Mars is a Grimoire card from the original Destiny (Destiny 1), found under Places → Mars. 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
In the years since the Collapse, much of Mars has returned to the desolation that came before the Traveler. But buried beneath its surface are traces of the glimmering mega-cities that once sprawled across the planet, swallowed by dunes and Darkness.
Grimoire text written by Bungie, shown as it appears in the Destiny 1 game files.