Extraterrestrial arid surface processes - Arid Zone Geomorphology - page 66

et al. , 1997). The spacecraft did not remain operational
for long enough to detect surface changes.
The Spirit Rover (Crumpler et al. , 2005; Haskin et al. ,
2005) was deployed after landing in Gusev crater. From
its landing site on the basaltic crater floor covered mostly
by very small loose rocks and scattered wind ripples, the
Rover drove across to the Columbia Hills, composed of
bedrock predating the crater fill. Spirit then climbed Hus-
band Hill and descended past some very dark basaltic
dunes to the volcanic feature known as Home Plate. At
the time of writing Spirit had travelled more than 7 km
but was badly bogged, a not unusual experience for a
desert traveller. Noteworthy images have been obtained
of tafoni-style weathering, insolation fracturing and well-
developed ventifacts that illustrate a range of processes
operating on the surface that are also common in ter-
restrial arid environments. Spirit has also observed dust
devils that have removed dust from the surface, as have
short-lived wind gusts (Greeley et al. , 2006). Heightened
dust deposition during and after regional and global dust
storms has also been observed.
The Opportunity Rover landed on almost the opposite
side of the planet at Meridiani Planum (Soderblom et al. ,
2004; Sullivan et al. , 2005). The landscape here is very
different, comprising an extremely flat plain of horizontal
evaporitic sediments punctuated by variably eroded im-
pact craters. The surface is mantled by a lag of ironstone
concretions eroding out of the sedimentary bedrock and
veneered by small dunes or large ripples of basaltic sand.
At the time of writing Opportunity has covered more than
12 km in its traverse from its landing site, visiting suc-
cessively larger craters that expose progressively deeper
parts of the stratigraphy. In the course of its journey the
Rover has been twice bogged, once seriously and once
briefly, but was successfully extricated on both occasions.
Noteworthy images reminiscent of terrestrial arid envi-
ronments have included dedo-like projections (Thomas,
Clarke and Pain, 2005) attributable to wind erosion, sur-
ficial soil crusts, probably from salt cementation, and a
range of wind ripples and small dunes. As with Spirit ,
dust devils and wind gusts have removed dust from the
surface (and the Rover), and heightened dust deposition
during and after regional and global dust storms has been
measured. Winter frosts have been imaged on the Rover.
Phoenix , in the most recent mission to land on Mars,
touched down on the martian arctic plains at 68 ◦ N (Smith
et al. , 2009). It revealed a very flat landscape dominated
by patterned ground with polygonal cracking, mounding
and stone sorting (Mellon et al. , 2009), similar to those
of terrestrial polar dry deserts (Levy, Head and Marchant,
2009). The efflux from the landing rocket engines re-

table at a few centimetres depth. The polygonal patterned
ground occurred at two scales, 20-25 m large polygons
and small 2-3 m polygons. The properties of the soil were
largely unexpected. Rather than being loose and acidic, it
proved to be exceptionally sticky and alkaline, containing
traces of perchlorate salts, which significantly lowered
the eutectic temperature of the water and may control
regolith-atmosphere water exchange (Smith et al. , 2009).
A number of active processes were observed over the
course of the mission, despite its relatively short dura-
tion. These included the partial sublimation of water ice
exposed by the rocket efflux and in excavations, frost de-
position as winter approached, dust storms and water ice
snow.

5.5.5 The geomorphology of Mars

Martian surface features are diverse (Figures 5.5 and 5.6)
and are only summarised below with respect to the most
significant geomorphological elements related to aridity.
Other processes - cratering, tectonics and volcanism - are
also highly important but are not considered here.

5.5.5.1 Lakes

Mars presently has no bodies of standing water, frozen
or unfrozen. In the past, especially in the Noachian and
Hesperian eras when the hydrological system appears to
have been more active, there were lakes (Irwin et al. ,
2005) and possibly a northern sea (Carr and Head, 2003).
The Opportunity Rover has imaged a diverse suite of sul-
fate evaporites associated with ancient lake deposits on
Meridiani Planum, including lake bed, lake margin and
lake derived dune facies (Grotzinger et al. , 2005). The
complex facies mosaic of these sediments suggests depo-
sition in groundwater-fed discharge complexes, of which
the Australian boinkas may provide a terrestrial analogue
(Baldridge et al. , 2010).

5.5.5.2 Soils

The complex processes of the martian surface, especially
the potential role of ephemeral moisture (Thomas, Clarke
and Pain, 2005), can allow the formation of differentiated
surface regolith with some affinities to soils of both hot
and cold deserts. Soil-like features have been observed at
all landing sites to date, including resistant layers ( Viking 1
and Viking 2 , Mutch et al. , 1976a, 1976b), highly variable
mechanical properties of fine-grained surface materials
from unconsolidated to high compressive to strong resis-