Moon's bright streaks caused by space weathering

Sept. 11 (UPI) -- The moon's bright streaks, or crater rays, are caused by a combination of space weathering and impact ejecta.

Planetary scientists at Purdue University published a physical explanation of the phenomena this week in the journal Icarus.

"Rays of larger craters are created when primary crater ejecta excavate bright, unweathered material from below the dark lunar soil," researchers wrote in their paper on the subject.

The new research provides the first mathematical characterization of the relationship between impact craters and the length of crater rays.

Analysis of several dozen craters and their crater rays revealed a predictive relationship between radii and ray length among large craters. But scientists found smaller craters yielded unexpectedly long crater rays.

The anomaly suggests the streaks can be formed two different ways. The predominant way involves evacuation of weathered surface material by ejecta.

"The intense radiation and impact environment of space changes the composition of the thin upper layer of the moon over time, causing it to darken," David Minton, a professor of earth, atmospheric and planetary sciences at Purdue, said in a news release. "Just underneath that is fresher, brighter material. When a crater impact throws up ejecta, it splashes that material onto the surface, and that becomes a crater ray."

Larger craters produce ejecta capable of gouging the surface and allowing subsurface material to become exposed. In their paper, scientists suggest smaller impacts yield smaller, slower ejecta. Instead of gouging the surface, exposing fresh material, the ejecta simply form the crater rays themselves.

"We basically say that this crater has to be a certain size in order for its ejected debris to break the dark layer of the surface. This will bring bright material to the surface, creating the rays that we see," Elliott said.