The Moon Guide
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The phase of the Moon right now

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< About impacts
Ray craters
Anyone glancing at a photo of the full Moon spots one feature above all: the giant system of rays that spread out from a point in the southern hemisphere. These look a bit like grid lines on a map – so much so that people sometimes ask whether this is the Moon's south pole. But of course the real south pole is not marked by any such grid lines.
Ful Moon
Rays spread out from a point
in the southern hemisphere.

A closer look shows that there are several other ray systems on the Moon. So what causes them?

When a large body hits the Moon, a huge amount of material is thrown out from the resulting impact crater. Material that is travelling slowly overturns as it is ejected from the crater and builds up the crater rim, while slightly faster moving material falls as secondary impacts further away, producing what are known as secondary crater chains.

Other material gets strewn across the surface, extending tens, hundreds or maybe even thousands of kilometres from the crater itself as bright crater ray systems. The rays are typically brighter than the surrounding surface as they are fresher and younger than the older surface into which the material was thrown.

The rays are very useful to lunar geologists as they can tell us about the relative age of the crater compared with other craters and geological features, since if they overlie another ray system, they must be younger than that other system. The composition of the rays can also tell us about the material that existed below the surface, since it was once buried prior to the impact.  Over time the rays will become degraded due to exposure to space weathering or being covered up by other impact events.
Tyclo closeup
Tycho closeup. Photo: Dave Tyler


The most prominent ray system was caused by the impact that created the crater Tycho, a mere 108 million years ago, according to data collected by the Apollo 17 mission. Had the dinosaurs that roamed the Earth at the time been interested in the Moon, they would have witnessed a dramatic event as a body many kilometres across crashed into the Moon. In fact is is possible that this body was a chunk from the same asteroid that hit Earth some 40 million years later, resulting in their own demise.

Tycho’s ray system, like others, is very obvious at full Moon because of the way the debris reflects light back whence it came, rather like reflective road signs. But the crater is less spectacular under lower illumination, though it is evidently much more recent than the surrounding terrain.

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Lunar geologist

Lunar geology

Moon rocks on Earth

Moon quakes

How the Moon was formed

Geology of the Moon's features

What is the Moon made of?

Lunar gardening

About impacts

Ray craters

Ageing wrinkles

Meandering channels

 
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