Geology Reference
In-Depth Information
can result from larger grains, such as sand. Although
composition must also be considered, bright wind streak
features are considered to be dust deposits, while dark
wind streaks are sand deposits or exposures of a dark
substrate from which dust has been eroded. Thousands
of variable features have been mapped, and their orienta-
tions are used to assess the prevailing wind directions at
the time of their formation.
these clay minerals form in the presence of alkaline
water, he and his colleagues proposed that the early cli-
mate on Mars was much warmer and wetter than present-
day conditions. The integrated valley networks suggest
the presence of surface run-off in this period of Mars
'
history, which would be consistent with Bibring
'
s
interpretation.
The Hesperian Period (3.5 to 1.8 Ga) is marked by
the eruptions of vast sheets of lava that formed the
ridged plains, including the type example, Hesperian
Planum. Central volcanism included the formation of the
highland paterae, Syria Planum, and early eruptions in the
Elysium and Tharsis provinces, coupled with the contin-
ued development of the Tharsis rise. In addition to exten-
sive volcanism, most of the large out
ow channels were
cut in this period.
The extensive Hesperian volcanism that occurred in the
Hesperian Period could account for the formation of sul-
fate minerals and related materials revealed in remote
sensing data obtained from orbit. These minerals might
have formed from volcanic sulfur dioxide combined with
acidic water. The transition from Noachian to Hesperian
times also saw the end of the martian magnetic
field. It is
thought that the loss of the magnetic
field would have
exposed Mars to the solar wind, which would have strip-
ped away Mars
7.6 Geologic history
The first global geologic maps of Mars were made fol-
lowing the Mariner 9 mission and included a geologic
time scale. Derived by Mike Carr and the late Dave
Scott, both of the US Geological Survey, the time
scale includes three periods, the Noachian (oldest),
Hesperian, and Amazonian (youngest). Although this
time scale is relative, impact crater size
-
frequency dis-
tributions for the major units associated with these peri-
ods enable estimates of
“
absolute
”
dates. As discussed
in
Section 7.5.2
, the dates are based on extrapolations
from lunar crater counts that are calibrated against radio-
genic ages from samples returned from the Moon. The
ages of various events have been coupled with the
wealth of new data for Mars since Mariner 9 to re
ne
the general geologic sequence, as discussed by Mike
Carr and Jim Head (
2010
).
The Noachian Period (4.6 to 3.5 Ga) began with the
solidi
cation of the martian crust from an inferred
magma ocean. In this time, the global crustal dichotomy
formed and the uplift of the Tharsis rise was initiated.
The southern uplands preserve impact craters that rep-
resent the
final stages of heavy bombardment in the
inner Solar System and include the Hellas and Argyre
basins. This record is suggested to underlie the northern
lowlands, where vestiges of large circular structures are
seen in the subsurface by radar sounding from orbit. The
discovery of the remnant magnetic
eldinNoachian-
age rocks (but not in younger materials) suggests that
the interior of Mars was able to generate a magnetic
field.
Noachian-age surfaces have been heavily modified by a
wide variety of processes, including volcanism, tectonic
modi
cation, and gradation by wind and water, but the
exact timing of these modi
cations is poorly constrained.
Jean-Pierre Bibring, the lead on the French OMEGA near-
infrared spectrometer on Mars Express, found that phyl-
losilicates are common in Noachian materials; because
atmosphere, leading to a colder, drier
environment. Thus, the out
ow channels would re
ect
release of water from the subsurface rather than from
surface runoff, as suggested by their association with the
chaos terrain.
The Amazonian Period (1.8 Ga to the present) is the
youngest subdivision of time onMars. However, one must
remember that this time corresponds to all of the
Phanerozoic Eon and much of the Proterozoic Eon of the
Precambrian on Earth. Tectonic deformation and volcan-
ism continued from the Hesperian Period but were much
less pronounced. Very young lava flows of limited areal
extent are seen in many areas, and active volcanism can-
not be ruled out with currently available data. Gradation of
all types modi
ed the surface through the agents of wind,
ice, gravity, and local liquid water; many of these pro-
cesses continue today, along with the formation of small
impact craters. In the absence of abundant volcanism and
liquid water on the surface in this period, slow chemical
weathering takes place through oxidation of the iron-rich
basaltic materials by atmospheric peroxides (which have
been detected by the Phoenix lander), which leads to the
formation of iron oxides (i.e.,
“
rust
”
) that gives Mars its
red color.
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