Further to this post, and this webpage which it links to, discussions with potential entrants to the Iris Wildthyme of Mars open submissions competition have thrown up a few points of clarification.
1. I'd like the competition (and I really should have been clear about this) to be for unpublished authors. If you're a published author of fiction, you're very welcome to pitch a story for the anthology, but it'll go through a rather different route, to keep the field clear for the newcomers. Contact me at firstname.lastname@example.org for further instructions.
2. Multiple entries per person are allowed, though not especially encouraged. They'll make my life a bit of a pain, but I didn't explicitly rule them out when I had the chance, and it would be unfair to now. Don't say I never do anything for you.
3. While using anyone else's copyrighted characters or settings is a definite no-no, fiction which is in the public domain in the UK (where Obverse operates) is fair game for inclusion in the anthology. The UK's copyright laws are specific, though, that all works remain in copyright for 70 years after the author's death, so some material which is in the public domain in the USA is off limits -- most notably HG Wells's (1866-1946) The War of the Worlds. (The two most significant early Mars texts which are out of copyright in the UK are probably Gulliver of Mars and Edison's Conquest of Mars.)
Hope that all makes sense. Happy typing...