Today's Image of Mars is a topographic map of the "highland-lowland dichotomy boundary," one of the most mysterious features on the Red Planet. This map shows the drastic difference in elevation between the northern and southern hemisphere.
The northern hemisphere on Mars is 1-3km lower than the southern hemisphere. The crust in the northern hemisphere averages 32 km thick, but the crust in the southern hemisphere averages 58 km thick. The crust in the southern hemisphere is much older than that in the northern hemisphere as evidence by the much greater density of impact craters in the south.
There are a few theories as to how this dichotomy on Mars developed.
- One involves a Pluto-sized object impacting with the northern hemisphere of Mars around four billion years ago. Scientists believe the impact created the North Polar Basin (also called the Borealis Basin), which is a depression that covers 40% of Mars, measuring 10,600 km and 8,500 km. If this was proven true it would be the largest impact crater ever discovered, beating out the Moon's South Pole-Aitken Basin.
- Another theory is less interesting; it involves tectonic processes. Scientists theorize that one half of Mars could have gone through an upwelling, when the other half went through a downwelling, as the result of mantle convection.
- The third theory suggests that the Borealis Basin is actually the result of multiple impacts. This theory has the least evidence to back it up and is statistically very unlikely to have occurred.
The highland-lowland boundary is what separates the northern lowlands from the southern highlands. It is full of mesas, knobs, and flat valleys with walls about one mile high. Surrounding these features are lobate debris aprons, which have been determined to be rock-covered glaciers. Many large river valleys cut through the highland-lowland boundary. There are many regions in the boundary that are of great interest to scientists because they contain landforms which were likely formed by the movement of ice.
Clicking on this image will take you to the very thorough Wikipedia article on the Martian dichotomy.