Comet 67P/CG: surface temperature maps from Rosetta/VIRTIS during the pre-landing phase

It was seldom possible, with observations carried out from spaceborne facilities, to derive spatially-resolved thermal maps of small bodies, and even more rarely this result was achieved in the case of close observations of comets. The Visible InfraRed Thermal Imaging Spectrometer (VIRTIS) onboard the Rosetta Orbiter Coradini (2007) is able to obtain hyperspectral images of the observed targets in 864 wavelengths simultaneously, in the overall spectral range 0.25-5.1 μm, with the major goal of inferring and mapping the surface composition and temperature of comet 67P/Churyumov-Gerasimenko. VIRTIS spectra acquired on the dayside of the comet’s nucleus show the thermal emission of the surface at wavelengths ¿ 3.5 μm, which can be ex- ploited to derive and map the surface temperature at different spatial scales and under changing lighting conditions. To do this, we rely on a Bayesian approach that was previously adopted to derive surface temperature maps of the two asteroids 2678 Steins and 21 Lutetia, encountered by Rosetta during its long cruise phase towards the comet Coradini (2011); Keihm (2012), and of the large asteroid Vesta from the entire infrared dataset acquired by the VIR instrument onboard the Dawn spacecraft Tosi (2014). In this paper we summarize the main results concerning the thermal mapping of comet 67P, obtained by VIRTIS in the first months of observation at a reso- lution between 1000 and 1 m, and at a heliocentric distance between 3.6 and 3.4 AU. Comet 67P was shown to be everywhere rich in organic materials with little to no water ice visible on the surface Capaccioni (2015). In the range of heliocentric distances from 3.59 to 2.74 AU, daytime surface temperatures were overall comprised in the range between 180 and 220 K Tosi (2015), which is incompatible with large exposures of water ice and is consistent with a low-albedo, organics-rich surface. Maximum temperature values as high as 230 K were recorded in very few places Tosi (2015). In the above period, the highest values of surface temperature were obtained with observations carried out at small phase angles, implying that the observed surface has a large predominance of small incidence angles, and local solar times centered around the max- imum daily insolation. In all cases, direct correlation with topographic features was observed, i.e. largest temperature values were generally associated with the smallest values of illumination angles, while no evidence was found of thermal anomalies, i.e. places of the surface that are intrinsically warmer or cooler than surrounding terrains observed at the same local solar time and under similar solar illumination

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

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

Mapping of thermal properties of comet 67P/C-G and temporal variations

The long-term evolution of the surfaces of comets depends mainly on the erosion rate that is driven by the thermal properties of the regolith and the sub-surface material. Following the diurnal and the seasonal thermal cycles, dust and gas are released progressively, increasing the erosion process. The amount of dust released depends on the surface and subsurface temperatures and thus on thermal inertia and bulk composition.The ESA's Rosetta spacecraft has followed the comet 67P/Churyumov-Gerasimenko over several months from 4 AU to 1.28 AU heliocentric distance, and the VIRTIS/Rosetta imaging infrared spectrometer was capable of detecting the thermal emission of the surface longward of 3 microns.The surface temperature was mapped over a large fraction of the nucleus and was previously used to derive thermal inertia of the main geomorphological units.In this presentation, we now focus on two different aspects: (1) We aim to present a complete detailed map of the thermal inertia by combining measurements of similar areas obtained at different viewing angles ; and (2) we track the evolution of the local thermal properties derived over months when the comet was moving towards perihelion. We then discuss and compare our results with the textural features observed at the surface

Sulfides and refractory organic matter at the surface of 67P/Churyumov-Gerasimenko: evidence from VIRTIS data and laboratory measurements

International audienceFrom Aug. 2014 to Sept. 2016, Rosetta has been orbiting comet 67P and has obtained informations on the origin and evolution of comets. The imaging spectrometer VIRTIS collected reflectance spectra of the surface within the range 0.25-5.1 µm that revealed a low albedo and a homogeneous surface (Capaccioni et al., 2015; Ciarniello et al., 2015). The spectra are also characterized by red slopes in the visible and in the near infrared. These properties have been interpreted to be due to the presence of an organic polyaromatic material mixed with opaque minerals, presumably troilite-like sulfides according to the composition of presumed cometary grains (Quirico et al., 2016).In order to test this proposition, we have run a series of experimental measurements of granular mixtures of an analog of cometary polyaromatic organic matter (an immature coal) and different sulfides (pyrite, pyrrhotite and troilite). Bi-directional reflectance spectra were obtained at IPAG in the range 0.4-4 µm and under a range of viewing geometries. For the first time we are performing measurements on materials with sub-micrometer grains relevant to what is expected for cometary grains. Produced with a planetary grinder operating on colloidal solutions, these grains were characterized with SEM, X-ray diffraction and an electronic microprobe.The experiment confirms that the low albedo in the near infrared is controlled by the abundance of pyrrhotite or troilite, while pyrite is not a viable candidate. These sulfides also account very well for the red slopes in the visible and the near infrared ranges. Excellent match with VIRTIS spectra is obtained for coal pyrrhotite mixtures with a pyrrhotite abundance ranging from 30 to 50 wt%. Although the cometary grains composition include silicates and other organic compounds (see IDPs and Wild2 samples analysis), these results offer another interpretation of the reddish nature of some small bodies' surfaces, which has been interpreted so far as the presence of red organics or the consequence of space weathering. Finally, the suggestion that sulfides are major contributors to 67P dust might be generalized to other cometary nuclei that have similar VNIR spectra, as well as to P & D type asteroids

Comet 67P/CG as Seen by VIRTIS-M Onboard Rosetta: Photometric Correction

International audienc

Sulfides and refractory organic matter at the surface of 67P/Churyumov-Gerasimenko: evidence from VIRTIS data and laboratory measurements

International audienceFrom Aug. 2014 to Sept. 2016, Rosetta has been orbiting comet 67P and has obtained informations on the origin and evolution of comets. The imaging spectrometer VIRTIS collected reflectance spectra of the surface within the range 0.25-5.1 µm that revealed a low albedo and a homogeneous surface (Capaccioni et al., 2015; Ciarniello et al., 2015). The spectra are also characterized by red slopes in the visible and in the near infrared. These properties have been interpreted to be due to the presence of an organic polyaromatic material mixed with opaque minerals, presumably troilite-like sulfides according to the composition of presumed cometary grains (Quirico et al., 2016).In order to test this proposition, we have run a series of experimental measurements of granular mixtures of an analog of cometary polyaromatic organic matter (an immature coal) and different sulfides (pyrite, pyrrhotite and troilite). Bi-directional reflectance spectra were obtained at IPAG in the range 0.4-4 µm and under a range of viewing geometries. For the first time we are performing measurements on materials with sub-micrometer grains relevant to what is expected for cometary grains. Produced with a planetary grinder operating on colloidal solutions, these grains were characterized with SEM, X-ray diffraction and an electronic microprobe.The experiment confirms that the low albedo in the near infrared is controlled by the abundance of pyrrhotite or troilite, while pyrite is not a viable candidate. These sulfides also account very well for the red slopes in the visible and the near infrared ranges. Excellent match with VIRTIS spectra is obtained for coal pyrrhotite mixtures with a pyrrhotite abundance ranging from 30 to 50 wt%. Although the cometary grains composition include silicates and other organic compounds (see IDPs and Wild2 samples analysis), these results offer another interpretation of the reddish nature of some small bodies' surfaces, which has been interpreted so far as the presence of red organics or the consequence of space weathering. Finally, the suggestion that sulfides are major contributors to 67P dust might be generalized to other cometary nuclei that have similar VNIR spectra, as well as to P & D type asteroids

Sulfides and refractory organic matter at the surface of 67P/Churyumov-Gerasimenko: evidence from VIRTIS data and laboratory measurements

International audienceFrom Aug. 2014 to Sept. 2016, Rosetta has been orbiting comet 67P and has obtained informations on the origin and evolution of comets. The imaging spectrometer VIRTIS collected reflectance spectra of the surface within the range 0.25-5.1 µm that revealed a low albedo and a homogeneous surface (Capaccioni et al., 2015; Ciarniello et al., 2015). The spectra are also characterized by red slopes in the visible and in the near infrared. These properties have been interpreted to be due to the presence of an organic polyaromatic material mixed with opaque minerals, presumably troilite-like sulfides according to the composition of presumed cometary grains (Quirico et al., 2016).In order to test this proposition, we have run a series of experimental measurements of granular mixtures of an analog of cometary polyaromatic organic matter (an immature coal) and different sulfides (pyrite, pyrrhotite and troilite). Bi-directional reflectance spectra were obtained at IPAG in the range 0.4-4 µm and under a range of viewing geometries. For the first time we are performing measurements on materials with sub-micrometer grains relevant to what is expected for cometary grains. Produced with a planetary grinder operating on colloidal solutions, these grains were characterized with SEM, X-ray diffraction and an electronic microprobe.The experiment confirms that the low albedo in the near infrared is controlled by the abundance of pyrrhotite or troilite, while pyrite is not a viable candidate. These sulfides also account very well for the red slopes in the visible and the near infrared ranges. Excellent match with VIRTIS spectra is obtained for coal pyrrhotite mixtures with a pyrrhotite abundance ranging from 30 to 50 wt%. Although the cometary grains composition include silicates and other organic compounds (see IDPs and Wild2 samples analysis), these results offer another interpretation of the reddish nature of some small bodies' surfaces, which has been interpreted so far as the presence of red organics or the consequence of space weathering. Finally, the suggestion that sulfides are major contributors to 67P dust might be generalized to other cometary nuclei that have similar VNIR spectra, as well as to P & D type asteroids

Sulfides and refractory organic matter at the surface of 67P/Churyumov-Gerasimenko: evidence from VIRTIS data and laboratory measurements

International audienceFrom Aug. 2014 to Sept. 2016, Rosetta has been orbiting comet 67P and has obtained informations on the origin and evolution of comets. The imaging spectrometer VIRTIS collected reflectance spectra of the surface within the range 0.25-5.1 µm that revealed a low albedo and a homogeneous surface (Capaccioni et al., 2015; Ciarniello et al., 2015). The spectra are also characterized by red slopes in the visible and in the near infrared. These properties have been interpreted to be due to the presence of an organic polyaromatic material mixed with opaque minerals, presumably troilite-like sulfides according to the composition of presumed cometary grains (Quirico et al., 2016).In order to test this proposition, we have run a series of experimental measurements of granular mixtures of an analog of cometary polyaromatic organic matter (an immature coal) and different sulfides (pyrite, pyrrhotite and troilite). Bi-directional reflectance spectra were obtained at IPAG in the range 0.4-4 µm and under a range of viewing geometries. For the first time we are performing measurements on materials with sub-micrometer grains relevant to what is expected for cometary grains. Produced with a planetary grinder operating on colloidal solutions, these grains were characterized with SEM, X-ray diffraction and an electronic microprobe.The experiment confirms that the low albedo in the near infrared is controlled by the abundance of pyrrhotite or troilite, while pyrite is not a viable candidate. These sulfides also account very well for the red slopes in the visible and the near infrared ranges. Excellent match with VIRTIS spectra is obtained for coal pyrrhotite mixtures with a pyrrhotite abundance ranging from 30 to 50 wt%. Although the cometary grains composition include silicates and other organic compounds (see IDPs and Wild2 samples analysis), these results offer another interpretation of the reddish nature of some small bodies' surfaces, which has been interpreted so far as the presence of red organics or the consequence of space weathering. Finally, the suggestion that sulfides are major contributors to 67P dust might be generalized to other cometary nuclei that have similar VNIR spectra, as well as to P & D type asteroids

Sulfides and refractory organic matter at the surface of 67P/Churyumov-Gerasimenko: evidence from VIRTIS data and laboratory measurements

International audienceFrom Aug. 2014 to Sept. 2016, Rosetta has been orbiting comet 67P and has obtained informations on the origin and evolution of comets. The imaging spectrometer VIRTIS collected reflectance spectra of the surface within the range 0.25-5.1 µm that revealed a low albedo and a homogeneous surface (Capaccioni et al., 2015; Ciarniello et al., 2015). The spectra are also characterized by red slopes in the visible and in the near infrared. These properties have been interpreted to be due to the presence of an organic polyaromatic material mixed with opaque minerals, presumably troilite-like sulfides according to the composition of presumed cometary grains (Quirico et al., 2016).In order to test this proposition, we have run a series of experimental measurements of granular mixtures of an analog of cometary polyaromatic organic matter (an immature coal) and different sulfides (pyrite, pyrrhotite and troilite). Bi-directional reflectance spectra were obtained at IPAG in the range 0.4-4 µm and under a range of viewing geometries. For the first time we are performing measurements on materials with sub-micrometer grains relevant to what is expected for cometary grains. Produced with a planetary grinder operating on colloidal solutions, these grains were characterized with SEM, X-ray diffraction and an electronic microprobe.The experiment confirms that the low albedo in the near infrared is controlled by the abundance of pyrrhotite or troilite, while pyrite is not a viable candidate. These sulfides also account very well for the red slopes in the visible and the near infrared ranges. Excellent match with VIRTIS spectra is obtained for coal pyrrhotite mixtures with a pyrrhotite abundance ranging from 30 to 50 wt%. Although the cometary grains composition include silicates and other organic compounds (see IDPs and Wild2 samples analysis), these results offer another interpretation of the reddish nature of some small bodies' surfaces, which has been interpreted so far as the presence of red organics or the consequence of space weathering. Finally, the suggestion that sulfides are major contributors to 67P dust might be generalized to other cometary nuclei that have similar VNIR spectra, as well as to P & D type asteroids