There are lots of stories, even on SOL where residents of Mars are "Martians". How does the "t" get into a resident of Mars? Marsian would make more sense to me. Or Marsan, like a resident of America. Maybe even Marsish like a resident of Britain is British. And England has English. Germany has German, I could go for Marsan, oops I already voted for Marsan in connection with American. France and French are hard to follow. But Mars and Marsh would make more sense than adding a "t". We could add "er" to Mars to make Marser, it would be like New Zealander. I prefer something like Greenies, from a current serial on SOL following an earlier highly rated story here.
If you are from a Mart, Martian would make some sense. Someone who farts is a farter. If you barter you are a barterer. Barter is to trade. You are either a trader or a barterer. There are lots of historical uses of Martian in stories from hundreds of years ago. Why?