You discover a new moon orbiting saturn. it has liquid water, energy, and carbon. which essential ingredient for life as we know it is missing?

The next missing ingredient or element here is nitrogen. The next possible answers would be phosphorus and sulfur, respectively in their average concentration in living things.

There are many reasons why nitrogen is necessary. One importance is that nitrogen is one of the components of amino acids. Amino acids are the units needed to make proteins. And proteins is pretty much what makes up all living things.