Mercury 123
FIGURE 5 Artist’s rendition of
Mercury’s dipole magnetic field
showing the bow shock,
magnetosheath,magnetopause,
and a possible plasma sheet. The
diameter of the Earth is also shown
for comparison. (Courtesy of Jim
Slavin, Goddard Spaceflight
Center, Greenbelt, Maryland.)6. Geology and Planet Evolution
Mercury has heavily cratered upland regions and large
areas of younger smooth plains that surround and fill
impact basins (Fig. 6). Thermal infrared measurements
fromMariner 10indicate that the surface is a good insulator
and, therefore, consists of a porous cover of fine-grained re-
golith. Earth-based microwave measurements indicate that
this layer is a few centimeters thick and is underlain by
a highly compact region extending to a depth of several
meters. Mercury’s heavily cratered terrain contains large
areas of gently rolling intercrater plains, the major terrain
type on the planet. Mercury’s surface is also traversed by
a unique system of contractionalthrust faultscalled lo-
bate scarps. The largest well-preserved structure viewed
byMariner 10is the Caloris impact basin some 1300 km in
diameter. Antipodal to this basin is a large region of broken-
up terrain called thehilly and lineated terrain,probably
caused by focused seismic waves from the Caloris impact.
6.1 Geologic Surface Units
The origin of some of the major terrains and their inferred
geologic history are somewhat uncertain because of the
limited photographic coverage and resolution and the poor
quality or lack of other remotely sensed data. In general,
the surface of Mercury can be divided into four major ter-
rains: (1) heavily cratered regions, (2) intercrater plains,
(3) smooth plains, and (4) hilly and lineated terrain. Other
relatively minor units have been identified, such as ejecta
deposits exterior to the Caloris and other basins.
6.1.1 IMPACT CRATERS AND BASINS
The heavily cratered uplands probably record the period
of late heavy meteoroid bombardment that ended about
3.8 billion years ago on the Moon, and presumably at about
the same time on Mercury. Thisperiod of late heavy
bombardmentoccurred throughout the inner solar system
and is also recorded by the heavily cratered regions on the
Moon and Mars. Based on chemical evidence from Apollo
samples from the Moon, the bombardment may have been
catastrophic lasting only about 100 million years or less. It
appears to have peaked about 3.9 billion years ago. In the
heavily cratered terrain on Mercury, there is an increasing
paucity of craters with decreasing crater diameter relative to
heavily cratered terrain on the Moon. This paucity of craters
is probably due to obliteration of the smaller craters by
emplacement of intercrater plains during the period of late
heavy bombardment. Below a diameter of about 20 km, the
abundance of craters increases sharply. These craters may
represent secondary impact craters from large craters or
basins. The crater population superimposed on the smooth
plains within and surrounding the Caloris Basin has a size
distribution intermediate between the heavily cratered and
lightly cratered plains. This suggests that unlike the lunar
maria the Caloris smooth plains formed near the end of late
heavy bombardment.
Fresh impact craters on Mercury exhibit similar mor-
phologies as those on the other terrestrial planets. Small
craters are bowl-shaped, but with increasing size they de-
velop central peaks, flat floors, and terraces on their inner
walls. The transition from simple (bowl-shaped) to com-
plex (central peak and terraces) craters is about 10 km. At