There are holes in Curiosity wheels. There have always been holes — the rover landed with twelve holes deliberately machined in each wheel to aid in rover navigation. But there are new holes now: punctures, fissures, and ghastly tears. The holes in Curiosity’s wheels have become a major concern to the mission, affecting every day of mission operations and the choice of path to Mount Sharp.
One of the Saturn program greybeards would always talk about designing for the unknown unknowns. In this case, the Curiosity team designed the wheels to counteract some of the problems encountered by the Spirit and Opportunity rovers. What they didn’t know is how much damage would be done by fatigue and punctures. They didn’t know the rover would encounter ventifacts (wind-eroded pyramidal rocks) that are embedded in bedrock, so they don’t move as the wheels roll over them.
The good thing is that they’ve been checking the condition of the rover, they understand the cause, and they can adjust the drive path or drive backwards to minimize further damage. The designers for the next rover, Mars 2020 should take note.