Part of my 2009 flag proposal series. SVG files of the flag and its construction sheet are available on request.
Note: My choice of design does not reflect my political views.
The current design
I think that the current de facto flag of Mars looks horrible and mainly represents terraforming which is only a small aspect of the planet Mars. Therefore, here is my proposal.
First off, this design is unique enough to be distinguishable, yet simple enough to be remembered by a child.
I will not explain the design because I think that I have made it intuitive enough. The design is neutral so that it can represent the planet Mars in any aspect. Note that I have not included symbolism of terraforming and/or space exploration because they represent terraforming and/or space exploration, not the planet Mars.