Image: NASA/JPL-Caltech/Univ. of Arizona
A rolling stone in Mars gathers no moss, because there's no moss in Mars, silly! But there are boulders that roll down the slopes of Mars, as this HiRISE camera on board NASA's Mars Reconnaissance Orbiter show.
Phil Plait of Bad Astronomy wrote:
What started it up? A Marsquake, a nearby impact, the erosion of its underpinning due to the relentless Martian winds?
You can see the boulder at the bottom of the image, its shadow stretching off to the upper right. The rock is not quite large enough to clearly see any shape or features on it; it’s probably only a few of meters across.
Link | Larger pic over at HiRISE's Beautiful Mars Tumblr