An impact across time
The bright, web-like streaks emanating from Tycho crater record a violent impact that occurred 108 million years ago. At that time, dinosaurs of the Cretaceous Period roamed the Earth. The impactor, likely an asteroid between 8 and 10 kilometers wide, struck the Moon's southern highlands and excavated the 85-kilometer-diameter crater we see today. The explosion launched vast quantities of pulverized rock, or ejecta, across the lunar surface at high velocity. This material settled into the long, linear patterns called crater rays.
These rays are composed of fresh, high-albedo (highly reflective) material from beneath the Moon's surface. Their brightness comes from the fact that they are geologically young and have not been exposed to the space environment for long. Over billions of years, the lunar surface is gradually darkened by a process called "space weathering," which involves constant bombardment by micrometeorites and charged particles from the solar wind. This process creates a darker, mature soil, or regolith. Tycho's rays are bright because they are a thin blanket of fresh ejecta sitting on top of this older, darker surface. a long ray stretches over 1,500 kilometers, reaching the Apollo 17 landing site in the Taurus-Littrow Valley. Samples of impact melt glass collected by the Apollo 17 astronauts were radiometrically dated to 108 million years, dating the Tycho impact event.
A pristine geological record
The rays of Tycho are so well-preserved because the Moon lacks the atmospheric and geological forces that erase such features on Earth. Without wind, water, or significant tectonic activity, the delicate patterns of ejecta remain almost unchanged. The crater itself is a complex crater, with terraced inner walls and a central peak that rises 2 kilometers from the crater floor. The crater floor is approximately 4.8 kilometers deep.
The best time to view the crater rays from Earth is during a full moon. At this phase, the Sun is directly overhead on the Moon, eliminating shadows and making the differences in surface brightness the clearest feature. While the 85-kilometer crater itself is too small to be resolved by the naked eye, the vast and bright ray system makes its location easy to spot as a brilliant point of light in the Moon's southern hemisphere. Early lunar mappers noted the feature, with 17th-century astronomer Pierre Gassendi referring to it as Umbilicus Lunaris—the Moon's belly button. Studying the composition of the rays shows scientists the makeup of the lunar crust at the impact site, giving a geological survey without having to land and drill.