Effects of Nitrogen Photoabsorption Cross Section Resolution on Minor Species Vertical Profiles in Titan's Upper Atmosphere

The significant variations in both measured and modeled densities of minor species in Titan's atmosphere call for the evaluation of possible influencing factors in photochemical modeling. The effect of nitrogen photoabsorption cross section selection on the modeled vertical profiles of minor species is analyzed here, with particular focus on C2H6 and HCN. Our results show a clear impact of cross sections used on all neutral and ion species studied. Affected species include neutrals and ions that are not primary photochemical products, including species that do not even contain nitrogen. The results indicate that photochemical models that employ low-resolution cross sections may significantly miscalculate the vertical profiles of minor species. Such differences are expected to have important implications for Titan's overall atmospheric structure and chemistry.NASA Outer Planet Research program NNH12ZDA001NInstitute for Computational Engineering and Sciences (ICES

Isotope Geochemistry for Comparative Planetology of Exoplanets

Isotope geochemistry has played a critical role in understanding processes at work in and the history of solar system bodies. Application of these techniques to exoplanets would be revolutionary and would allow comparative planetology with the formation and evolution of exoplanet systems. The roadmap for comparative planetology of the origins and workings of exoplanets involves isotopic geochemistry efforts in three areas: (1) technology development to expand observations of the isotopic composition of solar system bodies and expand observations to isotopic composition of exoplanet atmospheres; (2) theoretical modeling of how isotopes fractionate and the role they play in evolution of exoplanetary systems, atmospheres, surfaces and interiors; and (3) laboratory studies to constrain isotopic fractionation due to processes at work throughout the solar system

A protosolar nebula origin for the ices agglomerated by Comet 67P/Churyumov-Gerasimenko

The nature of the icy material accreted by comets during their formation in the outer regions of the protosolar nebula is a major open question in planetary science. Some scenarios of comet formation predict that these bodies agglomerated from crystalline ices condensed in the protosolar nebula. Concurrently, alternative scenarios suggest that comets accreted amorphous ice originating from the interstellar cloud or from the very distant regions of the protosolar nebula. On the basis of existing laboratory and modeling data, we find that the N$_2$/CO and Ar/CO ratios measured in the coma of the Jupiter family comet 67P/Churyumov-Gerasimenko by the ROSINA instrument aboard the European Space Agency's Rosetta spacecraft match those predicted for gases trapped in clathrates. If these measurements are representative of the bulk N$_2$/CO and Ar/CO ratios in 67P/Churyumov-Gerasimenko, it implies that the ices accreted by the comet formed in the nebula and do not originate from the interstellar medium, supporting the idea that the building blocks of outer solar system bodies have been formed from clathrates and possibly from pure crystalline ices. Moreover, because 67P/Churyumov-Gerasimenko is impoverished in Ar and N$_2$, the volatile enrichments observed in Jupiter's atmosphere cannot be explained solely via the accretion of building blocks with similar compositions and require an additional delivery source. A potential source may be the accretion of gas from the nebula that has been progressively enriched in heavy elements due to photoevaporation.Comment: The Astrophysical Journal Letters, in pres

Characterization of the Subsurface of 67P/Churyumov-Gerasimenko's Abydos Site

We investigate the structure of the subsurface of the Abydos site using a cometary nucleus model with parameters adapted to comet 67P/Churyumov-Gerasimenko and the Abydos landing site. We aim to compare the production rates derived from our model with those of the main molecules measured by Ptolemy. This will allow us to retrieve the depths at which the different molecules still exist in solid form

Origin of molecular oxygen in Comet 67P/Churyumov-Gerasimenko

Molecular oxygen has been detected in the coma of comet 67P/Churyumov-Gerasimenko with abundances in the 1-10% range by the ROSINA-DFMS instrument on board the Rosetta spacecraft. Here we find that the radiolysis of icy grains in low-density environments such as the presolar cloud may induce the production of large amounts of molecular oxygen. We also show that molecular oxygen can be efficiently trapped in clathrates formed in the protosolar nebula, and that its incorporation as crystalline ice is highly implausible because this would imply much larger abundances of Ar and N2 than those observed in the coma. Assuming that radiolysis has been the only O2 production mechanism at work, we conclude that the formation of comet 67P/Churyumov-Gerasimenko is possible in a dense and early protosolar nebula in the framework of two extreme scenarios: (1) agglomeration from pristine amorphous icy grains/particles formed in ISM and (2) agglomeration from clathrates that formed during the disk's cooling. The former scenario is found consistent with the strong correlation between O2 and H2O observed in 67P/C-G's coma while the latter scenario requires that clathrates formed from ISM icy grains that crystallized when entering the protosolar nebula.Comment: The Astrophysical Journal Letters, in pres

Evolution of the subsurface of 67P/Churyumov-Gerasimenko’s Abydos Site

On November 12, 2014, Rosetta's descent module Philae landed on the Abydos site of comet 67P/Churyumov-Gerasimenko (67P). Here we investigate the structure of the subsurface of the Abydos site by making use of a cometary nucleus model [1] employing an updated set of thermodynamic parameters relevant for 67P and an appropriated parameterization of the illumination of the Abydos site. The model considers an initially homogeneous sphere composed of a predefined porous mixture of crystalline ices (H2O, CO and CO2) and dust in specified proportions, and uses parameters derived from recent 67P studies [2], [3] and [4]. The comparison of the production rates derived from our model with those of the main molecules measured by Ptolemy (the mass spectrometer performing the analysis of several samples collected from the surface and atmosphere of the comet) should allow us to place important constraints on the structure (layering and composition) of the subsurface of Philae’s landing site

Science Opportunities offered by Mercury's Ice-Bearing Polar Deposits

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On the origin and evolution of the material in 67P/Churyumov-Gerasimenko

International audiencePrimitive objects like comets hold important information on the material that formed our solar system. Several comets have been visited by spacecraft and many more have been observed through Earth- and space-based telescopes. Still our understanding remains limited. Molecular abundances in comets have been shown to be similar to interstellar ices and thus indicate that common processes and conditions were involved in their formation. The samples returned by the Stardust mission to comet Wild 2 showed that the bulk refractory material was processed by high temperatures in the vicinity of the early sun. The recent Rosetta mission acquired a wealth of new data on the composition of comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) and complemented earlier observations of other comets. The isotopic, elemental, and molecular abundances of the volatile, semi-volatile, and refractory phases brought many new insights into the origin and processing of the incorporated material. The emerging picture after Rosetta is that at least part of the volatile material was formed before the solar system and that cometary nuclei agglomerated over a wide range of heliocentric distances, different from where they are found today. Deviations from bulk solar system abundances indicate that the material was not fully homogenized at the location of comet formation, despite the radial mixing implied by the Stardust results. Post-formation evolution of the material might play an important role, which further complicates the picture. This paper discusses these major findings of the Rosetta mission with respect to the origin of the material and puts them in the context of what we know from other comets and solar system objects

Vacuum ultraviolet photoabsorption spectroscopy of space-related ices: 1 keV electron irradiation of nitrogen- and oxygen-rich ices

Context. Molecular oxygen, nitrogen, and ozone have been detected on some satellites of Saturn and Jupiter, as well as on comets. They are also expected to be present in ice-grain mantles within star-forming regions. The continuous energetic processing of icy objects in the Solar System induces physical and chemical changes within the ice. Laboratory experiments that simulate energetic processing (ions, photons, and electrons) of ices are therefore essential for interpreting and directing future astronomical observations. Aims. We provide vacuum ultraviolet (VUV) photoabsorption spectroscopic data of energetically processed nitrogen- and oxygen-rich ices that will help to identify absorption bands and/or spectral slopes observed on icy objects in the Solar System and on ice-grain mantles of the interstellar medium. Methods. We present VUV photoabsorption spectra of frozen O2 and N2, a 1:1 mixture of both, and a new systematic set of pure and mixed nitrogen oxide ices. Spectra were obtained at 22 K before and after 1 keV electron bombardment of the ice sample. Ices were then annealed to higher temperatures to study their thermal evolution. In addition, Fourier-transform infrared spectroscopy was used as a secondary probe of molecular synthesis to better identify the physical and chemical processes at play. Results. Our VUV data show that ozone and the azide radical (N3) are observed in our experiments after electron irradiation of pure O2 and N2 ices, respectively. Energetic processing of an O2:N2 = 1:1 ice mixture leads to the formation of ozone along with a series of nitrogen oxides. The electron irradiation of solid nitrogen oxides, pure and in mixtures, induces the formation of new species such as O2, N2, and other nitrogen oxides not present in the initial ice. Results are discussed here in light of their relevance to various astrophysical environments. Finally, we show that VUV spectra of solid NO2 and water can reproduce the observational VUV profile of the cold surface of Enceladus, Dione, and Rhea, strongly suggesting the presence of nitrogen oxides on the surface of the icy Saturn moons