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  • Articles  (3)
  • Comets  (2)
  • Satellites  (1)
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  • Articles  (3)
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  • 1
    Electronic Resource
    Electronic Resource
    The atmosphere of Titan (1982)
    Springer
    Journal of molecular evolution 18 (1982), S. 150-156 
    Details
    ISSN: 1432-1432
    Keywords: Titan ; Planetary atmoshperes ; Chemical evolution ; Planets ; Satellites
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The discovery that Titan had an atmosphere was made by the identification of methane in the satellite's spectrum in 1944. But the abundance of this gas and the identification of other major constituents required the 1980 encounter by the Voyager 1 spacecraft. in the intervening years, traces of C2H2, C2H4, C2H6 and CH3D had been posited to interpret emission bands in Titan's IR spectrum. The Voyager infrared Spectrometer confirmed that these gases were present and added seven more. The atmosphere is now known to be composed primarily of molecular nitrogen. But the derived mean molecular weight suggests the presence of a significant amount of some heavier gas, most probably argon. It is shown that this argon must be primordial, and that one can understand the evolution of Titan's atmosphere in terms of degassing of a mixed hydrate dominated by CH4, N2 and36Ar. This model satisfactorily explains the absence of neon and makes no special requirements on the satellite's surface temperature. The organic chemistry taking place on Titan today invites comparision with chemical evolution on the primitive Earth prior to the origin of life.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01733040
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  • 2
    Electronic Resource
    Electronic Resource
    The Comet Nucleus Tour (Contour); A NASA Discovery Mission (2000)
    Springer
    Earth, moon and planets 89 (2000), S. 289-300 
    Details
    ISSN: 1573-0794
    Keywords: Comets ; missions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract In 1997, the COmet Nucleus TOUR (CONTOUR) was selected byNASA for a new start as part of the Discovery line. In this paper, we review the status of the mission, the mission timeline and the instruments to be flown. Detail is given of the science goals and how they are to be accomplished.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1021567007750
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  • 3
    Electronic Resource
    Electronic Resource
    Cometary Deuterium (1999)
    Springer
    Space science reviews 90 (1999), S. 33-43 
    Details
    ISSN: 1572-9672
    Keywords: Deuterium ; Origin ; Gas ; Composition ; Planets ; Water ; HCN ; Interstellar Medium ; Comets
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Deuterium fractionations in cometary ices provide important clues to the origin and evolution of comets. Mass spectrometers aboard spaceprobe Giotto revealed the first accurate D/H ratios in the water of Comet 1P/Halley. Ground-based observations of HDO in Comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp), the detection of DCN in Comet Hale-Bopp, and upper limits for several other D-bearing molecules complement our limited sample of D/H measurements. On the basis of this data set all Oort cloud comets seem to exhibit a similar $$\left( {{{\text{D}} \mathord{\left/ {\vphantom {{\text{D}} {\text{H}}}} \right. \kern-\nulldelimiterspace} {\text{H}}}} \right)_{{\text{H}}_{\text{2}} {\text{O}}} $$ ratio in H2O, enriched by about a factor of two relative to terrestrial water and approximately one order of magnitude relative to the protosolar value. Oort cloud comets, and by inference also classical short-period comets derived from the Kuiper Belt cannot be the only source for the Earth's oceans. The cometary O/C ratio and dynamical reasons make it difficult to defend an early influx of icy planetesimals from the Jupiter zone to the early Earth. D/H measurements of OH groups in phyllosilicate rich meteorites suggest a mixture of cometary water and water adsorbed from the nebula by the rocky grains that formed the bulk of the Earth may be responsible for the terrestrial D/H. The D/H ratio in cometary HCN is 7 times higher than the value in cometary H2O. Species-dependent D-fractionations occur at low temperatures and low gas densities via ion-molecule or grain-surface reactions and cannot be explained by a pure solar nebula chemistry. It is plausible that cometary volatiles preserved the interstellar D fractionation. The observed D abundances set a lower limit to the formation temperature of (30 ± 10) K. Similar numbers can be derived from the ortho-to-para ratio in cometary water, from the absence of neon in cometary ices and the presence of S2. Noble gases on Earth and Mars, and the relative abundance of cometary hydrocarbons place the comet formation temperature near 50 K. So far all cometary D/H measurements refer to bulk compositions, and it is conceivable that significant departures from the mean value could occur at the grain-size level. Strong isotope effects as a result of coma chemistry can be excluded for molecules H2O and HCN. A comparison of the cometary $$\left( {{{\text{D}} \mathord{\left/ {\vphantom {{\text{D}} {\text{H}}}} \right. \kern-\nulldelimiterspace} {\text{H}}}} \right)_{{\text{H}}_{\text{2}} {\text{O}}} $$ ratio with values found in the atmospheres of the outer planets is consistent with the long-held idea that the gas planets formed around icy cores with a high cometary D/H ratio and subsequently accumulated significant amounts of H2 from the solar nebula with a low protosolar D/H.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005269208310
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