One wonders where else in the solar system moss from Earth may have taken hold. Spores and such could be making the journey randomly from gravity well to gravity well.

A mechanism by which this could happen:

>A Martian meteorite is a rock that formed on Mars, was ejected from the planet by an impact event, and traversed interplanetary space before landing on Earth as a meteorite.

https://en.wikipedia.org/wiki/Martian_meteorite