Apollo 15: The View from Lunar Orbit
The Scientific Instrument Module on the Apollo 15 Service Module. The Metric and Panoramic Cameras, along with the Gamma-ray Spectrometer, X-ray Fluorescence Spectrometer, and Alpha Particle Spectrometer were located there.
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 (SIM) 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. On Apollo 15, the Metric Camera was used on 18 orbits and the early hours of the return to Earth, obtaining 2240 usable photographs. The Panoramic Camera was used on 11 orbits and the early hours of the return to Earth, obtaining 1529 usable photographs. This covered virtually all of the Moon that was visible in sunlight to the Apollo 15 crew.
The Apollo 15 Landing Site
This photograph shows part of the Apennine Mountains, which form part of the rim of the Imbrium impact basin. In this region, the Apennines are more than 4 kilometers higher than the adjacent mare plains. The sinuous feature running roughly vertically through the center of the image is Hadley Rille, which is a volcanic channel. The Apollo 15 mission landed near the center of the photo on the flat mare plains to the right (east) of the rille and explored both the mountains and the rille at close range. The region shown is about 150 km across.
Mare Imbrium Lava Flows
This oblique photograph looks north across the southern portion of Mare Imbrium. The low sun angle and long shadows accentuate details of the surface structure. The surface in this region is mare basalt. A good example of a lava flow crosses the image from center left to upper right. The prominent ridges running from upper left to lower right are wrinkle ridges, formed when the mare surface sagged under the weight of several kilometers of basalt. Similar ridges are seen in other mare regions, such as Mare Serenitatis. The prominent peak in the lower left is Mt. Lahire, which is 1.7 km high.
The Aristarchus Plateau
This oblique photograph looks south across the Aristarchus Plateau, which is 1.5 km higher than the surrounding mare basalt plains. There are two prominent impact craters in the photograph. On the left is Aristarchus, 40 km in diameter, and on the right is Herodotus, 35 km in diameter. In the center of the photograph, between the two craters, is a feature known as the Cobra’s Head. Beginning at the Cobra’s Head, the sinuous valley that snakes its way to the right is Schröter’s Valley. Schröter’s Valley, which is typically 8-10 km wide and more than 150 km long, most likely formed when the Aristarchus Plateau was uplifted nearly 4 billion years ago. Within Schröter’s Valley is a narrow sinuous rille, which formed as a volcanic channel similar to Hadley Rille.
Tsiolkovsky Crater on the lunar farside is about 180 km in diameter. The view shown here shows the floor of the crater and portions of the crater rim. Unlike most large craters, the floor of Tsiolkovsky is covered with mare basalt and is quite smooth. This is a rare example of mare material on the Moon’s farside. Although Tsiolkovsky, like most large impact craters, is relatively circular, the mare-covered region is distinctly non-circular. The complex terracing and slump blocks on the inner wall of the crater rim are visible in the bottom of the photo and typical of large craters. The prominent central peak is also typical of large craters.
On Apollos 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 Apollos 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.
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 15, an enhanced level of 222Ra was detected near Aristarchus crater and enhanced 210Po was detected between Mare Crisium and Van de Graaf crater on the farside.