Mg(ClO4)2

The magnesium of the magnesium perchlorate which helps keep the water flowing on Mars. The first extra is the sodium of the sodium perchlorate -  NaClO4 - also frees up the water. Let's hear it for these unassuming potential harbingers of life. And thank you also to Theodore Gray and Nick Mann for their magnificent book, The Elements: A Visual Exploration of Every Known Atom in the Universe. Highly recommended to the armchair scientist.

Meet The Elements - They Might Be Giants