Mercury is a Grimoire card from the original Destiny (Destiny 1), found under Places → Mercury. Grimoire cards held most of the game's lore and were read on Bungie.net and in the companion app, not in the game itself.
Card text
Legends say that Mercury was once a garden world. Now its surface, baked in the Sun's glare, is honeycombed with forgotten Vex structures. No life stirs here. Not anymore.
Grimoire text written by Bungie, shown as it appears in the Destiny 1 game files.