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.