Left: This microscopic view of the Apollo 17 orange soil 74220 is about 1 millimeter across. The orange particles are volcanic glass and the black particles are the mineral ilmenite. Photograph by Graham Ryder, Lunar and Planetary Institute.
Center: Apollo 17 norite 78236. This sample has a mass of 93 grams and is up to 7 centimeters across. NASA/Johnson Space Center photograph S73-15393.
Right: Apollo 17 troctolite 76535. This sample has a mass of 156 grams and is up to 5 centimeters across. NASA/Johnson Space Center photograph S73-19456.
Apollo 17 landed in the Taurus-Littrow Valley on the eastern edge of Mare Serenitatis. There were two main geology objectives for this site: to obtain samples of ancient rocks from the lunar highlands and to look for evidence of young volcanic activity on the valley floor. The Apollo 17 crew collected 741 individual rock and soil samples, including a deep drill core that included material from 3 meters below the lunar surface, with a total mass of 111 kilograms. These samples addressed both of the pre-mission objectives.
Mare Volcanism
Rocks from the floor of the Taurus-Littrow Valley are mostly mare basalts. Basalt consists primarily of the minerals plagioclase and pyroxene and is formed from molten lava. These basalts formed from material that melted at depths of at least 130 to 220 kilometers and then rose to the surface before solidifying. Most of the Apollo 17 mare basalts formed between 3.7 and 3.8 billion years ago. Like the Apollo 11 basalts, the Apollo 17 basalts generally contain large amounts of the element titanium. However, observations by the Clementine spacecraft indicate that the basalts in most of Mare Serenitatis have lower titanium abundances than is typically seen at the Apollo 17 landing site. A few rare Apollo 17 basalts have very low titanium abundances. Seismic and gravity observations indicate that the basalt layer is between 1.0 and 1.4 kilometers thick near the Apollo 17 landing site.
Mare basalts were emplaced as fluids that flowed easily across the Moon's surface. However, photographs taken from lunar orbit suggested that some explosive volcanic activity had also occurred in this region, and some geologists thought this activity might have occurred recently in lunar history. Shorty Crater was explored to determine if it was actually a volcanic vent. Orange and black volcanic glass (the famous "orange soil") was found near the rim of Shorty Crater and did form in an explosive volcanic eruption. On Earth, such eruptions are sometimes called fire fountains. However, the relationship between Shorty Crater and the volcanic glass is just coincidental. The glass formed 3.64 billion years ago from material that melted about 400 kilometers below the surface. Shorty Crater turns out to be an ordinary impact crater, and the lack of degradation of its features indicates that the crater is much younger than the glass. Volcanic glass was also found at the Apollo 15 landing site.
Lunar Highland Rocks
A variety of very old rocks were collected from the mountains to the north and south of the landing site. Some of these rocks, known as impact melts, were melted by the heat of a very large impact event. Studies of such rocks indicate that the impact that formed the Serenitatis basin occurred 3.89 billion years ago. Other rocks, including norite, troctolite, and dunite, formed even earlier in the Moon's history. Norite consists primarily of the minerals plagioclase and pyroxene. Troctolite consists primarily of plagioclase and olivine, but small amounts of pyroxene are also present. Dunite is nearly pure olivine. Many of these rocks originally formed in the lower half of the Moon's crust and were later brought to the Moon's surface by large meteorite impacts. These rocks formed between 4.2 and 4.5 billion years ago (the solar system formed about 4.56 billion years ago).
Tycho Crater
Tycho Crater is 85 kilometers in diameter and located in the southern highlands of the Moon's nearside, about 2000 kilometers from the Apollo 17 landing site. Tycho has an extremely fresh appearance and for that reason is believed to be one of the youngest large impact craters on the Moon. The impact that formed Tycho distributed material across much of the nearside of the Moon. A ray of material ejected from Tycho can be traced across the Apollo 17 landing site. Some Apollo 17 samples show evidence of being disrupted by a large impact about 100 million years ago, which may indicate when Tycho formed.
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