MATISSE (Multi-purpose Advanced Tool for the Solar System Exploration) [1] is a tool that allows the visualization of observations from space missions and datasets derived from these observations on a three-dimensional model of the selected target body. The second version of the tool (named MATISSE 2.0 –https://tools.ssdc.asi.it/Matisse) will, among other things, include algorithms developed by partner research teams; in this work we focalize our attention on the MATISSE inclusion of two codes developed for atmospheric retrieval and thermophysical modeling. The retrieval code is used for the analysis of the spectra provided by the JIRAM instrument (Jovian Infrared Auroral Mapper [2]) onboard the NASA’s Juno mission, whose main purpose is the study of the upper regions of Jupiter’s atmosphere in the 2-5 μm wavelength range and pressure up to 5-7 bar. The spectra provided by the instrument are processed with the retrieval code that calculates, for each pixel of a hyperspectral image, the chemical and physical parameters in the corresponding points of the atmosphere [3]. The code processes all pixels of a hyperspectral image, so parallelization is convenient in order to reduce the computation time; this is possible by using the Python language tools, which allow the execution of a code written in its own language (FORTRAN in this case) by providing the required parallelization. As a further optimization step, the code has been converted into a Docker image to make it portable and easy to run on heterogeneous architectures. The second code included in MATISSE is a thermophysical model that calculates the surface temperature of airless bodies as function of thermal conductivity [4,5] and other physical properties; the calculated temperature can be compared with the measured ones, if any, in order to retrieve the thermal properties of the soil, or can be used to compute other temperature-dependent quantities. At the present time this code is going to be used for Mercury and Ceres and is almost ready to be included in MATISSE 2.0. [1] Zinzi, A., et al. (2016), Astronomy & Computing, 15, 16-28 [2] Adriani, A., et al. (2017), Space Science Reviews, 213, 393-446 [3] Grassi et al. (2010), Planetary and Space Science, 58, 1265-1278 [4] Capria, M. T. et al (2014), Geophysical Research Letters, 41, 1438-1443 [5] Rognini et al. (2019), Journal of Geophysical Research, https://doi.org/10.1029/2018JE005733

Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA’s F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum V capability of 600 ms^-1 . Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes – B1, provided by the Japanese space agency, JAXA, and B2 – that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission’s science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule.

IntroductionSome 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). MethodsWe 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 discussionWe 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. AcknowledgementsThis 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. ReferencesBelton et al. (2013) Icarus, 222, 477-486Besse et al. (2015) EPSC conference, id.EPSC2015-114Besse et al. (2017) ACM conference Brownlee et al. (2004) Science, 304, 1764-1769Bruck Syal et al. (2013) Icarus, 222, 610-624De Sanctis et al. (2005) A&A, 444, 605-614De Sanctis et al. (2010) Icarus, 207, 341-358Guilbert et al. (2016) MNRAS, 462, 146-155Ip et al. (2016) A&A, EDP Sciences, 591, A132Keller et al. (2015) A&A, 583, A34Lamy 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, A33Sierks et al. (2015) Science, 347, aaa1044 Tosi et al. (2019) Nat. Astron, 3, 649-658Vincent et al. (2015) Nature, 523, 63-

IntroductionRecent high-resolution ALMA observations of protoplanetary disks have raised interest in the study of solid bodies in disks at different scales, from sub-micrometric grains up to solid bodies hundreds of meters in size, for which dynamic evolution is governed by the interaction between the gas and the dust in the disk. We propose a novel research in the field of dust evolution and dynamics in protoplanetary disks. One of the main goals of this research is to address the dust dynamics evolution, the dynamical interaction between proto-planetary bodies, preparing the path for the SKA Key Programmes. We aim at contributing to this field twofold: (i) modeling of non-spherical dust dynamics in rarefied gas field present in the disk gaps and (ii) to interpret and reconstruct protoplanetary disk observations through numerical simulations, extending the PHANTOM code [1] to non-spherical dust setup, i. e. a particular focus is how the dynamics is influenced by dust dimension and shape. One of the main objectives is to reproduce the image of ALMA data and understand how different disk initial dust parameters and dust characteristics may influence the disk evolution and face-on or edge-on appearance.Using the state-of-the-art non-spherical dust dynamical model [2], developed for analysis of the ESA comet mission ROSETTA data, we determine the region in the gaps where the settling could occur as a function of the grain non-sphericity (shape, elongation), size, density etc. We investigate the terminal velocities and rotational frequencies of the non-spherical particles using different physical particle parameters, to show that the dust accommodation in the ring structures can be at distances different than what predicted by spherical dust models. The second focus of this work is to study the vertical settling with the SPH approach taking into account the dusty-gas interaction through the whole disk including possible planet-formation gaps.Dust dynamics models We use two models to simulate dust dynamics in protoplanetary disks: 1) a non-spherical dust dynamical model to simulate dust settling in Epstein regime and 2) the-state-of-the-art SPH code PHANTOM to simulate dust motion in both the Epstein and Stokes regimes. The first code is a 3D+t non-spherical dust model that solves the Euler dynamical and kinetic equations. Considering free-collisional dust regime we study the effects on the particle dynamics provided with different particle shapes, initial orientation and velocities, as well as torque. Torque is computed from the law of variation of the angular momentum by using the Euler dynamic equations. The particles are assumed to be homogeneous, isothermal convex bodies. The dust motion is governed by the gas drag and gravity of the host star and/or a planet in the gaps. The second model is a modified version of the PHANTOM code. We introduced non-spherical ellipsoidal particles and averaged drag coefficients for those non-spherical shapes. The code is a 3D+t SPH and MHD code and includes modules for self-gravity, dust-gas mixtures, viscosity, photo-evaporation, etc. A description of the parameters set used in these simulations with the results of simulations of dust settling are reported in Fig. 2. The plot is an example of dust settling in dusty disks, each of them having only one of three selected types of particles that are of the same mass and density, but of different shape. Dust settling with non-spherical particles Here we discuss two example cases produced by the two codes. Fig.1 shows rotational frequency (number of rotation per second) of two spheroids of different aspect ratio calculated with our non-spherical dust model in the Epstein regime. We simulate the dynamical properties and evolution of μm to mm sized particles in the case study with a planet in a disk gap at 60 AU, as observed in the HD163296 data [3, 4]. We assess the structure of the hypothesized vertically-extended dust layer. We investigated the influence of different dust sizes and initial dust speeds. Different spheroids of the same mass lead to different dynamics (e.g. velocity, angular velocity, etc) in the vertical settling phenomena.In the second group of simulations our non-spherical dust setup of the PHANTOM code has been used. By utilizing the pre-calculated averaged drag coefficients of spheroid particles in Epstein regime we simulated dust vertical settling in a disk using the SPH approach. The non-spherical dust particles in Epstein regime settle down slower than their spherical equivalent particles (see Fig 2). Moreover, the settling of larger non-spherical dust particles in Stokes regime shows a stratified structure: a denser sub-disk near the mid-plane for dust in Stokes regime and suspended particles in Epstein regime.Conclusions and future work We used two different models to simulate non-spherical dust settling in protoplanetary disks. The first one computes dusty-gas motion in Epstein regime using free molecular expression for rarefied field [1]. The second one uses a newly implemented feature of the state-of-the-art PHANTOM code, namely non-spherical dust shapes. Both models reveal that the interpretation of the ALMA observations can be biased by the spherical particle approximation. In our future work we will describe what we found applying the two dust models with non-spherical particles to study the dust evolution of edge-on disks. Usually the mm-sized particles settling in protoplanetary disks are aggregates of fractals formed from the dusty-gas interactions during the protostellar nebula phase. Such aggregates could be approximated with compact porous particles if dynamical (e.g. drag coefficients) and physical (e.g. porosity, density, fractal dimension) parameters are taken into account.AcknowledgmentsThis research was supported by the National Institute for Astrophysics, Italy (INAF) within the Mainstream project “Non-spherical dust dynamics in protoplanetary disks - how realistic dust particle shapes change the dust evolution timescales”.References[1] Price D. J. et al. (2018), PASA 35, E031[2] Ivanovski S. et al. (2017), Icarus 282, p. 333 - 350.[3] Isella et al. 2016, PRL, 117;[4] ALMA Partnership et al. 2015, ApJ Lett. 808:L3,10pp;

On 30 August 2019 the amateur Borisov discovered a new comet; after few days it was clear from the characteristics of its orbit (eccentricity > 3 and high hyperbolic excess velocity) that the second interstellar object had been detected and the object received the name of 2I/Borisov. It appears to be very different from 1I/’Oumuamua and can be considered as the first interstellar comet.According to the first observations the comet had a nucleus with a radius of few km and a dust coma and tail due to the activity started in June 2019 (Jewitt et al., 2019).At the beginning of October we submitted the Discretionary Director Time (DDT) proposal to the TNG in order to monitor the comet. Some images have been acquired, in November and December 2019, with the DOLORES instrument in the R filter.We have applied the dust model described in Fulle et al. (2010), that has been tested on the comet 67P/Churyumov-Gerasimenko and validated with the Rosetta measurements.According to the results of our dust model and the activity model (Fulle et al., 2020) we derived a water flux from the nucleus of 8x10-6 kg m-2 s-1 and a dust loss rate of 35 and 30 kg s-1 in November and December 2019 respectively (Cremonese et al., 2020). This slight decrease has been observed around the perihelion on 8 December, few months later the comet fragmented.In this work we will describe the dust tail observations and the dust model results, even comparing them with the Jupiter family comet 67P.References:G.Cremonese, et al., 2020, ApJL, 893, L12M.Fulle et al., 2010, A&A, 522, A63.M.Fulle et al., 2020, MNRAS, 493, 4039.Jewitt et al., 2019, ApJ, 886, L29.

The characterization of the surface material of airless bodies is important for the comprehension of the geology and evolution, and also for mission decisions as the selection of the sample site. The thermophysical properties affect the surface temperature curve, that is the temperature as function of time; the thermal inertia, defined by TI=(k ρ c)0.5 (where k is the thermal conductivity, ρ the density and c the specific heat), is the key parameter that controls the maximum daytime temperature and the time at wich the maximum occurs. A thermophysical model requires to properly model the thermal conductivity of the soil; common thermophysical models assume that surface material (regolith or rock) can be approximated as a continuous, non-discretized material whose physical properties are constant or vary with depth in some mathematical way, e.g. the density increases exponentially with depth. These assumptions are valid when the particles sizes are smaller than the thermal skin depth; however, high-resolution images of the Nightingale Crater on Bennu (DellaGiustina et al., 2019) revealed a particle size distribution in which the particles are smaller/larger than, and comparable to, the diurnal skin depth (that is of order of cm). Furthermore, the Bennu's moderate thermal inertia appears to be inconsistent with the large number of boulders, suggesting that the link between thermal inertia and particle size is not adeguately captured by standard models. We have included in our thermophysical model (Rognini et al., 2019; Capria et al., 2014) the effects of the particle size distribution, using the results of Ryan et al. (2020); numerical simulations by these authors indicate that the thermal conductivity of a polydisperse soil is approximated by the thermal conductivity of a monodisperse soil with a particle diameter equal to Sauter mean D32, that is the diameter of a particle with the same volume-to-area ratio. Corrections for non-isothermality of the particles are also included. These modifications allow us to properly model the temperature of rubble pile asteroids and analyze the thermal data from future missions; in particular, we want to apply our model to Ryugu and Bennu, the goals of JAXA and NASA missions Hayabusa 2 and Osiris-Rex, respectively, although the model can be used for the general case. We investigate how to use the simulation results of our thermophysical model as input for the non-spherical dust dynamics model (Ivanovski et al., 2017) that has been modified to study dust motion in near asteroid environments after recent and future impacts on rubble-pile asteroids performed by JAXA and NASA missions. Furthermore, we plan to constrain the dust distribution based on the data and simulations obtained by these missions. This will allow to foresee what kind of dust environment the ESA mission HERA could find at the encounter with the Didymos binary system. A version of the thermophysical code is almost ready to be available to the scientific community through MATISSE, the webtool developed at the SSDC in ASI (Zinzi et al., 2016); the modifications reported in this work will be included. Figure 1: examples of temperature curves calculated with different average particle sizes D=D32 (meters), for a point located on the equator of an asteroid with emissivity 0.9, heliocentric distance 1.6 A.U., albedo 0.1, density 2146 kg/m3, specific heat 600 J kg-1 K-1, rotation period 11.92 h. References:Capria, M. T., Tosi, F., De Sanctis, C., et al. (2014), Vesta surface thermal properties map, AGU, DOI:10.1002/2013GL059026DellaGiustina, D.N., Emery, J.P., et al. (2019), Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis, Nature Astronomy, DOI:10.1038/s41550-019-0731-1Ivanovski, S. L., Zakharov, V. V., et al. (2017), Dynamics of aspherical dust grains in a cometary atmosphere: I. axially symmetric grains in a spherically symmetric atmosphere, Icarus, DOI:10.1016/j.icarus.2016.09.024Rognini, E., Capria, M. T., Tosi, F., De Sanctis, C., et al. (2019), High Thermal Inertia Zones on Ceres From Dawn Data, JGR, DOI:10.1029/2018JE005733Ryan, A., Pino Muñoz, D., Bernacki, M., Delbo, M. (2020), Full-Field Modeling of Heat Transfer in Asteroid Regolith: Radiative Thermal Conductivity of Polydisperse Particulates, JGR:Planets. DOI:10.1029/2019JE006100Zinzi, A., et al. (2016), MATISSE: A novel tool to access, visualize and analyse data from planetary exploration missions, Astronomy and Computing, DOI:10.1016/j.ascom.2016.02.

The BepiColombo mission is the first European mission to Mercury; the spacecraft will reach its destination in December 2025, and will study in detail the surface, the exosphere and the magnetosphere of the planet. We have developed a thermophysical model with the aim to analyze the dependence of the temperature of the surface and of the layers close to it on the assumptions on the thermophysical properties of the soil. The code solves the one-dimensional heat equation, assumes purely conductive heat propagation and no internal heat sources; the surface is assumed to be composed of a regolith layer with high porosity and density increasing with depth. The illumination conditions are calculated by using a Mercury shape model and the SPICE routines [1]. The model will help us to interpret the data that will be provided by the instruments onboard the BepiColombo mission. Preliminary calculations have been carried out to analyze the thermal response of the soil as a function of thermal conductivity. The model is currently also used to study the sodium content in the planet's exosphere, whose origin is under investigation [2]; the MESSENGER mission has measured the exospheric sodium content as a function of time, detecting an increase at the "cold poles" (so called because of their lower than average temperature). We therefore want to study the effect of surface temperatures on the sodium content in the exosphere; for this purpose, the temperature distribution calculated with the code is used together with an atmospheric circulation model that calculates the exospheric sodium content [3]. A simplified version of the thermophysical code is almost ready to be available to the scientific community through MATISSE [4], the software developed at the SSDC in ASI and available at https://tools.ssdc.asi.it/Matisse. [1] Acton, C. H. (1996), Planetary and Space Science, 44, 65-70 [2] Cassidy, T., et al. (2016), GRL, 43, 11 121-128 [3] Mura, A., et al. (2009), Icarus, 1, 1-11 [4] Zinzi, A., et al. (2016), Astronomy & Computing, 15, 16-28

After a brief introductory historical perspective, this chapter reviews the role played by international collaboration in the implementation of the four pillars. It addresses the particularly important perspective of enabling the ambitious set of representative missions identified in Pillar 2, many of which are out of reach by a single space agency or accessible to only a small subgroup of them: international cooperation appears as one of the most promising avenues to accomplish these missions and to provide a valuable role to each space-faring nation in planetary exploration. It then reviews some of the mechanisms for international collaboration and describes some of the most successful ones. Finally, it describes the roles that international cooperation and public–private collaborations are expected to play at the 2061 horizon.

Gas-surface interactions at the Moon, Mercury and other massive planetary bodies constitute, alongside production and escape, an essential element of the physics of their gravitationally bound exospheres. From condensation and accumulation of exospheric species onto the surface in response to diurnal and seasonal changes of surface temperature, to thermal accommodation, diffusion and ultimate escape of these species from the regolith back into space, surface-interactions have a drastic impact on exospheric composition, structure and dynamics. The study of this interaction at planetary bodies combines exospheric modeling and observations with a consideration of fundamental physics and laboratory experimentation in surface science. With a growing body of earth-based and spacecraft observational data, and a renewed focus on lunar missions and exploration, the connection between the exospheres and surfaces of planetary bodies is an area of active and growing research, with advances being made on problems such as topographical and epiregolith thermal effects on volatile cold trapping, among others. In this paper we review current understanding, latest developments, outstanding issues and future directions on the topic of exosphere-surface interactions at the Moon, Mercury and elsewhere.

Context. The observation of pits at the surface of comets offers the opportunity to take a glimpse into the properties and the mechanisms that shape a nucleus through cometary activity. If the origin of these pits is still a matter of debate, multiple studies have recently suggested that known phase transitions (such as volatile sublimation or amorphous water ice crystallization) alone could not have carved these morphological features on the surface of 67P/Churyumov-Gerasimenko (hereafter 67P). Aims. We want to understand how the progressive modification of 67P’s surface due to cometary activity might have affected the characteristics of pits and alcoves. In particular, we aim to understand whether signatures of the formation mechanism of these surface morphological features can still be identified. Methods. To quantify the amount of erosion sustained at the surface of 67P since it arrived on its currently observed orbit, we selected 380 facets of a medium-resolution shape model of the nucleus, sampling 30 pits and alcoves across the surface. We computed the surface energy balance with a high temporal resolution, including shadowing and self-heating contributions. We then applied a thermal evolution model to assess the amount of erosion sustained after ten orbital revolutions under current illumination conditions. Results. We find that the maximum erosion sustained after ten orbital revolutions is on the order of 80 m, for facets located in the southern hemisphere. We thus confirm that progressive erosion cannot form pits and alcoves, as local erosion is much lower than their observed depth and diameter. We find that plateaus tend to erode more than bottoms, especially for the deepest depressions, and that some differential erosion can affect their morphology. As a general rule, our results suggest that sharp morphological features tend to be erased by progressive erosion. Conclusions. This study supports the assumption that deep circular pits, such as Seth_01, are the least processed morphological features at the surface of 67P, or the best preserved since their formation.

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 ESA mission Comet Interceptor will target an Oort or interstellar comet during its first approach to the Sun. Meanwhile, the Vera Rubin LSST Survey will observe hundreds of active comets per month beyond 4 au from the Sun, where water vapour pressure is expected to be too low to eject dust. We discuss observations of dust tails at heliocentric distances larger than 4 au in order to retrieve the physical parameters driving cometary activity beyond Jupiter by means of a probabilistic tail model, which is consistent with the activity model defining the gas coma parameters due to the sublimation of carbon monoxide, molecular oxygen, methane, ethane, and carbon dioxide since the activity onset at 85 au from the Sun. We find that: (i) All the observed dust tails are consistent with the adopted activity model; (ii) The tail fits depend on three free parameters only, all correlated to the nucleus size; (iii) Tail fits are always improved by anisotropic dust ejection, suggesting activity of Oort nuclei dominated by seasons; (iv) Inbound seasons suggest cometary activity before the ejection of protocomets into the Oort cloud, as predicted by the activity model; (v) Oort nuclei larger than 1 km may be characterized by a fallout up to approximate to 100 m thick deposited during approximate to 60 yr inbound; (vi) On the other side, Oort nuclei smaller than 1 km may appear more pristine than Jupiter Family Comets when observed at 1 au from the Sun.

Comets evolve due to sublimation of ices embedded inside porous dust, triggering dust emission (that is, erosion) followed by mass loss, mass redistribution and surface modifications. Surface changes were revealed by the Deep Impact and Stardust NExT missions for comet 9P/Tempel 1 (ref. 1 ), and a full inventory of the processes modifying cometary nuclei was provided by Rosetta while it escorted comet 67P/Churyumov–Gerasimenko for approximately two years 2 – 4 . Such observations also showed puzzling water-ice-rich spots that stood out as patches optically brighter and spectrally bluer than the average cometary surface 5 – 9 . These are up to tens of metres large and indicate macroscopic compositional dishomogeneities apparently in contrast with the structural homogeneity above centimetre scales of pebble-made nuclei 10 . Here we show that the occurrence of blue patches determines the seasonal variability of the nucleus colour 4 , 11 , 12 and gives insight into the internal structure of comets. We define a new model that links the centimetre-sized pebbles composing the nucleus 10 and driving cometary activity 13 , 14 to metre-sized water-ice-enriched blocks embedded in a drier matrix. The emergence of blue patches is due to the matrix erosion driven by CO 2 -ice sublimation that exposes the water-ice-enriched blocks, which in turn are eroded by water-ice sublimation when exposed to sunlight. Our model explains the observed seasonal evolution of the nucleus and reconciles the available data at micro (sub-centimetre) and macro (metre) scales.

Remote sensing data of comets 9P/Tempel 1 and 67P/Churyumov-Gerasimenko (67P hereafter) indicate the occurrence of water-ice-rich spots on the surface of cometary nuclei [1-5]. These spots are up to tens of metres in size and appear brighter and bluer than the average surface at visible wavelengths. In addition, the extensive observation campaign performed by the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS, [6]) and the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS, [7]) during the Rosetta escort phase at 67P revealed a seasonal cycle of the nucleus colour. This is characterised by blueing of the surface while approaching perihelion followed by progressive reddening and restoral of the original colour along the outbound orbit. The temporal evolution of the colour has been interpreted in previous studies as the result of increasing exposure of water ice at smaller heliocentric distances [8, 9], however, an explanation of such seasonal cycle in the context of a quantitative cometary activity model was not yet been provided. Recently, in [10] we showed that the seasonal colour cycle observed on comet 67P is determined by the occurrence of the above-mentioned water-ice-rich spots (referred to as Blue Patches – BPs –, given their colour). This can be explained in the context of activity models [11, 12] of pebble-made cometary nuclei [13], i.e. in terms of nucleus surface erosion induced by H2O and CO2 ices sublimation, driving the cometary activity. According to the scenario proposed in [10] (Fig. 1), the presence of the BPs is due to the exposure of subsurface sub-metre-sized Water-ice-Enriched Blocks (WEBs) thanks to surface erosion triggered by CO2 sublimation ejecting decimetre-sized chunks [12]. The WEBs are composed of ice-rich pebbles (dust-to-ice mass ratio δ=2, [14]), embedded in a matrix of drier pebbles (δ>>5) forming most of the nucleus. Once exposed to illumination as BPs, the WEBs are eroded by water-ice sublimation ejecting sub-cm dust [11]. By means of dedicated spectral and thermophysical modelling, we match the nucleus colour temporal evolution measured by the VIRTIS Mapping channel in the 0.55-0.8 µm spectral range. In doing this, we take into account the competing effects of CO2- and H2O-driven erosion that expose and remove the BPs, respectively, and are seasonally modulated by the insolation conditions, primarily depending on the heliocentric distance. The new nucleus model proposed in [10], implying an uneven distribution of water ice in cometary nuclei, reconciles the compositional dishomogeneities observed on comets (the BPs) at macroscopic (up to tens of metres) scale, with a structurally homogeneous pebble-made nucleus at small (centimetre) scale, and with the processes determining the cometary activity at microscopic (sub-pebble) scales. Figure 1. 67P surface gets bluer approaching perihelion as a consequence of the progressive exposure to sunlight of subsurface WEBs (from Figure 4 in Ciarniello et al., 2022, Nature Astronomy, https://doi.org/10.1038/s41550-022-01625-y). The comet nucleus is made of two types of pebbles, both including refractories and CO2 ice, with different water ice content: pebbles with high content of H2O ice form the WEBs, while H2O-ice-poor pebbles represent the rest of the nucleus. CO2 ice is stable beneath the CO2 sublimation front at depths >0.1 m [12]. Approaching perihelion, the CO2 ice sublimation rate increases, eroding the surface by chunk ejection and exposing the WEBs. Once exposed, WEBs lose CO2 and are observable as BPs. Water-ice sublimation erodes the BPs ejecting sub-cm dust from their surface and preventing the formation of a dry crust [11]. The BPs survive until their water-ice fraction is sublimated, producing the observed surface blueing. Please refer to ref. [10] for complete details. References [1] Sunshine, J. M. et al. (2006) Science 311, 1453–1455. [2] Filacchione, G. et al. (2016) Nature 529, 368–372. [3] Raponi, A. et al. (2016) Mon. Not. R. Astron. Soc. 462, S476-S490. [4] Barucci, M. A. et al. (2016) Astron. Astrophys. 595, A102. [5] Oklay, N. et al. (2017) Mon. Not. R. Astron. Soc. 469, S582–S597. [6] Coradini, A. et al. (2007) Space Sci. Rev. 128, 529–559. [7] Keller, H. U. et al. (2007) Space Sci. Rev. 128, 433–506. [8] Fornasier, S. et al. (2016) Science 354, 1566–1570. [9] Filacchione, G. et al. (2020) Nature 578, 49-52. [10] Ciarniello, M. et al. (2022) Nat. Astron. doi:10.1038/s41550-022-01625-y. [11] Fulle, M. et al. (2020) Mon. Not. R. Astron. Soc. 493, 4039–4044. [12] Gundlach, B. et al (2020). Mon. Not. R. Astron. Soc. 493, 3690–3715. [13] Blum, J. et al. (2017) Mon. Not. R. Astron. Soc. 469, S755–S77. [14] O’Rourke, L. et al. (2020) Nature 586, 697–701. Acknowledgements We thank the Italian Space Agency (ASI, Italy; ASI-INAF agreements I/032/05/0 and I/024/12/0), Centre National d’Etudes Spatiales (CNES, France), and Deutsches Zentrum für Luft-und Raumfahrt (DLR, Germany) for supporting this work. VIRTIS was built by a consortium from Italy, France and Germany, under the scientific responsibility of IAPS, Istituto di Astrofisica e Planetologia Spaziali of INAF, Rome, which also led the scientific operations. The VIRTIS instrument development for ESA has been funded and managed by ASI (Italy), with contributions from Observatoire de Meudon (France) financed by CNES and from DLR (Germany). The VIRTIS instrument industrial prime contractor was former Officine Galileo, now Leonardo Company, in Campi Bisenzio, Florence, Italy. Part of this research was supported by the ESA Express Procurement (EXPRO) RFP for IPL-PSS/JD/190.2016. D.K. acknowledges DFG-grant KA 3757/2-1. This work was supported by the International Space Science Institute (ISSI) through the ISSI International Team "Characterization of cometary activity of 67P/Churyumov-Gerasimenko comet". This research has made use of NASA’s Astrophysics Data System.

Context Circular depressions and alcoves were observed on the surface of some JFCs visited by spacecrafts: 81P/Wild 2 (Brownlee et al., 2004), 9P/Tempel 1 (Belton et al., 2013), 103P/Hartley 2 (Syal et al., 2013), and 67P/Churyumov- Gerasimenko (Vincent et al., 2015). These features are characterized by different shapes and sizes ranging from few tens to few hundreds of meters (Ip et al., 2016). Several studies investigated the thermal processing in relation to their formation and evolution (Guilbert-Lepoutre et al., 2016), and found that recent thermal activity in the inner solar system orbits is not sufficient to carve them. Ip et al. (2016) found that the size frequency distribution of the depressions on 67P, 81P and 9P has the same power law distribution, implying that they might have the same origin and formation mechanism. Dynamical simulations show that the thermal history of 81P and 9P is likely shorter than 67P’s and 103P’s, suggesting a younger surface. In this work, we investigate the thermally-induced evolution of depressions at the surface of 81P, 9P, 103P, and 67P under each of their current illumination conditions. Methods For these four nuclei, we select more than 10 surface features (i.e. depressions or alcoves). From their shape models, we select multiple facets on different sides of each feature (plateaux, bottom and walls) and consider the complete thermal environment for each facet, including self-heating and shadowing, either by neighboring facets or due to the complex global morphology of the nucleus. We compute the energy input for each facet during a full recent orbit, with a time step of ∼ 8 min. The total energy received at the surface is used as an input of a 1D thermal evolution model, which accounts for heat diffusion, phase transitions (sublimation of various ices and crystallization of amorphous water ice), gas diffusion, erosion, and dust mantling (Lasue et al., 2008). The thermal behaviour of each surface feature is investigated in detail. Results * We find that self-heating can be important in deep pits and steep cliffs of 67P and 81P (~65% and ~30% of the total energy input, respectively). In comparison, it is very low for 9P and 103P’s (<10%), where surface features are shallower. • Plateaux tend to erode more than the shadowed bottoms of sharp features, found on 67P and 81P: i.e. circular depressions become shallower with time. On 9P and 103P, erosion is more uniform since depressions are already shallower (as in the southern hemisphere of 67P). Overall, sharp depressions are likely erased by cometary activity. • Erosion sustained after the multiple perihelion passages is not able to carve depressions with the observed size and shape. It is therefore very unlikely that current illumination conditions were able to carve them. • We have, however, performed our simulations with a uniform set of thermo-physical parameters for all facets. Therefore, we cannot exclude that local or regional heterogeneities may yield different erosion rates. • A comparison between simulation outcomes for all nuclei allows to consider 103P as having the most altered surface. 9P could be an intermediate state. 81P would thus represent the least altered, or best preserved surface of these nuclei. Finally, 67P display a variety of surface ages, with areas as preserved as 81P, and a southern hemisphere as altered as 9P. 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. We thank the European Space Research and Technology Centre (ESAC) and the European Space Astronomy Centre faculty council for supporting this research. References Belton, M. J., Thomas, P., Carcich, B., et al. 2013, Icarus, 222, 477 Brownlee, D. E., Horz, F., Newburn, R. L., et al. 2004, Science, 304, 1764 Guilbert-Lepoutre, A., Rosenberg, E. D., Prialnik, D., & Besse, S. 2016, Monthly Notices of the Royal Astronomical Society, 462, S146 Ip, W.-H., Lai, I.-L., Lee, J.-C., et al. 2016, Astronomy & Astrophysics, 591, A132 Lasue, J., De Sanctis, M. C., Coradini, A., et al. 2008, Planetary and Space Science, 56, 1977 Syal, M. B., Schultz, P. H., Sunshine, J. M., et al. 2013, Icarus, 222, 610 Vincent, J.-B., Bodewits, D., Besse, S., et al. 2015, Nature, 523, 63

Introduction. The ASI Space Science Data Center (SSDC) has a long-standing experience in space data management. Among its tools, MATISSE (Multi-purpose Advanced Tool for Instruments for the Solar System Exploration [1]) was created in 2013 to search, visualize and analyze data from planetary exploration missions. MATISSE, whose v2.0 update [2] is available at https://tools.ssdc.asi.it/Matisse/, allows users to analyze data from different missions, such as NASA Messenger and NASA Dawn, and the possibility of visualizing the data directly on the 3D shape of the targets greatly helped in reaching stunning scientific results. The work here presented points at improving and expanding the functionalities of the MATISSE tool for the Mercury explorations, by including the possibility to merge together the outputs of a thermophysical code of Mercury surface and the study of planet’s surface from a geological point of view. MATISSE for Mercury exploration. The ESA-JAXA BepiColombo mission is the first European mission to Mercury; the spacecraft will study in detail the surface, the exosphere and the magnetosphere of the planet. We have developed a thermophysical model with the aim to analyze the dependence of the temperature of the surface and of the layers close to it on the assumptions on the thermophysical properties of the soil. The code solves the one-dimensional heat equation, assumes purely conductive heat propagation and no internal heat sources; the surface is assumed to be composed of a regolith layer with high porosity and density increasing with depth. Calculations have been carried out to analyze the thermal response of the soil as a function of thermal conductivity. The model has been also used to study the sodium content in the planet's exosphere, whose origin is under investigation [3]; the MESSENGER mission has measured the exospheric sodium content as a function of time, detecting an increase at the "cold poles" (so called because of their lower than average temperature). In order to study the effect of surface temperatures on the sodium content in the exosphere, the temperature distribution calculated with the code has been used together with an atmospheric circulation model that calculates the exospheric sodium content [4]. Figure 1. Total sodium exospheric content as function of time and true anomaly angle, calculated in two cases: surface temperature from the thermophysical code (red line), and reference temperature (T proportional to ¼ power of cosine of illumination angle, blue line). A simplified version of the thermophysical code is available to the scientific community through MATISSE and, therefore, it could be crucial to interpret the data acquired by the instruments on board the BepiColombo mission, especially if it is taken into account that, through MATISSE, it is possible to analyze the surface of Mercury with data from NASA Messenger based on queries looking for specific geological units (as is already possible for Mars, Mercury and Ceres). Thanks to the data from the MDIS (Mercury Dual Imaging System [5]) camera on board the MESSENGER, it was possible to create a global map of the planet's surface. The identified lithologies can be analyzed and subsequently uploaded to our MATISSE tool in order to have the possibility to study the surface of Mercury directly from the site. On MATISSE it will be possible to select the data not only with the usual geographical coordinate, but on the basis of geological maps, so that the user can analyze observational and modeled data collected on areas with well-defined geology that allow to study the effects of the heterogeneity of surface. Figure 2. MATISSE homepage and selection of parameters to be observed. Example of selection of the Hokusay crater on Mercury's surface. The possibility of studying the Mercury surface directly from the tool will allow not only to make a detailed study of the terrain but understanding its formation will help science to understand how the solar wind affects some materials. For this reason our work will allow planetologists and astrophysicists to have all the available data for the study of the planet. In collaboration with the PlanMap and GMap teams, we are currently working to include the geological units identified on the surface of Mercury in order to be ready with the data that will come from the Bepi Colombo mission in order to have everything you need to study this planet in depth. Next steps. We plan to add in the tool all geological units mapped by GMAP team on Mercury surface. Moreover we will expand in the tool the possibility of searching for data based on the morphologies that the user wants to analyze (e.g. craters, valleys). Another goal will be to expand the use of the tool, making it similar to the geographic information system (GIS). We will expand the possibility of selecting specific areas to be analyzed, having clear the geographical position of the data. It will also be possible to obtain topographic profiles, select more data to be observed. All these analyses will be performed directly on the 3D models. The inclusion of these functionalities in the tool could produce a sensible step forward in the study of planetary geology, with the possibility of better exploiting different datasets and taking also into account the collaboration of different teams already leaders in this field. References. [1] Zinzi A. et al. (2016) Astron. Comput., 15, 16-28 [2] Zinzi A. et al. (2019) EPSC-DPS Joint Meeting 2019, id. EPSC-DPS2019-1272 [3] Rognini, E., et al. (2022), Effects of mercury surface temperature on the sodium abundance in its exosphere, Planetary and Space Science, 212 [4] Mura, A., et al. (2009), The sodium exosphere of Mercury: Comparison between observations during Mercury’s transit and model results, Icarus, 200, 1-11 [5] Hawkins et al., (2007). The Mercury Dual Imaging system on the MESSENGER spacecraft, Space Science Reviews, 131, 247–338

The link between the surface temperature of Mercury and the exosphere sodium content has been investigated. Observations show that, along the orbit of Mercury, two maxima of total Na content are present: one at aphelion and one at perihelion. Previous models, based on a simple thermal map, were not able to reproduce the aphelion peak. Here we introduce a new thermophysical model giving soil temperatures as an input for the IAPS exospheric model already used in the past with the input of a simple thermal map. By comparing the reference model output with the new one, we show that such improved surface temperature map is crucial to explain the temporal variability of Sodium along the orbit.

Linear features are very common on asteroid surfaces. They are generally formed after impact and provide information about asteroid evolution. This work focuses on a mineralogical and spectral analysis of the main linear features on the 1/Ceres surface, having both tectonic (Samhain Catena’s pit chains) and geomorphic origins, i.e., generated by ejecta material (Occator ejecta, Dantu’s secondary radial chains, secondary radial chains generated from the Urvara impact). The analysis is based on spectral parameters defined by the Dawn’s VIR imaging spectrometer data, as albedo and depths of the bands centered at approximately 2.7, 3.1, 3.4 and 3.9 mm. The geomorphic linear features show spectral variations with respect to the surroundings, i.e., ammoniated phyllosilicates band depth shallowing is caused by the presence of material originating in a different region or dehydration caused by impact. The Samhain Catena does not show any mineralogical variation, due to its tectonic origin. The spectral behavior of Ceres’ linear features is similar to that observed on other asteroids (Vesta, Eros) and can be diagnostic in discerning the origin of linear features. Then, we searched spectral signatures of organics in the Samhain Catena region, since they are expected to form at depth due to internal processes: the absence of such signatures indicates that either they form at a larger depth or that their subsurface distribution is uneven.

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(-1/2) 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 (similar to 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.