Mars

Mars is the fourth planet from the Sun and the second smallest planet in the Solar System, after Mercury. Named after the Roman god of war, it was once described as the "Red Planet" because the iron oxide once prevalent on its surface gave it a reddish appearance. Mars is a terrestrial planet with a thinner atmosphere than Earth, but thick enough to support a wide variety of Terran life. The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, as is the tilt that produces the seasons.

Mars is the site of Olympus Mons, the second highest known mountain within the Solar System (the tallest on a planet), and of Valles Marineris, one of the largest canyons. Mars has two known moons, Phobos and Deimos, which are small and irregularly shaped.

Mars was a hostile, dead world, until terraforming efforts beginning in the late 21st century melted the polar ice caps, releasing huge amounts of CO2 into the atmosphere, and heating the planet up even more until liquid water was able to flow freely on the surface. Life was introduced, starting with lichens and mosses, and followed by more complex species. In the 2300s, Mars's atmosphere was boosted with Nitrogen from the outer planets.

Mars can easily be seen from Earth with the naked eye.