Mercury 125
radii on the Moon. All of these differences are probably
due to the larger surface gravity of Mercury (3.70 m/s^2 )
compared to the Moon (1.62 m/s^2 ).
Twenty-two multiring basins have been recognized on
the part of Mercury viewed byMariner 10.However, high-
resolution radar images of the side not viewed byMariner 10
show several large circular features about 1000 km in diam-
eter that may be impact basins. Based on the pattern and
extent ofgrabenson the floor of Caloris, it is estimated
that Mercury’s lithosphere under Caloris was thicker
(>100 km) than the Moon’s (between 25 and>75 km de-
pending on location) at the end of late heavy bombardment.
The 1300-km-diameter Caloris impact basin is the largest
well-preserved impact structure (Fig. 7), although the much
more degraded Borealis Basin is larger (1530 km). The floor
structure of the Caloris Basin is like no other basin floor
structure in the solar system. It consists of closely spaced
ridges and troughs arranged in both a concentric and ra-
dial pattern (Fig. 8a and 8b). The ridges are probably due
to contraction, while the troughs are probably extensional
grabens that postdate the ridges. The fractures get progres-
sively deeper and wider toward the center of the basin. Near
the edge of the basin there are very few fractures. This pat-
tern may have been caused by subsidence and subsequent
uplift of the basin floor.
6.1.2 HILLY AND LINEATED TERRAIN
Directly opposite the Caloris Basin on the other side of
Mercury (the antipodal point of Caloris) is the unusual
hilly and lineated terrain that disrupts preexisting land-
forms, particularly crater rims (Fig. 9a and 9b). The hills
are 5–10 km wide and about 0.1–1.8 km high. Linear de-
pressions that are probably extensional fault troughs form a
roughly orthogonal pattern. Geologic relationships suggest
that the age of this terrain is the same as that of the Caloris
Basin. Similar, but smaller, terrains occur at theantipodes
of the Imbrium and Orientale impact basins on the Moon.
The hilly and lineated terrain is thought to be the result of
shock waves generated by the Caloris impact and focused
at the antipodal region (Fig. 10). Computer simulations of
shock wave propagation indicate that focused shock waves
from an impact of this size can cause vertical ground mo-
tions of about 1 km or more and tensile failure to depths
of tens of kilometers below the antipode. Although the lu-
nar Imbrium Basin (1400 km diameter) is larger than the
Caloris Basin, the disrupted terrain at its antipode is much
smaller than that at the Caloris antipode. The larger dis-
rupted terrain on Mercury may be the result of enhanced
shock wave focusing due to the large iron core.
6.1.3 INTERCRATER PLAINS
Mercury’s two plains units have been interpreted as either
impact basin ejecta or as lava plains. The older intercrater
plains are the most extensive terrain on Mercury (Figs. 11
FIGURE 7 Photomosaic of the 1300-km-diameter Caloris
impact basin showing the highly ridged and fractured nature of
its floor. (Courtesy of NASA.)and 12). They both partially fill and are superimposed by
craters in the heavily cratered uplands. Furthermore, they
have probably been responsible for obliterating a significant
number of craters as evidenced by the paucity of craters
less than about 40 km diameter compared to the highlands
of the Moon. Therefore, intercrater plains were emplaced
over a range of ages contemporaneous with the period of
late heavy bombardment. There are no definitive features
diagnostic of their origin. Because intercrater plains were
emplaced during the period of late heavy bombardment,
they are probably extensively fragmented and do not retain
any signature of their original surface morphology. Although
no landforms diagnostic of volcanic activity have been dis-
covered, there are also no obvious source basins to pro-
vide ballistically emplaced ejecta. The global distribution
of intercrater plains and the lack of source basins for ejecta
deposits are indirect evidence for a volcanic origin. Addi-
tional evidence for a volcanic origin is recentMariner 10en-
hanced color images showing color boundaries that coincide
with geologic unit boundaries of some intercrater plains
(Fig. 13). If intercrater plains are volcanic, then they are