Mars' Caves? Or even Mars's Caves?
Both are correct.
Here is a good discussion
I've just finished creating a short movie about the caves of mars and would like to name it "Mars Caves".
Is it grammatically correct?
Or should it be: Mars' Caves? Or even Mars's Caves?
Does the fact that it is a proper noun influence the name?
Otherwise, whether you get the apostrophe in the right place or not, others will write it incorrectly in reviews etc. People will always have trouble doing a search for it, and other people will keep asking you how it's spelt.