In April 2020, the United States Geological Survey (USGS), in collaboration with NASA and the Lunar Planetary Institute, released the first ever comprehensive geologic map of the entire Moon. This high-resolution map, called the “Unified Geologic Map of the Moon”, will serve as the definitive blueprint of the Moon’s 4.5 billion-year history for future scientific research and exploration missions.
The map is a synthesis of six Apollo-era regional geologic maps from the 1960s and 70s, updated and supplemented with data from recent satellite missions like the Lunar Reconnaissance Orbiter (LRO). It shows the Moon’s surface geology in incredible detail at a scale of 1:5,000,000, or about 1 mm on the map equals 5 km on the lunar surface.
Importance of Lunar Geologic Mapping
Detailed geologic maps of planetary bodies like the Moon are crucial tools for both scientific research and planning future exploration missions. They provide the “big picture” context needed to understand a planet’s origin, evolution, and geologic processes.
For the Moon specifically, geologic maps help locate potential resources like water ice, identify safe landing sites, and plan traverses for both robotic and future human missions. The Apollo astronauts relied heavily on early geologic maps of their landing sites for mission planning and real-time decision making on the lunar surface.
Creating planetary geologic maps is a unique challenge compared to terrestrial mapping. With no ability to make direct field observations, planetary mappers must rely entirely on orbital images and remote sensing data to interpret the geologic units and stratigraphy (rock layers) of an extraterrestrial surface. The nature of impact cratering also makes deciphering the geologic history of planetary surfaces difficult, as each new impact event disrupts and reworks the existing surface.
Unifying Apollo-Era Maps with Modern Data
To create the new unified geologic map, the USGS team led by geologist Corey Fortezzo had to first reconcile the patchwork of existing maps from the Apollo era. The six 1:5,000,000 scale regional maps, created in the 1970s based on analysis of Apollo surface photography and early orbital images, covered most of the Moon’s near side and a small portion of the far side.
However, these maps had inconsistencies in their approach to describing and naming geologic units. The first step was to align the old maps to a modern coordinate system and redraft them while preserving the original unit boundaries and descriptions as much as possible.
The team then supplemented and revised the maps with modern remote sensing datasets, primarily from the Lunar Reconnaissance Orbiter Camera (LROC) and Lunar Orbiter Laser Altimeter (LOLA) instruments aboard LRO. High resolution images from LROC revealed details of the lunar surface down to scales of meters, while LOLA provided precise topographic data.
Other key datasets included gravity measurements from NASA’s GRAIL mission which mapped the Moon’s interior structure, and mineralogical data from instruments like the Moon Mineralogy Mapper (M3) which flew on India’s Chandrayaan-1 spacecraft. Together, these modern datasets enabled mappers to characterize geologic units with much greater precision than the Apollo-era maps.
Standardizing the Lunar Stratigraphy
In addition to spatially aligning the maps, the USGS team also had to address inconsistencies in how geologic units were defined and described between the different Apollo-era maps. Over the decades of lunar mapping, a variety of schemes were used to classify units based on their inferred origin (volcanic, impact, etc.), relative age, and other characteristics.
To resolve this, the team developed a new, consistent classification scheme and unit descriptions spanning the entire Moon. This required careful analysis to correlate units across the different maps based on their characteristics and stratigraphic relationships.
The resulting unified stratigraphy organizes the Moon’s geologic history into 43 distinct units in 5 broad categories:
- Copernican craters and impact materials (<1.1 billion years old)
- Eratosthenian craters and impact materials (1.1-3.2 billion years old)
- Imbrian craters, impact materials, and volcanic plains (3.2-3.9 billion years old)
- Nectarian craters and impact materials (3.9-4.1 billion years old)
- Pre-Nectarian craters and impact materials (>4.1 billion years old)
This establishes a global framework for the relative ages and origins of different regions of the lunar surface. For example, the prominent bright rays extending from Copernicus crater immediately identify it as one of the youngest lunar features, while the dark volcanic plains of Oceanus Procellarum are classified as ancient Imbrian-aged units.
New Insights into Lunar Geology
In addition to its practical use for mission planning, the unified geologic map also provides new scientific insights into the Moon’s complex and dynamic geologic history. Some key findings include:
Evidence for recent volcanic activity: Analysis of small volcanic deposits called “irregular mare patches” suggests the Moon may have been volcanically active as recently as 100 million years ago, far later than the 1-2 billion years ago end of volcanism previously assumed.
Improved mapping of tectonic features: The map shows the distribution and orientations of tectonic faults and folds in much greater detail than previous maps. This sheds light on the history of global contraction and expansion that shaped the Moon’s crust.
Refined crater counts and ages: Detailed counts of impact craters of different sizes and degradation states were used to refine the age boundaries between the major geologic periods. For example, the number of 1 km or larger craters was used to define the base of the Copernican period as terrain younger than 1.1 billion years.
Identification of coldspot volcanoes: The map reveals several enormous volcanoes, not associated with the familiar dark mare plains, which may represent a distinct type of intrusive magmatism. The irregular shapes and locations of these “coldspot” volcanoes may reflect eruption of magma through unusually thick crust.
Characterization of the Procellarum KREEP Terrane (PKT): The unique composition of the PKT region, enriched in elements like potassium (K), rare earth elements (REE), and phosphorus (P), is now mapped in detail. Understanding the PKT provides clues to the Moon’s magmatic evolution and asymmetry between the near and far sides.
A Roadmap for Future Exploration
The unified geologic map is not just an academic exercise – it will serve as a key reference for planning the next generation of lunar missions. Both robotic precursor missions and the crewed Artemis missions will rely on the map to identify targets of high scientific interest and resources needed to support a sustainable presence on the Moon.
Some priority targets revealed by the new map include:
- Young coldspot volcanoes as locations to sample mantle materials
- Polar craters that may harbor water ice deposits
- Silicic volcanic domes as sources of useful minerals
- Magnetic anomalies and swirls which may represent remnants of an ancient lunar dynamo
- The South Pole-Aitken basin, the largest and oldest impact structure, as a window into the lunar interior
The map will also inform decisions about landing site safety, accessibility, and trafficability based on detailed topography, roughness, and crater and boulder distributions. Exploration traverses can be planned to visit multiple geologic units and address key scientific objectives.
As a publicly available resource, the unified geologic map enables lunar scientists around the world to share a common reference and language for discussing the Moon’s history and planning investigations. It will also serve as an important educational tool for students and the public to understand Earth’s nearest neighbor.
Summary
The USGS’s “Unified Geologic Map of the Moon” represents a new milestone in lunar science and exploration. By combining the pioneering work of Apollo-era geologic mappers with the wealth of modern remote sensing data, the map reveals the Moon’s 4.5 billion year history in unprecedented detail.
As NASA and other space agencies prepare to return humans to the Moon for the first time in over 50 years, this new map will be an invaluable guide. From choosing landing sites to planning exploration traverses to identifying resources, the map provides the geologic context needed for safe and successful missions.
But beyond its practical applications, the map is also a scientific triumph that sheds new light on the complex and dynamic evolution of Earth’s only natural satellite. It reminds us that the Moon, though familiar in our skies, still holds countless secrets waiting to be discovered. As the map sparks new questions and debate within the planetary science community, it will help set the course for a new age of lunar exploration and discovery in the years to come.
