365 Days of the Moon -- August 13, 2014
Copernicus ray system
Thank goodness for crater rays! Without them, the Moon would be far less interesting to look at. Here we see the magnificent fan of rays
extending in all directions around Copernicus, once called the "Monarch of the Moon" because its spectacular ray system is so
enchanting. For hundreds of years, astronomers did not understand the nature of crater rays. One prominent early 20th century German lunar
expert called rays a "riddle wrapped in an enigma".
Today we understand rays are actually a combination of material splashed out of a crater by the impact that created it, and thousands of
small unseen (from Earth) secondary craters that have churned the surface of the Moon. The heat of a large crater-forming impact melts
thousands of tons of rock and creates millions of glassy particles that are very efficient at reflecting light. These particles are much
like the material in highway signs that reflects your car headlights. These particles are splashed across the Moon by the explosion that
excavates the ray's parent crater.
The layer of material creating the rays is very thin. In fact, the Apollo 12 crew landed on a Copernican ray and could not detect it.
"Space weathering", or the infall of micrometeorites, erases crater rays after about a billion years. Rays are thus useful in dating how
old some craters are. In the case of Copernicus, the consensus averages about 800 million years old.
Image taken through a Celestron 11 Edge HD with a Skyris 274M camera
Image copyright Robert Reeves 2014
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