Descartes Highlands

Lore & Background

The Descartes Highlands is characterized by an undulating landscape covered with old and some new, sharp-rimmed craters. Two major formations dominate the area: the Cayley Formation and the Descartes Formation, the latter composed primarily of highland plateau material possibly debris from the impact that formed Mare Nectaris. North Ray and South Ray craters revealed a layering sequence, possibly an overlap of the two formations.

Before Apollo 16 sampled the area, it was believed that volcanic material would be abundant based on visual analysis. This was later disproved after sample analysis revealed that many rocks are breccias composed of fragments from several lunar impacts, not volcanic in origin. Astronauts Young and Duke visited Sudbury, Ontario, Canada in July 1971 to examine shatter cones before the mission.

The Descartes Highlands was selected for Apollo 16 to sample the prominent Descartes and Cayley formations, which cover much of the near side of the Moon. Priority was also given to locating old highland material older than the Imbrium impact and to sampling North Ray and South Ray craters, where material from the formations had been naturally excavated.

Reader's Guide

The Descartes Highlands holds significance as the first Apollo landing site in the lunar highlands, shifting focus from mare sampling to highland geology. The mission disproved the pre-mission hypothesis that the area's formations were volcanic, instead revealing that the topography was formed by meteor impact and that the rocks are breccias. This corrected earlier interpretations based on visual analysis and provided key insights into the Moon's impact history. The sampling of the Cayley and Descartes formations, which cover much of the near side, helped establish a geologic timeline and composition for the highlands. The discovery of glass on crater floors, similar to that at the Taurus-Littrow site, added to understanding of impact processes. The Descartes Highlands thus represents a pivotal site in lunar science, where direct sampling resolved a major geological debate.

Did You Know?

• The Descartes Highlands is named after the nearby Descartes crater.
• Before Apollo 16, scientists believed the area was formed by viscous lavas, but samples later showed the rocks are breccias from impacts.
• Astronauts John Young and Charles Duke visited Sudbury, Ontario, Canada in July 1971 to study shatter cones before the mission.
• Some crater floors in the Descartes Highlands were covered with glass similar to that found at the Taurus-Littrow landing site on Apollo 17.

Geological Architecture of the Highlands

The Descartes Highlands presents a landscape shaped by deep time, dominated by two principal geological formations: the Cayley Formation and the Descartes Formation. The latter is thought to consist largely of highland plateau material, potentially representing debris flung outward during the cataclysmic impact that created Mare Nectaris. Together, these two formations blanket a substantial portion of the Moon's near side, making them central to understanding lunar crustal history. The terrain itself is far from flat. Instead, it undulates across a surface peppered with craters of varying ages—some ancient and softened, others bearing sharp, well-defined rims that speak to more recent strikes. Two particularly notable features, North Ray and South Ray craters, sit in the immediate vicinity of the landing area. When examined, they exposed a layered sequence of material that may represent an overlap between the Cayley and Descartes formations, offering a natural cross-section of the region's stratigraphy. This layering provided a window into how the two formations interact and which predates the other, a question that remained unresolved until astronauts walked the surface.

From Volcanic Myth to Impact Reality

Before the Apollo 16 crew set foot in the Descartes Highlands, the prevailing scientific consensus painted a very different picture of the terrain. Visual analysis of the region's features from orbit led researchers to expect an abundance of volcanic material. The formations were assumed to have been sculpted by lavas considerably more viscous than those responsible for the smooth Lunar mare basins. This volcanic narrative seemed reasonable given the rugged, varied topography on display. The samples told a different story entirely. Laboratory analysis revealed that many of the rocks collected were not volcanic in origin at all. Instead, they were breccias—aggregates of fragments hurled and welded together by multiple separate impact events. The region's dramatic topography, it turned out, was the product of meteor strikes rather than lava flows. The connection to impact geology was reinforced before the mission itself. In July 1971, astronauts John Young and Charles Duke traveled to Sudbury, Ontario, Canada, to study shatter cones—distinctive rock formations produced by the shock waves of a large meteor impact. That terrestrial analog helped them recognize the same processes at work beneath their boots on the Moon.

Choosing the Highlands: Site Selection for Apollo 16

Every Apollo landing mission before the sixteenth had targeted the Lunar mare—the dark, basalt plains that cover much of the Moon's visible face. By the time planners were designing the next mission, a clear gap remained in humanity's geological understanding: no crew had yet sampled the ancient highland terrain. The objective was to close that gap, and two candidate sites rose to the top of the list: the Descartes Highlands and the crater Alphonsus. Descartes ultimately won the selection. The rationale was straightforward and ambitious. The area concentrates both the Descartes and Cayley formations, which together drape a large fraction of the near side, making them essential to any comprehensive model of lunar crust. Beyond simply collecting rock, the mission carried the priority of recovering highland material that predated the Imbrium impact, a find that would anchor the geologic timeline of the Moon. North Ray and South Ray craters earned special attention in the site plan. The impacts that carved them had naturally excavated and exposed material from the underlying formations, effectively providing astronauts with pre-dug sample pits right in the landing zone.

Glass, Craters, and the Crew's Walk

Once John Young and Charles Duke stepped onto the Descartes Highlands in early 1972, the landscape confirmed and then complicated expectations. The undulating terrain was studded with craters of all ages, their sharp rims catching the harsh sunlight. But it was what lay in the floors of some of these craters that caught the crew's eye: a layer of glass spread across the surface, strikingly similar to material later encountered at the Taurus-Littrow site during Apollo 17. Commander Young described the arrangement of the glass as resembling dried mud—cracked, patterned, and oddly terrestrial in its appearance. This observation added a new dimension to the region's geology, suggesting processes of melting and rapid cooling that tied the highlands to the broader impact history of the Moon. The Ray craters, both prominent features in the immediate landing area, became focal points of the surface operations. North Ray was sampled directly by the crew, yielding material from the layered sequence that may represent the overlap of the Cayley and Descartes formations. These samples, combined with the glass observations and the breccia rocks, rewrote the geological narrative of the highlands from one of volcanic construction to one of layered impact debris.

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