Identification of Ammonium Salts on Comet 67P/C-G Surface from Infrared VIRTIS/Rosetta Data Based on Laboratory Experiments. Implications and Perspectives

The nucleus of comet 67P/Churyumov-Gerasimenko exhibits a broad spectral reflectance feature around 3.2 $\mu$m, which is omnipresent in all spectra of the surface, and whose attribution has remained elusive since its discovery. Based on laboratory experiments, we have shown that most of this absorption feature is due to ammonium (NH4+) salts mixed with the dark surface material. The depth of the band is compatible with semi-volatile ammonium salts being a major reservoir of nitrogen in the comet, which could dominate over refractory organic matter and volatile species. These salts may thus represent the long-sought reservoir of nitrogen in comets, possibly bringing their nitrogen-to-carbon ratio in agreement with the solar value. Moreover, the reflectance spectra of several asteroids are compatible with the presence of NH4+ salts at their surfaces. The presence of such salts, and other NH4+-bearing compounds on asteroids, comets, and possibly in proto-stellar environments, suggests that NH4+ may be a tracer of the incorporation and transformation of nitrogen in ices, minerals and organics, at different phases of the formation of the Solar System

Ion-irradiation of complex hydrocarbons: implications for small Solar System bodies

International audienc

Ion-irradiation of complex hydrocarbons: implications for small Solar System bodies

International audienc

Ion-irradiation of complex hydrocarbons: implications for small Solar System bodies

International audienc

Comet 67P/CG Nucleus Composition and Comparison to Other Comets

International audienceWe review our current knowledge of comet 67P/Churyumov-Gerasimenko nucleus composition as inferred from measurements made by remote sensing and in-situ instruments aboard Rosetta orbiter and Philae lander. Spectropho-tometric properties (albedos, color indexes and Hapke parameters) of 67P/CG derived by Rosetta are discussed in the context of other comets previously explored by space missions. Composed of an assemblage made of ices, organic materials and minerals, cometary nuclei exhibit very dark and red surfaces which can be described by means of spectrophotometric quantities and reproduced with laboratory measurements. The presence of surface water and carbon dioxide ices was found by Rosetta to occur at localized sites where the the activity driven by solar input, gaseous condensation or exposure of G. Filacchione 2 G. Filacchione et al. pristine inner layers can maintain these species on the surface. Apart from these specific areas, 67P/CG's surface appears remarkably uniform in composition with a predominance of organic materials and minerals. The organic compounds contain abundant hydroxyl group and a refractory macromolecular material bearing aliphatic and aromatic hydrocarbons. The mineral components are compatible with a mixture of silicates and fine-grained opaques, including Fe-sulfides, like troilite and pyrrhotite, and ammoniated salts. In the vicinity of the perihelion several active phenomena, including the erosion of surface layers, the localized activity in cliffs, fractures and pits, the collapse of overhangs and walls, the transfer and redeposition of dust, cause the evolution of the different regions of the nucleus by inducing color, composition and texture changes

Correlation of Comet 67P/CG'S Morphology with the Occurrence of Exposed Water Ice Patches

International audienceIntroduction: Comet 67P's surface is quite homogeneously covered by dark refractory materials rich in organics [1,2]. Rare water ice expo- sures on the surface, most likely originating from sub-surface layers, have recently been discovered [3,4]. Such H2O ice patches on 67P's Imhotep region in the pre-perihelion phase were ex- amined and related to the local morphology to understand the exposure mechanisms [5]. Methods: H2O ice was identified in two study areas using characteristic H2O spectral features observed by the VIRTIS-M instrument [1]: absorption bands at 1.04, 1.25, 1.52, 2.02, 2.96 mum, and the VIS spectral slope (0.5-0.8 mum). Corresponding normalized spectral indicators were projected onto a 3D digital shape model (DSM) of 67P [6], along with high spatial resolution images acquired by OSIRIS [7] for morphological context. Results and conclusions: The 2.0-mum absorption band proved to be the most sensitive H2O indicator in the IR. Flat (bluer) normalized VIS slopes correlate very well with depths of H2O ice absorption bands. The DSM projections show a significant spatial correlation between spectral H2O indicators and morphological features. H2O ice deposits were identified in two areas, each extending over hundreds of square meters. Both are located at the bases of steep-sloped (>60°) walls of Consolidated Cometary Material (CCM) on debris falls that came to rest on moderately inclined (20°-30°) terrain, pointing towards gravitational lows. Both deposits are located in poorly illuminated areas due to shadowing from close-by steep walls. The morphological and photometrical properties of these deposits appear to be stable over months. Spectral modeling [3,4] indicated the presence of large (mm-sized) H2O ice grains. Such grains form through vapor diffusion in ice-rich colder layers or by sintering and are exposed by erosion [3]. The CCM in both study areas was fractured and weakened by thermal fatigue and sublimation, leading to the collapse of overhangs in one single event or in small steps over a longer time. For study area 1 this interpretation is supported by a small remnant H2O ice patch in the upper part of the steep wall indicating the original location of the collapse overhang. Sinkhole-structures on the CCM in study area 2 indicate sub-surface activity connected to ice sublimation, thermal stress, and occasional outbursts. Even though the nucleus structure probably is mostly homogeneous and primordial, the variable size and irregular distribution of sink-holes and erosional features strongly imply a highly active and heterogeneous sub-surface layer of at least tens of meters [8,9]. References: [1] Capaccioni F. et al. (2015) Sci- ence, 347, 628. [2] Quirico E. et al. (2016) Ica- rus, 272, 32-47. [3] Filacchione G. et al. (2016) Nature, 529, 368-372. [4] Barucci A. et al. (2016) A&A, 595, A102. [5] Weller D. (2016) Master thesis, University of Potsdam. [6] Preusker F. et al. (2015) A&A, 583, A33. [7] Keller H. et al. (2007) Space Sci. Rev., 128(1), 433-506. [8] Thomas N. et al. (2015) Science, 347, 440. [9] Vincent J.B. et al. (2015) Nature, 523, 63-66

Correlation of Comet 67P/CG'S Morphology with the Occurrence of Exposed Water Ice Patches

International audienceIntroduction: Comet 67P's surface is quite homogeneously covered by dark refractory materials rich in organics [1,2]. Rare water ice expo- sures on the surface, most likely originating from sub-surface layers, have recently been discovered [3,4]. Such H2O ice patches on 67P's Imhotep region in the pre-perihelion phase were ex- amined and related to the local morphology to understand the exposure mechanisms [5]. Methods: H2O ice was identified in two study areas using characteristic H2O spectral features observed by the VIRTIS-M instrument [1]: absorption bands at 1.04, 1.25, 1.52, 2.02, 2.96 mum, and the VIS spectral slope (0.5-0.8 mum). Corresponding normalized spectral indicators were projected onto a 3D digital shape model (DSM) of 67P [6], along with high spatial resolution images acquired by OSIRIS [7] for morphological context. Results and conclusions: The 2.0-mum absorption band proved to be the most sensitive H2O indicator in the IR. Flat (bluer) normalized VIS slopes correlate very well with depths of H2O ice absorption bands. The DSM projections show a significant spatial correlation between spectral H2O indicators and morphological features. H2O ice deposits were identified in two areas, each extending over hundreds of square meters. Both are located at the bases of steep-sloped (>60°) walls of Consolidated Cometary Material (CCM) on debris falls that came to rest on moderately inclined (20°-30°) terrain, pointing towards gravitational lows. Both deposits are located in poorly illuminated areas due to shadowing from close-by steep walls. The morphological and photometrical properties of these deposits appear to be stable over months. Spectral modeling [3,4] indicated the presence of large (mm-sized) H2O ice grains. Such grains form through vapor diffusion in ice-rich colder layers or by sintering and are exposed by erosion [3]. The CCM in both study areas was fractured and weakened by thermal fatigue and sublimation, leading to the collapse of overhangs in one single event or in small steps over a longer time. For study area 1 this interpretation is supported by a small remnant H2O ice patch in the upper part of the steep wall indicating the original location of the collapse overhang. Sinkhole-structures on the CCM in study area 2 indicate sub-surface activity connected to ice sublimation, thermal stress, and occasional outbursts. Even though the nucleus structure probably is mostly homogeneous and primordial, the variable size and irregular distribution of sink-holes and erosional features strongly imply a highly active and heterogeneous sub-surface layer of at least tens of meters [8,9]. References: [1] Capaccioni F. et al. (2015) Sci- ence, 347, 628. [2] Quirico E. et al. (2016) Ica- rus, 272, 32-47. [3] Filacchione G. et al. (2016) Nature, 529, 368-372. [4] Barucci A. et al. (2016) A&A, 595, A102. [5] Weller D. (2016) Master thesis, University of Potsdam. [6] Preusker F. et al. (2015) A&A, 583, A33. [7] Keller H. et al. (2007) Space Sci. Rev., 128(1), 433-506. [8] Thomas N. et al. (2015) Science, 347, 440. [9] Vincent J.B. et al. (2015) Nature, 523, 63-66

Interpretation of VIRTIS/Rosetta surface spectra of comet 67P from laboratory reflectance measurements of cometary analogues, including iron sulfides

International audienceThis work provides clues about the interpretation of the surface spectrum of 67P as acquired by VIRTIS/Rosetta imaging-spectrometer. Fine powders (<0.4µm) of coal, iron sulphide and silicates have been produced and measured in reflectance. Grain sizes effects, mixing modalities and various compositions have been tested in order to reproduce the VIRTIS spectrum

Interpretation of VIRTIS/Rosetta surface spectra of comet 67P from laboratory reflectance measurements of cometary analogues, including iron sulfides

International audienceThis work provides clues about the interpretation of the surface spectrum of 67P as acquired by VIRTIS/Rosetta imaging-spectrometer. Fine powders (<0.4µm) of coal, iron sulphide and silicates have been produced and measured in reflectance. Grain sizes effects, mixing modalities and various compositions have been tested in order to reproduce the VIRTIS spectrum

VIS-IR Spectroscopy of Mixtures of Water Ice, Organic Matter, and Opaque Mineral in Support of Small Body Remote Sensing Observations

International audienceVisual-to-infrared (VIS-IR) remote sensing observations of different classes of outer solar system objects indicate the presence of water ice and organics. Here, we present laboratory reflectance spectra in the 0.5–4.2 μm spectral range of binary particulate mixtures of water ice, organics analogue (kerite), and an opaque iron sulphide phase (pyrrhotite) to investigate the spectral effects of varying mixing ratios, endmember grain size, and mixing modality. The laboratory spectra are also compared to different implementations of the Hapke reflectance model (Hapke, 2012). We find that minor amounts (≲1 wt%) of kerite (investigated grain sizes of 45–63 μm and <25 μm) can remain undetected when mixed in coarse-grained (67 ± 31 μm) water ice, suggesting that organics similar to meteoritic insoluble organic matter (IOM) might be characterized by larger detectability thresholds. Additionally, our measurements indicate that the VIS absolute reflectance of water ice-containing mixtures is not necessarily monotonically linked to water ice abundance. The latter is better constrained by spectral indicators such as the band depths of water ice VIS-IR diagnostic absorptions and spectral slopes. Simulation of laboratory spectra of intimate mixtures with a semi-empirical formulation of the Hapke model suggests that simplistic assumptions on the endmember grain size distribution and shape may lead to estimated mixing ratios considerably offset from the nominal values. Finally, laboratory spectra of water ice grains with fine-grained pyrrhotite inclusions (intraparticle mixture) have been positively compared with a modified version of the Hapke model from Lucey and Riner (2011)