Mars is the 4th planet of the Solar System and was featured prominently during Season Twenty.
After having all of his electronic devices broken, Eric Cartman forms a close relationship with Heidi Turner. She briefly shows him her vagina, prior to "Wieners Out", and in that instant he realizes that humans can colonize Mars.
In "Fort Collins", Heidi and Eric are at M Burgers eating, when Eric has a daydream about Mars. In his dream, he imagines a Mars colony with rollercoasters and numerous buildings. Later, after realizing Heidi may be able to access his internet history, Cartman's daydream about Mars becomes clouded with a severe dust storm.
In "Members Only", Heidi and Eric are given a tour of SpaceX by Elon Musk. They are joined by Butters Stotch, who not only has a change of heart, but is also interested in going to Mars. Much to their chagrin, they do not get to go to Mars.
Mars was seen once again in Cartman's daydream in "The End of Serialization as We Know It". Cartman points out that all of the habitants of Mars are female, and that all the males are kept underground being harvested for their semen and comedy. In the dream, Cartman is also seen being forced into the same harvesting devices.