Mars Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 143

Fig. 12.22 Curiosity Mastcam panorama of the floor of Gale crater, showing layered mounds in Mount Sharp (aka Aeolus Mons) with dark
dunes in the foreground. An aerial view of these dunes was obtained during the parachute descent of the rover Fig. ( 18.3 )

temporarily stuck. Sullivan et al. (2005) review aeolian
processes observed along the traverse to Endurance. Gol-
ombek et al. (2010) consider the interaction of ripples with
small impact craters to constrain relative timescales of
movement.
Victoria crater, Meridiani Planum (*800 m diameter).
Victoria crater is substantially larger than Endurance crater,
and thus the ripple field on its floor (Fig. 12.24 ) is corre-
spondingly larger as well. Opportunity drove around
roughly one-quarter of the rim of the crater, searching for
the best path to gain access into it. During this drive,
Opportunity crossed the surface of a broad dark wind streak
that emanates from the northern side of the crater (Fig. 8.2 ),
becoming the first rover to investigate a wind streak on
Mars. Inspection of the crater walls (Figs. 5.21 and 5.22 )at
Cape St. Mary and Cape St. Vincent indicated crossbedding
suggesting of an aeolian origin. Opportunity documented an
increased quantity of basaltic sand on the surface of the
streak (relative to outside of the streak), supporting the idea
that the sand in the streak is derived from sand within the
crater, blown out of the crater in a northerly direction (at
present), and possibly eroding the rim in the process to form
the distinctive scalloped appearance of the crater rim
(Geissler et al. 2008). Opportunity eventually entered the
crater on the northwest rim, but it only descended to the
point where the bedrock became covered by fine detritus,
beyond which posed a threat to the rover being able to leave
the crater. The cross-bedding in the walls of Victoria crater
has been studied by Hayes et al. (2011) to infer wind
transport of sediments prior to the formation of the crater.
Endeavour crater, Meridiani Planum (22 km diameter).
After nearly three years of driving south from Victoria
crater, in 2012 Opportunity began to explore the rim
deposits of by far the largest crater yet examined by a Mars

rover. Several separate dune patches are present on the floor
of this broad crater, some of which are visible in distant
views from Opportunity, but orbiter imaging provides the
best viewing option thus far. The dunes in Endeavour crater
have been documented to move at a rate that is substantially
greater than what has been observed in HiRISE images
from elsewhere on the planet (Chojnacki et al. 2011). As
long as Opportunity remains healthy, the rover might

eventually drive closer to the dune fields.
'Purgatory dune', Meridiani Planum. Megaripples on
Meridiani Planum steadily increased in size as Opportunity
traveled south from Victoria crater. Finally the rover got stuck
in one of the megaripples for about six weeks—we discuss the
challenges of driving on sand and dust on Earth and Mars in
Sect. 5.3 . Termed 'Purgatory dune', the rover became trap-
ped when several of its wheels broke through an indurated
upper surface on the megaripple, and the fine-grained sedi-
ments inside the feature provided little traction to the wheels.
Opportunity eventually extracted itself by repeated com-
mands for drives that should have covered many meters but
actually only moved the vehicle a few centimeters; engineers
determined that the rover wheels were experiencing [99 %
slippage during the extraction drives. Another ripple on
which slippage was subsequently encountered (but detected
onboard) is shown in Fig. 22.12 . Jerolmack et al. (2006) have
considered the sorting of sediments on ripples at Meridiani.
Spirit also got stuck in fine drifts that are visually quite
different from Purgatory dune, so not all collections of fines
must necessarily be associated with megaripples. Indeed,
the demise of Spirit came about because it became stuck
while attempting to drive to a site where it could orient its
solar panels toward the sun during local winter; while it was
stuck, the solar panels received insufficient sunlight to
maintain heaters to warm the spacecraft instruments and