Mars rovers use solid aluminum wheels because rubber tires would shatter in the planet's extreme cold.

The Martian environment poses a severe engineering challenge due to extreme thermal cycling and sharp volcanic terrain. Temperatures at the Martian equator can reach 70°F (20°C) during the day but plummet to −100°F (−73°C) at night, while polar regions drop to −195°F (−125°C). At these temperatures, the polymers in rubber reach their glass transition point, becoming as brittle as glass and shattering upon impact with rocks.To overcome this challenge, NASA's Jet Propulsion Laboratory (JPL) designed wheels for the Curiosity (2012) and Perseverance (2021) rovers using 7075-T7351 aluminum, a high-strength alloy that maintains structural integrity and ductility even in cryogenic conditions. Each wheel is approximately 20 inches (50 centimeters) in diameter and is machined from a single solid cylinder of metal, eliminating seams or weak points that could fail under stress.The wheels are remarkably thin, with Curiosity's wheels featuring a skin only 0.03 inches (0.75 millimeters) thick to minimize weight. This thinness led to unexpected punctures from sharp rocks in Gale Crater, prompting engineers to reinforce the design for the Perseverance rover. The 'JPL' Morse code pattern stamped into the tracks serves a functional purpose called visual odometry. By photographing these marks, the rover's computer can calculate precisely how far it has traveled and detect whether the wheels are slipping in the Martian sand.