TakeruK: Could you clarify that? From the website, "Students who are not sure whether they want to work in astrophysics or in other areas of physics should apply to the Physics Department." That's why I thought I needed to apply to the physics department. Is this true from your experience?
When I was applying to PhD programs, I already knew that I was 100% certain that I wanted to be in the Planetary Sciences department at Caltech, and not Astronomy nor Physics. So it made sense for me to apply directly to PS. In your case, since you are considering physics vs. other forms of planetary science then it might keep more options for you to apply to the Physics department. However, if you are certain you want to work on Mars data then your best bet really is the GPS division (Geological & Planetary Science). There is a ton of data available now from Curiosity/MSL. The 2020 rover would probably be out of the typical timescale of a PhD though, but maybe you can find something!
It's definitely true that you need a strong PGRE score to apply to the Physics department. It is not true for Planetary Sciences. I would imagine it's somewhere in between for the Astronomy department. By the way, if you didn't already know it, Planetary Science is in the GPS division while Physics and Astronomy are both in the PMA (Physics, Math, Astronomy) division, so the structure of our programs are fairly different!
Again, in your case, your listed research interests are really broad! This is not necessarily a bad thing, but the only thing in common with all your interests is Physics, so it does make the most sense for you to apply there. Unless, you know for certain you want to do Mars related things -- then you should go to GPS. The PS department works a lot with the Geology and Geophysics department (all within GPS) on Mars related things. The PS department also works a lot with Astronomy on exoplanets and PS works with Chemistry on astrochemistry. As far as I know, there isn't a lot of astrobiology going on -- the closest we have is studying atmospheres of planets in our Solar System and in extrasolar systems, and again, all of this is going on in Planetary Science or Environmental Sciences (also in the GPS division). However, I think we actually have fewer connections with the Physics department. I don't really know anyone who works with anybody in Physics but maybe I just don't know the right people!
If you want to talk further about the PS department / GPS division, send me a PM!
My adviser recommended Caltech because of my fluorimeter work and because one of his good postdocs came from there. My other adviser recommended Berkeley physics (hoping I could get around the EPS bias against Berkeley undergrads so I could do work on MAVEN results) or University of Colorado -- Boulder physics (where I can also do MAVEN research).
I find Caltech interesting because it is close to JPL and has research connections to a broad swath of projects (from astrobiology to plasma to astrophysics, plus the 2020 Mars rover will be designed there).
Those are all great reasons to go to Caltech. I didn't mean to question your choice of Caltech, I was just curious that you only mentioned one single school.
I'll assume that you know most people choose a wide range of schools that meet their research and other interests too! Good luck!