Mars Climate, Now and In the Past
The Cooling of Mars
Mars is small. This means that it cooled off very fast.
Mars cooled from the outside, inward, like hardening candy. This caused the crust of Mars to thicken and get hard. This means the planet couldn't have movable plates like Earth.
Much later, Mars finally warmed a little from inside. Then a bubble of material rose from the deep interior of Mars and raised the crust, and created volcanoes and valleys.