A comparative analysis of water ice on the surface of comets Tempel 1 and 67P/Churyumov-Gerasimenko

In this work we compare data of two spectrometers onboard space missions directed to comets: HRI (High Resolution Imager) on board Deep Impact \citep{AHearn2005} which overflown Tempel 1 on 2005 July 4th, and VIRTIS (Visible InfraRd and Thermal Imaging Spectrometer) onboard Rosetta which nowadays is orbiting around Comet Churyumov-Gerasimenko \citep{Coradini2007}. This work is focused on the detection of water ice on the surface, which seems to be present on both comets in two distinct modalities: - small grain size (1-2 mu m), as derived in the material ejected from the surface of Tempel 1 after the impact \citep{Sunshine2007}, and on the surface of 67P/C-G as result of vapour recondensation \citep{DeSanctis2015}. - large grain size (>30 mu m), in minor amounts, detected as exposed ice on both comets \citep{Sunshine2006, Raponi2013, Filacchione2015}. These two modalities are related to different spectral features. To retrieve the physical properties of the surface we apply the Hapke scattering model to the measured spectra. The data are corrected for artifacts and thermal emission before comparing them with the model. Moreover the estimated signal to noise ratio is taken into account by a least square optimization algorithm in the fitting procedure. This comparative analysis could reveal common processes for comets, which have implication on their formation and evolution. Authors acknowledge the funding from Italian, French and German Space Agencies

Photometric properties of Comet 67P/CG as seen by VIRTIS-M onboard Rosetta: light curves and disk-integrated phase curves

VIRTIS-M is the Visible InfraRed Thermal Imaging Spectrometer onboard the Rosetta Mission orbiter (Coradini et al. 2007) devoted to investigate the spectrophotometric properties of the comet 67P/Churyumov-Gerasimenko in the 0.25-5.1 μm spectral range. Here we present data acquired during the first observations of the comet, starting from mid-July 2014, when the spacecraft-target distance was approximately 13000 km and the comet nucleus as seen by the instrument was pixel-size, up to more recent acquisitions in which the surface was resolved (Ciarniello et al. 2015). The far-approach data have allowed us to produce both light curves of the nucleus at different solar phase angles and disk-integrated phase curves over the entire instrument spectral range within the 1.2°-14.9° phase angle interval. The light curve is characterized by two asymmetric peaks due to the elongated, e.g. double-lobed, shape of the nucleus. The disk-integrated phase curves exhibit a back -scattering behavior and a well defined surge at low phase angle due to the Shadow Hiding Opposition Effect. The derived geometric albedo at 0.55 μm is Ageo = 0.062 ± 0.002. Dependence of color ratios on rotational phase and phase angle will be discussed in order to exploit large scale surface compositional variations. We also present a retrieval of the main photometric parameters by means of Hapke model (Hapke 1993) from analysis of disk-resolved images. The derived single scattering albedo at 0.55 μm is w=0.052 while the asymmetry parameter of the single particle phase function is b=-0.42. These values indicate a very dark and back-scattering surface, common also to other comets (Li et al. 2007a,b, 2009, 2013). Hapke photometric parameters have been used to perform the photometric correction of the whole investigated dataset and albedo maps of the nucleus have been produced in the visible and infrared range. Albedo appears fairly homogeneous across the surface with Api and Imothep regions showing relatively larger values. Color maps have been produced as well, indicating that Hapi region has a more neutral spectral slope with respect to the rest of the surface. Authors acknowledge the funding from Italian Space Agency

Cassini-VIMS observations of Saturn's main rings: II. A spectrophotometric study by means of Monte Carlo ray-tracing and Hapke's theory

This work is the second in a series of manuscripts devoted to the investigation of the spectrophotometric properties of Saturn's rings from Cassini-VIMS (Visible and Infrared Mapping Spectrometer) observations. The dataset used for this analysis is represented by ten radial spectrograms of the rings which have been derived in Filacchione et al. (2014) by radial mosaics produced by VIMS. Spectrograms report the measured radiance factor I/F of the main rings of Saturn as a function of both radial distance (from 73500 to 141375 km) and wavelength (0.35-5.1 μm) for different observation geometries (phase angle ranging in the 2°-132° interval). We take advantage of a Monte Carlo ray-tracing routine (Ciarniello et al., 2014) to characterize the photometric behavior of the rings at each wavelength and derive the spectral Bond albedo of ring particles. This quantity is used to infer the composition of the regolith covering ring particles by applying Hapke's theory. Four different regions, characterized by different optical depths, and respectively located in the C ring, inner B ring, mid B ring and A ring, have been investigated. Results from photometric modeling indicate that, in the VIS-NIR spectral range, B ring particles are intrinsically brighter than A and C ring particles, with the latter having the lowest albedo, while the single particle phase function of the ring's particles is compatible with an Europa-like or Callisto-like formulation, depending on the investigated region. Spectral modeling of the inferred Bond albedo indicates that ring spectrum can be reproduced by water ice grains with inclusion of organic materials (tholin) as a UV absorber intimately mixed with variable amounts of other compounds in pure form (carbon, silicates) or embedded in water ice grains (nanophase hydrated iron oxides, carbon, silicates, crystalline hematite, metallic iron, troilite). The abundance of tholin decreases with radial distance from C ring (0.2-0.6%) to A ring (0.06%) for the selected regions. Its distribution is compatible with an intrinsic origin and is possibly related to the different plasma environment of the different ring regions. The identification of the other absorber(s) and its absolute volumetric abundance is uncertain, depending on the adopted grain size and mixing modality (intraparticle or intimate). However, assuming a common composition of the other absorber in the ring plane, we find that its abundance anti-correlates with the optical depth of the investigated regions, being maximum in the thinnest C ring and minimum in the thickest mid B ring. In the case of the C ring, an additional population of low-albedo grains is required to match the positive spectral slope of the continuum in the 0.55-2.2 μm interval, represented by an intraparticle mixture of water ice and a spectrum similar to troilite or metallic iron. The distribution of the darkening compounds is interpreted as the result of a contamination by exogenous material, which is more effective in the less dense regions of the rings because of their lower surface mass density of pure water ice

The Surface Composition and Thermal Properties of the Organic-Rich Surface of Comet 67P/Churyumov-Gerasimenko : VIRTIS/Rosetta Results (Invited)

The paper will describe the major results obtained by the instrument VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer), the dual channel spectrometer onboard Rosetta, on the surface composition and thermal properties of the nucleus of comet 67P/Churyumov-Gerasimenko. VIRTIS is a dual channel spectrometer; VIRTIS-M (M for Mapper) is a hyper spectral imager covering a wide spectral range from 0.25 through 5μm. VIRTIS-M uses a slit and a scan mirror to generate images with spatial resolution of 250 μrad over a FOV of 3.7°. The second channel is VIRTIS-H (H for High-resolution), a point spectrometer with high spectral resolution (λ/∆λ=3000 @3μm) in the range 2-5 μm. The nucleus observations are performed in a wide range of conditions with spatial resolution varying from the initial 500m down to 2.5m. The surface temperature has been determined since the first distant observations when the nucleus filled one single VIRTIS-M pixel. On the mid of July 2014 from a distance of 15000km the mean surface temperature has been measured as 205±5K. This pointed to a surface structure largely covered by a porous crust, mainly devoid of water ice. Maximum temperature determined so far are as high as 230K on the subsolar point. The VIRTIS composition analysis has showed 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 μm), the spectral slopes in VIS and IR ranges (5-25 and 1.5-5 % kÅ-1) and the broad absorption feature in the 2.9-3.6 µm 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 C-H and/or O-H chemical groups, with little contribution of N-H. Authors acknowledge the support from national funding agencies

Compositional maps of 67P/CG nucleus after perihelion passage by VIRTIS-M aboard Rosetta

Comet 67P/Churyumov-Gerasimenko undergoes periherm transit at 1.2 AU in August 2015. During this passage the illumination conditions above the south hemisphere of the nucleus rapidly improve becoming optimal for the retrieval of the surface properties by VIRTIS-M [1] onboard Rosetta. A similar mapping of the surface at about 12.5 m/pixel and at solar phases below 40 deg has been already performed during the Philae prelanding phase (August-September 2014, heliocentric distance 3.6 AU) allowing us to build compositional maps of the entire north hemisphere and equatorial regions down to latitudes -50 deg on a limited part of the Hapi, or the comet's neck, region [2]. One year after this first mapping campaign the illumination geometry becomes favorable to complete the coverage above the South polar region. Since comet's activity is rapidly increasing and the Rosetta spacecraft cannot orbit on low trajectories like during the prelanding phase, the south hemisphere maps shall be reasonably observed by VIRTIS-M with a spatial resolution of about 25 m/pixel. Global scale data have shown that the nucleus' double-lobe surface is characterized by morphologically different units [3] uniformly covered by a very dark, low-albedo, dehydrated organic-rich material [4]. Compositional properties across the different regions of the nucleus are mapped by measuring visible and infrared spectral slopes, calculated on the best linear fit to the reflectance spectra between 0.5-0.8 μm and 1-2.5 μm, respectively. As pre-landing data have clearly shown, spectral slopes are highly diagnostic to identify active areas, like in the Hapi area [5], and exposed water ice deposits where the spectra appear less red. As heliocentric distance decreases and diurnal temperatures increase, the 3-5 μm spectral range becomes affected by thermal emission from the surface [6]. This emission is overlapping with the 3 μm feature previously observed by VIRTIS during the pre-landing period making more difficult to retrieve the distribution of the organic material. A summary of the spectral characteristics observed on the south hemisphere region during the perihelion passage is given. Activity-driven spectral changes observed before and after perihelion passage on some specific areas of the surface are discussed. <P /

Spectroscopic classification of icy satellites of Saturn II: Identification of Terrain Units on Rhea

Rhea is the second largest icy satellites of Saturn and it is mainly composed of water ice. Its surface is characterized by a leading hemisphere slightly brighter than the trailing side. The main goal of this work is to identify homogeneous compositional units on Rhea by applying the Spectral Angle Mapper (SAM) classification technique to Rhea’s hyperspectral images acquired by the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini Orbiter in the infrared range (0.88-5.12 μm). The first step of the classification is dedicated to the identification of Rhea’s spectral endmembers by applying the k-means unsupervised clustering technique to four hyperspectral images representative of a limited portion of the surface, imaged at relatively high spatial resolution. We then identified eight spectral endmembers, corresponding to as many terrain units, which mostly distinguish for water ice abundance and ice grain size. In the second step, endmembers are used as reference spectra in SAM classification method to achieve a comprehensive classification of the entire surface. From our analysis of the infrared spectra returned by VIMS, it clearly emerges that Rhea’ surface units shows differences in terms of water ice bands depths, average ice grain size, and concentration of contaminants, particularly CO2 and hydrocarbons. The spectral units that classify optically dark terrains are those showing suppressed water ice bands, a finer ice grain size and a higher concentration of carbon dioxide. Conversely, spectral units labeling brighter regions have deeper water ice absorption bands, higher albedo and a smaller concentration of contaminants. All these variations reflect surface’s morphological and geological structures. Finally, we performed a comparison between Rhea and Dione, to highlight different magnitudes of space weathering effects in the icy satellites as a function of the distance from Saturn

High Resolution North and South Polar Maps of the Moon with AMIE/SMART-1

International audienc

Colour Images Of The Moon From Amie On Smart-1: A Preliminary Analysys Of The Region Of Oppenheimer.

International audienc

Colour Images Of The Moon From Amie On Smart-1: A Preliminary Analysys Of The Region Of Oppenheimer.

International audienc

High Resolution North and South Polar Maps of the Moon with AMIE/SMART-1

International audienc