Mars is divided into two fundamentally different geological provinces of approximately hemispherical extent1–3. The more southerly province is heavily cratered, contains relatively old geological units, and superficially resembles the lunar and mercurian highlands. The northern province is relatively lightly cratered and contains younger geological units, including extensive plains, volcanic edifices, and volcanic calderas. Except for the Tharsis and Elysium regions and other large volcanoes, most of the younger, northern province consists of lowlands, which lie an average of 3 km below the highlands. Lowlands occupy about one-third of Mars. They are separated from the highlands by a distinct scarp or by a sloping transitional zone as much as 700km wide in which highland materials have been disrupted and partly replaced by lowland deposits (Fig. 1). The transition is characterized by a variety of landforms unknown on other planets. The highlands and lowlands are in isostatic equilibrium across the transitional zone4. No generally accepted explanation for the cause of the highland-lowland dichotomy has been proposed although thinning of the lithosph¨re in the northern hemisphere by internal processes has been suggested5. Large impacts, which were a feature of early Solar System history, have also been suggested as an important cause of such early planetary-surface heterogeneities3,6,7. Subcircular lowlands such as Argyre, Chryse, Hellas, and Isidis Planitia and the plains south of Tharsis occupy previously recognized impact basins8–10. We propose here that the largest expanse of lowlands also results from formation of a large impact basin early in Mars' history that has profoundly shaped the planet's surface.
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Wilhelms, D., Squyres, S. The martian hemispheric dichotomy may be due to a giant impact. Nature 309, 138–140 (1984). https://doi.org/10.1038/309138a0
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