Lunar Atmospheric Composition Experiment (LACE)

Description

To study the composition and variations in the lunar atmosphere, a miniature magnetic deflection mass spectrometer was deployed on the lunar surface and oriented to intercept and measure the downward flux of gases. This instrument, the Lunar Atmospheric Composition Experiment (LACE) was part of the Apollo Lunar Surface Experiments Package (ALSEP). It was deployed by the Apollo 17 astronauts on 12 December 1972 at roughly 5;00 UTith a nylon dust-screen covering hte upward-facing aperture to protect it during mission surface activities. The dust screen was pulled back by radio command afte rthe crew had taken off and the seismic charges had been detonated. The instrument was turned on by ground command at 18:07 UT on 27 December 1972, approximately 50 hours after the first sunset following deployment. Performance was very good in genenral and continued throughout the lunar night. At sunrise, heating of the site and instrument resulted in high rates of outgassing, necessitating cessation of operation during the day except for a brief check near noon. High daytime outgassing rates severely curtailed instrument operation throughout its history because of the fear that high background rates would degrade instrument sensitivity over time. During April and September 1973 operation was continued 4 or 5 hours after sunrise to track argon. All housekeeping data remained in bounds. Unplanned slow evaporation of the tungsten filament, producing peaks in the 91 to 93 amu range, provided a constant check on instrument sensitivity, which remained stable.

The ion source contained two filaments selectable by command. Three collector assemblies were placed to collect ion beams in the ratio 1 to 12 to 27.4, so that three mass ranges were scanned simultaneously, viz., 1 to 4, 12 to 48, and 27.4 to 110 amu. Resolution of the analyzer was set at about 100 for the high-mass channel at 82. Electron multipliers, pulse amplifiers, discriminators, and counters were used, one system for each mass range. In normal operation, the electron bombardment energy was fixed at 70 eV, and then the instrument sensitivity to n2 of 5.0E-5 amp/torr was sufficient to measure the concentration of gas species in the 1.0E-5 torr range. An alternate operating mode provided four different electron energies of 70, 27, 20, and 18 Ev, which were cycled by successive sweeps of the mass spectrum. Voltage scan of the mass spectrum was employed using a high-voltage stepping power supply. The sweep voltage varied through 1330 steps from 320 to 1420 volts, with a dwell time of 0.6 sec per step. Since each step was synchronized to a main frame of the telemetry format, the telemetry word position served as the identifier of atomic mass number. The sweep time was 13.5 min. The instrument was calibrated inside a molecular beam facility.