A. Raponi

The organic-rich surface of comet 67P/Churyumov-Gerasimenko as seen by VIRTIS/Rosetta

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.

Ammoniated phyllosilicates with a likely outer Solar System origin on (1) Ceres

Studies of the dwarf planet (1) Ceres using ground-based and orbiting telescopes have concluded that its closest meteoritic analogues are the volatile-rich CI and CM carbonaceous chondrites. Water in clay minerals, ammoniated phyllosilicates, or a mixture of Mg(OH)2 (brucite), Mg2CO3 and iron-rich serpentine have all been proposed to exist on the surface. In particular, brucite has been suggested from analysis of the mid-infrared spectrum of Ceres. But the lack of spectral data across telluric absorption bands in the wavelength region 2.5 to 2.9 micrometres—where the OH stretching vibration and the H2O bending overtone are found—has precluded definitive identifications. In addition, water vapour around Ceres has recently been reported, possibly originating from localized sources. Here we report spectra of Ceres from 0.4 to 5 micrometres acquired at distances from ~82,000 to 4,300 kilometres from the surface. Our measurements indicate widespread ammoniated phyllosilicates across the surface, but no detectable water ice. Ammonia, accreted either as organic matter or as ice, may have reacted with phyllosilicates on Ceres during differentiation. This suggests that material from the outer Solar System was incorporated into Ceres, either during its formation at great heliocentric distance or by incorporation of material transported into the main asteroid belt.

Bright carbonate deposits as evidence of aqueous alteration on (1) Ceres

The typically dark surface of the dwarf planet Ceres is punctuated by areas of much higher albedo, most prominently in the Occator crater. These small bright areas have been tentatively interpreted as containing a large amount of hydrated magnesium sulfate, in contrast to the average surface, which is a mixture of low-albedo materials and magnesium phyllosilicates, ammoniated phyllosilicates and carbonates. Here we report high spatial and spectral resolution near-infrared observations of the bright areas in the Occator crater on Ceres. Spectra of these bright areas are consistent with a large amount of sodium carbonate, constituting the most concentrated known extraterrestrial occurrence of carbonate on kilometre-wide scales in the Solar System. The carbonates are mixed with a dark component and small amounts of phyllosilicates, as well as ammonium carbonate or ammonium chloride. Some of these compounds have also been detected in the plume of Saturn’s sixth-largest moon Enceladus. The compounds are endogenous and we propose that they are the solid residue of crystallization of brines and entrained altered solids that reached the surface from below. The heat source may have been transient (triggered by impact heating). Alternatively, internal temperatures may be above the eutectic temperature of subsurface brines, in which case fluids may exist at depth on Ceres today.

Distribution of phyllosilicates on the surface of Ceres

INTRODUCTION The surface of the dwarf planet Ceres is known to host phyllosilicate minerals, but their distribution and origin have not previously been determined. Phyllosilicates are hydrated silicates, and their presence on the surface of Ceres is intriguing given that their structure evolves through an aqueous alteration process. In addition, some phyllosilicates are known to bear NH4, which places a constraint on the pH and redox conditions during the evolution of Ceres. We studied the distribution of phyllosilicates across the planet’s surface to better understand the evolutionary pathway of Ceres. RATIONALE Using the data acquired by the mapping spectrometer (VIR) onboard the Dawn spacecraft, we mapped the spatial distribution of different minerals on Ceres on the basis of their diagnostic absorption features in visible and infrared spectra. We studied the phyllosilicates through their OH-stretch fundamental absorption at about 2.7 µm and through the NH4 absorption at about 3.1 µm. From our composition maps, we infer the origin of the materials identified. RESULTS We found that Mg- and NH4-bearing phyllosilicates are ubiquitous across the surface of Ceres and that their chemical composition is fairly uniform. The widespread presence of these two types of minerals is a strong indication of a global and extensive aqueous alteration—i.e., the presence of water at some point in Ceres’ geological history. Although the detected phyllosilicates are compositionally homogeneous, we found variations in the intensity of their absorption features in the 3-µm region of the reflectance spectrum. Such variations are likely due to spatial variability in relative mineral abundance (see the figure). CONCLUSION The large-scale regional variations evident in the figure suggest lateral heterogeneity in surficial phyllosilicate abundance on scales of several hundreds of kilometers. Terrains associated with the Kerwan crater (higher concentration of phyllosilicates) appear smooth, whereas the Yalode crater (lower concentration of phyllosilicates) is characterized by both smooth and rugged terrains. These distinct morphologies and phyllosilicate concentrations observed in two craters that are similar in size may reflect different compositions and/or rheological properties. On top of this large-scale lateral heterogeneity, small-scale variations associated with individual craters could result from different proportions of mixed materials in a stratified upper crustal layer that has been exposed by impacts. Variations associated with fresh craters, such as the 34-km-diameter Haulani, indicate the presence of crustal variations over a vertical scale of a few kilometers, whereas much larger craters, such as the 126-km-diameter Dantu, suggest that such stratification may extend for at least several tens of kilometers. Abundance maps. Qualitative maps of the abundances of (top) phyllosilicates and (bottom) NH4, based on the depth of their absorption features. The two maps have a similar global pattern, although they differ in some localized regions such as Urvara. The scale bar is valid at the equator. The dwarf planet Ceres is known to host phyllosilicate minerals at its surface, but their distribution and origin have not previously been determined. We used the spectrometer onboard the Dawn spacecraft to map their spatial distribution on the basis of diagnostic absorption features in the visible and near-infrared spectral range (0.25 to 5.0 micrometers). We found that magnesium- and ammonium-bearing minerals are ubiquitous across the surface. Variations in the strength of the absorption features are spatially correlated and indicate considerable variability in the relative abundance of the phyllosilicates, although their composition is fairly uniform. These data, along with the distinctive spectral properties of Ceres relative to other asteroids and carbonaceous meteorites, indicate that the phyllosilicates were formed endogenously by a globally widespread and extensive alteration process.

Detection of local H2O exposed at the surface of Ceres

INTRODUCTION Dwarf planet Ceres’ low average-density (2162 ± 3 kg m−3) indicates that it must contain considerable water. Water is likely a key component in the chemical evolution and internal activity of Ceres, possibly resulting in a layer of ice-rich material and perhaps liquid in the mantle. Mineral hydroxides (OH-bearing) and hydrates (H2O-bearing), such as clays, carbonates, and various salts, would be created. These hypotheses were supported by the detection of hydroxyl (OH)–rich materials, OH-bearing molecule releases, H2O vapor molecules, and haze. However, the presence of H2O on the surface has not previously been confirmed. The detection and mapping of H2O on Ceres is one objective of the Dawn spacecraft, in orbit around Ceres since March 2015. RATIONALE The purpose of the Dawn space mission at Ceres is to study the geology, geophysics, and composition remotely by means of high-resolution imagery and spectrometry. Dawn’s Visible and InfraRed Mapping Spectrometer (VIR) measures the sunlight scattered by the surface of Ceres in a range of wavelengths between 0.25 and 5.1 μm. The position and shape of absorption features in VIR reflectance spectra are sensitive to the surface mineral and molecular composition. In spectroscopy, absorption bands at 2.0, 1.65, and 1.28 μm are characteristic of vibration overtones in the H2O molecule. RESULTS Dawn has detected water-rich surface materials in a 10-km-diameter crater named Oxo, which exhibit all absorption bands that are diagnostic of the H2O molecule (see the figure). These spectra are most similar to those of H2O ice, but they could also be attributable to hydrated minerals. Oxo crater appears to be geologically very young (~1 million to 10 million years); it has sharp rims and its floor is almost devoid of impacts, suggesting a recent exposure of surface H2O. The high latitude and morphology of the Oxo crater protects much of the surface area from direct solar illumination for most of the cerean day, presenting favorable conditions for the stability of water ice or heavily hydrated salts. CONCLUSION Four ways to create or transport H2O on Ceres are considered: (i) Exposure of near-surface H2O-rich materials by a recent impact or an active landslide seems most consistent with the presence of both mineral hydrates and water ice. (ii) Release of subsurface H2O may occur on Ceres, similar to release on comet nuclei, but may never recondense on the surface. (iii) Infall of ice-bearing objects is not likely to deposit water on Ceres, because the H2O molecule likely would dissociate upon impact. (iv) Implantation of protons from the solar wind on the surface is not a probable origin of OH on Ceres because of the low flux of solar wind charged particles. We therefore conclude that surface H2O or hydrated minerals are the most plausible explanation. Dawn VIR infrared observations of Oxo crater on Ceres demonstrate the detection of H2O at the surface. (A) Reflectance spectrum collected where absorption bands of H2O at 1.28, 1.65, and 2 μm are the strongest (in blue) compared with a laboratory spectrum of H2O ice (black). The lab spectrum is scaled and vertically shifted for clarity. (B) Perspective view of Oxo crater observed by the Dawn Framing Camera (FC), where the two high-albedo areas right next to the scarps contain H2O-rich materials. The surface of dwarf planet Ceres contains hydroxyl-rich materials. Theories predict a water ice-rich mantle, and water vapor emissions have been observed, yet no water (H2O) has been previously identified. The Visible and InfraRed (VIR) mapping spectrometer onboard the Dawn spacecraft has now detected water absorption features within a low-illumination, highly reflective zone in Oxo, a 10-kilometer, geologically fresh crater, on five occasions over a period of 1 month. Candidate materials are H2O ice and mineral hydrates. Exposed H2O ice would become optically undetectable within tens of years under current Ceres temperatures; consequently, only a relatively recent exposure or formation of H2O would explain Dawn’s findings. Some mineral hydrates are stable on geological time scales, but their formation would imply extended contact with ice or liquid H2O.

The diurnal cycle of water ice on comet 67P/Churyumov–Gerasimenko

Observations of cometary nuclei have revealed a very limited amount of surface water ice, which is insufficient to explain the observed water outgassing. This was clearly demonstrated on comet 9P/Tempel 1, where the dust jets (driven by volatiles) were only partially correlated with the exposed ice regions. The observations of 67P/Churyumov–Gerasimenko have revealed that activity has a diurnal variation in intensity arising from changing insolation conditions. It was previously concluded that water vapour was generated in ice-rich subsurface layers with a transport mechanism linked to solar illumination, but that has not hitherto been observed. Periodic condensations of water vapour very close to, or on, the surface were suggested to explain short-lived outbursts seen near sunrise on comet 9P/Tempel 1. Here we report observations of water ice on the surface of comet 67P/Churyumov–Gerasimenko, appearing and disappearing in a cyclic pattern that follows local illumination conditions, providing a source of localized activity. This water cycle appears to be an important process in the evolution of the comet, leading to cyclical modification of the relative abundance of water ice on its surface.

Exposed water ice on the nucleus of comet 67P/Churyumov–Gerasimenko

Although water vapour is the main species observed in the coma of comet 67P/Churyumov–Gerasimenko and water is the major constituent of cometary nuclei, limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far. The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far. The nucleus of 67P/Churyumov–Gerasimenko appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material. Here we report the identification at infrared wavelengths of water ice on two debris falls in the Imhotep region of the nucleus. The ice has been exposed on the walls of elevated structures and at the base of the walls. A quantitative derivation of the abundance of ice in these regions indicates the presence of millimetre-sized pure water-ice grains, considerably larger than in all previous observations. Although micrometre-sized water-ice grains are the usual result of vapour recondensation in ice-free layers, the occurrence of millimetre-sized grains of pure ice as observed in the Imhotep debris falls is best explained by grain growth by vapour diffusion in ice-rich layers, or by sintering. As a consequence of these processes, the nucleus can develop an extended and complex coating in which the outer dehydrated crust is superimposed on layers enriched in water ice. The stratigraphy observed on 67P/Churyumov–Gerasimenko is therefore the result of evolutionary processes affecting the uppermost metres of the nucleus and does not necessarily require a global layering to have occurred at the time of the comet’s formation.

Spectrophotometric properties of dwarf planet Ceres from the VIR spectrometer on board the Dawn mission

Aims. We present a study of the spectrophotometric properties of dwarf planet Ceres in the visual-to-infrared (VIS-IR) spectral range by means of hyper-spectral images acquired by the VIR imaging spectrometer on board the NASA Dawn mission. Methods. Disk-resolved observations with a phase angle within the 7 ◦ < α < 132 ◦ interval were used to characterize Ceres’ phase curve in the 0.465-4.05 µm spectral range. Hapke’s model was applied to perform the photometric correction of the dataset to standard observation geometry at VIS-IR wavelength, allowing us to produce albedo and color maps of the surface. The V-band magnitude phase function of Ceres as been computed from disk-resolved images and fitted with both the classical linear model and H-G formalism. Results. The single- scattering albedo and the asymmetry parameter at 0.55 µm are w = 0.14±0.02 and ξ = −0.11±0.08, respectively (two-lobe Henyey-Greenstein phase function); at the same wavelength, Ceres’ geometric albedo as derived from our modeling is 0.094±0.007; the roughness parameter is ¯ θ = 29 ◦ ±6 ◦ . Albedo maps indicate small variability on a global scale with an average reflectance at standard geometry of 0.034 ± 0.003. Nonetheless, isolated areas such as the Occator bright spots, Haulani, and Oxo show an albedo much higher than average. We measure a significant spectral phase reddening, and the average spectral slope of Ceres’ surface after photometric correction is 1.1%kA −1 and 0.85%kA −1 at VIS and IR wavelengths, respectively. Broadband color indices are V−R = 0.38±0.01 and R−I = 0.33±0.02. Color maps show that the brightest features typically exhibit smaller slopes. The H-G modeling of the V-band magnitude phase curve for α < 30 ◦ gives H = 3.14±0.04 and G = 0.10±0.04, while the classical linear model provides V(1,1,0 ◦ ) = 3.48±0.03 and β = 0.036±0.002. The comparison of our results with spectrophotometric properties of other minor bodies indicates that Ceres has a less back-scattering phase function and a slightly higher albedo than comets and C-type objects. However, the latter represents the closest match in the usual asteroid taxonomy.

Photometric properties of comet 67P/Churyumov-Gerasimenko from VIRTIS-M onboard Rosetta

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.

Spectral analysis of Ahuna Mons from Dawn mission's visible-infrared spectrometer

Ahuna Mons is the highest mountain on Ceres. A unique complex in terms of size, shape, and morphology, Ahuna is bordered by flanks of the talus around its summit. Recent work by Ruesch et al. [2016] based on Dawns Framing Camera images shed light on the possible origin of Ahuna Mons. According to Ruesch et al. [2016], Ahuna Mons is formed by a volcanic process involving the ascent of cryomagma and extrusion onto the surface followed by dome development and subsequent spreading. Here we analyzed in detail the composition of Ahuna Mons, using data acquired by the Visible and InfraRed spectrometer aboard Dawn. The spectral analysis reveals a relatively high abundance of carbonates and a non-homogeneous variation in carbonates composition and abundance along Ahunas flanks, associated with a lower amount of the Ceress ubiquitous NH4-phyllosilicates over a large portion of the flanks. The grain size is coarser on the flanks than in the surrounding regions, suggesting the presence of fresher material, also compatible with a larger abundance of carbonates. Thermal variations are seen in Ahuna, supporting the evidence of different compactness of the surface regolith in specific locations. Results of the spectral analysis are consistent with a possible cryovolcanic origin which exposed fresher material that slid down on the flanks.