F. Mancarella

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.

Detection of exposed H2O ice on the nucleus of comet 67P/Churyumov-Gerasimenko

Since the orbital insertion of the Rosetta spacecraft, comet 67P/Churyumov-Gerasimenko (67P/C-G) has been mapped by OSIRIS camera and VIRTIS spectro-imager, producing a huge quantity of images and spectra of the comet’s nucleus. The aim of this work is to search for the presence of H 2 O on the nucleus which, in general, appears very dark and rich in dehydrated organic material. After selecting images of the bright spots which could be good candidates to search for H 2 O ice, taken at high resolution by OSIRIS, we check for spectral cubes of the selected coordinates to identify these spots observed by VIRTIS. Methods. The selected OSIRIS images were processed with the OSIRIS standard pipeline and corrected for the illumination condi- tions for each pixel using the Lommel-Seeliger disk law. The spots with higher I/F were selected and then analysed spectrophotomet- rically and compared with the surrounding area. We selected 13 spots as good targets to be analysed by VIRTIS to search for the 2 μm absorption band of water ice in the VIRTIS spectral cubes. Results. Out of the 13 selected bright spots, eight of them present positive H 2 O ice detection on the VIRTIS data. A spectral analysis was performed and the approximate temperature of each spot was computed. The H 2 O ice content was confirmed by modeling the spectra with mixing (areal and intimate) of H 2 O ice and dark terrain, using Hapke’s radiative transfer modeling. We also present a detailed analysis of the detected spots.

Detection of exposed H2O ice on the nucleus of comet 67P/Churyumov-Gerasimenko: As observed by Rosetta OSIRIS and VIRTIS instruments

Since the orbital insertion of the Rosetta spacecraft, comet 67P/Churyumov-Gerasimenko (67P/C-G) has been mapped by OSIRIS camera and VIRTIS spectro-imager, producing a huge quantity of images and spectra of the comet’s nucleus. The aim of this work is to search for the presence of H 2 O on the nucleus which, in general, appears very dark and rich in dehydrated organic material. After selecting images of the bright spots which could be good candidates to search for H 2 O ice, taken at high resolution by OSIRIS, we check for spectral cubes of the selected coordinates to identify these spots observed by VIRTIS. Methods. The selected OSIRIS images were processed with the OSIRIS standard pipeline and corrected for the illumination condi- tions for each pixel using the Lommel-Seeliger disk law. The spots with higher I/F were selected and then analysed spectrophotomet- rically and compared with the surrounding area. We selected 13 spots as good targets to be analysed by VIRTIS to search for the 2 μm absorption band of water ice in the VIRTIS spectral cubes. Results. Out of the 13 selected bright spots, eight of them present positive H 2 O ice detection on the VIRTIS data. A spectral analysis was performed and the approximate temperature of each spot was computed. The H 2 O ice content was confirmed by modeling the spectra with mixing (areal and intimate) of H 2 O ice and dark terrain, using Hapke’s radiative transfer modeling. We also present a detailed analysis of the detected spots.

Seasonal exposure of carbon dioxide ice on the nucleus of comet 67P/Churyumov-Gerasimenko

Rosetta observes sublimating surface ices Comets are “dirty snowballs” made of ice and dust, but they are dark because the ice sublimates away, leaving some of the dust behind on the surface. The Rosetta spacecraft has provided a close-up view of the comet 67P/Churyumov-Gerasimenko as it passes through its closest point to the Sun (see the Perspective by Dello Russo). Filacchione et al. detected the spectral signature of solid CO2 (dry ice) in small patches on the surface of the nucleus as they emerged from local winter. By modeling how the CO2 sublimates, they constrain the composition of comets and how ices generate the gaseous coma and tail. Fornasier et al. studied images of the comet and discovered bright patches on the surface where ice was exposed, which disappeared as the ice sublimated. They also saw frost emerging from receding shadows. The surface of the comet was noticeably less red just after local dawn, indicating that icy material is removed by sunlight during the local day. Science, this issue p. 1563, p. 1566; see also p. 1536 Solid carbon dioxide on the surface of a comet nucleus has been found by the Rosetta spacecraft. Carbon dioxide (CO2) is one of the most abundant species in cometary nuclei, but because of its high volatility, CO2 ice is generally only found beneath the surface. We report the infrared spectroscopic identification of a CO2 ice–rich surface area located in the Anhur region of comet 67P/Churyumov-Gerasimenko. Spectral modeling shows that about 0.1% of the 80- by 60-meter area is CO2 ice. This exposed ice was observed a short time after the comet exited local winter; following the increased illumination, the CO2 ice completely disappeared over about 3 weeks. We estimate the mass of the sublimated CO2 ice and the depth of the eroded surface layer. We interpret the presence of CO2 ice as the result of the extreme seasonal changes induced by the rotation and orbit of the comet.

Search for Martian methane with TES data: development of a dedicated radiative transfer code: first results

In the present work we describe a dedicated and fast monochromatic radiative transfer code, developed for computing Martian radiance spectra as seen by the Thermal Emission Spectrometer (TES), and its Jacobians with respect to gas, dust aerosol and ice concentrations, atmospheric temperatures, and surface emissivity. The code accuracy has been tested comparing its results with a state-of-art line-by-line radiative transfer model, and it has been optimized for simulating nadir-viewing spectra. The model is well-suited to simulate spectra with di erent amounts of methane in the atmosphere, whose detection is currently one of the most fascinating issues concerning the Martian atmospheric chemistry and planetary dynamics.

SEM morphological studies of carbonates and the search for ancient life on Mars

Next space missions will investigate the possibility of extinct or extant life on Mars. Studying the infrared spectral modifications, induced by thermal processing on different carbonate samples (recent shells and fossils of different ages), we developed a method able to discriminate biogenic carbonates from their abiogenic counterparts. The method has been successfully applied to microbialites, i.e. bio-induced carbonates deposits, and particularly to stromatolites, the laminated fabric of microbialites, some of which can be ascribed to among the oldest traces of biological activity known on Earth. These results are of valuable importance since such carbonates are linked to primitive living organisms that can be considered as good analogues for putative Martian life forms. Considering that the microstructures of biogenic carbonate are different from those of abiogenic origin, we investigated the micromorphology of shells, skeletal grains and microbialites at different scale with a scanning electron microscope. The results show that this line of research may provide an alternative and complementary approach to other techniques developed in the past by our group to distinguish biotic from abiotic carbonates. In this paper, we present some results that can be of valuable interest since they demonstrate the utility for a database of images concerning the structures and textures of relevant carbonate minerals. Such data may be useful for the analysis of Martian samples, coming from sample return missions or investigated by future in situ explorations, aimed to characterize the near-subsurface of Mars in search for past or present life.

Simultaneous physical retrieval of Martian geophysical parameters using Thermal Emission Spectrometer spectra: the φ-MARS algorithm

In this paper, we present a new methodology for the simultaneous retrieval of surface and atmospheric parameters of Mars. The methodology is essentially based on similar codes implemented for high-resolution instruments looking at Earth, supported by a statistical retrieval procedure used to initialize the physical retrieval algorithm with a reliable first guess of the atmospheric parameters. The methodology has been customized for the Thermal Emission Spectrometer (TES), which is a low-resolution interferometer. However, with minor changes to the forward and inverse modules, it is applicable to any instrument looking at Mars, and with particular effectiveness to high-resolution instruments. The forward module is a monochromatic radiative transfer model with the capability to calculate analytical Jacobians of any desired geophysical parameter. In the present work, we describe the general methodology and its application to a large sample of TES spectra. Results are drawn for the case of surface temperature and emissivity, atmospheric temperature profile, water vapor, and dust and ice mixing ratios. Comparison with climate models and other TES data analyses show very good agreement and consistency.