"Mars. The red planet. Cold, dry, inhospitle to life. But billions and billions of years ago, Mars look like this. There were lakes, rivers, oceans, the perfect condition for life to fall. So...what happened; Why was life unable to take hold? Where did the water go? We may never know."
―Neil DeBuck Weasel explaining the mystery behind Mars.[src]
Earth-like Mars
Inhabitants Mars Creatures

Mars is a planet in The Solar System and 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 is often referred to as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth.

History (Ice Age: Collision Course)

Mars used to be very similar to Earth, having rivers and supporting life, such as strange fish-like creatures. All was well on the planet, until, Scrat, a saber-tooth squirrel, crashed a UFO into Mars, destroying all life on Mars. It made the planet red as it is today.