Apollo 15 Mission
Science Experiments - Cold Cathode Gauge
The Cold Cathode Gauge measured the abundance of gases in the lunar atmosphere.
The Moon has an atmosphere, but it is very tenuous. Gases in the lunar atmosphere are easily lost to space. Because of the Moon's low gravity, light atoms such as helium receive sufficient energy from solar heating that they escape in just a few hours. Heavier atoms take longer to escape, but are ultimately ionized by the Sun's ultraviolet radiation, after which they are carried away from the Moon by solar wind. This process takes a few months. Because of the rate at which atoms escape from the lunar atmosphere, there must be a continuous source of particles to maintain even a tenuous atmosphere. Sources for the lunar atmosphere include the capture of particles from solar wind and the material released from the impact of comets and meteorites. For some atoms, particularly helium-4 and argon-40, outgassing from the Moon's interior may also be a source.
The Cold Cathode Gauge Experiment measured the total pressure of the lunar atmosphere. Some of the electronics for this experiment were contained in the Suprathermal Ion Detector Experiment, and the two experiments were connected by a short cable. The Cold Cathode Gauge Experiment was deployed on Apollo 12, 14, and 15. The Apollo 12 instrument operated for only a brief time. The Apollo 14 and 15 instruments radioed data back to Earth from 1971 until 1975. The Cold Cathode Gauge did not measure the composition of the atmosphere. Compositional information was obtained by the Lunar Atmospheric Composition Experiment on Apollo 17.
The measured abundance of the lunar atmosphere is about 200,000 molecules per cubic centimeter at night. This is about 100 trillion (14 orders of magnitude) times less than in Earth's atmosphere. For most gases, the abundance is lower in the day than at night. Because of this very low abundance, the Moon's atmosphere is easily susceptible to contamination by human activities. The total mass of the Moon's atmosphere is estimated to be just 10,000 kilograms, which is comparable to the amount of gas released (primarily as rocket exhaust) by each Apollo landing. The Cold Cathode Gauge easily detected outgassing from the astronauts' space suits whenever they were near the instrument. The effects of landing-site contamination decay with time, so measurements were made for several years in order to estimate the abundance of the normal (uncontaminated) lunar atmosphere.