A
ray system comprises radial streaks of fine
ejecta thrown out during the
formation of an
impact crater, looking
a bit like many thin spokes coming from the hub of a wheel. The
rays can extend for lengths
up to several times the
diameter of their
originating crater, and are often accompanied by small secondary
craters formed by larger chunks of ejecta. Ray systems have been
identified on the
Moon,
Mercury, and some
moons of the outer planets. Originally it
was thought that they existed only on
planets
or moons lacking an
atmosphere, but more recently
they've been found on
Mars in infrared images
taken from orbit by
Mars Odyssey's
thermal
imager.
appear at visible, and in some cases infrared wavelengths, when
ejecta are made of material with different reflectivity (i.e.,
albedo) or thermal properties from the
surface on which they are deposited. Typically, visible rays have a
higher albedo than the surrounding surface. More rarely an impact
will excavate low albedo material, for example
basaltic-
lava deposits on the
lunar maria. Thermal rays, as seen on
Mars, are especially apparent at night when slopes and shadows do
not influence the infrared energy emitted by the Martian
surface.
The layering of rays across other surface features can be useful as
an indicator of the relative age of the impact crater, because over
time various processes obliterate the rays. On non-atmosphered
bodies such as the Moon,
space
weathering from exposure to
cosmic
rays and
micrometeorite causes a steady
reduction of the differential between the ejecta's albedo and that
of the underlying material. Micrometeorites in particular produce a
glassy melt in the
regolith that lowers the
albedo. Rays can also become covered by
lava flows, or by other impact craters or
ejecta.
Lunar rays
The physical nature of lunar rays has historically been a subject
of speculation. Early hypotheses suggested that they were deposits
of salt from evaporated water. Later they were thought to be
deposits of 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.
Recent studies suggest that the relative brightness of a lunar ray
system is not always a reliable indicator of the age of a ray
system. Instead the albedo also depends on the portion of
iron oxide (FeO). Low portions of FeO result in
brighter materials, so such a ray system can retain its lighter
appearance for longer periods. Thus the material composition needs
to be factored into the albedo analysis to determine age.
Among the
lunar craters on the near side with pronounced ray systems are
Aristarchus
, Copernicus
, Kepler
, Proclus
, and
Tycho
.
Similar
ray systems also occur on the far
side of the Moon, such as the rays radiating from the craters
Giordano
Bruno
and Ohm
.
References
See also
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