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| Titel |
Isotopic Ratios in Titan’s Atmosphere from Cassini CIRS |
| VerfasserIn |
C. A. Nixon, D. E. Jennings, B. Bezard, S. Vinatier, A. Coustenis, N. A. Teanby, P. G. J. Irwin, R. K. Achterberg, F. M. Flasar |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250020654
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| Zusammenfassung |
| Isotopic ratios in planetary atmospheres are valuable sources of information regarding the
formation and evolution of the body. Their present-day values may also be ambiguous,
reflecting the sum total of many processes that can alter the ratio from primordial: physical
and chemical processes in the nebula, during primary or secondary atmospheric formation,
and subsequently. For this reason, deducing a unique past history for a given isotopic ratio is
rarely straightforward.
Since Cassini’s entry to Kronian orbit in 2004 the spacecraft has made more than 50 close
flybys of Titan, during which the Composite Infrared Spectrometer (CIRS) instrument has
been active in mapping the thermal emissions from stratospheric gases. The large
improvement in spectral resolution and sensitivity over Voyager IRIS, combined with the
ability to make repeated long-path limb observations to increase signal-to-noise, has
proved CIRS to be a powerful tool for measuring isotopic variants of stratospheric
gases.
These measurements provide an ideal complement to those of the Huygens probe mass
spectrometer (GCMS). Whereas GCMS provided a single, high-precision value for the
isotopic ratios in the most abundant species (e.g. D/H in H2), CIRS instead provides multiple,
lower-precision measurements of the same isotopic ratios in different gases. So far,
CIRS has published values for four ratios (D/H, 12C/13C, 14N/15N, 18O/16O) in six
gases (CH4, C2H2, C2H6, HCN, HC3N, CO2), totalling ten different molecular
ratios in all (six 13C variants, two deuterated gases, and one of each 18O and 15N).
Divergence of the values between species (e.g. D/H is different in H2 and CH4;
14N/15N is different in HCN than in N2) can then be used to constrain models of
fractionation.
In this paper we summarize the molecular isotopic measurements by CIRS to date, and
their current interpretations; we also outline which other ratios will potentially be detected by
CIRS during the Cassini extended mission. |
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