The Mass Spectrometer of the Galileo Probe
The Atmospheric Experiments Laboratory developed the mass spectrometer on the Galileo Probe. Shortly before arrival at Jupiter, the Probe separated from the Galileo Orbiter, entered the upper atmosphere, and then descended on a parachute into the deep Jovian atmosphere. The Galileo Probe Mass Spectrometer (GPMS) was one of six instruments on the Probe and was the primary Probe instrument to measure chemical and isotopic composition of the atmosphere of Jupiter. Data transmission from the descending Probe lasted 58 minutes, during which time the ambient gas pressure increased from 0.4 to 23 times the sea level surface pressure of the atmosphere of Earth.
The Galileo Probe Mass Spectrometer (GPMS) was highly integrated quadrupole mass spectrometer and electronics package designed to be lightweight and compact. The instrument weight was 13.2 kg. (29.1 lb.) and average power consumption was 22 watts. An intricate system of microvalves, gas plumbing, sample enrichment cells, and chemical getter pumps were utilized to obtain a series of chemical composition measurements throughout the descent sequence. The instrument housing, made of titanium, was sealed with just over one atmosphere of nitrogen inside, in order to protect the sensor and electronics from the wide variations of pressure and temperature during the descent. The housing was designed to survive to an external pressure of approximately 22 times earth sea level pressure (i.e., 330 pounds per square inch), at which time it was expected to implode. For one of the tests performed to insure instrument survival at that pressure, a test housing was constructed and imploded.
Elements of the GPMS are shown in this cutaway illustration.
The sensor was maintained under vacuum by chemical getter pumps and periodic operation of a sputter ion pump. After entry into the Jovian atmosphere, the gas inlet ports were opened and the incoming gases analyzed by the quadrupole mass spectrometer for major and minor constituents. Neutral gas molecules were ionized, focused and finally separated in a quadrupole rod assembly. Ions exiting the quadrupole rods were focused into a pulse counting electron multiplier. Atmospheric species with molecular weights from 2 to 150 Atomic Mass Units were analyzed, with the sampling rate limited by the communication link at two samples per second. More than 6000 samples were taken during the descent.
The instrument was designed by the Atmospheric Experiments Laboratory in collaboration in the electronics design and fabrication with the Space Physics Research Laboratory at the University of Michigan. Numerous vendors participated in the fabrication of various sections of the hardware and electronics. The frames below give a cutaway view of the components of this mass spectrometer and show the details of the inlet system used to process gas before its introduction into the ion source of the mass spectrometer. The chemical enrichment system shown allowed certain species present at less than parts per billion (volume mixing ratio) to be detected and analyzed by this mass spectrometer.
