Geoscience Reference
In-Depth Information
Noachian (younger limits 3.50-3.85 Ga; older limit
>
The
Viking 1
spacecraft was the first to operate suc-
cessfully on Mars. It landed on the western slopes of the
Chryse Planitia basin (Mutch
et al.
, 1976a) and revealed a
rough surface with possible bedrock surfaces, numerous
rocks of 20 cm size, scattered blocks and boulders up to
1 m by 3 m in size, and small aeolian dunes. Many of the
rocks are pitted through differential erosion or weather-
ing or because of their original texture. The exhaust jets of
the landing engines locally exposed a polygonally cracked
resistive horizon just beneath the surface. This was incor-
rectly termed 'duricrust' (Mutch
et al.
, 1976a) when in
reality it more closely resembles the lightly cohesive or
weakly indurated surface crusts of terrestrial arid environ-
ments (Thomas, Clarke and Pain, 2005). Over time, minor
rearrangement by the wind of fine-grained surface mate-
rial was noted and there was a small slump on the face of
a dune. This showed the presence of weak induration on
that dune (Jones
et al.
, 1979).
Viking 2
landed on the northern plains of Mars at
Utopia Planitia on the ejecta blanket of Mie crater (Mutch
et al.
, 1976b). A flat landscape supported large numbers
of blocks, some more than 20 cm across, scattered with
a much higher density than at the
Viking 1
site. Rock
surfaces were heavily pitted. Actual lithological variabil-
ity appeared lower than at the previous site and there
were no small dunes or bedrock outcrops. The surface
was composed of a weakly indurated crust broken up by
large polygons
4.60 Ga).
The alteration eras correspond loosely to the cratering
eras - Noachian with the Phyllocian, Hesperian with the
Theiikian and Amazonian with the Siderikian.
5.5.3
Martian water cycle
The expression of the water cycle on Mars has therefore
varied greatly through the planet's history, reflecting the
planetary scale geophysical evolution and in turn driving
the extant geomorphic processes.
The presence of a thick, greenhouse atmosphere in the
Noachian appears to have led to a hydrological cycle simi-
lar to that of Earth. Precipitation (not necessarily rainfall)
led to runoff and the formation of dendritic valley net-
works like Warrego Vallis (Ansan and Mangold, 2006)
that discharged ultimately into a northern ocean (Carr and
Head, 2003). This active, Earth-like hydrologic cycle may
have been accompanied by at least local deep weathering
with low-temperature clay-carbonate alternation (Wray
et al.
, 2009). The martian atmosphere may have been lost
with the global magmatic after the cessation of the core
dynamo (Kargel, 2004; Cattermole, 2001).
The Hesperian marks a transition from the active water
cycle of the Noachian to the largely inactive cycle of
the Amazonian. Water, released by magmatic or impact
events formed outflow channels, but was refrozen into the
upper martian crust and regolith in what was essentially a
one-way process. Atmospheric water was also deposited
at the poles forming ice deposits, though probably not the
ice deposits seen today.
The Amazonian appears to have been characterised by
a water cycle consisting of the solid and vapour phases
only, and the transfer of that between latitudes where sub-
limation is dominant and its trapping where condensation
dominates (Head
et al.
, 2003). The water may be deposited
as surface ice deposits and subsequently buried.
On Earth an active water cycle involving all three phases
has been operating for its entire geologic history. As a
result the whole hydrosphere has been cycled perhaps
1000 times (Kargel, 2004). On Mars the cycling has been
far less since it ended sometime between 3 and 4 Ga.
8 m across, suggesting subsurface ice.
No ice was encountered during the Lander's excavation
programme (Jones
et al.
, 1979) but data from hydrogen
mapping and studies of ice distribution
(
Mitrofanov
et al.
,
2003
)
shows that there was probably an ice table within a
metre of the surface. Ripples of wind-deposited material
occurred in the polygonal troughs. Over the operational
period of the mission small changes were observed (Jones
et al.
, 1979), consisting of minor aeolian reworking and
the build-up and sublimation of water ice frosts.
The
Pathfinder
Lander touched down on Ares Vallis
and soon after deployed a small rover,
The Sojourner
,
which allowed data to be collected over a much wider
area. The landing site was where a large outflow chan-
nel debouched on to the southern part of Chryse Planitia
(Ward
et al.
, 1999). The surface relief was complex, with
probable water-eroded bedrock hills and crater rims in
the distance, rounded cobbles and boulders, some possi-
bly imbricate, scattered across an irregular sedimentary
surface that showed possible small channels, and a sur-
face deposit of small aeolian dunes. Rock surfaces showed
numerous ventifacts and there were signs of relatively re-
cent stripping by wind of significant thicknesses of soil.
Boulders ranged from 5-20 cm up to
∼
5.5.4
Surface images
Many thousands of images have been returned from six lo-
cations on Mars (Figure 5.4), four of which are essentially
point sites and two are multikilometre traverses.
