Mars is a planet inhabited by alien cows. An alien is sent to come and "abduct" cows from you and gives you rubies in return. The amount of rubies you receive depends on what cow the alien asks for. You can then click the mars icon near the bottom right and warp to mars.
On Mars you can get an egg for 3 rubies. It then acts like a crate and opens to reveal an alien baby cow. As you progress, larger cows can also hatch from the eggs.
Alien Cows EditFarm