Positive ion composition in the polar D and E regions measured during moderate ionospheric absorption

During the MAC/Epsilon campaign a mass spectrometer probe was flown on a rocket launched from Andoya (Norway) on 12 November 1987 at 0021 UT providing partial ion density profiles in the altitude range between less than 50 to 125 km. Due to the short sampling period of 0.17 seconds structural features could be observed at approx. 150 m height resolution in the regimes where metal ions occur and where cluster ions are dominant. The observations were made during stable ionospheric absorption of 1 to 1.5 dB. Preliminary results are presented and discussed

Evidence for methane and ammonia in the coma of comet P/Halley

Methane and ammonia abundances in the coma of Halley are derived from Giotto IMS data using an Eulerian model of chemical and physical processes inside the contact surface to simulate Giotto HIS ion mass spectral data for mass-to-charge ratios (m/q) from 15 to 19. The ratio m/q = 19/18 as a function of distance from the nucleus is not reproduced by a model for a pure water coma. It is necessary to include the presence of NH_3 , and uniquely NH_3 , in coma gases in order to explain the data. A ratio of production rates Q(NH_3)/Q(H20) = 0.01-Q.02 results in model values approximating the Giotto data. Methane is identified as the most probable source of the distinct peak at m/q = 15. The observations are fit best with Q(CH_4)/Q(H_20) = 0.02. The chemical composition of the comet nucleus implied by these production rate ratios is unlike that of the outer planets. On the other hand, there are also significant differences from observations of gas phase interstellar material

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

Thymus vulgaris, un cas de polymorphisme chimique pour comprendre l'écologie évolutive des composés secondaires

MONTPELLIER-SupAgro La Gaillarde (341722306) / SudocSudocFranceF

Temperature dependence of ozone rate coefficients and isotopologue fractionation in ¹⁶O-¹⁸O oxygen mixtures

The temperature dependence of five ozone isotope-specific rate coefficient ratios and of isotopologue fractionation has been determined. Large formation rate coefficient ratios of 1.5 like 16O + 18O18O vs. 16O + 16O16O show no temperature dependence while small ratios such as 18O + 16O16O vs. 16O + 16O16O with a value of 0.92 decrease with decreasing temperatures. Temperature-related changes of isotopologue fractionation values for 50O3 and 52O3 are explained in terms of changes in rate coefficient ratios and contributions from isotope exchange reactions. The latter reactions exclusively control the large isotope fractionation of 54O3 while the rate coefficient ratio 18O+ 18O18O vs. 16O+ 16O16O remains constant at 1

Temperature dependence of ozone rate coefficients and isotopologue fractionation in O-16-O-18 oxygen mixtures

The temperature dependence of five ozone isotope-specific rate coefficient ratios and of isotopologue fractionation has been determined. Large formation rate coefficient ratios of 1.5 like O-16 + (OO)-O-18-O-18 vs. O-16 + (OO)-O-16-O-16 show no temperature dependence while small ratios such as O-18 + (OO)- O-16-O-16 vs. O-16 + (OO)-O-16-O-16 with a value of 0.92 decrease with decreasing temperatures. Temperature-related changes of isotopologue fractionation values for O-50(3) and O- 52(3) are explained in terms of changes in rate coefficient ratios and contributions from isotope exchange reactions. The latter reactions exclusively control the large isotope fractionation of O-54(3) while the rate coefficient ratio O-18 + (OO)-O-18-O-18 vs. O-16 + (OO)-O-16-O-16 remains constant at 1.02. (C) 2002 Elsevier Science B.V. All rights reserved

Temperature dependence of ozone rate coefficients and isotopologue fractionation in O-16-O-18 oxygen mixtures

The temperature dependence of five ozone isotope-specific rate coefficient ratios and of isotopologue fractionation has been determined. Large formation rate coefficient ratios of 1.5 like O-16 + (OO)-O-18-O-18 vs. O-16 + (OO)-O-16-O-16 show no temperature dependence while small ratios such as O-18 + (OO)- O-16-O-16 vs. O-16 + (OO)-O-16-O-16 with a value of 0.92 decrease with decreasing temperatures. Temperature-related changes of isotopologue fractionation values for O-50(3) and O- 52(3) are explained in terms of changes in rate coefficient ratios and contributions from isotope exchange reactions. The latter reactions exclusively control the large isotope fractionation of O-54(3) while the rate coefficient ratio O-18 + (OO)-O-18-O-18 vs. O-16 + (OO)-O-16-O-16 remains constant at 1.02. (C) 2002 Elsevier Science B.V. All rights reserved