A prefabricated mining colony existed on the surface, which held a weak but breathable atmosphere due to terraforming. Individuals could bounce about five metres in one step. The First Doctor, Oliver Harper and Steven Taylor encountered the Vardans on Grace Alone when they attempted to invade Earth.
Presumably, the TARDIS travellers were meant to die on Grace Alone as they read that their bodies were found there. Before they could be killed by a Vardan, Oliver absorbed its remaining energy, dispersing himself to save the Doctor and Steven. (AUDIO: The First Wave)