Spin Temperatures of Ammonia and Water Molecules in Comets

The nuclear spin temperature, which is derived from the ortho-to-para abundance ratio of molecules measured in cometary comae, is a clue to the formation conditions of cometary materials, especially the physical temperature at which the molecules were formed. In this paper we present new results for the nuclear spin temperatures of ammonia in comets Hale-Bopp (C/1995 O1) and 153P/Ikeya-Zhang based on observations of NH2 at 26 and 32 K, respectively. These results are similar to previous measurements in two other comets, and the nuclear spin temperatures of ammonia in the four comets are concentrated at about 30 K. We emphasize that the nuclear spin temperatures of water measured thus far have also been about 30 K. In particular, the spin temperatures of ammonia and water are equal to each other within ±1 σ error bars in the case of comet Hale-Bopp. These nuclear spin temperatures of ammonia and water were measured under quite different conditions (heliocentric distances and gas production rates). There is no clear trend between the nuclear spin temperatures and the heliocentric distances, the gas production rates, or the orbital periods of the comets. The possibilities of the ortho-to-para conversion in the coma and in the nucleus are discussed. The present data set implies that the ortho-to-para ratios were not altered after the molecules were incorporated into the cometary nuclei. It appears that cometary ammonia and water molecules formed on cold grains at about 30 K

Ceres water regime: surface temperature, water sublimation and transient exo(atmo)sphere

Recent observations of water emission around Ceres suggest the presence of an ice layer on or beneath the surface of this asteroid. Several mechanisms have been suggested to explain these plumes, among which cometary-like sublimation seems to be plausible, since there is a correlation between the magnitude of the emission and the change in the heliocentric distance along the orbit. In this work, we applied a comet sublimation model to study the plausible scenarios that match with Herschel observations of the water flux (1026 molecules s-1). Each scenario is characterized by a well-defined set of physical and orbital parameters. Moreover, a study of the dynamic evolution of the H2O plume has been performed, showing that an optically thin transient atmospheric envelope, with a typical timescale of some tens of days, can be maintained by the H2O surface emission. Our simulations could be useful theoretical support for the Dawn NASA mission by giving a better understanding of the physical conditions for water sublimation and ice stability

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

IDIS Small Bodies and Dust Node: Technical innovation and science

This work was supported by the EUROPLANET RI FP7 grant agreement 228319It is not trivial, nowadays, to be fully aware of the impressive amount of astrophysical resources that are at hand. Virtual Observatories (VOs) were therefore created to provide a simple access to what astronomers look for. In this paper we focus on the original data access services developed specifically, in a VO perspective, for the "Small Bodies and Dust Node" (SBDN) in the framework of the Integrated and Distributed Information System (IDIS) initiative of the Europlanet Research Infrastructure project. We describe the scientific goals, along with the innovative technical aspects, of the tools that SBDN presently provides to the scientific community, namely the Comet Emission Lines service, and the Cosmic Dust Catalog service. In the former, an algorithm for the detection of unidentified emission lines has been implemented

Water vapor emission mechanism for 67P/Churyumov-Gerasimenko

In this work we study the water vapor emission by the the comet 67P/CG, the target of Rosetta mission. In this work we investigate the physical conditions required to generate short-lived outbursts in cometary nuclei. We applied a thermo-physical model [1, 2, 3] in order to evaluate the temperature of surface and subsurface layers and the water flux. Cyclic sublimation and water condensation in the sub-surface layers, due to the change of the illumination condition on the surface, is a likely mechanism to explain part of the water outgassing [5]

Variations in the amount of water ice on Ceres' surface suggest a seasonal water cycle.

The dwarf planet Ceres is known to host a considerable amount of water in its interior, and areas of water ice were detected by the Dawn spacecraft on its surface. Moreover, sporadic water and hydroxyl emissions have been observed from space telescopes. We report the detection of water ice in a mid-latitude crater and its unexpected variation with time. The Dawn spectrometer data show a change of water ice signatures over a period of 6 months, which is well modeled as ~2-km2 increase of water ice. The observed increase, coupled with Ceres' orbital parameters, points to an ongoing process that seems correlated with solar flux. The reported variation on Ceres' surface indicates that this body is chemically and physically active at the present time

Water Sublimation and Surface Temperature Simulations of Ceres

Ceres is one of the major objects of the main belt. Using a cometary-like model, we study the water sublimation and the surface temperature

Ceres: ice stability and water emission

Recent observations of H2O vapor plumes in localized regions [1] suggest the presence of ice on surface and/or on sub-surface regions of asteroid Ceres. In the hypothesis of a cometary-like emission mechanism (as already suggested by [2]), we performed several simulations in order to establish what are the likely physical conditions (in particular ice depth and thermal conductivity of crust) to fit Herschel observations [1]