[ChadGore]

Re: Look at the tracks. Every few feet there is a hashmark. What are those?

The rover wheels have no "axle" in the classic sense of an automobile.

Each of the rover wheels are attached to their "drive train" on the tread side, inside the wheel. Thus, their fasteners leave marks in the sand as the rover wheel turns.

Note that in the above picture, the guy is holding the wheel up in his left hand, over his left shoulder . . it's as big as a human head.

More here: http://www.2020hindsight.org/2002/09/22.html

[Don Joe]

Each of the rover wheels are attached to their "drive train" on the tread side, inside the wheel. Thus, their fasteners leave marks in the sand as the rover wheel turns.

I looked at that site (http://www.2020hindsight.org/2002/09/22.html), and it says that "This rover wheel is constructed from a single slug of aluminum."

That being the case, and looking at the photo (which bears out the "single-billet theory" :), it looks like the "hash marks" visible at the bottom part of the tread are milled into the surface to give a visual distance indicator in the tracks it leaves in the dirt. (i.e., they can see the marks in the tracks, and calculate actual distance, since the wheel's diameter is known.)