GCMS: Gas Chromatograph and Mass Spectrometer
The Gas Chromatograph and Mass Spectrometer (GCMS) will measure the chemical composition of Titan's atmosphere from 170 km altitude (approx. 1 mbar) to the surface (approx. 1.5 bar) and determine the isotope ratios of the major gaseous constituents. GCMS will also analyse gas samples from the Aerosol Collector Pyrolyser (ACP) and will be able to investigate the composition (including isotope ratio) of several candidate surface materials. GCMS is a quadrupole mass filter with a secondary electron multiplier detection system and a gas sampling system providing direct atmospheric composition measurements and batch sampling through three Gas Chromatograph (GC) columns.
GCMS Scientific Objectives
Analysis of the composition of the atmosphere of Titan is one of the most important goals of the Cassini-Huygens Mission. The scope of this problem is unusually large, including the determination of noble gas abundance, isotopic ratios, and the identification of high molecular weight organic compounds in trace quantities. Even the relative abundance of major constituents are poorly known, while the opportunities for minor constituent formation in this evolving atmosphere are so rich that it is not possible to predict with certainty just what substances to seek.
The payload addresses this challenge with a variety of instruments on the probe and the orbiter. This instrument provides in situ data along the track of the probe descent and a context for these results by remote sensing of the atmosphere. A versatile, sensitive Gas Chromatograph-Mass Spectrometer (GC-MS) on the probe is essential to the success of this strategy.
With a dynamic range of 108 the GCMS can identify atmospheric constituents over a mass range from 2 to 146 amu. Using the best current models for the atmosphere of Titan, the instrument will be able to measure the major isotopes of carbon, nitrogen, hydrogen, oxygen and argon. It can detect neon and the other noble gases to levels of 10 - 100 parts per billion. The abundance of CO can be tracked, resolving present uncertainties about possible altitude variations in the mixing ratio of this important oxygen containing gas and gathering the data required to determine its source(s) and sink(s).
Similarly, the vertical variations in mixing ratios of various organic compounds can be used to delineate the chemical processes leading to their formation. A search for new constituents will be conducted (even unpredicted ones) to a level of 10 parts per billion, identifying them with the aid of a computer-accessed library of mass spectra of thousands of organic compounds. By analyzing the output of the Aerosol Collector Pyrolyser (ACP), it will also be possible to study the end products of the atmospheric chemistry of Titan. These data will provide new insights into the mechanisms for chemical evolution on Titan and its possible relevance to prebiotic synthesis on the early Earth.
GCMS Instrument Overview
The main elements of the instrument are:
- A mass spectrometer system consisting of ion sources, mass analyzer and ion detector to permit species concentration measurements to be made
- A gas sample inlet system to reduce the ambient atmospheric pressure to a lower value which is acceptable for proper operation of the mass spectrometer ion source
- A sample enrichment system to increase the measurement capability of the instrument by selective physical and chemical treatment of atmospheric samples prior to their introduction into the ionization region of the ion source
- A gas chromatograph to increase the measurement capability of the instrument by batch sampling at specific points in the atmosphere and subsequent time separation of species with different chemical properties for detection and identification and analyses by the mass spectrometer
- A vacuum pump system to maintain proper operating pressures in the mass spectrometer while atmospheric gas samples, aerosol Pyrolyser products or gases eluting from the gas chromatograph are introduced into the ionization regions of the ion sources
- A sample transfer system for the gas mixtures generated by the Aerosol Collector Pyrolyser (ACP) to the mass spectrometer sample inlet systems
The mass spectrometer employs five ion sources selectably feeding a common mass analyzer. One of the five sources (IS1) is connected to the atmospheric input manifold. The second ion source (IS2) is connected to the output of the Aerosol Collector Pyrolyser (ACP), and three ion sources (IS3, IS4 and IS5) are connected to three gas chromatographic columns (GC columns).
During part of the descent the source choice is prescribed but for most of the time the choice will be determined in-flight based on a measurement of the effluent output of the three chromatographic columns. During the intervals when effluent from the GC columns is expected, the presence of a gas peak at the output, as determined by a measurement of the total abundance of gas with mass greater than the carrier gas, will signal priority status for the source connected to the eluting column. In the absence of such a peak, the source connected directly to the atmospheric manifold will be selected. During the intervals set aside for the ACP direct analysis, the associated source (IS2) will be selected and during the GC analysis of the ACP pyrolysis products, the priority interrupt will be identical to that associated with the atmospheric sample.
Last Update: 07 December 2005