Moving on Sand - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 255

Fig. 22.11 Rover tracks on a ripple on Meridiani Planum, Mars, identified by on-board software on Sol 603 of Opportunity's mission as having
dangerous wheel slippage. The rover was recovered by backing out. (Note that due to a wheel actuator failure, the rover was driving backwards,
hence the rear wheel reaches the maximum extent up the ripple.) Image credit NASA

including vast fields of ripples (or TARs, 'Transverse Aeo-
lian Ridges'—see Chap. 5 ) in which sinkage or wheel-slip-
page often occurred (e.g., see Figs. 1.7 and 22.11 ).
On the 446th sol of operation on Mars, during which it was
supposed to perform a 50 m autonomous ('blind') driving
run, the Opportunity rover achieved only 2 m of forward

progress, as it buried its wheels in a ripple. It then spent almost
40 days stuck on this ripple that was nicknamed, for obvious
reasons, 'Purgatory' (e.g., Arvidson et al. 2011).
The wheel slippage could be documented by comparing
the number of wheel revolutions measured by a counter
with the forward progress measured onboard by comparing