One of the enduring enigmas of Mercury is why such a small planet, roughly 38 percent of Earth’s diameter, is so dense. From Mariner 10’s 1974–5 flybys of the innermost world and now the orbiting MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, we know Mercury has an average density of 5.43 grams per cubic centimeter. (Earth’s average density is 5.52 g/cm3.) This value means the planet . . .Astronomy magazine subscribers can read the full answer for free. Just make sure you’re registered with the website.