The SSDC contribution to the improvement of knowledge by means of 3D data projections of minor bodies

The latest developments of planetary exploration missions devoted to minor bodies required new solutions to correctly visualize and analyse data acquired over irregularly shaped bodies. ASI Space Science Data Center (SSDC-ASI, formerly ASDC-ASI Science Data Center) worked on this task since early 2013, when started developing the web tool MATISSE (Multi-purpose Advanced Tool for the Instruments of the Solar System Exploration) mainly focused on the Rosetta/ESA space mission data. In order to visualize very high-resolution shape models, MATISSE uses a Python module (vtpMaker), which can also be launched as a stand-alone command-line software. MATISSE and vtpMaker are part of the SSDC contribution to the new challenges imposed by the "orbital exploration" of minor bodies: 1) MATISSE allows to search for specific observations inside datasets and then analyse them in parallel, providing high-level outputs; 2) the 3D capabilities of both tools are critical in inferring information otherwise difficult to retrieve for non-spherical targets and, as in the case for the GIADA instrument onboard Rosetta, to visualize data related to the coma. New tasks and features adding valuable capabilities to the minor bodies SSDC tools are planned for the near future thanks to new collaborations

How Comets Work

Two major questions regarding comets have been up to now far from any solution. (i) How is it possible that water-ice sublimation from the nucleus surface does not lead to an insulating crust, stopping every gas and dust ejection within a few days? (ii) How is it possible that the gas flow crossing the refractory surface crust ejects dust particles bonded by tensile strengths larger than tens of Pa when the perihelion gas pressure at the nucleus-coma interface is less than one Pa? We have developed a simple but rigorous analytical model, assuming that the cometary nucleus consists of agglomerates of ice and dust (“clusters”). As soon as the clusters become exposed to sunlight, gas diffusion from their inside leads to their dehydration. We find that (i) the gas diffusing from the interior to the surface of a sunlit cluster has a steep density gradient at the cluster surface, which blasts the cluster into particles of sizes larger than or equal to those actually observed by Rosetta dust instruments; (ii) the heat-conduction and diffusion timescales are much shorter than the dehydration timescale, ensuring that the described process prevents any dumping of the nucleus activity driven by water-ice sublimation from 4 au inbound to 4 au outbound; and (iii) the clusters are in fact cm-sized pebbles, so that a cometary nucleus made of pebbles is confirmed to be the only one consistent with cometary gas and dust activity, a process unexplained until now

On the electronic structure of small carbon grains of astrophysical interest

In a previous paper Mennella et al. (1995a) studied the evolution of the UV spectrum of small carbon grains due to thermal annealing in the range 250-800°C. The spectral variations were interpreted in terms of internal structural rearrangement of the grains caused by hydrogen loss. The electronic transitions (Ï-Ï* and Ï-Ï*) of the sp2 clusters forming the grains were indicated as the major factors responsible for determining their extinction properties. In this paper we present the results of new measurements aimed at probing the heat-induced structural changes. The thermal evolution of the optical gap and of the Raman spectrum, both sensitive to the sp2 clustering degree, confirms that the observed spectral changes do depend on structural variations. In fact, the Ï electron delocalization of the sp2 clusters determines a link between structural and electronic properties in carbons. We find a basic correlation between the UV peak position and the optical gap. It is interpreted in terms of a dependence of the dipole matrix momentum of Ï transitions on the sp2 cluster size. The attribution of the spectral changes to the grain internal structure is corroborated by morphological analyses. Scanning and transmission electron microscope images show that the fluffy structure of the samples as well as the dimension and the shape of the single grains do not change after the annealing process. In the astrophysical context, the present results can be relevant for the attribution of the 217.5 nm feature, as they show that the internal structure of carbon grains, having sizes similar to those expected for the "bump" carriers, controls the interaction with UV photons

The dust environment of comet 67P/Churyumov–Gerasimenko: results from Monte Carlo dust tail modelling applied to a large ground-based observation data set

We present an extensive data set of ground-based observations and models of the dust environment of comet 67P/Churyumov–Gerasimenko covering a large portion of the orbital arc from about 4.5 au pre-perihelion through 3.0 au post-perihelion, acquired during the current orbit. In addition, we have also applied the model to a dust trail image acquired during this orbit, as well as to dust trail observations obtained during previous orbits, in both the visible and the infrared. The results of the Monte Carlo modelling of the dust tail and trail data are generally consistent with the in situ results reported so far by the Rosetta instruments Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) and Grain Impact Analyser and Dust Accumulator (GIADA). We found the comet nucleus already active at 4.5 au pre-perihelion, with a dust production rate increasing up to ∼3000 kg s−1 some 20 d after perihelion passage. The dust size distribution at sizes smaller than r = 1 mm is linked to the nucleus seasons, being described by a power law of index −3.0 during the comet nucleus southern hemisphere winter but becoming considerably steeper, with values between −3.6 and −4.3, during the nucleus southern hemisphere summer, which includes perihelion passage (from about 1.7 au inbound to 2.4 au outbound). This agrees with the increase of the steepness of the dust size distribution found from GIADA measurements at perihelion showing a power index of −3.7. The size distribution at sizes larger than 1 mm for the current orbit is set to a power law of index −3.6, which is near the average value of in situ measurements by OSIRIS on large particles. However, in order to fit the trail data acquired during past orbits previous to the 2009 perihelion passage, a steeper power-law index of −4.1 has been set at those dates, in agreement with previous trail modelling. The particle sizes are set at a minimum of r = 10 μm, and a maximum size, which increases with decreasing heliocentric distance, in the 1–40 cm radius domain. The particle terminal velocities are found to be consistent with the in situ measurements as derived from the instrument GIADA on board Rosetta

Laboratory experiments on cosmic dust analogues: the structure of small carbon grains

In this paper we present new results of our experiments aimed to study the internal structure of cosmic analogue carbon grains. The samples, produced by arc discharge between two carbon electrodes in an argon atmosphere, were annealed in the temperature range 250-780°C in order to produce modification of the internal grain structure. These changes were monitored by analysing the variations of the extinction profile between 190 and 2600 nm and of the optical gap as a function of the annealing temperature. The shift of the UV peak position towards longer wavelengths. the overall increase of the extinction coefficient and the closing of the gap as the temperature increases are all consistent with the evolution of carbon grains outlined by Mennella et al. (Astrophys. J., 444, 288, 1995 ; Astrophys. J. Suppl. Ser., 100, 149, 1995). It provides a growth in number and size of the sp2 clusters forming the grains during the annealing. The relevance of the electronic structure of the aromatic clusters in the extinction processes and the dependence of the energy Ï transitions on their size are confirmed by the present results. These results may be relevant in the context of interstellar bump attribution, as they show that the internal structure of small carbon grains is dominant in extinction processes. © 1995

Ultraviolet Spectral Changes in Amorphous Carbon Grains Induced by Ion Irradiation

Small carbon grains, processed by UV radiation and cosmic rays, have been proposed as carriers of the 217.5 nm bump present in the interstellar extinction curves (Hecht 1986; Sorrell 1990). In this paper, we present the results of an experiment aimed at simulating, in a -rst approximation, the cosmic-ray irradiation active in space. We have studied the e†ects induced by 3 keV Heions on the UV spectrum of small cosmic analog carbon grains. Two di†erent kinds of grains have been analyzed. They were produced by vapor conden- sation in hydrogen and argon quenching atmospheres. Spectrophotometric measurements have been carried out on grains as they were produced and after ion irradiation in the spectral range 0.19E2 km. Relevant UV spectral changes are observed after ion irradiation: while the UV absorption band shifts from 203 to 215 nm in hydrogenated amorphous carbon grains, an opposite trend is observed for the samples produced in the argon atmosphere. In this case the UV band moves from 240 to 218 nm. These spectral changes are well correlated with the optical gap variations and are therefore interpreted in terms of grain microstructure changes induced by the interactions with ions. At the highest ion Nuence considered, the two carbons tend to have a similar microstructure, as testi-ed by the UV peak position and optical gap values because of a saturation e†ect of the two competitive processes, amorphization and graphitization, which occur in carbon samples during ion irradiation (Compagnini & Calcagno 1996). The results of the present experiment suggest that hydrogenated amorphous carbon grains cannot be transformed into graphite grains by cosmic-ray irradiation. Moreover, the efficiency of ion irradiation in destroying well-ordered aromatic structures poses the problem of the survival itself of polycrystalline or pure graphite particles in the interstellar medium. Subject headings: cosmic rays E dust, extinction E methods: laboratory E ultraviolet: IS

Analysis of cosmic materials: Results on carbon and silicate laboratory analogues

Carbon and silicates are two of the main components of cosmic dust. They change nature through different evolutionary phases, according to the cosmic environment and the experienced processing. To understand the evolution of cosmic materials the study of "laboratory analogues" represents a powerful tool. In this context, systematic analyses are performed at the cosmic physics laboratory of Naples on solid particles, synthesised and processed under carefully controlled conditions. Different kinds of carbon and silicate samples are produced under various environmental conditions and exposed to processes (e.g. thermal annealing, UV irradiation and ion bombardment). The comparative analysis of the results allows us to link intrinsic properties (such as chemical composition and structure) to the optical behaviour of grains. This study offers the opportunity to interpret observations concerning the composition of small bodies in the Solar System, such as spectroscopic results obtained for comets by the Infrared Space Observatory (ISO). Several open questions remain, however, unsolved and await results from new laboratory experiments. ©1999 COSPAR. Published by Elsevier Science Ltd

Exploring Refractory Organics in Extraterrestrial Particles

The origin of organic compounds detected in meteorites and comets, some of which could have served as precursors of life on Earth, remains an open question. The aim of the present study is to make one more step in revealing the nature and composition of organic materials of extraterrestrial particles by comparing infrared spectra of laboratory-made refractory organic residues to spectra of cometary particles returned by the Stardust mission, interplanetary dust particles, and meteorites. Our results reinforce the idea of a pathway for the formation of refractory organics through energetic and thermal processing of molecular ices in the solar nebula. There is also the possibility that some of the organic material had formed already in the parental molecular cloud before it entered the solar nebula. The majority of the IR “organic” bands of the studied extraterrestrial particles can be reproduced in the spectra of the laboratory organic residues. We confirm the detection of water, nitriles, hydrocarbons, and carbonates in extraterrestrial particles and link it to the formation location of the particles in the outer regions of the solar nebula. To clarify the genesis of the species, high-sensitivity observations in combination with laboratory measurements like those presented in this paper are needed. Thus, this study presents one more piece of the puzzle of the origin of water and organic compounds on Earth and motivation for future collaborative laboratory and observational projects