Apollo 16 Mission
Apollo 16 landed in the Cayley Plains in the central highlands of the Moon's nearside. The geology objectives for this mission were to study two geologic units, the Cayley Formation and the Descartes Formation. These two formations cover more than 10% of the Moon's nearside. Prior to the mission, they were thought to be volcanic plains that are comparable in age to the Imbrium impact basin. The Apollo 16 crew collected 731 individual rock and soil samples, including a deep drill core that included material from 2.2 meters below the Moon's surface, with a total mass of 96 kilograms. These samples provided the biggest scientific surprise of the entire Apollo program.
Although it was thought that this region of the Moon was covered by volcanic plains, this turned out not to be the case. Instead, virtually every rock collected on Apollo 16 was a breccia. Breccias are rocks that are composed of fragments of other, older rocks. Over its long history, the Moon has been bombarded by countless meteorites. These impacts have broken many rocks up into small fragments. Sometimes, the heat and pressure of these impacts fuses small rock fragments into new rocks, called breccias. Many such fragments are visible in the breccia shown above. In some cases, the rock fragments that form a breccia are themselves even older breccias. Such rocks have experienced complex histories involving multiple generations of impact events, and deciphering these histories is a great challenge.
The most likely source for all of the impact debris found at the Apollo 16 landing site is large impact basins, which can distribute ejecta over very large distances. This ejecta can churn up preexisting material when it impacts the surface at high speeds. Most of the samples collected on Apollo 16 probably represent churning of old rocks rather than the addition of new material from the distant basin. The Nectaris and Imbrium basins were probably important contributors to the geology of the Apollo 16 landing site. Nectaris is the nearest large impact basin, a distance of less than 200 kilometers from the basin rim to the landing site. Although Imbrium is much further away, with a distance of 1000 kilometers from the basin rim to the landing site, Imbrium was also a very large impact event, and ejecta from this impact can be found near the landing site. One interpretation is that the older Descartes Formation is related to Nectaris basin ejecta and that the slightly younger Cayley Formation is related to Imbrium basin ejecta.
The Nectaris Basin
Mare Imbrium was studied on Apollo 15 and its ejecta was studied on Apollo 14. As a result, Apollo 16 samples do not significantly alter our understanding of Imbrium basin geology. Mare Nectaris, on the other hand, was not studied on any other Apollo mission, so Apollo 16 samples are important contributors to our understanding of the history of the Nectaris region. A number of rocks were melted by the force of a large impact 3.92 billion years ago. This is believed to be the age of the Nectaris basin-forming impact.
A few tiny basalt samples were collected on Apollo 16. These basalts are rich in the elements titanium (as on Apollo 11) and aluminum. The basalts formed 3.79 billion years ago. The basalts were most likely hurled to this location by the impact that produced a large crater. The most likely source crater is Theophilus, which is 100 kilometers in diameter and 250 kilometers from the landing site. This would imply that these basalts are fragments of Mare Nectaris.
More Samples From the Magma Ocean
Very early in the Moon's history, much of its outer region was molten, a stage in lunar history known as the magma ocean. As the magma ocean cooled and solidified, a type of rock known as anorthosite which consists mostly of the mineral plagioclase floated to the surface. Small fragments of anorthosite were found beginning on Apollo 11, and a larger sample was found on Apollo 15. Two anorthosite samples collected on Apollo 16 are much larger than those found on any other Apollo mission. The one shown above formed between 4.44 and 4.51 billion years ago. It is one of the oldest rocks collected during the Apollo program and is nearly as old as the Moon itself (based on studies of meteorites, the solar system formed 4.56 billion years ago, and the Moon must have formed very soon thereafter).
Collecting Moon Rocks
This document describes the tools and procedures used by the Apollo astronauts to collect lunar samples.
The Lunar Sample Laboratory at the Johnson Space Center stores the lunar samples and distributes them to qualified researchers for study. This link connects to a website at the Johnson Space Center that provides a virtual tour of the Lunar Sample Laboratory.