Apollo 17 Mission
Science Experiments - Lunar Neutron Probe
The lunar regolith, or the uppermost few meters of the Moon, consists of soil and highly fragmented rocks formed by repeated impacts of meteoroids with the Moon's surface. These impacts eject material from the craters that are formed, and the ejecta are usually deposited nearby. The overall effect of many such impacts is a continual overturn of the regolith, which keeps it well mixed. This can be thought of as a sort of gardening, analogous to the mixing of soil performed by a farmer's plow. The Lunar Neutron Probe Experiment was performed on Apollo 17 to determine the rate at which the lunar regolith is overturned or mixed. It consisted of a 2-meter-long rod with several detectors that was inserted into the hole left from drilling a deep core sample. These detectors measured the number of neutrons penetrating to different depths in the regolith. The Neutron Probe was deployed on the first EVA and retrieved at the end of the third EVA after being exposed to the lunar regolith for 49 hours. It was then returned to Earth for analysis.
When cosmic-ray protons strike the lunar surface, nuclear reactions between the protons and atoms in the soil sometimes create neutrons, which may penetrate a short distance into the regolith. These neutrons are ultimately absorbed by other atoms, and the resulting nuclear reactions sometimes lead to the formation of radioactive isotopes, which later break down to form more stable isotopes. These radioactive isotopes provide a way to measure the rate at which the regolith is mixed. Because neutrons only penetrate a short distance into the regolith, finding radioactive elements at deeper depths means that material has been mixed to that depth in less time than it takes for the radioactive isotopes to break down. Measuring the regolith's mixing rate requires knowing three things: (1) the current abundance of radioactive elements, which is measured in core samples returned by the astronauts to Earth; (2) the rate at which radioactive elements decay, which is known from laboratory studies on Earth; and (3) the rate at which radioactive elements are created by neutron bombardment, which was determined from Lunar Neutron Probe data.
The degree of mixing in the regolith depends on the size of an impacting body, with larger impacts mixing material to greater depths. Small impacts are much more common than larger impacts, and so mixing to small depths occurs more often than mixing to large depths. Current estimates are that mixing to a depth of 1 centimeter occurs on average every million years, while mixing to a depth of 1 meter occurs only about once every billion years. These estimates are based on measurements of radioactive element abundances in lunar core tubes as well as on a variety of other chemical measurements of these cores.