Geology Reference
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Figure 5.6. AMariner 10 image of typical intercrater plains and their
embayment of a 90 km impact crater (arrow), showing that some
intercrater plains post-date the heavily cratered terrain. The lobate
scarp, Santa Maria Rupes, is in the middle of the image and cross-
cuts the terrain, indicating that this tectonic deformation post-dated
the emplacement of intercrater plains. The area shown is about
400 km across (from Strom, 1979 ; NASA Mariner 10 FDS 27448).
Figure 5.8. An oblique view of smooth plains showing the
characteristic ridges (similar to mare ridges on the Moon) and the
relative lack of superposed impact craters; the area shown is about
600 km wide (NASA MESSENGER NAC 162744209).
Smooth plains constitute about 40% of the surface of
Mercury. They are generally flat or gently rolling and are
relatively sparsely cratered, making them the youngest of
the principal physiographic units. Smooth plains are
found as irregular-shaped patches within the heavily cra-
tered terrain and intercrater plains, as well as within
impact craters and basins. Most smooth plains are charac-
terized by ridges that resemble lunar mare ridges
( Fig. 5.8 ). Embayment by smooth plains into heavily
cratered terrains suggests that they are of volcanic origin
( Fig. 5.9 ) and are comparable in size to flood lavas on
Earth and the Moon. Alternatively, some investigators
suggest that the Mercury smooth plains are ejecta deposits
similar to the Cayley Plains on the Moon; however, as
with the intercrater plains, there are no obvious primary
source craters and the smooth plains are also thought to be
volcanic.
Figure 5.7. A circular ridge, or
similar to those seen
on the Moon and Mars that are considered to represent impact
craters buried by lava ows. In this model, Head et al.( 2008 ) estimate
that flows of ~2.7 km thickness would be required in order to form
the feature shown here on Mercury (NASA MESSENGER image
EN108827047M).
ghost crater,
5.5.2 Impact craters
terrain shows that some plains are relatively young.
Impact craters on Mercury range in size from the
1,560 km in diameter Borealis basin down to the limit
of recognition on MESSENGER images of tens of
meters. No doubt even smaller craters exist in the airless
environment of Mercury. As on the Moon, mercurian
craters increase in complexity with size, from simple
craters, similar to those seen on the Moon, sug-
gest that some intercrater plains represent a relatively thin
mantle that buries craters ( Fig. 5.7 ), perhaps analogous to
lunar volcanic flood lavas. Thus, the intercrater plains are
likely to be of volcanic origin.
Ghost
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