I am not natively English speaking and I was wondering about this spelling when I saw the title of the movie “The Martian”.
This pattern also seems to apply to other things ending on an “s” sound, like
- Venus -> Venutian (though it’s less common than Venusian)
- Venice -> Venetian
On the other hand, there is examples showing the exact opposite, like
- Paris -> Parisian
- France -> French
- Los Angeles -> Angeleno (probably derived from its hispanic origin)
Why is it not “Marsian” and “Venecian” like it is in German for instance (“Marsianer”, “Venezianer”)?
Is there a rule for how demonyms are spelled or do they evolve historically?
There is no general rule about this kind of alternate word stem used when forming demonyms. Each pair you list has a different history that explains the form used. The different stems or suffixes used in the words for the place and the inhabitant are usually the result of historical evolution in other languages, so the rules are different depending on the source language and the time the word entered English. (For example, as AP mentioned, the t in martian is taken from Latin forms of words relating to Mars. Angeleno appears to be derived from Spanish angeleño with the ñ converted to an n. French is derived from a fusion of a root meaning France plus a native English suffix that has a completely different etymology from -ian.)