The above image taken my
NASAs MESSENGER satellite is the Kuiper Crater on Mercury, with its prominent
rays similar to those of our Moon's Aristarchus and Tycho craters.
It seems to be mostly a
matter of age. The rayed craters like Copernicus, Tycho and Kepler are believed
to be very young, perhaps only a million years old or less. The material
excavated by an impact has to go somewhere and ejection fans are 'it'. Young
rocks and rock surfaces have a higher reflectivity than old rocks, and on the
moon, micrometeoroid impacts and even particles from the solar wind can
significantly age exposed rock changing their reflectivity significantly. The
rays are ejecta from craters which have not been weathered very much.
The rays that we can see from some lunar craters are very spectacular and
next to the lunar maria, one of the characteristic features of the lunar
topography as seen from Earth. For a long time it has been recognized that
these rays represent material ejected from the crater upon impact by the
object that created the crater. Studies of volcanic ejecta on the Earth show
that the ejecta often comes out as narrow, fan shaped plumes which lead to
narrow 'fans' extending outwards from the caldera. This explains why rays exist
at all, in terms of the tendency of impact and volcanic ejecta to form these
narrow, vertical plumes.
If you carefully study the orientations of the rays, you will discover that
they do not exactly point back to the center of the crater, however. In July
1959, Louis Giamboni of the Rand Corporation published an article in the
Astrophysical Journal describing his detailed study of lunar rays, and proposed
that their non-radial orientation is caused by the Moon having rotated on its
axis during the time that the more distant ejects was in its ballistic
trajectory over its hundred-mile journey. Material in low trajectories would
fall sooner and have less deviation from radial. His model of the 68 rays in
the Tycho crater indicated that the pattern of deviations could be fit only if
the lunar 'day' were between 0.5 and 6.8 days long, not the 27 days that it
currently is. According to Giamboni, this means that the rayed craters were
created when the Moon was only about 100 million years old. This also explains
why only a few craters have rays, because they are the oldest, and were formed
when the crust was very very young. Since the end of the
Apollo program, however, the rays are widely interpreted as the 'splash marks'
from very YOUNG craters, and the deviations seen are not considered
statistically significant.
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