Saturn's largest moon, Titan, remains an enigma, explored only by remote sensing from Earth, and by the Voyager and Cassini spacecraft. The most puzzling aspects include the origin of the molecular nitrogen and methane in its atmosphere, and the mechanism(s) by which methane is maintained in the face of rapid destruction by photolysis. The Huygens probe, launched from the Cassini spacecraft, has made the first direct observations of the satellite's surface and lower atmosphere. Here we report direct atmospheric measurements from the Gas Chromatograph Mass Spectrometer (GCMS), including altitude profiles of the constituents, isotopic ratios and trace species (including organic compounds). The primary constituents were confirmed to be nitrogen and methane. Noble gases other than argon were not detected. The argon includes primordial 36Ar, and the radiogenic isotope 40Ar, providing an important constraint on the outgassing history of Titan. Trace organic species, including cyanogen and ethane, were found in surface measurements.

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.