The Mars 2020 plans to create a cache with samples for a future mission to grab and bring back. What are the advantages of doing one mission to create the cache and one mission to bring the cache back to Earth, rather than doing a single mission that collects the samples and brings them back?
At first, I thought that it might be more convenient to have one rover gather everything into one spot. Then you have all the samples in a single spot, so that you just have to land a second non-roving spacecraft, grab and take off again. But from further reading, it appears that the caches will not be all left in the same place. So you would still need a rover to roll around the surface to pick the caches up.
What are the advantages of separating the sample return mission into two missions ?