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[2.03] Results from the Gas Chromatograph Mass Spectrometer (GCMS) Experiment on the Cassini-Huygens Probe

H. Niemann (NASA GSFC), Cassini-Huygens GCMS Team

The GCMS was part of the instrument complement on the Cassini-Huygens Probe to measure in situ the chemical composition of the atmosphere during the probe descent and to support the Aerosol Collector Pyrolyser (ACP) experiment by serving as detector for the pyrolization products. The GCMS employed a quadrupole mass filter with a secondary electron multiplier detection system and a gas sampling system providing continuous direct atmospheric composition measurements and batch sampling through three gas chromatographic (GC) columns. The GCMS gas inlet was heated to prevent condensation, and served to evaporate surface constituents after impact.

The GCMS collected data from an altitude of 146 km to ground impact. The Probe and the GCMS survived impact and collected data for 1 hour and 9 minutes on the surface. Mass spectra were collected during descent and on the ground over a range of m/z from 2 to 141. Eight gas chromatograph samples were taken during the descent and two on the ground.

The major constituents of the lower atmosphere were confirmed to be N2 and CH4. The methane mole fraction was fairly uniform in the stratosphere. It began to increase below the tropopause, at about 32 km altitude, monotonically toward the surface, reaching a plateau at about 8 km at a level near saturation. After surface impact a steep increase of the methane signal was observed, suggesting evaporation of surface condensed methane due to heating by the GCMS sample inlet heater. The presence of Argon 40 was confirmed. The other primordial noble gases, except Argon 36, were below 10^-8 mixing ratio. The isotope ratios of 12C/13C and of 14N/15N were determined. Carbon dioxide, ethane, and cyanogen were also evaporating from the surface. Isotope ratios for the major carbon and nitrogen isotopes were computed from methane and molecular nitrogen measurements.

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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.