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

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 (N 2 ), methane (CH 4 ), and molecular hydrogen (H 2 ) were determined. Traces were detected on the surface of evaporating methane, ethane (C 2 H 6 ), acetylene (C 2 H 2 ), cyanogen (C 2 N 2 ), and carbon dioxide (CO 2 ). 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 14 N/ 15 N in molecular nitrogen, 91.1 ± 1.4 for 12 C/ 13 C in methane, and (1.35 ± 0.30) × 10 −4 for D/H in molecular hydrogen. The mole fractions of 36 Ar and radiogenic 40 Ar are (2.1 ± 0.8) × 10 −7 and (3.39 ± 0.12) × 10 −5 , respectively. 22 Ne 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 Results previously published from the GCMS experiment are superseded by this publication.