Mike Caplinger, Malin Space Science Systems February 1994
The simplest way to see the martian crustal dichotomy is to take a globe of Mars (I'm partial to "Hugg-a-Planet Mars", a stuffed globe of Mars available in many science stores, museum shops, and from the Planetary Society) and try to orient it so that you see as few large craters as possible. On "Hugg-a-Planet Mars," you can orient it such that you can't see any red (which is the color assigned to the cratered terrain geologic unit). This happens when you're roughly looking at the crater Milankovic just east of Arcadia Planitia at about 55 N latitude, 150 W longitude. You will now be looking at almost none of the heavily-cratered terrain of the southern highlands, and all of the northern lowlands and the Tharsis Bulge.
You can also see the dichotomy in the topography alone. I've taken a topographic map of Mars and digitally colored all the low areas with blues and all the high areas with yellows and reds, and then projected the images so that we're looking down at latitude 55 north, longitude 150 west and its antipodes:
The red that you can see in the north is mostly the Tharsis and Elysium "bulges" where hot material pushing up from the interior of Mars has raised the elevation of the planet, and led to volcanic eruptions that have further built up the topography. The blue you can see in the south are the two impact basins Argyre and Hellas. Otherwise, the north is low and mostly uncratered and the south is high and largely heavily cratered.
The dichotomy can also be seen easily in MARS by Kieffer et al, from University of Arizona Press. Color Plate 8 shows the relative ages of the martian surface based on crater distributions. One can easily see that the northern lowlands are young, and the southern highlands are old, but that the boundary is tilted with respect to the equator. We can also see this on our simplified maps of the larger craters on Mars.
The source of the dichotomy is not known, although there are several theories. Some suggest that the lowlands were formed by single or multiple impacts, while others propose that internal processes like those that formed the continents on Earth are responsible. We simply need more data to know for sure.
Mike Caplinger (firstname.lastname@example.org)