Taurus–Littrow

• 1. Introduction

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  Taurus-Littrow is a lunar valley on the near side of the Moon, and in December 1972 it became the last place human beings have ever stood on another world. The Apollo 17 astronauts Eugene Cernan and Harrison Schmitt touched down there amid mountains formed between 3.8 and 3.9 billion years ago, when a massive impactor struck the Moon and threw rock outward and upward to create the rim of the Serenitatis basin. The valley they landed in is deeper than the Grand Canyon. The samples they carried home included a rock 4.25 billion years old that would later challenge everything scientists thought they knew about the Moon's inner life. What made this particular valley the final destination of the Apollo program? What secrets did it hold that no other candidate site could offer? And what do the orange soil, the light mantle, and a stone called Troctolite 76535 tell us about a world that formed before life existed on Earth?

• 2. A Valley Forged by Impact

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  Between 3.8 and 3.9 billion years ago, a large object struck the Moon with enough force to push rock outward and upward in a ring of mountains that still defines the southeastern edge of Mare Serenitatis today. That collision formed the Serenitatis basin, and Taurus-Littrow sits along its edge, flanked by the Taurus mountain range. Several million years after the basin formed, lavas began welling up from the Moon's interior, flooding the low-lying terrain and ultimately creating what we now see from Earth as the dark, smooth expanse of Mare Serenitatis. Those lava flows were sometimes accompanied by lava fountains, which scattered tiny glass beads across the surrounding landscape. Most of those beads are dark in color, which is why Mare Serenitatis appears so dark when viewed from Earth. A smaller fraction of the glass beads, however, settled into what became the orange soil that the Apollo 17 crew discovered at Shorty crater, one of the mission's more striking finds. The valley's name was proposed by the Apollo 17 crew themselves, drawing on the Taurus mountains to the north and Littrow crater to the south. The International Astronomical Union formally approved the name in 1973.

• 3. The Shape of the Place

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  The valley floor is, by general observation from Apollo 17, a gently rolling plain elongated along an axis that roughly points toward the center of Mare Serenitatis. On either side rise the North and South massifs, their combined height giving the valley a depth that exceeds the Grand Canyon. A scarp cuts across the valley floor to the west, rising about 2 km above it. To the east, sculptured hills and an East massif complete the boundary. The North and South massifs funnel into the main outlet of the valley, which opens onto Mare Serenitatis, that gap partially blocked by a formation called Family Mountain. Bear Mountain lies along the South Massif. It was named after a mountain near Silver City, New Mexico, the hometown of astronaut Harrison Schmitt, who was a geologist by training and the only professional scientist to walk on the Moon. Boulders of varying sizes are scattered across the valley floor. At the ALSEP lunar experiment deployment area, west of the landing site, those boulders average roughly four meters in size and are more densely concentrated than elsewhere in the valley.

• 4. Tycho's Fingerprints

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  Tycho crater is hundreds of kilometers away, yet its influence reaches deep into Taurus-Littrow. Analysis of secondary crater clusters from the Tycho impact, which occurred somewhere between 70 and 95 million years ago, reveals that most of them bear a distinctive 'birdsfoot' pattern in their downrange ejecta blankets. The central crater cluster on the Taurus-Littrow valley floor shows exactly that pattern, pointing toward Tycho. The unconsolidated regolith layer on the valley floor is roughly 14 meters thick and contains ejecta from many impacts, but Tycho's contribution stands out. Because of that, Apollo 17 was able to retrieve sample material from the Tycho impact without traveling to the crater itself. The light mantle, a deposit of lightly colored material extending roughly six kilometres from the South massif across the valley floor, adds another thread to this story. Its debris pattern points directly toward the South massif, supporting the hypothesis that it originated as an avalanche from the massif's northern slope, perhaps triggered by secondary impacts from Tycho. Craters located close to the South massif and as wide as 75 meters do not appear to penetrate through the light mantle at all, while craters farther away do reach the darker material beneath. The age of the light mantle is estimated at roughly 70-95 million years, consistent with the Tycho impact timeline.

• 5. What the Rock Remembers

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  Among all the samples Apollo 17 returned, one stands apart. Troctolite 76535 is a coarse-grained rock composed primarily of olivine and plagioclase, and it is 4.25 billion years old. Scientists have called it the most interesting sample ever returned from the Moon. Thermochronological analysis of the sample was undertaken to determine whether the Moon once generated a core dynamo, meaning an active, churning metallic core that produced a magnetic field. The magnetism preserved in Troctolite 76535 appeared to support exactly that conclusion. Further analysis by Garrick-Bethell and colleagues revealed that the magnetism in the sample is nearly unidirectional, consistent with alignment to a larger, sustained magnetic field rather than a singular shock event. The broader composition of the valley fills in the geological picture. Seismic studies conducted during Apollo 17 suggest the basalt layer below the valley floor is greater than 1,400 meters thick. The basalt samples recovered from near the Lunar Module are primarily vesicular and coarse-grained, composed mainly of plagioclase but also containing clinopyroxene and other minerals. Above the basalt sits the 14-meter unconsolidated regolith layer, a mixed record of volcanic activity and repeated impacts stretching back billions of years.

• 6. Why This Valley, Why Now

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  Apollo 17 was the final lunar mission of the Apollo program, and planners approached the site selection with an awareness that there would be no second chance. Landing sites that had been considered and rejected for earlier missions received fresh scrutiny. Among the candidates were Tycho crater, Copernicus crater, and Tsiolkovskiy crater on the far side of the Moon. Tycho was ruled out because its rough terrain was judged to exceed mission safety constraints. A far-side landing at Tsiolkovskiy would have required communications satellites to maintain contact between the crew and mission control, adding cost and logistical complexity the program could not absorb. Copernicus lost its appeal because data from Apollo 12 had already allowed scientists to gauge the timing and history of that impact. Taurus-Littrow offered something no other finalist could: the chance to sample ancient highland material and geologically young volcanic material at the same site. The highland samples would come in the form of ejecta from the Tycho impact. The young volcanic material was tied to crater-like features on the valley floor that planners believed might have a volcanic origin. That dual objective, one site, two radically different geological epochs, made Taurus-Littrow the choice that ended the Apollo program's scientific ambitions on the highest possible note.

• 7. What Comes Next

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  NASA issued guidelines in 2011 for the protection of Apollo lunar landing sites, recommending that future missions keep a respectful distance from the aging hardware that Cernan and Schmitt left behind in Taurus-Littrow. In 2019, aerospace company PTScientists announced that its ALINA lunar lander was planned to set down 3 to 5 km from the Apollo 17 Lunar Module within the valley, initially targeting early 2020. That date was later postponed to an indefinite time no earlier than the second half of 2021. The craters dotting the valley carry names that reflect the breadth of human curiosity: Charlotte Bronte, Sherlock Holmes, Cochise, Fridtjof Nansen, and a crater called Shorty, named after a fictional character from the novel Trout Fishing in America. One of those craters, Nansen-Apollo, measures 0.86 km in diameter, the largest named feature in the valley. Meanwhile, Troctolite 76535 remains in active scientific use, still yielding data on questions about the Moon's earliest history that no Earth-based instrument could have uncovered on its own.