Each wheel tire was machined from a single piece of aluminum. It is 50 centimeters in diameter and 40 centimeters wide. It has grousers (treads) that protude 7.5 millimeters from the wheel skin. Grousers are spaced 15 degrees apart. Unlike Spirit and Opportunity, the grousers are not straight; they have chevron features designed to prevent sideways slip. The skin of the wheel is 0.75 millimeters thick -- the absolute thinnest that could be machined. The grousers provide structural strength; the skin is for floating the rover atop loose sand. There is a slight crown to the wheel to give it strength. There is a vertical rim on each edge of the wheel, again for structural strength. There is another, double rim located about one-third of the way into the tire, the structural stiffener to which the wheel flexures (spokes) are attached. One section of the tire has a set of odometry marker holes drilled into it, which provide a way for the rover's navigational software to measure its driving progress across soft surfaces by photographing the tracks. For fun, these holes spell out "J P L" in Morse code.