Like most of Garden's moons, Yerthad was larger than its parent. In fact, it was two size categories larger, with a diameter of 395 miles (636 kilometers). It was a spherical body.
Yerthad was notable for having an extremely poisonous atmosphere that was also very thick. The surface of the moon was covered with a sea of vitriol, and fumes from the acid rose to an altitude of about 100 miles (160 kilometers).