The TES and THEMIS instruments (MGS and Odyssey respectively) transform the browns and greys of the rocks you see on Earth or on Mars into all the colors of the rainbow, blues, pinks and greens, and we use those colors to tell what the rocks are actually made of. But THEMIS is going to have much better spatial resolution than TES, and so we hope to solve some of the puzzles left even after MGS's fantastically successful mission. Specifically, since TES saw traces of hematite, which we think formed in water, we want to see if we can find carbonates, which also form in water and which we've not really seen on Mars before. Some say that on Mars carbonates just break down in the UV radiation, but if we saw them that's another clue that there really might have been lots of water and even maybe life on Mars.
I guess my interest in science is also partly personal. In the back of my mind is the idea that someday, perhaps, I'll get to go to Mars--and if I can't go then understanding the geology of Mars is the next best thing! If humans go, I don't think we should colonize Mars, terraform it, and make it earth-like. I think it should be like going to a National Park, like the Grand Canyon which I recently visited. You go to learn and wonder, not to make it like "back home"!
My advice to middle schoolers who think science sounds like fun? Whatever you do, do what you enjoy: if you have a hobby that you really love, try and find a career that builds on that hobby. But I know that when I was in middle school, we did all the equations and we never had much perspective on what it all meant. We never knew why the equations mattered--and we certainly never knew that people had careers that they enjoyed in which they were using this knowledge. In actuality, most working scientists are trying to find out answers to questions they find personally particularly interesting, and they really care about what they are doing. Students should find out more about that...
One of the reasons I think that geology spoke to me is that I realized geology was why we were doing science: geology synthesizes physics and chemistry and you can see them in action. In fact, you can go anywhere and see my subject, you can just walk outside your classroom and there it is! That's why I like my job and my career.