Mars Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 144

Fig. 12.23 Close-up view of Martian soil with the Mars Hand Lens Imager MAHLI. At left is the surface of the 'Rocknest' sand shadow, while
at right is a sample of sieved sand (\150 lm) from the interior of the feature. Image PIA16570. NASA/JPL-Caltech/MSSS

Fig. 12.24 A mosaic of Opportunity images showing the view across
Victoria crater, with the ripple pattern on the crater floor. An orbital
view of the crater and the ripples is seen in Fig. ( 8.2 ) , and a close-up of

the rock exposures in the crater wall (which reveals bedding patterns
suggesting the bedrock is an aeolian sandstone) are given in Figs. ( 5.21
and ( 5.22 ) . Credit NASA/JPL PIA08779

computers through the cold Martian nights. The physical
properties of Purgatory dune and the drifts in which Spirit
became bogged down are dramatically different from what
Spirit encountered at 'El Dorado', discussed next.
'El Dorado', Gusev crater. The only large patch of low
albedo (dark) sand visited so far by a rover is the site
called 'El Dorado', on the northern side of Husband Hill,
the largest of the Columbia Hills within Gusev crater.
During its descent from the summit of Husband Hill, Spirit
drove well into the margin of the El Dorado sand deposit
(Fig. 12.25 ). Wheel mobility was fine while driving over
the well-sorted black basaltic sand that comprised the
small dunes and ripples present throughout the deposit.
Excellent microscopic imager data were obtained of sand
(Fig. 2.5 ) , as well as APXS measurements of a basaltic

composition (Gellert et al. 2006; Sullivan et al. 2008; see
also Sect. 4.2 ) . Where the rover wheels dug into ripples,
good assessments could be made of the subsurface, which
was comprised of the same 200-300 lm, well rounded,
coarse sand grains that were visible on the surface of the
sand. Subsequent monitoring of the sand patch by Spirit,
following its visit to the site, revealed that dust devil
tracks were at times visible on the sand, but eventually all
tracks were erased, presumably by a wind event that was
strong enough to activate (even briefly) most of the sur-
face sand. It appears that the mobility maintained by the
sand in deposits like El Dorado allows the wind to move
the grains sufficiently to disturb most dust that might settle
onto the sand, thus maintaining the low reflectance of the
entire deposit. El Dorado likely is the most representative