Geoscience Reference
In-Depth Information
Venus and Titan, despite the radically different conditions
and compositions on the surface of each, or the similarity
in dune patterns on Earth and Titan. Furthermore, plane-
tary landscapes provide natural laboratories that can test
a range of hypotheses, including the role of ice complex-
ity in landscape processes, the significance of moisture in
extremely low abundances, and the degree to which vege-
tation influences the expression of geomorphic processes.
Ultimately there is the possibility that, through under-
standing palaeoclimatic records across the solar system,
e.g. in the polar deposits of Mars, this will assist in clarify-
ing the role of solar variability in climate change. Lastly,
an appreciation of extraterrestrial geomorphology can en-
courage research into specific terrestrial analogues and
spur study of previous underresearched aspects (Clarke,
2008).
face (Kargel, 2004). Frozen CO
2
passes back into the
gaseous phase without going through a liquid phase but the
pressure and temperatures are high enough for ephemeral
moisture films and adsorbed water in many locations on
Mars (Mohlmann, 2004; Richardson and Mischna, 2005).
Evaporation rates are very high, creating a largely dry
surface, although shallow ice extends over much of the
surface in the mid and high latitudes (Mitrofanov
et al.
,
2003). In these respects Mars resembles terrestrial cold
deserts.
Mars appears to undergo orbitally forced climate
changes analogous to those of Earth (Head
et al.
, 2003).
Large changes in obliquity from 15 to 35 degrees occur
at intervals of
40 ka. This, in conjunction with the or-
bital eccentricity and changes to areocentric longitude at
perihelion, may lead to 'glacial' periods of
∼
1.75 Ma du-
ration and 'interglacial' periods of 0.65 Ma. Polar ice is
most stable at periods of low obliquity and equatorial ice
during periods of high obliquity (Head
et al.
, 2005). Water
is therefore shuttled between the poles and the equator, be-
ing lost by sublimation from one and deposited at the other
as snow. At present Mars is in an intermediate state, with
ice stable at the poles but having being lost through sub-
limation at the equator, except where protected by thick
regolith.
Over a much longer time frame Mars appears to have
evolved from earlier epochs with much more abundant
liquid water, including lakes and seas, through a transi-
tional phase to the present cold and dry regime. These
different epochs of water activity may have led to char-
acteristic mineralogical signatures in the martian surface
(Bibring
et al.
, 2006). The earliest epoch is dominated by
phyllosilicates formed during water-rich conditions and
has been termed the 'Phyllocian' era. Sulfates formed in a
second era (the 'Theiikian'), which saw increased aridity.
The 'Siderikian' era of the last
∼
5.5
Mars: water-based aridity
5.5.1
Overview
Mars is the third largest rocky body in the solar system. It
has a diameter that is 53 % of Earth's, a mass that is 10.7 %
of Earth's and the surface gravity is consequently 38 %
of the terrestrial value. The atmosphere is predominantly
(95.7 %) CO
2
with minor (2.7 %) N
2
. The surface pressure
is 6.36 millibars at the mean radius, varying from 4.0 to 8.7
millibars depending on season. Pressure varies between
11.6 and 0.3 millibars over an altitude range of
−
8.2 km
below the Mars areoid and
21.2 km above Mars areoid.
The mean surface temperature is about
+
46
◦
C with highs
of 20
◦
C at the ground surface at midday on the equator and
lows of
−
130
o
C during the polar winters. The atmosphere
is active with global and regional dust storms, local dust
devils and with clouds, snow and frost composed of water
and carbon dioxide ices.
The surface of Mars (Figure 5.3) is the most stud-
ied of any solar system body after the Earth and Moon.
The data sources and literature are therefore very exten-
sive. However, interpretations of some surface features are
controversial; see, for example, the discussion of recent
gully forms (Malin
et al.
, 2006; McEwan
et al.
, 2007;
Heldmann
et al.
, 2009). This brief section will therefore
only summarise some of the more salient aridity-related
features.
−
3.5 Ga is dominated by
anhydrous ferric oxides in a slow, dry weathering environ-
ment. Kargel (2004) gives a good summary of the ideas
associated with the evolution of the surface environment
with respect to the availability of water and whether this
evolution was a simple linear process or one of greater
complexity involving longer-term cycles of wetter and
drier epochs or episodic water release events driven by
volcanism or impacts. These eras can be compared with
the more widely used martian timescale based on crater
densities (Cattermole, 2001):
∼
Amazonian (younger limits 0.70-1.80 Ga; older limits
2.3-3.55 Ga).
5.5.2
The history of atmosphere-surface
interactions
Hesperian (younger limits 1.80-3.10 Ga; older limits
