Why is Mars similar to Earth?

Both have solid, rocky surfaces, an atmosphere and water. However, Mars has a very much thinner atmosphere, almost no free oxygen, and most of the water is frozen.

The length of a day on both planets is around 24 Earth hours (Mars 24.62 hours) even though Mars is only about half the size of Earth.

Because Mars has no oceans, the available land surface there (practically all of the surface) is about the same as the dry-land surface on Earth.