G. Piccioni

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.

Three-dimensional direct simulation Monte-Carlo modeling of the coma of comet 67P/Churyumov-Gerasimenko observed by the VIRTIS and ROSINA instruments on board Rosetta

Since its rendezvous with comet 67P/Churyumov-Gerasimenko (67P), the Rosetta spacecraft has provided invaluable information contributing to our understanding of the cometary environment. On board, the VIRTIS and ROSINA instruments can both measure gas parameters in the rarefied cometary atmosphere, the so-called coma, and provide complementary results with remote sensing and in situ measurement techniques, respectively. The data from both ROSINA and VIRTIS instruments suggest that the source regions of H2O and CO2 are not uniformly distributed over the surface of the nucleus even after accounting for the changing solar illumination of the irregularly shaped rotating nucleus. The source regions of H2O and CO2 are also relatively different from one another. Aims. The use of a combination of a formal numerical data inversion method with a fully kinetic coma model is a way to correlate and interpret the information provided by these two instruments to fully understand the volatile environment and activity of comet 67P. Methods. In this work, the nonuniformity of the outgassing activity at the surface of the nucleus is described by spherical harmonics and constrained by ROSINA-DFMS data. This activity distribution is coupled with the local illumination to describe the inner boundary conditions of a 3D direct simulation Monte-Carlo (DSMC) approach using the Adaptive Mesh Particle Simulator (AMPS) code applied to the H2O and CO2 coma of comet 67P. Results. We obtain activity distribution of H2O and CO2 showing a dominant source of H2O in the Hapi region, while more CO2 is produced in the southern hemisphere. The resulting model outputs are analyzed and compared with VIRTIS-M/-H and ROSINA-DFMS measurements, showing much better agreement between model and data than a simpler model assuming a uniform surface activity. The evolution of the H2O and CO2 production rates with heliocentric distance are derived accurately from the coma model showing agreement between the observations from the different instruments and ground-based observations. Conclusions. We derive the activity distributions for H2O and CO2 at the surface of the nucleus described in spherical harmonics, which we couple to the local solar illumination to constitute the boundary conditions of our coma model. The model presented reproduces the coma observations made by the ROSINA and VIRTIS instruments on board the Rosetta spacecraft showing our understanding of the physics of 67P’s coma. This model can be used for further data analyses, such as dust modeling, in a future work.

First observations of H2O and CO2 vapor in comet 67P/Churyumov-Gerasimenko made by VIRTIS onboard Rosetta

Context. Outgassing from cometary nuclei involves complex surface and subsurface processes that need to be understood to investigate the composition of cometary ices from coma observations. Aims. We investigate the production of water, carbon dioxide, and carbon monoxide from the nucleus of comet 67P/Churyumov-Gerasimenko (67P). These species have different volatility and are key species of cometary ices. Methods. Using the high spectral-resolution channel of the Visible InfraRed Thermal Imaging Spectrometer (VIRTIS-H), we observed the ν3 vibrational bands of H2O and CO2 at 2.67 and 4.27 μm, respectively, from 24 November 2014 to 24 January 2015, when comet 67P was between 2.91 and 2.47 AU from the Sun. Observations were undertaken in limb-viewing geometry at distances from the surface of 0 to 1.5 km and with various line-of-sight (LOS) orientations in the body-fixed frame. A geometry tool was used to characterize the position of the LOS with respect to geomorphologic regions and the illumination properties of these regions. Results. The water production of 67P did not increase much from 2.9 to 2.5 AU. High water column densities are observed for LOS above the neck regions, suggesting they are the most productive in water vapor. While water production is weak in regions with low solar illumination, CO2 is outgassing from both illuminated and non-illuminated regions, which indicates that CO2 sublimates at a depth that is below the diurnal skin depth. The CO2/H2O column density ratio varies from 2 to 60%. For regions that are in sunlight, mean values between 2 and 7% are measured. The lower bound value is likely representative of the CO2/H2O production rate ratio from the neck regions. For carbon monoxide, we derive column density ratios CO/H2O < 1.9% and CO/CO2< 80%. An illumination-driven model, with a uniformly active surface releasing water at a mean rate of 8 × 1025 s-1, provides an overall agreement to VIRTIS-H data, although some mismatches show local surface inhomogeneities in water production. Rotational temperatures of 90–100 K are derived from H2O and CO2 averaged spectra.

Concurrent observations of the ultraviolet nitric oxide and infrared O2 nightglow emissions with Venus Express

Two prominent features of the Venus nightside airglow are the nitric oxide δ and γ bands produced by radiative association of O and N atoms in the lower thermosphere and the O2 infrared emission generated by three-body recombination of oxygen atoms in the upper mesosphere. The O2 airglow has been observed from the ground, during the Cassini flyby, and with VIRTIS on board Venus Express. It now appears that the global structure of the two emissions shows some similarities, but the statistical location of the region of strongest emission is not coincident. The Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus (SPICAV) ultraviolet spectrograph has collected a large number of spectra of the Venus nitric oxide nightside airglow. Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) images have been obtained at the limb and in the nadir-viewing mode and have provided new information on the horizontal and vertical distribution of the emission. We present the first concurrent observations of the two emissions observed with Venus Express. We show that nadir observations generally indicate a low degree of correlation between the two emissions observed quasi-simultaneously at a common location. A statistical study of limb profiles indicates that the altitude and the brightness of the two airglow layers generally do not covary. We suggest that this lack of correlation is explained by the presence of strong horizontal winds in the mesosphere-thermosphere transition region. They carry the downflowing atoms over large distances in such a way that regions of enhanced NO emission generally do not coincide with zones of bright O2 airglow.

On-ground characterization of Rosetta/VIRTIS-M. II. Spatial and radiometric calibrations

After having considered the spectral and geometrical performances of the Rosetta/VIRTIS-M experiment, we complete here the analysis by evaluating quantitatively the flat-field and radiometric responses. The purpose of this work is to retrieve the flat-field matrix necessary to homogenize the focal plane response. Moreover, the most important result is the determination of the instrument transfer function that allows to convert digital numbers in physical units of spectral radiance (Wm−2μm−1sterad−1). The strategy adopted to organize measurement sequence, a basic description of the on-ground experimental setups and the analysis of the collected data, is included in this article. An analysis of the instrumental stability has been performed as well by examining how the internal calibration data are affected by environmental conditions. These data allow to evaluate the cumulative effects of thermal and vibrational stresses on the instrumental performances: up to now we have verified that this effect is neglig...

The near-infrared nitric oxide nightglow in the upper atmosphere of Venus

The v′ = 0 progressions of the C → X and A → X band systems of nitric oxide dominate the middle-UV spectrum of the night-time upper atmospheres of the Earth, Mars, and Venus. The C(0) → A(0)+hν radiative transition at 1.224 μm, the only channel effectively populating the A(0) level, must therefore occur also. There have been, however, no reported detections of the C(0) → A(0) band in the atmospheres of these or any other planets. We analyzed all available near-infrared limb observations of the dark-side atmosphere of Venus by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument on the Venus Express spacecraft and found 2 unambiguous detections of this band at equatorial latitudes that seem to be associated with episodic events of highly enhanced nightglow emission. The discovery of the C(0) → A(0) band means observations in the 1.2–1.3 μm region, which also contains the a(0) → X(0) emission band of molecular oxygen, can provide a wealth of information on the high-altitude chemistry and dynamics of the Venusian atmosphere.

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.

Evolution of CO2, CH4, and OCS abundances relative to H2O in the coma of comet 67P around perihelion from Rosetta/VIRTIS-H observations

Infrared observations of the coma of 67P/Churyumov–Gerasimenko were carried out from 2015 July to September, i.e. around perihelion (2015 August 13), with the high-resolution channel of the Visible and Infrared Thermal Imaging Spectrometer instrument onboard Rosetta. We present the analysis of fluorescence emission lines of H2O, CO2, 13CO2, OCS, and CH4 detected in limb sounding with the field of view at 2.7–5 km from the comet centre. Measurements are sampling outgassing from the illuminated Southern hemisphere, as revealed by H2O and CO2 raster maps, which show anisotropic distributions, aligned along the projected rotation axis. An abrupt increase of water production is observed 6 d after perihelion. In the meantime, CO2, CH4, and OCS abundances relative to water increased by a factor of 2 to reach mean values of 32, 0.47, and 0.18 per cent, respectively, averaging post-perihelion data. We interpret these changes as resulting from the erosion of volatile-poor surface layers. Sustained dust ablation due to the sublimation of water ice maintained volatile-rich layers near the surface until at least the end of the considered period, as expected for low thermal inertia surface layers. The large abundance measured for CO2 should be representative of the 67P nucleus original composition, and indicates that 67P is a CO2-rich comet. Comparison with abundance ratios measured in the Northern hemisphere shows that seasons play an important role in comet outgassing. The low CO2/H2O values measured above the illuminated Northern hemisphere are not original, but the result of the devolatilization of the uppermost layers.

MA_MISS: Mars multispectral imager for subsurface studies

Abstract The Italian drill “DEEDRI” is going to be the lander based sample acquisition system for the Mars Surveyor Program of the Mars Sample Return mission. DEEDRI is capable to collect core/sand sample of the martian soil down to 50 cm in depth. The MA_MISS experiment belongs to the DEEDRI system and it will be dedicated to observe the wall of the excavated hole in terms of infrared spectral reflectance in the range 0.8–2.8 μm. The spectral sampling is about 20 nm while the spatial sampling is 100 μm over the target. The optical window of MA_MISS is placed very close to the drill tip so that the target view to be observed can span from the soil down to 50 cm. The proximity optics and electronics of MA_MISS have to be very miniaturized since they will be collocated inside the drill tool in a very limited volume of about 25 mm in diameter. On the other side the main electronics will be on the lander and it will communicate through an interface based on slip rings devices. MA_MISS can acquire in different observation modes. The images are scanned by moving the DEEDRI itself. One image ring is built up by acquiring contiguous images of the MA_MISS slit. The study of the Martian subsurface will provide important constraints on the nature, timing and duration of alteration and sedimentation processes on Mars, as well as on the complex interactions between the surface and the atmosphere. This study will permit to infer the history of erosion, transport and deposition of loose material. Alteration processes can dominate the mineralogy of the Martian surface: it will be essential to study the mineralogy of deeper layers, where a more limited alteration took place. MA_MISS can provide very important scientific return from the subsurface of Mars along with a selection criteria for the samples collection.

Clusters of cyclones encircling Jupiter’s poles

The familiar axisymmetric zones and belts that characterize Jupiter’s weather system at lower latitudes give way to pervasive cyclonic activity at higher latitudes. Two-dimensional turbulence in combination with the Coriolis β-effect (that is, the large meridionally varying Coriolis force on the giant planets of the Solar System) produces alternating zonal flows. The zonal flows weaken with rising latitude so that a transition between equatorial jets and polar turbulence on Jupiter can occur. Simulations with shallow-water models of giant planets support this transition by producing both alternating flows near the equator and circumpolar cyclones near the poles. Jovian polar regions are not visible from Earth owing to Jupiter’s low axial tilt, and were poorly characterized by previous missions because the trajectories of these missions did not venture far from Jupiter’s equatorial plane. Here we report that visible and infrared images obtained from above each pole by the Juno spacecraft during its first five orbits reveal persistent polygonal patterns of large cyclones. In the north, eight circumpolar cyclones are observed about a single polar cyclone; in the south, one polar cyclone is encircled by five circumpolar cyclones. Cyclonic circulation is established via time-lapse imagery obtained over intervals ranging from 20 minutes to 4 hours. Although migration of cyclones towards the pole might be expected as a consequence of the Coriolis β-effect, by which cyclonic vortices naturally drift towards the rotational pole, the configuration of the cyclones is without precedent on other planets (including Saturn’s polar hexagonal features). The manner in which the cyclones persist without merging and the process by which they evolve to their current configuration are unknown.