Geology Reference
In-Depth Information
5.5.6 Gradation features
An extremely thin cloud of sodium, oxygen, potassium,
and calcium is found above Mercury ' is surface, constitut-
ing an
(perhaps more properly called an
exosphere) around the planet. However, it is so thin
that in terms of geologic processes it is insigni cant.
Moreover, there is no evidence to suggest that Mercury
has ever had an atmosphere that could support surface
processes related to wind or liquid water. Consequently,
Mercury ' is surface is the result primarily of impact, vol-
canic, and tectonic processes, much like the surface of
Earth ' s Moon. However, some gradation does occur by
space weathering as described in Section 3.5.1 , a process
that is ubiquitous on airless bodies. Moreover, images
show features resembling landslides in some areas, such
as on the walls of some impact craters.
Impact cratering at all scales leads to gradation, espe-
cially in the airless environment of Mercury. As on the
Moon, impact-generated debris forms a fragmental sur-
face layer of regolith.
atmosphere
Figure 5.32. Vostock Rupes (arrow) cuts across the 65 km in diameter
crater Guido D
'
Arezzo, foreshortening the rim and suggesting
5
7 km thrust-fault displacement resulting from compression (NASA
Mariner 10 FDS 27380).
-
conclusion that cooling and shrinkage of Mercury
s mas-
sive iron core led to a 1 - 2 km decrease in planetary radius
and the
'
subsequent
foreshortening of
the
crust.
Alternatively, it has been suggested that tidal
despin-
ning
could have generated compressional stresses to
form the scarps. In this case, compressional stresses
would have been east - west, with northwest - northeast
linear shear and north - south thrusting in the crust.
Ridges occur in many regions and geologic settings on
Mercury. They are positive relief features (in contrast with
the
5.6 Geologic history
Geologic mapping of Mercury led to the establishment
of a formal five-fold time - stratigraphic sequence
( Tabl e 5 . 1 ). From oldest to youngest, the sequence con-
sists of the Pre-Tolstojan System, Tolstojan System,
Calorian System, Mansurian System, and Kuiperian
System, with names derived from the type localities for
each system.
Mercury, along with its sibling terrestrial planets,
formed by accretion of smaller bodies, leading to global
heating, the likely formation of a magma ocean, and sub-
sequent differentiation into a core, crust, and possible
mantle. The accretion of a high percentage of iron led to
Mercury ' s very large core. As on the Moon, the earliest
geologic record on Mercury (pre-Tolstojan) consists of the
heavy cratered terrain, re ecting suf cient solidi cation
of the crust over the inferred magma ocean to record the
heavy bombardment. Some intercrater plains were prob-
ably emplaced at this time. Cooling and solidi cation of
Mercury could have established some of the earliest tec-
tonic patterns of the evolving crust. The Tolstoj impact
basin marks the base of the Tolstojan System. Throughout
this system, the formation of numerous large craters and
basins, such as Beethoven, re ects the waning stages of
heavy bombardment in the inner Solar System.
scarps) that are as long as 400 km, with
heights that can exceed 700 m and widths of up to
35 km, and can closely resemble mare ridges on the
Moon ( Fig. 5.8 ). Although some ridges are found in
intercrater plains, most occur in smooth plains deposits
and within the oor-lling materials of impact basins.
Like scarps, ridges also are considered to represent tec-
tonic compression, although perhaps on a more local
scale. For example, ridges within Caloris and other basins
could re ect settling of lava or impact melt toward the
center of the basin, leading to concentric ridge patterns.
Ridges not associated with basin interiors could have
formed in response to the proposed global shrinkage of
Mercury.
Many investigators have mapped the distribution of the
types and ages of the various scarps and ridges using
Mariner 10 data in attempts to place constraints on the
interior evolution of Mercury. These studies, however,
have been hampered by the lack of a global data set and
the low resolution of the data. No doubt, these studies will
be revisited using MESSENGER global data and the abil-
ity to derive better age relations among the structures and
the associated units.
stepped
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