Average speed of light travel time longer than block time.
Not a huge issue but should probably wait for multiple confirmations on transactions.
Discussion
Actually now that I think about it chain on Mars and chain on Earth would deviate pretty quickly being that many minutes out of sync if there's significantly more hashpower on one or the other.
What I think is that if we want to adopt Bitcoin on other planets, the one way is probably with another layers or side chains. The distances are just too great.
Expect more shitcoins to be made for space travel industry. Bitcoin won't work as it is.