There isn't evidence of cannonballs on Mars, despite recent reports by UFO hunters and conspiracy theorists.
Images of seemingly spherical balls taken by the Curiosity rover are showing rocks created through a known process call concretion. NASA says were formed when liquid that used to sit on the surface of the red planet dried up.
They've been spotted numerous times over the years and NASA even has a nickname for them: blueberries. Conspiracy theorists have been talking about the phenomenon since 2014.