The Mercer Sphere is one of Satisfactory’s alien collectibles, found by exploring rather than building. It looks like a dark, cracked rock hanging in the air with a glowing purple core, so it’s easy to spot on the cliffs and floating islands where it turns up.
Picking one up drops it straight into your inventory. You don’t use it on the spot; like the other alien finds, a Mercer Sphere is spent later. They don’t spoil and they stack in your inventory, so grab each one you spot and store it until you need it.
They’re easy to walk past, so take the short detour when you spot that purple glow.
Keep going: the walkthrough overview lists every part of the series in order.