Unpublished images from NASA's Lunar Reconnaissance Orbiter Camera reveal that the Moon's ancient crust was torn apart billions of years ago, creating enormous cracks known as grabens. These grabens form a broken ring around a lunar sea called Mare Humorum, which is located on the Moon's near side. The research, led by Thomas Watters, a planetary scientist at the Smithsonian's National Air and Space Museum, focuses on how tectonic scars on airless worlds, like the Moon, reflect the slow stretching and squeezing of their crust. Mare Humorum, filled with dark basalt, is a heavy load on the crust, causing the basin floor to sag downward and inward. This stress, combined with the cooling and contracting lava, led to the formation of the graben system, which includes three main valleys: Rimae Hippalus. These valleys trace a broken ring around the basin's rim, with each valley marking a different stage in the crust's response to the sinking lava sea. The analysis of these grabens reveals that they are the largest tensional linear structures on the Moon, clustering along the margins of mare basins. A dedicated mapping campaign identified over 1800 separate graben segments on the Moon's nearside, with many running for hundreds of kilometers. Most large grabens formed between 3.7 and 3.4 billion years ago, with activity peaking around 3.6 billion years ago. This global extension caused the Moon's radius to grow by approximately 400 feet. However, smaller grabens formed much more recently, less than 50 million years ago, indicating that the Moon's crust continues to adjust over deep time. The graben system around Mare Humorum, including Rimae Hippalus, showcases a complex interplay between global contraction and local extension. The Moon is slowly cooling and shrinking, but in certain regions, the crust is still opening along old weak zones, echoing stresses from when basins like Humorum were loaded with lava. Lunar geologists use cross-cutting relationships to determine the order of these events, helping them build a timeline of the Moon's volcanic era and the formation of its surface features. Understanding the Moon's grabens is crucial for mission planning, as it helps identify safe landing and construction sites, and for scientific research, as it aids in deciding where to place seismometers and deep drills. Future orbiters and landers will continue to refine the stress map, revealing the Moon's ancient history and ongoing changes.
