Mars and Earth compared. Computer artwork comparing the size of Mars (right) with that of the Earth (left). Mars' diameter is half that of the Earth's, it has ten percent the mass and is 1.5 times the distance from the Sun. While the Earth is host to oceans of liquid water and has a relatively dense water-rich atmosphere, Mars is exceedingly dry, cold and, as far as we know, sterile. It is thought to have lost most of its water to space a long time ago, and any that remains is frozen at the poles and locked deep in the ground as permafrost.