There's an old penny on Curiosity that the robotic probe uses to calibrate the Mars Hand Lens Imager at the end of its arm. The rover recently took a hi-res close-up of the coin, and as you can see, it's getting a little dusty.
As the Planetary Science Institute reports, the image, which was snapped on October 2nd, shows that "during the penny's 14 months (so far) on Mars, it has accumulated Martian dust and clumps of dust, despite its vertical mounting position."
The image of the coin, a 1909 VDB penny minted in Philadelphia during the first year that Lincoln cents became available, was beamed back by Curiosity in ultra high-resolution; at 14 micrometers per pixel, it's the highest resolution image that MAHLI can take.
"This image was obtained as part of a test," writes PSI. "It was the first time that the rover's robotic arm placed the MAHLI close enough to a target to obtain MAHLI's highest-possible resolution. The previous highest-resolution MAHLI images, which were pictures of Martian rocks, were at 16-17 micrometers per pixel. A micrometer, also known as a micron, is about 0.000039 inches."
Here's what the coin looked like on September 9th, 2012 — day 34 of the mission:
Related: Curiosity takes her own upskirt shot.