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Lunar Atmospheric Composition Experiment

The Lunar Atmospheric Composition Experiment (LACE) was a miniature magnetic deflection mass spectrometer (neutral mass spectrometer). The experiment's aim was to study the composition and variations of the lunar atmosphere. The only deployment of LACE was as part of the Apollo Lunar Surface Experiments Package (ALSEP) on Apollo 17 within the Taurus–Littrow valley.[1][2]

Not to be confused with LACE, the Low-power Atmospheric Compensation Experiment.

LACE was a follow-on to the Cold Cathode Gauges that were flown on Apollo 14 and Apollo 15.[3] Those experiments proved the existence of a tenuous lunar atmosphere and determined the upper bounds on the lunar atmospheric density during the lunar day and night, but left its composition unknown.[3]

Results[edit]

The experiment positively identified that the tenuous lunar atmosphere consisted of helium, neon and argon. Helium concentrations matched predictions that assumed most of the lunar helium was derived from the solar winds and that helium does not freeze on the lunar surface.[3][4] Argon (36Ar and 40Ar) was detected. Since the increase of argon concentrations occurred just prior to dawn, it was shown that argon was likely a condensible gas. It was proposed that the argon freezes out and is adsorbed on the lunar surface at night. As night transitions into day, this frozen argon becomes mobile and migrates ahead of, and in tandem with, the sunrise terminator. This was colloquially referred to in the Apollo 17 preliminary science report as a "pre-dawn breeze".[4] Since the source of 40Ar was likely radioactive decay of potassium (40K), its presence detected by LACE provided evidence of a true native lunar gas.[3]


The total density of all the known gases detected by LACE matches that found by the Cold Cathode Gauges.[3]


Other species were identified including molecular hydrogen, chlorine, oxygen, hydrogen chloride, and carbon dioxide. Concentrations of these declined throughout the operation of the experiment and it is suspected these constituted instrument contaminants.[4] This conclusion was reached due to the fact that, unlike argon, the detection of these contaminants rose sharply contemporaneously with the local sunrise, rather than leading it.[4] Neon concentrations were 20 times lower than anticipated and the reason for this was not understood at the time.[4]

Instrument failure[edit]

During LACE's tenth lunar month of operation, the experiment developed a problem with the instrument's high-voltage section. The sweep high voltage dropped to zero on 17 October 1973 at 17:32 UTC. The normal 2900 volt output had reduced to several hundred volts, and the instrument could no longer operate. Numerous corrective measures were attempted, but none were successful.[2]