Re: How was zero meridian deturmined for the planet Mars.

Excellent question!

I can only find a partial answer to the question, but it should give you a good idea. In some sense it is an arbitrary distinction. The prime meridian is defined to pass through the center of a particular crater, Airy-0. All other things being equal, it must pass through the north and south poles, so the meridian is defined. This definition was adopted by the IAU---International Astronomical Union which has the task of choosing names for all features of the other planets and objects in the solar system, besides Earth.

I hope that this helps.

Steve