The reddish plains of Arabia Terra and Terra Sabaea in the centre of the image are notable for the presence of many large impact craters, indicating that they are among the oldest regions on Mars. Along their northern border is a striking escarpment, with a difference of several kilometres in height. This separates the flat, barely cratered plains of the northern lowlands from the southern highlands, which have many more craters. This remarkable change in terrain, referred to as the Martian dichotomy, marks a fundamental topographical and regional division on Mars. This is reflected, most importantly, in the different crust thicknesses, but also extends to the magnetic properties of the crust and its gravitational field. There is still a certain amount of scientific debate over how this crust dichotomy came about. It could have originated from endogenous forces in the Martian interior and thus been caused by mantle convection or tectonics. If exogenous (external) forces were responsible, this effect could perhaps be traced back to one or more major asteroid impacts.