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

Mapping olivine abundance on asteroid (25143) Itokawa from Hayabusa/NIRS data

Olivine is one of the main abundant mineral in the Solar System, and the determination of its abundance on a surface may give fundamental information about its evolution. The study of surface distribution of olivine on asteroid (25143) Itokawa through near-Infrared reflectance spectroscopy is a difficult goal because olivine and pyroxene bands centred at 1 μm and 2 μm are not entirely included in Hayabusa/NIRS' spectral range. In this work, the retrieval of olivine abundance has been performed by applying two different methods: the first one uses some spectral indices to retrieve olivine abundance, whilst the second one consists of the application of the Hapke's theory in order to create synthetic spectra aimed at fitting a selection of NIRS' spectra. The analysis performed with the first method brought to an approximately homogeneous distribution of olivine content (60 ± 15% on average) on Itokawa's surface, with the exception of Sagamihara region, which has a slightly (up to 10%) lower olivine content. The second method brought to an average 60 ± 7.5% olivine content within 5 selected spectra, with the same reduction found in the spectrum from the Sagamihara region. All these values are in agreement with literature values on this topic, especially with the ones retrieved from particles sampled in Muses Sea by the Hayabusa probe

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

Visible-Infrared spectral characterization of 3200 Phaethon at its closest approach to Earth

The asteroid 3200 Phaethon is a peculiar object with a very eccentric orbit with a perihelion located at only 0.14 AU and it has been dynamically associated with the Geminid meteor stream (Gustafson 1989; Williams & Wu 1993; Jenniskens 2006, and references therein). Phaethon is a B-class asteroid and it is linked with carbonaceous species and hydrated silicates such as phyllosilicates. Good match of the Phaeton spectra are the aqueously altered CI/CM meteorites (Licandro, 2007) and the CK meteorites (Clark et al. 2010). The asteroid 3200 Phaethon is the target of the Destiny + Space mission, managed by Japanese Space Agency (Jaxa), which will perform a close rendevouz with this asteroid, with the scientific objectives of studying its surface properties and assessing its cometary activity in terms of release of dust and volatiles. The December 2017 Phaethon Earth approach have been a very important event since it was about 10 times closer than any other future approach predicted at least for the next 20 years. In that occasion, we observed the asteroid at the 3.5 mt Telescopio Nazionale Galileo in the spectral interval (0.4-2-5 μm).We obtained three spectra in the Visible from 0.4 to 0.8 μmm, with a strong fringing longward of 0.8 μmm which doesn't allow to use the data between 0.8 and 1 μmm. The spectra are featureless, however, the slope of two spectra agrees well with many previous observations of Phaeton (see e.g. Licandro et al, 2006). One spectra show, instead, a bluer behaviour, similarly to the unique previous observation by Luu and Jewitt (1993). The IR spectrum is almost featureless, with very weak features at the limit of the S/N which at present are under investigation

Ceres' opposition effect observed by the Dawn framing camera

The surface reflectance of planetary regoliths may increase dramatically towards zero phase angle, a phenomenon known as the opposition effect (OE). Two physical processes that are thought to be the dominant contributors to the brightness surge are shadow hiding (SH) and coherent backscatter (CB). The occurrence of shadow hiding in planetary regoliths is self-evident, but it has proved difficult to unambiguously demonstrate CB from remote sensing observations. One prediction of CB theory is the wavelength dependence of the OE angular width. The Dawn spacecraft observed the OE on the surface of dwarf planet Ceres. We characterize the OE over the resolved surface, including the bright Cerealia Facula, and to find evidence for SH and/or CB. We analyze images of the Dawn framing camera by means of photometric modeling of the phase curve. We find that the OE of most of the investigated surface has very similar characteristics, with an enhancement factor of 1.4 and a FWHM of 3{\deg} (broad OE). A notable exception are the fresh ejecta of the Azacca crater, which display a very narrow brightness enhancement that is restricted to phase angles $< 0.5${\deg} (narrow OE); suggestively, this is in the range in which CB is thought to dominate. We do not find a wavelength dependence for the width of the broad OE, and lack the data to investigate the dependence for the narrow OE. The prediction of a wavelength-dependent CB width is rather ambiguous. The zero-phase observations allow us to determine Ceres' visible geometric albedo as $p_V = 0.094 \pm 0.005$. A comparison with other asteroids suggests that Ceres' broad OE is typical for an asteroid of its spectral type, with characteristics that are primarily linked to surface albedo. Our analysis suggests that CB may occur on the dark surface of Ceres in a highly localized fashion.Comment: Credit: Schr\"oder et al, A&A in press, 2018, reproduced with permission, \copyright ES

Multi-instrument analysis of 67P/Churyumov-Gerasimenko coma particles: COPS-GIADA data fusion

The European Space Agency's Rosetta mission to comet 67P/Churyumov-Gerasimenko has offered scientists the opportunity to study a comet in unprecedented detail. Four instruments of the Rosetta orbiter, namely, the Micro-Imaging Dust Analysis System (MIDAS), the Grain Impact Analyzer and Dust Accumulator (GIADA), the COmetary Secondary Ion Mass Analyser (COSIMA), and the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) have provided information on cometary dust particles. Cross-instrument comparisons are crucial to characterize cometary dust particles beyond the capabilities of individual sensors, as they are sensitive to different dust components. We present the first comparison between detections of the ROSINA COmet Pressure Sensor (COPS) and GIADA. These two instruments are complementary as the former is sensitive solely to volatiles of icy particles, while the latter measured the dust particle as a whole, including refractories and condensed (semi)volatiles. Our goal is to correlate the particles detected by COPS and GIADA and to assess whether they belong to a common group. We statistically analyzed the in situ data of COPS and GIADA by calculating Pearson correlation coefficients. Among the several types of particles detected by GIADA, we find that COPS particles are significantly correlated solely with GIADA fluffy agglomerates (Pearson correlation coefficient of 0.55 and p-value of $4.6\cdot 10^{-3}$). This suggests that fluffy particles are composed of both refractories and volatiles. COPS volatile volumes, which may be represented by equivalent spheres with a diameter in the range between 0.06 $\mu$m and 0.8 $\mu$m, are similar to the sizes of the fractal particle's subunits identified by MIDAS (i.e., 0.05-0.18 $\mu$m).Comment: 6 pages, 3 figures, accepted for publication in A&

Variations in the amount of water ice on Ceres' surface suggest a seasonal water cycle.

The dwarf planet Ceres is known to host a considerable amount of water in its interior, and areas of water ice were detected by the Dawn spacecraft on its surface. Moreover, sporadic water and hydroxyl emissions have been observed from space telescopes. We report the detection of water ice in a mid-latitude crater and its unexpected variation with time. The Dawn spectrometer data show a change of water ice signatures over a period of 6 months, which is well modeled as ~2-km2 increase of water ice. The observed increase, coupled with Ceres' orbital parameters, points to an ongoing process that seems correlated with solar flux. The reported variation on Ceres' surface indicates that this body is chemically and physically active at the present time

PFS/MEX limb observations of 4.3-μm CO2 non-LTE emission in the atmosphere of Mars

We present PFS-MEX limb observations of the CO2 non-local thermodynamic equilibrium (non-LTE) emission at 4.3 μm in the atmosphere of Mars collected in more than six Martian years. With unprecedented spatial and temporal coverage, and relatively high spectral resolution, this unique dataset promises to improve our understanding of the upper atmosphere of Mars. The former allows analyses of the emission as a function of tangent altitude, solar zenith angle, season, latitude, local time, and thermal condition of the atmosphere. The latter allows unambiguous identification of several emission bands of different isotopologues. We selected observations in the altitude range 50-200 km. No emission was detected for altitudes higher than 170 km. The spectral shape of the non-LTE emission changes dramatically with the altitude of the tangent point, reflecting the different contribution of the major and minor CO2 bands and isotopologues to the total emission at different heights. For altitudes higher than 130 km the observed spectrum is dominated by the second hot (SH) bands of the main isotopologue 12C16O2 (also referred to as 626 SH). At lower altitudes, the contribution of the isotopic 13C16O2 second hot bands (636 SH) to the observed spectrum gradually increases, and is maximum around 70-80 km. Similar consideration apply to the fourth hot bands of the 12C16O2 (626 FRH), and particularly those from the (2001x) levels, whose contribution is maximum around 80-90 km. The 626 SH bands can be observed up to an altitude 160-170 km, and their emission is peaked around 120-130 km. The 626 FRH and 636 SH bands are not observed above 130-140 km. Both the first hot (FH) and the fundamental band (FB) of the main isotopologue show a peculiar behavior. Indeed, these emissions can be observed at all altitudes, from 50 km up to 170 km. The intensity of the FH band increases linearly with decreasing height, while the intensity of the FB band is essentially constant at all altitudes, and rapidly decreases above 150 km. For a fixed altitude, the solar zenith angle (SZA) is the main parameter affecting the intensity and the spectral shape of the non-LTE emission. For SZA between 0 and 40° the intensity of the emission does not show significant variations. For SZAs larger than 40° the observed emission decreases rapidly with increasing SZA, following a cosine-like relation. The different illumination also affects the spectral shape of the non-LTE emission spectrum. High incidence angles tend to increase the relative contribution of weaker bands compared to stronger/optically thicker bands. For a fixed SZA, we found variation of the intensity of the emission with local time, in response to variations of the thermal structure of the atmosphere. Latitudinal variation of the intensity of the CO2 non-LTE are also investigated. The maximum intensity is observed around the sub-solar latitudes, where the solar flux is maximum. The intensity of the emission and the altitude at which the maximum emission is observed also changes with the season. The altitude where the maximum intensity of the 626 SH bands is observed decreases from 120-130 km at the perihelion (Ls = 251°), down to ∼85 km at the southern winter solstice (Ls = 90°). This is explained by the variability of the thermal structure (scale heights) of the Martian atmosphere with the season, as a response to the changing solar flux. The altitude of a given pressure level depends on the thermal structure of the atmosphere which, in turn, depends on the season. On the contrary, the pressure level of the peak emission does not depend on the scale heights, as it is mainly controlled by the CO2 column density above the peak. These results, while on one hand confirm and provide more insights and constraints to some aspects of the non-LTE processes on Mars, on the other hand further stimulate and challenge current theoretical models, possibly bringing closer the moment in which the measurements could be inverted to derive important information about the upper mesosphere and lower thermosphere of Mars

Spectrophotometric Modeling and Mapping of Ceres

We report a comprehensive analysis of the global spectrophotometric properties of Ceres using Dawn Framing Camera images collected from April to June 2015 during the RC3 and Survey mission phases. The single-scattering albedo of Ceres at 555 nm is 0.14$\pm$0.04, the geometric albedo is 0.096$\pm$0.006, and the Bond albedo is 0.037$\pm$0.002. The asymmetry factors calculated from the best-fit two-term Henyey-Greenstein (HG) single-particle phase function (SPPF) show a wavelength dependence, suggesting that the phase reddening of Ceres is dominated by single-particle scattering rather than multiple scattering or small-scale surface roughness. The Hapke roughness parameter of Ceres is derived to be 20$^\circ\pm$6$^\circ$ with no wavelength dependence. The phase function of Ceres shows appreciably strong scattering around 90$^\circ$ phase angle that cannot be fitted with a single-term HG SPPF, suggesting possible stronger forward scattering than other asteroids previously analyzed with spacecraft data. We speculate that such a scattering characteristic of Ceres might be related to its unique surface composition. We grouped the reflectance data into a 1$^\circ$ latitude-longitude grid and fitted each grid independently to study the spatial variations of photometric properties. The albedo and color maps are consistent with previous studies. The SPPF over the surface of Ceres shows stronger backscattering associated with lower albedo and vice versa, consistent with the general trend among asteroids. The Hapke roughness parameter does not vary much across the surface of Ceres, except for the ancient Vendimia Planitia region that has a slightly higher roughness. Based on the wavelength dependence of the SPPF of Ceres, we hypothesize that its regolith grains either contain a considerable fraction of $\lessapprox\mu$m-sized particles, or are strongly affected by internal scatterers of this size.Comment: 43 pages, 3 tables, 17 figures, accepted by Icaru