Curiosity spotted a mysterious ‘ball‘ on Mars. It looks like a pretty perfect sphere. Image © NASA/JPL-Caltech/MSSS
The Martian ball pictured by Curiosity in sol 746, it’s actually only about a centimeter across. According to NASA scientists it is likely an example of a Martian concretion.
As Ian O’Neill explains for Discovery News, “this cannonball-like object is almost certainly something called a concretion.”
Concretions exist on Earth, too.
A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word ‘concretion’ is derived from the Latin con meaning ‘together’ and crescere meaning ‘to grow’.