128 Encyclopedia of the Solar System
FIGURE 12 High-resolution
view of the intercrater plains.
The chains and clusters of
small craters are secondaries
from younger craters. The
90-km-diameter crater in the
upper right-hand corner has been
embayed by intercrater plains.
The lobate scarp that diagonally
crosses the image is a thrust fault.
(Courtesy of NASA.)probably lava flows erupted from fissures early in Mercurian
history. Intercrater plains are probably≥3.9 billions years
old.
6.1.4 SMOOTH PLAINS
The younger smooth plains cover almost 40% of the total
area imaged byMariner 10.About 90% of the regional ex-
posures of smooth plains are associated with large impact
basins. They also fill smaller basins and large craters. The
largest occurrence of smooth plains fill and surround the
Caloris Basin (Fig. 7), and occupy a large circular area in
the north polar region that is probably an old impact basin
(Borealis Basin). They are similar in morphology and mode
of occurrence to the lunar maria. Craters within the Bore-
alis, Goethe, Tolstoy, and other basins have been flooded
by smooth plains (Fig. 14). This indicates the plains are
younger than the basins they occupy. This is supported by
the fact that the density of craters superimposed on the
smooth plains that surround the Caloris Basin is substan-
tially less than the density of craters superimposed on the
floors of all major basins including Caloris. Furthermore,
several irregular rimless depressions that are probably of
volcanic origin occur in smooth plains on the floors of the
Caloris and the Tolstoy basins. The smooth plains’ youth
relative to the basins they occupy, their great areal extent,
and other stratigraphic relationships suggest they are vol-
canic deposits erupted relatively late in Mercurian history.
Mariner 10enhanced color images show the boundary of
smooth plains within the Tolstoy Basin is also a color bound-
ary, further strengthening the volcanic interpretation for
the smooth plains. Based on the shape and density of the
size/frequency distribution of superimposed craters, the
smooth plains probably formed near the end of late heavy
bombardment. They may have an average age of about
3.8 billion years as indicated by crater densities. If so, they
are, in general, older than the lava deposits that constitute
the lunar maria.
Three large radar-bright anomalies have been identi-
fied on the unimaged side of Mercury. They are desig-
nated as A (347◦W longitude,− 34 ◦latitude), B (343◦W
longitude, 58◦ longitude), and C (246◦W longitude,
11 ◦N latitude). All features are relatively fresh impact
craters with radar-bright ejecta blankets and/or rays similar
to Kuiper crater (60 km diameter) on the imaged portion
of Mercury. Feature A is 85 km in diameter with an exten-
sive ray system and a rough radar-bright floor, consistent
with a fresh impact crater (Figure 15). Feature B is 95 km
diameter with radar-bright rays and a radar-dark floor (Fig-
ure 15). Unlike feature A, the radar-dark floor indicates it
is smooth at the 12.6 cm wavelength of the image. Fea-
ture C is a fresh crater about 125 kilometers in diameter.
Water-rich comets or asteroids responsible for one or more