The simple explanation is that the planet's soil is rich in iron oxide, but there's much debate as to why the mineral is so ubiquitous in the Martian environment. The old theory is that the oxidization process began early in Mars' life cycle, when warm water flowed on the planet—water that may have carved out the long, now-barren channels that snake through portions of the planet. Rocks containing iron would have slowly been worn away by rivers and seas, and the oxygen in the water would have combined with the iron to create iron oxide—or, in lay terms, the iron would have rusted into red dust. Flecks of the reddish mineral would then have been dispersed all over the planet via raindrops. Scientists who believe that Mars was once flooded with water have pointed to the abundance of iron oxide as proof of their claims. And where there's water, of course, there may also be life.

Early in its history, Mars was much more like Earth. As with Earth almost all of its carbon dioxide was used up to form carbonate rocks. But lacking the Earth's plate tectonics, Mars is unable to recycle any of this carbon dioxide back into its atmosphere and so cannot sustain a significant greenhouse effect. The surface of Mars is therefore much colder than the Earth would be at that distance from the Sun.

Mars has a very thin atmosphere composed mostly of the tiny amount of remaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%). The average pressure on the surface of Mars is only about 7 millibars (less than 1% of Earth's), but it varies greatly with altitude from almost 9 millibars in the deepest basins to about 1 millibar at the top of Olympus Mons. But it is thick enough to support very strong winds and vast dust storms that on occasion engulf the entire planet for months. Mars' thin atmosphere produces a greenhouse effect but it is only enough to raise the surface temperature by 5 degrees (K); much less than what we see on Venus and Earth.

Mars' orbit is significantly elliptical. One result of this is a temperature variation of about 30 C at the subsolar point between aphelion and perihelion. This has a major influence on Mars' climate. While the average temperature on Mars is about 218 K (-55 C, -67 F), Martian surface temperatures range widely from as little as 140 K (-133 C, -207 F) at the winter pole to almost 300 K (27 C, 80 F) on the day side during summer.

Though Mars is much smaller than Earth, its surface area is about the same as the land surface area of Earth.