Apollo 16: The View from Lunar Orbit
The science return from the Apollo 15, 16, and 17 missions was enhanced by the inclusion of a set of orbital science instruments that were carried in the Scientific Instrument Module of the Service Module. On all three missions, this included high resolution photography using the Metric Camera and Panoramic Camera. The Metric Camera obtained photos that were typically 165 km on a side with a horizontal resolution of 20 meters. The Panoramic Camera obtained images in narrow strips, 20 km wide in the direction of orbital motion and 320 km long across the ground track, with a resolution of 1-2 meters directly below the spacecraft. Photos from both cameras were taken with substantial overlap to produce stereo images, which allowed lunar heights to be determined with an accuracy of better than 10 meters vertically. The film for these cameras was retrieved from the Service Module and stored in the Command Module during a spacewalk by the Command Module Pilot while on the return voyage from the Moon back to Earth. During Apollo 16, the Metric Camera was used on 16 orbits and during the early hours of the return to Earth, obtaining 2491 usable photographs. The Panoramic Camera was used on eight orbits and during the early hours of the return to Earth, obtaining 1586 usable photographs. This covered virtually all of the Moon visible in sunlight to the Apollo 16 crew.
This photograph shows King Crater on the Moon's farside. King Crater is 77 kilometers in diameter and more than 5 kilometers deep. It is the freshest crater in this size range on the farside of the Moon. Its overall form is generally typical of large lunar craters. The floor of the crater is relatively flat in places and has numerous small hummocks in other places. The central peak has a complex, Y-shaped form and is larger than normal for a crater of this size. The inside of the crater rim contains a series of terraces and slump blocks. Just north of the rim of King Crater, there is a dark, flat patch of ground that formed where molten material ponded in an old, degraded impact crater. This material might have been molten by the impact that formed King Crater; alternatively, it has also been suggested that it formed volcanically. The boom from the Gamma-ray Spectrometer is visible on the right side of the photo.
Ejecta from large impact basins can affect the surrounding terrain for hundreds of kilometers. This oblique photograph was taken looking north near the equator on the central portion of the Moon's nearside, about 650 kilometers from the rim of the Imbrium Basin. The many ridges and grooves in this photograph all point back toward the Imbrium Basin and give this region a scoured appearance. This type of terrain, termed "Imbrium sculpture," formed when material ejected during the formation of the Imbrium Basin impacted in this region. The smooth region in the upper portion of the photograph is Sinus Medii, a small mare unit in the center of the Moon's nearside.
This oblique photograph was taken looking north over the central part of the Moon's farside. This region is virtually saturated with craters – forming a new crater is likely to destroy one or more existing craters. This type of intensely cratered surface is typical of most of the Moon's farside and of those parts of the nearside that have not been flooded by mare basalt.
The Lunar Farside
This photo of the Moon's heavily cratered far side was obtained at the beginning of Apollo 16's return voyage to Earth. A small portion of the Moon's nearside is visible on the upper left side of the photo, including Mare Smythii and Mare Marginis, which are covered by dark basalt. In contrast to the nearside, the Moon’s farside has very little mare material.
On Apollo 15 and 16, there were also several orbital experiments, the Gamma-ray Spectrometer, the X-ray Fluorescence Spectrometer, and the Alpha Particle Spectrometer, that were designed to measure the chemical composition of the lunar rocks. The Gamma-ray Spectrometer mapped the abundance of iron, titanium, and thorium at the Moon’s surface. The X-ray Fluorescence Spectrometer mapped the abundance of magnesium, aluminum, and silicon.
These experiments allowed the detailed measurements of the Apollo samples to be generalized to a larger region of the Moon. For example, the mare are typically high in iron and low in aluminum, which is evidence that they are composed of basalt even in places that were not directly sampled by Apollo missions. Basalts from Apollo 11 and Apollo 17 are high in titanium due to the mineral ilmenite; the gamma-ray data shows that high Ti basalts also occur in western Oceanus Procellarum. Samples from the western part of the lunar nearside obtained by Apollo 12, 14, and 15 include a type of basalt known as KREEP. KREEP is a chemical acronym, denoting rocks that are rich in potassium (denoted chemically as K), rare earth elements (REE), and phosphorous (P). Thorium behaves chemically as a rare earth element, and the gamma-ray data maps out regions where KREEP may be abundant. Lunar highland rocks sampled by Apollo are anorthosites; they have abundant plagioclase, which is rich in aluminum. The orbital data shows that such rocks dominate the lunar highlands, even in regions that were not sampled by Apollo. Both the Gamma Ray Spectrometer and the X-ray Fluorescence Spectrometer were also used to make astronomical observations during the return voyage to Earth, which were coordinated with observations from several large telescopes on Earth.
The Alpha Particle Spectrometer mapped regions of the Moon with high abundances of uranium 238 (or high outgassing rates) by detecting concentrations of radon 222 and polonium 210 in lunar orbit. 238U undergoes radioactive decay to other elements. Two steps in this decay chain involve 222Ra and 210Po, which are both gases. The Alpha Particle Spectrometer detected these radioactive decays. On Apollo 16, a strong local concentration of 210Po was detected in Mare Fecunditatis.