The last nine years I have spent working on what became the Genesis mission with Don Burnett. It is typically a short-term position, lasting about two years. It is done right after getting your Ph.D. Typically in the old days you became a professor right after graduation. These days the job market often doesn't support that. You work a few places, not in the same place all your life.
Different people have different outlooks, they study different aspects of things.
If you are studying meteorites, you may start with studying their chemical composition, then do a postdoc stint studying their mineralogical composition. That's a rather simplistic example. Then I wanted to study these elements in the interior of the Earth. It is slightly related, but not totally related. If you understand the noble gas composition of Martian meteorites, you can understand something of how the interior of Mars has been processed.