

The Italian drill “DEEDRI” is going to be the lander based sample acquisition system for the Mars Surveyor Program of the Mars Sample Return mission. DEEDRI is capable to collect core/sand sample of the martian soil down to 50 cm in depth. The MA_MISS experiment belongs to the DEEDRI system and it will be dedicated to observe the wall of the excavated hole in terms of infrared spectral reflectance in the range 0.8–2.8 μm. The spectral sampling is about 20 nm while the spatial sampling is 100 μm over the target. The optical window of MA_MISS is placed very close to the drill tip so that the target view to be observed can span from the soil down to 50 cm. The proximity optics and electronics of MA_MISS have to be very miniaturized since they will be collocated inside the drill tool in a very limited volume of about 25 mm in diameter. On the other side the main electronics will be on the lander and it will communicate through an interface based on slip rings devices. MA_MISS can acquire in different observation modes. The images are scanned by moving the DEEDRI itself. One image ring is built up by acquiring contiguous images of the MA_MISS slit. The study of the Martian subsurface will provide important constraints on the nature, timing and duration of alteration and sedimentation processes on Mars, as well as on the complex interactions between the surface and the atmosphere. This study will permit to infer the history of erosion, transport and deposition of loose material. Alteration processes can dominate the mineralogy of the Martian surface: it will be essential to study the mineralogy of deeper layers, where a more limited alteration took place. MA_MISS can provide very important scientific return from the subsurface of Mars along with a selection criteria for the samples collection.

The present report of Commission 15 has been, as usual, prepared primarily by the chairpersons of the two working groups. E. Tedesco wrote the section about Asteroids and Meteorites, with the assistance of A. Cellino, G. Consolmagno and C.-I. Lagerkvist. W. F. Huebner prepared the section about Comets, with the assistance of J. Benkhoff, H. Boehnhardt, J. Brandt, M. T. Capria, A. Cochran, G. Cremonese, M. Duncan, W. Huntress, H. Levison, and G. P. Tozzi. Moreover, the whole document has been assembled by K. Muinonen, who did the final editing, to merge the two reports and fit the document into the allotted space. Material taken from both major areas regarding the relationship between comets and asteroids has been combined into a single section.

An improved unidimensional model of the heat transport and gas diffusion within a porous cometary nucleus is presented, in which the time-dependent gas diffusion equation is coupled with the heat diffusion equation to describe the energy transport due to sublimation and recondensation of volatiles, but is solved independently using a different discrete time step. Also, the erosion of interfaces within the nucleus, due to the sublimation of ices and the removal of dust, is now treated by means of a continuous adaptation of the discrete grid to the interfaces positions, removing numerical stability problems associated with the variation of structure and composition of the discrete layers. The results of this model are then compared with those of another unidimensional model which does not make use of the above-mentioned numerical methods, both computed for the same set of physical parameters describing comet P/Wirtanen, and the effects of the different modelling assumptions on the results are discussed. A new bidimensional model of the heat transport within a porous comet nucleus is presented, and its results are compared with those obtained from the above-mentioned unidimensional model (modified to include the same physics of the bidimensional model). The ability of bidimensional models to better describe the effects of variations in the local physical conditions on the comet activity is then discussed.

What is the ultimate fate of a comet? Excluding impacts with other bodies, two possibilities are foreseen: either long-lasting activity, accompanied by nucleus reduction, or the formation of a stable crust that inhibits dust emission and strongly reduces volatile emissions. In the first case the comet could disintegrate, whereas in the second case it could become dormant or extinct, assuming an asteroidal appearance; in both cases the comet could be reactivated. In this paper we present results of a comet evolution model trying to establish the conditions under which the nucleus becomes dormant or extinct and under which it continues its activity up to the consumption of the icy material. Our nucleus model is composed of a mixture of ices of water, CO 2 , and CO and dust particles. The H 2 O ice can be either amorphous or crystalline; the solid matrix is assumed to be porous. The evolution of the body is determined by the solar energy reaching its surface and by the heat transfer in the interior. The propagation of the heat through the nucleus is modeled by means of the heat transfer and gas diffusion equations, coupled via the condensation–sublimation terms that are seen as sinks or sources of energy and matter, respectively. Particular attention is given to the variations of porosity and to the changes in composition of the superficial layers due to sublimation–condensation phenomena, to gas diffusion processes through the pore system, and to the ejection of dust particles. At the beginning of the evolution of the nucleus the crust is never present and the interior of the comet is not differentiated. We have seen that the evolution can proceed essentially in two ways: (1) if the body is dark and rich in volatiles and the dust grains are fluffy, then the upper layers are usually removed at the same rate at which the CO 2 upper boundary sinks; (2) in the opposite case, sometimes a dusty crust is formed, sometimes not, but in any case the CO 2 interface sinks deeply. We conclude that in the first case the body will remain active for several orbits, while in the second case the gradual reduction of any activity can produce a dormant or extinct comet, maybe with the appearance of an asteroid.