Geology Reference
In-Depth Information
Figure 5.4. An oblique image taken by Mariner 10, showing heavily
cratered terrain and patches of more sparsely impacted intercrater
plains (NASA Mariner 10 FDS 27328).
Figure 5.5. Large craters on Mercury tend to have lower rims and
shallower
floors than do comparable-size craters on the Moon, as
seen in this oblique image of the 160 km in diameter crater Verdi in
the Shakespeare region of Mercury (NASA Mariner 10 FDS 166).
5.5 Geomorphology
these possibilities. Moreover, as shown in MESSENGER
altimeter data, the large craters (
Fig. 5.5
) have lower rims
and shallower
floors in comparison with craters of similar
size on the Moon. This can be explained in part by the
higher surface gravity of Mercury, which would inhibit
rim uplift during the excavation stage and result in lower
topographic relief. In addition, the
floors of many of the
large craters are
filled with smooth plains (described
below), which also contribute to the lower depth-to-
diameter ratio.
Intercrater plains occur as irregular-shaped patches of
level-to-gently rolling terrain within the heavily cratered
regions (
Fig. 5.6
). Unlike many of the lunar maria, most
mercurian intercrater plains are not found in circular
patches associated with obvious large impact basins.
What are the intercrater plains and how did they form?
These questions have puzzled planetary geologists for
more than three decades. Early ideas suggested that the
intercrater plains represent a primordial surface preserved
since the solidi
cation of the crust or that they are com-
parable to the highland plains seen on the Moon, which
are attributed to ejecta deposits from large impact basins.
However, speci
c large impacts have not been identi
ed
as sources on Mercury. Plains are now recognized to span
a wide range of ages. Some are relatively ancient, which is
evidenced by large superposed impact craters, while
embayment by intercrater plains into heavily cratered
At
first glance, Mercury can be confused with Earth
'
s
Moon. On a global scale, it is cratered with patches of
plains, and, as a small, airless body, it appears to have
experienced similar surface processes to the Moon. Upon
closer inspection, however, Mercury is seen to have its
own unique surface characteristics at all scales.
5.5.1 General physiography
The principal terrains on Mercury were de
ned by the
Mariner 10 imaging team as the heavily cratered high-
lands, intercrater plains (
Fig. 5.4
), and smooth plains.
While these super
cially resemble the lunar highlands
and mare regions, the team recognized that there are
fundamental differences between the lunar and mercurian
terrains.
The heavily cratered terrain on Mercury consists of
abundant large, overlapping impact craters representing
the final stages of intense bombardment in the inner Solar
System as are also seen on the Moon (
Fig. 4.4
). Although
initial Mariner 10 analysis suggested that there are fewer
large craters per unit area than on the Moon, subsequent
study shows this not to be the case.
Individual ejecta deposits and secondary craters are
rarely seen around the older mercurian craters in the
heavily cratered terrain. This could be due to gravitational
effects, mantling by younger deposits, or a combination of
