Apollo 15 Lunar Samples
The overall set of lunar samples collected during the Apollo program can be classified into three major rock types, basalts, breccias, and lunar highland rocks. Apollo 15 collected important examples of all three rock types. Apollo 15 landed in Mare Imbrium at the base of the Apennine Mountains, which forms part of the rim of the Imbrium impact basin. There were two main geology objectives for this site: to collect rocks from the Apennine mountains and to study Hadley Rille, a volcanic channel near the landing site. The Apollo 15 crew collected 370 individual rock and soil samples, including a deep drill core with material from 2.4 meters below the lunar surface, with a total mass of 77 kilograms.
The surface rocks in Mare Imbrium are basalt, as was the case for both the Apollo 11 and Apollo 12 samples. Basalts are dark-colored rocks that solidified from molten lava. They are rich in iron and consist primarily of the minerals pyroxene and plagioclase with some olivine. The basalts in Mare Imbrium formed about 3.3 billion years ago, 600 million years after the Imbrium impact basin formed. Like the Apollo 12 basalts from Oceanus Procellarum, the Imbrium basalts have low abundances of the element titanium. A few of these basalts include numerous holes, which geologists call vesicles. The vesicles formed as gas bubbles when the lava approached the Moon's surface. The gas later escaped, but it was most likely a combination of carbon dioxide and carbon monoxide. Numerous basalt samples without vesicles were also found on Apollo 15. Vesicles are also found in some basalts on Earth.
Hadley Rille is 135 kilometers long. Near the Apollo 15 landing site, the rille is 1.5 kilometers wide and 300 meters deep. Observations by the crew indicate that the rille formed as a volcanic feature, probably originating as a lava tube whose roof later collapsed. Volcanic channels and lava tubes are also known to occur at many basaltic volcanos on Earth, such as in Hawaii, but Hadley Rille is much larger than terrestrial volcanic channels.
Mare basalts were emplaced as fluids that flowed easily across the Moon's surface. Another type of volcanic material found at the Apollo 15 landing site is pyroclastic glass. In order for lava to form a glass rather than to crystallize into mineral grains, it must cool very quickly. This sort of rapid cooling can occur if an explosive volcanic eruption hurls material high above the Moon's surface and the material falls back down in the form of small beads. This type of explosive volcanic eruption is also known to occur on Earth and is called a pyroclastic eruption or fire fountain by geologists. Several types of volcanic glass occur in the Apollo 15 samples, the most common and famous of which is the green glass. This glass is very rich in the element magnesium, which causes the green color. Studies of the green glass indicate that it originated at about 400 kilometers below the Moon's surface. Pyroclastic glass was also collected on Apollo 17.
Left: Apollo 15 basalt 15555 has a mass of 9.6 kilograms and is one of the largest individual lunar samples collected during the Apollo program.
Right: Apollo 15 basalt 15016, informally known as the Seatbelt Basalt, has numerous vesicles. This sample has a mass of 923 grams and is up to 13 centimeters across.
The Apennine Front: Rocks from the Lunar Highlands
The Imbrium basin formed from the impact of a large asteroid or comet with the Moon. The Apennine Mountains were pushed up by the Imbrium basin impact. It was hoped that samples collected from these mountains would include ancient rocks that originated deep in the Moon's crust. The shock of the Imbrium impact melted many rocks. Studies of such rocks indicate that this impact occurred about 3.93 billion years ago.
The astronauts were also successful in returning material that was even older than the Imbrium impact. In particular, at Spur Crater on Mount Hadley Delta, they collected an anorthosite, a rock that is composed almost entirely of the mineral plagioclase. This rock is now known as "Genesis Rock." Early in the Moon's history, there was a stage known as the magma ocean in which the Moon's outer layers were molten. As the magma ocean cooled and solidified, the plagioclase-rich anorthosite floated on the surface like icebergs in the Earth's oceans. The existence of the magma ocean phase was first recognized from small fragments in the Apollo 11 samples, but Apollo 15's Genesis Rock is important because it is much larger than any previous sample of lunar anorthosite. Apollo 16 later collected anorthosite samples that are both larger and older than Genesis Rock. Studies of Genesis Rock indicate an age of about 4.1 billion years. However, it is believed that this represents a time when the rock experienced a metamorphic alteration and that the rock is actually older than 4.1 billion years. Norite and anorthositic norite samples, also collected at Spur Crater, are composed primarily of plagioclase and pyroxene and are about 4.5 billion years old, virtually as old as the Moon itself. They likely formed in the deeper part of the magma ocean and were brought to the lunar surface by the Imbrium basin-forming impact.
Left: Apollo 15 sample 15415, informally known as Genesis Rock, is an anorthosite with a mass of 269 grams.
Right: Apollo 15 sample 15445 is a breccia with clasts of norite (light color) and impact melt (dark color). This sample has a mass of 287 grams and is about 6 centimeters across.
- Collecting Moon Rocks 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.
- The Lunar Sample Compendium at the Johnson Space Center summarizes the scientific findings of studies of the Apollo lunar samples.