The Curiosity Rover recently sent home a picture of what looks like a pretty perfect sphere. Unfortunately, this isn't exactly evidence of an active Martian soccer club in the vicinity --  it's just a leftover from the days when Mars was a lot wetter than it is now.

When we spot Martian objects that look unusually familiar (like rocks masquerading as space squirrels, for example) it's easy to get excited. Look at that sphere! It's so smooth. Maybe somebody made it that way on purpose, right?

Eh. The answer is almost certainly no. For starters, the ball isn't as big as it looks in the image -- it's actually only about a centimeter across. And as Ian O'Neill explains for Discovery News, this cannonball-like object is almost certainly something called a concretion.

We have concretions on Earth, too, which is cool when you think about how different these two planets are. Back when Mars had water, it formed sedimentary rock --  and concretions -- the same way Earth does. Water carries and drops sediment, which builds up over time to form rock. But this process leaves pores between grains of sediment, and minerals seep down to form masses inside the growing rock.

A concretion is what's left when the outer layers of sedimentary rock get sloughed away by erosion, leaving just the tough minerals that have seeped inside. Sometimes, they're pretty round.

So while it's admittedly a bit jarring to see a round little ball among the rough Martian rocks, try to keep your imagination in check. And just remember that we've seen way weirder stuff (like perfect lava spirals) before, and will probably see even weirder stuff in the future.