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.
Rays spread out from a point
the southern hemisphere.
A closer look shows that
there are several other ray systems on the Moon. So what
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
the surface, extending tens, hundreds or maybe even thousands of
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.
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
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.
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.
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