Upper limits on carbon group ions near the orbit of Titan: Implications for methane escape from Titan
Abstract
High neutral methane escape rates from Titan (∼3x109 cm-2 s-1 ) have been inferred from Cassini Ion and Neutral Mass Spectrometer observations [Yelle et al., 2008]. This is much higher than past predictions (e.g. due to Jeans loss). To investigate this hypothesis, we have examined Cassini Cassini Plasma Spectrometer (CAPS) data obtained near Titan's orbit. We have used the CAPS linear electric field (LEF) mass spectra, which provide high resolution measurements of atomic ions and the atomic constituents of molecular ions. The expected lifetime of neutral methane is sufficiently long that escaping molecules would not ionize in Titan's immediate vicinity. However, ionospheric methane ions are observed in Titan's atmosphere. To distinguish between these two possible sources of methane ions, we have examined spectra obtained within five Saturn radii of Titan's orbit, but at distances of over one Saturn radius from Titan itself. Between March 2005 and Dec. 2009, 5466 LEF spectra were obtained in this region. These data show a clear oxygen peak, either from atomic O+ or from fragmentation of oxygen-bearing molecular ions. A weaker nitrogen peak, with 5.9% the amplitude of the oxygen peak, is also present. At the instrument's noise level, no carbon peak is present. This non-detection corresponds to an abundance of carbon ions and carbon-bearing molecular ions under 0.28% that of oxygen and oxygen-bearing ions. Estimates of the neutral and ion loss rates, and ion production rates, allow us to convert this upper limit into an upper limit on the escape rate of neutral methane from Titan. Unless there is some currently unknown and efficient loss process for neutral methane (i.e. other than ionization), this upper limit is several orders of magnitude lower than the escape rate determined by Yelle et al., 2008.
- Publication:
-
38th COSPAR Scientific Assembly
- Pub Date:
- 2010
- Bibcode:
- 2010cosp...38.1413C