Ray system

Lore & Background

The physical nature of lunar rays has historically been a subject of speculation. Early hypotheses suggested they were deposits of salt from evaporated water, later thought to be volcanic ash or streaks of dust. After the impact origin of craters became accepted, Eugene Shoemaker suggested during the 1960s that the rays were the result of fragmented ejecta material.

Reader's Guide

Ray systems are significant for determining the relative age of impact craters, as over time various processes obliterate the rays. On non-atmosphered bodies such as the Moon, space weathering from cosmic rays and micrometeorites reduces the albedo differential between ejecta and underlying material. Rays can also be covered by lava flows or other impact craters. However, recent studies suggest that the relative brightness of a lunar ray system is not always a reliable age indicator, as the albedo also depends on the portion of iron oxide (FeO); low FeO results in brighter materials that retain their appearance longer. Thus material composition must be factored into albedo analysis for age determination. Notable lunar craters with pronounced ray systems include Aristarchus, Copernicus, Kepler, Tycho, and others on both the near and far sides of the Moon.

Did You Know?

• Ray systems have been identified on Earth at Kamil Crater.
• Thermal rays on Mars are especially apparent at night when slopes and shadows do not influence infrared energy.
• Most lateral transport of primary ejecta is limited to a few crater radii, but impacts like Copernicus and Tycho launched ejecta halfway around the Moon.
• North Ray and South Ray craters, each with a clear ray system, were observed from the ground by Apollo 16 astronauts in 1972.