Geology Reference
In-Depth Information
Table 5.1. Time
-
stratigraphic sequence for Mercury (after Spudis and Guest,
1988
)
Age of base
of system
a
Major units
a
Lunar counterpart
b
System
Kuiperian
Crater materials
1.0Ga
Copernican
Mansurian
Crater materials, smooth plains
3.0
-
3.5Ga
Eratosthenian
Calorian
Caloris Group; plains, crater, small-basin materials
3.9Ga
Imbrian
Tolstojan
Goya Formation; crater, small-basin, plains materials
3.9
-
4.0Ga
Nectarian
Pre-Tolstojan
Intercrater plains, multi-ring basin, crater materials
pre-4.0Ga
Pre-Nectarian
a
Approximate ages based on the assumption of a lunar-type impact
flux history on Mercury.
b
Included for reference only; no implication of exact time correlation is intended.
Ejecta from the Caloris impact basin (the Van Eyck and
Odin formations) serves as the primary stratigraphic hori-
zon for Mercury
'
s geologic history (similarly to the Fra
Mauro Formation on the Moon) and the start of the
Calorian Period. The hilly and lineated terrain in the region
antipodal to the Caloris basin is considered to have resulted
from focusing of seismic energy from the impact. The
Caloris and Rembrandt basins re
ect the termination of
impacts by large objects. Continued cooling and contrac-
tion of Mercury generated additional compressional fea-
tures, such as the scarps (rupes)andmare-typeridges.
Fresh craters lacking bright rays and volcanic floods of
lava that formed many of the smooth plains on Mercury
record the Mansurian Period, the start of which is de
ned
by the impact of crater Mansur. The paucity of superposed
impact craters on the smooth plains indicates the relative
youth of Mansurian-age materials and is roughly analo-
gous to the lunar Eratosthenian System in its character-
istics. The youngest geologic units are keyed to young
bright-rayed craters, for which Kuiper is the type example,
and mark the initiation of the Kuiper Period. This period is
modeled on the lunar Copernicus Period.
This sketch of Mercury
s geologic history no doubt will
be re
ned with the analysis of MESSENGER data in the
coming years. It is likely that many of the units and
terrains identi
ed on Mariner 10 data will be seen to be
more complex and subject to subdivision. Not only will
units be better mapped and characterized using higher-
resolution images and compositional data, but also the
ability to obtain precise altimetry and crater counts will
provide a better understanding of the age relations among
the units.
'
Assignments
1.
Compare and contrast the mercurian smooth plains
with lunar maria.
units could be de
ned and placed in a stratigraphic
sequence.
2.
Discuss the missions that have returned data from
Mercury (
(flyby, orbiter, lander, etc., dates of operation,
scienti
c payload, and principal scienti
c results).
5.
Discuss the evidence for the hypothesis that Mercury
“
shrank
”
in size during its evolution and explain how
such shrinkage could have occurred.
3.
Describe the differences in morphology between
rupes on Mercury, ridges on Mercury, and
“
wrinkle
ridges
6.
Mercury and the Moon are both
“
airless
”
bodies, yet
impact craters of the same diameter have different
morphologies. Describe these differences and offer
an explanation for the differences.
”
on the Moon.
4.
Explain how a global geologic map could be derived
for Mercury with currently available data; include how
