Question: how dense are the rings of saturn?
Answer: Knowing its mass (about 2.8×109 kg) and its extend (from 1.527 to 1.951 Saturn radii, with a typical thickness of 100 m), you can deduce a mean density of about 0.0167 g.m−3 (about 105 times less dense than the terrestrial atmosphere at sea level). Just for fun: This gives you an idea of what means the optical depth.