For me, Marte sounds very youthful, bright and energetic, so that when I imagine a Marte, I see a young, smiling, courageous, feminine, but sporty, straight-forward, simple girl. Although I think it can grow and age with a woman, same way as Martha and Marta do. The difference is that for me, Marte doesn't have that little bit staid feel that Martha can have on an older woman for some people.