That were cast free from Mars

Can you please explain what is "cast free" in this context?

If Mars have life before earth had life, it is possible for there to have been bacterial stowaways in the nooks and crannies of the rocks that were cast into space. There's some hardy bacteria that we already know exists on earth - survives extreme temperatures, pressures, freeze-dried, reconstituted, radiation. The hostile environment of space would be nothing to some of these bacteria, it may be that life on Earth was seeded by bacterial stowaways on rocks that were cast free from Mars.