David Kappel

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.

Venus surface data extraction from VIRTIS/Venus Express measurements: Estimation of a quantitative approach

Nightside emission measurements of the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on the Venus Express (VEX) spacecraft were used to estimate the potential for surface data extraction. A selection of orbits over the northern hemisphere was performed for footprints that cover different scales of surface elevation variations. A correction method was used to remove straylight from the measured spectra that is due to direct sunlight striking the instrument. A preliminary radiative transfer calculation technique was applied to simulate Venus nightside radiation. The basic features of the measured spectra are well reproduced. Present limitations of the algorithm are discussed. The variability of the emission window radiances with respect to cloud opacity and surface elevation is modelled and discussed in direct comparison with the measurements. It is demonstrated that a multi-spectral analysis in the surface and deep atmosphere window ranges (1.0-2.3 µm) and the use of radiance ratios are well suited to de-cloud the data and to extract surface information from the VIRTIS measurements. This method allows a mapping of surface topography and the retrieval of the surface temperature. A preliminary topography retrieval at Beta Regio was performed and compared with Magellan radar data. Differences are possibly due to emissivity variations on the surface.

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.

VIRTIS/VEX observations of Venus: overview of selected scientific results

Abstract. After six years in a polar Venus orbit, the visible and infrared thermal imaging spectrometer (VIRTIS) on ESA’s Venus Express mission provided an enormous amount of new data, including a three-dimensional view of the atmosphere and information on global surface properties of the planet. An overview is given of selected scientific results achieved by use of VIRTIS data comprising atmospheric thermal structure, molecular and particulate composition, chemistry and dynamics, and surface features.

The temporal evolution of exposed water ice-rich areas on the surface of 67P/Churyumov-Gerasimenko: spectral analysis

Water ice-rich patches have been detected on the surface of comet 67P/Churyumov-Gerasimenko by the VIRTIS hyperspectral imager on-board the Rosetta spacecraft, since the orbital insertion in late August 2014. Among those, three icy patches have been selected, and VIRTIS data have been used to analyse their properties and temporal evolution while the comet was moving towards the Sun. We performed an extensive analysis of the spectral parameters, and we applied the Hapke radiative transfer model to retrieve the abundance and grain size of water ice, as well as the mixing modalities of water ice and the ubiquitous dark refractory terrains of the surface. Study of the spatial distribution of the spectral parameters within the ice-rich patches has revealed that water ice follows different patterns associated to a bimodal distribution of the grains: ~50 μm sized and ~2000 μm sized, respectively in intimate and areal mixture with the dark material. In all three cases we identified different stages of the evolution of abundance of ice in the selected patches after the first detections at about 3.5 AU heliocentric distance; the spatial extension and intensity of the water ice spectral features reached a maximum after 60-100 days at about 3.0 AU, was followed by an approximately equally timed decrease, and the features were no longer visible when observed again at about ~2.2 AU, before perihelion. The exposure of deeper layers is consistent with their occurrence in “active” areas where falls or landslides could have caused the occasional exposure of water ice-rich layers. After the initial exposure of the ice, the activity of the affected area increases thus causing dust removal powered by sublimation, which provides a positive feedback on the exposure itself. The process develops as the solar flux increases, and it reaches a turning point when the exposure rate is outweighed by the sublimation rate, until the complete sublimation of the patch. It is interesting to note that the behaviour of the analysed patches was very similar with a lifecycle of about 180 days, which can be ascribed to a seasonal cycle. In addition to the observed seasonal cycle we found evidence of short-term variability associated to a diurnal water cycle. Our analysis, in addition to previous work (Filacchione et al., 2016; Barucci et al., 2016; Ciarniello et al., 2016), indicates that water ice is rather evenly distributed in the subsurface and that no large water ice reservoirs are present.

VIRTIS on Rosetta: a unique technique to observe comet 67P/Churyumov-Gerasimenko – first results and prospects

VIRTIS aboard ESA’s Rosetta mission is a complex imaging spectrometer that combines three unique data channels in one compact instrument to study nucleus and coma of comet 67P/Churyumov-Gerasimenko. Two of the spectral channels are dedicated to spectral mapping (-M) at moderate spectral resolution in the range from 0.25 to 5.1 μm. The third channel is devoted to high resolution spectroscopy (-H) between 2 and 5 μm. The VIRTIS-H field of view is approximately centered in the middle of the -M image. The spectral sampling of VIRTIS-M is 1.8 nm/band below 1 μm and 9.7 nm/band between 1-5 μm, while for VIRTIS-H λ/Δλ= 1300-3000 in the 2-5 μm range. This paper describes selected findings during the pre-landing phase of Philae’s robotic subsystem and the comet’s escort phase as well as prospects of further observations. The preliminary results include studies of surface composition, coma analyses, and temperature retrieval for the nucleus surface-coma system demonstrating the capability of the instrument.

The Venus Emissivity Mapper (VEM) Concept

Based on experience gained from using the VIRTIS instrument on Venus Express to observe the surface of Venus and the new high temperature laboratory experiments, we have developed the multispectral Venus Emissivity Mapper (VEM) to study the surface of Venus. VEM imposes minimal requirements on the spacecraft and mission design and can therefore be added to any future Venus mission. Ideally, the VEM instrument will be combined with a high-resolution radar mapper to provide accurate topographic information, as it will be the case for the NASA Discovery VERITAS mission or the ESA EnVision M5 proposal.

The Venus Emissivity Mapper concept

The Venus Emissivity Mapper (VEM) is the first flight instrument specially designed with a sole focus on mapping the surface of Venus using the narrow atmospheric windows around 1μm. VEM will provide a global map of surface composition as well as redox state of the surface, providing a comprehensive picture of surface-atmosphere interaction on Venus. In addition, continuous observation of the thermal emission of the Venus will provide tight constraints on current day volcanic activity. These capabilities are complemented by measurements of atmospheric water vapor abundance as well as cloud microphysics and dynamic. Atmospheric data will allow for the accurate correction of atmospheric interference on the surface measurements and represent highly valuable science on their own. A mission combining VEM with a high-resolution radar mapper such as the NASA VOX or the ESA EnVision mission proposals in a low circular orbit will provide key insights in the divergent evolution of Venus.