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recognised in the high-resolution images collected by the
Mars orbital camera (MOC) onboard the
Mars Global
Surveyor
spacecraft (Sullivan
et al.
, 2001). Slope streaks
occur in equatorial regions with thick dust mantles. They
have an elongated fan-shaped plan and apparently orig-
inate at a point and extend downslope. Their termina-
tions are often wedge-shaped and sometimes branching
or braided. Slope streaks cross small topographic obsta-
cles but flow round larger ones. When newly formed they
are much darker than the surrounding material. They pro-
gressively lighten or fade, in some cases even becoming
brighter than their surrounds. Slope streaks at MOC res-
olution (
has not, however, been forthcoming to date. The same
applies to smaller surface ice accumulations in subpolar
craters, e.g. Louth Crater (Brown
et al.
, 2008). However,
ice accumulations at lower latitudes that are buried by
surface mantles do show signs of glacial flow.
Extant ice deposits (Neukum
et al.
, 2004) showing signs
of glacial flow have been found on the high mountains of
Tharsis (Head and Marchant, 2003). The temperatures
in these environments indicate that flow would resemble
those of terrestrial cold-based mountain glaciers. Else-
where on Mars, landforms apparently shaped by glacial
flow (Holt
et al.
, 2008) are commonplace. The ice is buried
too deep to be detected by neutron spectrometers but has
shown up in orbiting ground-penetrating radar profiling.
Diverse landforms are present including potential termi-
nal and ground moraines and apparent ice surface features
indicative of viscous flow.
Martian surface landforms show many similarities with
those of terrestrial cold deserts. As previously noted, the
landscape of the
Phoenix
landing site was characterised
by sorted stone patterns, polygonal terrain and a shallow
ice table (Mellon
et al.
, 2009; Levy, Head and Marchant,
2009). Polygons were also observed at the
Viking 2
land-
ing site (Mutch
et al.
, 1976b; Jones
et al.
, 1979). Some
of the most striking evidence for periglacial processes
was documented by Balme and Gallagher (2009) from
the equatorial regions. They reported a diverse suit of
features including pitted mound and cone structures (re-
sembling pingos) and scarps with downslope deposits,
dendritic channels, gullies, blocky debris and hummocky
terminal deposits (resembling retrogressive thaw slumps
in thermokarst environments). They postulate an evolu-
tionary sequence of morphologies beginning with sub-
sidence of polygonised surfaces and ending with scarp-
bounded basins fed by interpolygon channels. This appar-
ently active breakdown of inferred subsurface permafrost
in the martian tropics may be related to the instability of
equatorial ice deposits deposited under different climatic
conditions, as postulated by Head
et al.
(2003, 2005).
3 m) show no textural difference from the sur-
rounding terrain and appear neither raised or depressed
in contrast to it. Evidence suggests that the global rate
of formation has increased over an observation period of
seven years (Schorghofer
et al.
, 2007).
The most widely accepted explanation is that slope
streaks are dry flows, formed by mass flow of dust which
exposes the underlying darker bedrock. These dark streaks
are progressively lightened by deposition of new layers of
dust (Sullivan
et al.
, 2001). This explanation appears to
have been borne out by high-resolution (
∼
25 cm) images
from the HiRISE camera onboard the
Mars Global Sur-
veyor
spacecraft, which showed apparent textural changes
inside a streak as opposed to outside and the inside surface
was slightly depressed with a distinct scarp compared to
outside (Phillips, Burr and Beyer, 2007). Nevertheless, al-
ternative explanations for at least some slope streaks also
appear compelling, e.g. Head
et al.
's (2007) report of slope
streaks in Antarctic dry valleys which are morphologically
very similar and show similar behaviour. The Antarctic
slope streaks form by near-surface melting-derived saline
water travelling downslope along the top of the ice table,
wicking and dampening the surface to cause the streak.
These streaks are also initially dark and then brighten
over time. Slight differences in surface texture were also
reported. Salt efflorescence on the surface of the streak
can cause brightening over the surround terrain, a feature
reported from some martian streaks (Schorghofer
et al.
,
2007) and difficult to explain by removal of surface dust by
dry flow. Without ground truthing from a surface mission
it will be difficult to distinguish between these and other
hypotheses, but multiple formative processes for what is
superficially the same morphology cannot be discounted.
∼
5.5.5.7
Fluvial systems
The surface of Mars is characterised by many differ-
ent channel morphologies. They include dendritic chan-
nels showing high stream order, fretted terrain valleys
commonly inferred to be due to sapping (Harrison and
Grimm, 2005), outflow channels associated with catas-
trophic events (Baker, 2001) and inverted channels (Pain,
Clarke and Thomas, 2007). In addition there are the ex-
tremely young gullies found on crater walls, and occasion-
5.5.5.6 Glacial and periglacial processes
The polar caps of Mars are the most extensive deposits of
exposed ice on the surface. Radar soundings from orbit-
ing spacecraft (Phillips
et al.
, 2008) have revealed well-
