Named for the Roman god of War, Mars probably got this name due to its red color. It is sometimes referred to as the Red Planet. The name of the month March derives from Mars. Known since prehistoric times, Mars is still a favorite of choice for human exploration.
The density of Mars is about 30 percent less than that of Earth (3.94 g/cm3 vs. 5.52 g/cm3). Its core is probably similar to Earth's, mostly iron, with small amounts of nickel, but spacecraft mapping of its gravity field seem to indicate that its iron-rich core and mantle are a smaller portion of its volume than on Earth. Also, its smaller magnetic field than Earth, indicates a solid, rather than liquid core.
Mars' atmosphere is 95 percent carbon dioxide, nearly 3 percent nitrogen, and nearly 2 percent argon with trace quantities of oxygen, carbon monoxide, water vapor, ozone, and other trace gases.
