Nitrogen is so vital because it is a major component of chlorophyll, the compound by which plants use sunlight energy to produce sugars from water and carbon dioxide (i.e., photosynthesis). It is also a major component of amino acids, the building blocks of proteins. Without proteins, plants wither and die.
I believe studying the hydrosphere would be most important because we haven't explored a good majority of it yet so we need to explore our waters before we explore space
