I am not an expert in gender diversity in the sciences, but through my reading and talking about it with scientists, it appears that big strides have been made in the past decade or two, but the goal of gender equity is obviously still a ways off.
One thing I do know is that all disciplines need role models, and I just found out about an interesting web site called Women in Planetary Sciences (motto: "Women make up half the bodies in the solar system. Why not half the scientists?") which has a series of interviews with women planetary scientists. I know a few of these folks -- like Emily Lakdawalla, Heidi Hammel, and Sara Seager, names that may be familiar to regular BA readers -- and reading their stories is pretty interesting.
If you know a girl or woman interested in planetary science, or any science, then please send them that link. I think they'll find some encouragement and support from the words of these women who have been so successful in exploring the Universe.