Images captured on March 19 show two fresh breaks in the raised treads of the left middle wheel that were not present when the wheels were last checked on January 27. The landscape is strewn with rocks, and all those sharp bits are taking a toll on NASA's Curiosity rover. It has driven a total of 9.9 miles since the mission's August 2012 landing on Mars.
In addition to regular monitoring, a wheel-longevity testing program was started on Earth in 2013 using identical aluminum wheels.
Curiosity Project Manager Jim Erickson says the left middle wheel is "nearing a wheel-wear milestone", but "all six wheels have more than enough working lifespan remaining to get the vehicle to all destinations planned for the mission". Until now, two grousers of the left middle wheel have broken.
Information from NASA's Curiosity Rover has recently revealed that wind on Mars may be responsible for shaping the planet's Gale Crater.
Curiosity's wheels are each about 20 inches in diameter and are made of solid aluminium.
Ground-based tests conducted by NASA show that a wheel reaches 60% of its useful life when three of its grousers have broken. Each wheel also has 19 zigzag-shaped treads (the grousers) that bear most of the rover's weight and provide traction for drives.
As rover examines higher and younger layers, it raises concern that driving could become more damaging to Curiosity's wheels.
The Rover is now examining sand dunes partway up a geological unit called the Murray formation.
"This is an expected part of the life cycle of the wheels and, at this point, does not change our current science plans or diminish our chances of studying key transitions in mineralogy higher on Mount Sharp", Ashwin Vasavada of JPL, Curiosity's project scientist, said in the NASA statement.
Ten miles seems like a short distance for a wheel to break, but it took the rover a few years to travel that much - a few years of super slow rolling over sharp, jagged rocks. Planned destinations ahead include the hematite-containing "Vera Rubin Ridge", a clay-containing geological unit above that ridge, and a sulfate-containing unit above the clay unit.