The surface of Mars looks almost as if two different planets have been sutured together. The northern hemisphere consists of smooth, low-lying plains; the southern hemisphere is mostly rocky highlands. These highlands are heavily cratered and include several basins— vast circular impact craters that formed relatively early in Martian history. The basins had an important effect on the geological development of Mars and offer some of the best evidence that water once flowed across the surface.

From a distance, the Hellas Planitia basin is one of the most spectacular features on Mars—a circular crater, 1,300 miles across, with a brightly reflective floor and raised terrain all around it. Planitia is a Latin term often applied to Martian basins: It simply means "low plain/' The bottom of the Hellas basin is no exception, lying an average nine miles below the surrounding highlands.

Hellas has been known to astronomers for over a century—it is easily spotted as a bright circular feature through a small telescope. But it took close-up images from the Mariner space probes to reveal that Hellas is actually a crater formed by a massive meteorite impact. The impact site is circled by a rim of material that towers 1.25 miles above the surrounding highlands, which themselves extend for nearly 2,000 miles on every side.
Hellas is the most prominent of several basins on Mars. Others range from the well-preserved Argyre Planitia in the south, to the shallower depressions of Isidis and Utopia Planitia that border on—and in places are overlaid by—the northern plains.

It seems that the basins are among the oldest features on Mars, and date from a time when large, partially formed planetoids were still flying around the inner solar system. The sheer size of the craters, as well as their appearance, suggests that they were formed before the amount of meteorite bombardment in the solar system suddenly tailed off about 4 billion years ago. It seems likely that the surrounding highlands, too, are at least 4 billion years old, and that the northern plains are much younger. One theory is that the plains were resurfaced by lava from massive volcanic eruptions that occurred after the meteorite bombardment stopped.

Something similar may have happened in the basins themselves: The sheer force of the impacts appears to have opened cracks in the Martian crust, allowing lava to pour out and resurface the surrounding area. The impacts also threw up lumps of rock called ejecta that gave rise to many secondary craters, and today lie strewn amid vast fields of debris.

Ejecta from basins is one reason why the highlands might be so much higher than the northern plains, but it is far from the whole story. When scientists used variations in the orbit of the Mars Global Surveyor probe to map local changes in Martian gravity and reconstruct how mass is distributed within the planet, they found that the crust under the highlands extends further toward the center of the planet than it does beneath the plains—in other words, the crust of Mars' southern hemisphere is thicker than it is in the north, a difference that could only have arisen very early in Martian history.

The best explanation for why the crust should be lower, thinner and younger in the north is that it fell victim to a massive impact when Mars was in its infancy. This would have caused it to stay hot and liquefied for longer, covering the tracks whose absence baffled astronomers until so recently.