Does "Saint Seiya Omega" series have a prequel?
The only thing which comes between Elision and Omega is the movie
Saint Seiya: The Heaven Chapter - Overture. The Lost Canvas is a prequel.

However, according to Wikipedia, it is an anime-original story separate from the original continuity, and you aren't assumed to have watched all the previous series. As far as I know, Mars only appears in Omega, so there are definitely some unexplained points which might get answered in flashbacks or might remain forever unexplained.