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and Markiewicz, 2004) whose formation appears ongoing
although their genesis remains controversial.
Dendritic channels such as Warrego Vallis are of great
antiquity; they are found only on the oldest terrains and
are heavily cratered. Their formation is most likely due to
epochs early in martian history when precipitation-driven
runoff was more common (Harrison and Grimm, 2005).
The depositional termini of these channels are sometimes
marked by crater-filling fans and deltas, most spectacu-
larly that of Holden North East Crater (now known as
Eberswalde; Bhattacharya et al. , 2005). The meandering
channel forms with channel cut-offs and scroll bars of the
delta indicate multiple sustained flows similar to that of
terrestrial deltas (Bhattacharya et al. , 2005). Landscape
lowering has resulted in some of these channels being
exhumed and inverted, forming sinuous and often den-
dritic ridges on the martian landscape (Pain, Clarke and
Thomas, 2007).
The dendritic channels are often modified by the fretted
and outflow valleys, due to a shift to drier, colder climates
where water flows occurred after singular events, such as
large-scale impacts or volcanism and melting permafrost
(Baker, 2001). In most cases, these features can be traced
back to source areas, where collapse and removal of pre-
sumably ice-rich subsurface materials formed chaotic ter-
rain, with evidence for volcanism occurring in the last few
Myr (Neukum et al. , 2004); these events, although rare,
are likely to continue into the future. The scale of some
of the catastrophic outflow events was colossal, dwarf-
ing their terrestrial equivalents by two to three orders of
magnitude (Baker, 2001).
Like slope streaks, gullies are an ongoing process on
the surface of Mars, whose origin is highly controver-
sial (Malin et al. , 2006; McEwan et al. , 2007; Heldmann
et al. , 2009). As with terrestrial gully forms, it is likely
that these features are polygenic, formed by several pro-
cesses, some unique to Mars, such as dry mass flows and
CO 2 gasification. Of those that show sinuous channels and
lobate distributaries, many, although probably not all, are
related to fluvial or hyper-concentrated flows. (Heldmann
et al. , 2005, 2009). The mostly likely source of water is
the melting of shallowly buried snow or ice (Kossacki
and Markiewicz, 2004), given that the run-out distances
of gully-forming flows is close to that modelled for pure
water.
Lastly, it should recognised that there are many rela-
tively small mound and outflow features on the surface
of Mars whose origins are enigmatic but which may have
been formed through the discharge of artesian waters as
springs to form mounded deposits (Bourke et al. , 2007).
These deposits may form a continuum with both cold
mal systems. Like terrestrial desert springs, martian spring
deposits are of considerable potential interest to astrobi-
ologists as targets for sample return missions (Allen and
Oehler, 2008).
5.5.6
Summary
The surface of Mars is deceptively Earth-like in many
respects. Familiar landforms abound with analogues in
terrestrial cold and hot deserts. These should not blind
the geomorphologists to the fact that Mars is none the
less not Earth and the expression of familiar processes
may show subtle and perhaps profound differences. In
addition Mars will have its own unique set of processes,
especially those related to the condensation and volatili-
sation of CO 2 , that may mask, overprint or mimic more
familiar terrestrial processes. Lastly, the massive changes
in geomorphic style during to the large-scale evolution of
the planet have resulted in a unique landscape recording
immense climatic change.
5.6
Titan: methane-based aridity?
Titan is the largest satellite of Saturn and the second largest
in the solar system. Titan is larger than the planet Mercury
and its diameter is 74 % of Mars. The internal structure is
inferred to consist of a sepentinite core overlain by an icy
mantle and crust (Fortes et al. , 2007). Titan's density is
therefore low, resulting in a surface gravity only 14 % of
Earth's. Despite this, Titan has a very dense atmosphere,
with a surface pressure of 1.46 bars. The atmosphere is
very cold, with an average temperature of
180 C. The
atmosphere is composed of 98.4 % nitrogen and 1.6 %
methane and contains few clouds but a thick haze layer at
altitudes above 40 km (Tomasko et al. , 2005). The surface
temperatures on Titan are such that methane-ethane can
condense and fall as rain, acting in a manner analogous to
water on Earth (Lunine and Atreya, 2008).
Since 2004 the Cassini spacecraft in orbit round Sat-
urn has achieved multiple encounters with Titan. Infrared
imaging and spectroscopy have provided information on
the composition of the surface (e.g. Soderblom et al. ,
2007a) and radar on the morphology (e.g. Elachi et al. ,
2006). Some notable geomorphic features are shown in
Figure 5.7. However, only part of the surface of Titan has
been imaged in any detail and many unexpected features
may well emerge.
Fly-by data on the surface of Titan has been supple-
mented by a single Lander. The Huygens probe was re-
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