Composition of Titan's lower atmosphere and simple surface volatiles as measured by the Cassini‐Huygens probe gas chromatograph mass spectrometer experiment

Abstract

The Cassini‐Huygens probe gas chromatograph mass spectrometer (GCMS) determined the composition of the Titan atmosphere from ∼140 km altitude to the surface. After landing, it returned composition data of gases evaporated from the surface. Height profiles of molecular nitrogen (N2), methane (CH4), and molecular hydrogen (H2) were determined. Traces were detected on the surface of evaporating methane, ethane (C2H6), acetylene (C2H2), cyanogen (C2N2), and carbon dioxide (CO2). The methane data showed evidence that methane precipitation occurred recently. The methane mole fraction was (1.48 ± 0.09) × 10−2 in the lower stratosphere (139.8–75.5 km) and (5.65 ± 0.18) × 10−2 near the surface (6.7 km to the surface). The molecular hydrogen mole fraction was (1.01 ± 0.16) × 10−3 in the atmosphere and (9.90 ± 0.17) × 10−4 on the surface. Isotope ratios were 167.7 ± 0.6 for 14N/15N in molecular nitrogen, 91.1 ± 1.4 for 12C/13C in methane, and (1.35 ± 0.30) × 10−4 for D/H in molecular hydrogen. The mole fractions of 36Ar and radiogenic 40Ar are (2.1 ± 0.8) × 10−7 and (3.39 ± 0.12) × 10−5, respectively. 22Ne has been tentatively identified at a mole fraction of (2.8 ± 2.1) × 10−7. Krypton and xenon were below the detection threshold of 1 × 10−8 mole fraction. Science data were not retrieved from the gas chromatograph subsystem as the abundance of the organic trace gases in the atmosphere and on the ground did not reach the detection threshold. Results previously published from the GCMS experiment are superseded by this publication.

The structure of Titan’s atmosphere from Cassini radio occultations

“…N 2 is the dominant constituent, followed by CH 4 , which has a mole fraction of 0.015 in the stratosphere increasing at lower altitudes in the troposphere to several percent at the surface (Niemann et al, 2005(Niemann et al, , 2010. The most detailed measurements of temperatures in Titan's troposphere and middle atmosphere (stratosphere and mesosphere) have been made by instruments on the Cassini orbiter and Huygens probe.…”

Silence on Shangri-La: Attenuation of Huygens acoustic signals suggests surface volatiles

“…10. The mixing ratio evolution of some acoustically-absorbing gases sampled by the Huygens GCMS (Niemann et al, 2010). The rise post-impact is due to heating of the inlet, embedded in the surface material.…”

“…1553 s after landing. Additionally, material either inside or immediately adjacent to the heated inlet of the GCMS instrument progressively warmed up after landing and evolved methane, and later ethane and some other compounds (Niemann et al, 2005(Niemann et al, , 2010. The thermal interactions of the warm -but well-insulated and therefore cold-skinned -probe with the surface environment are discussed in Lorenz (2006).…”

“…The actual gas mixture that formed in the acoustic cavity on Huygens cannot in any case be uniquely determined: our only intent here is to show that methane, ethane and carbon dioxide (compounds known to be present on the Titan surface) and other organics (like ethylene and many others) can provide significant acoustic absorption at the frequencies discussed. Niemann et al (2005Niemann et al ( , 2010 show that the GCMS instrument detected several species after landing (see Fig. 10).…”

Constraints from Comets on the Formation and Volatile Acquisition of the Planets and Satellites

“…Notably, the Earth, the Moon and the interior of Mars and Venus all have comparable 15 N/ 14 N ratios that are also within the range of most values seen in primitive meteorites. The atmospheres of Titan and of Mars are exceptions (Mandt et al 2015): for the Titan atmosphere, the enrichment in 15 N in N 2 (Niemann et al 2010) is attributed to contribution of 15 N-rich material akin of cometary amines and nitrides (Mandt et al 2014), whereas the enrichment in 15 N of the atmosphere of Mars is regarded as resulting from isotope-selective non-thermal escape processes (e.g., Bogard et al 2001). In contrast to inner solar system bodies, all comets analyzed so far present strong enrichments (factors of about 2) in 15 N in HCN (Bockelée-Morvan et al 2008;Manfroid et al 2009) and NH 3 (Rousselot et al 2014;Shinnaka et al 2014).…”