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The northern part is an enormous topographic depression. About one-third of the surface (mostly in the northern hemisphere) lies 3–6 km lower in elevation than the southern two-thirds. This is a first-order relief feature on par with the elevation difference between Earth's continents and ocean basins. [12]
The same methodology was later applied to the Moon [1] and then to Mars. [2] Another stratigraphic principle used on planets where impact craters are well preserved is that of crater number density. The number of craters greater than a given size per unit surface area (usually a million km 2) provides a relative age for that surface. Heavily ...
Mars hosts many enormous extinct volcanoes (the tallest is Olympus Mons, 21.9 km or 13.6 mi tall) and one of the largest canyons in the Solar System (Valles Marineris, 4,000 km or 2,500 mi long). Geologically , the planet is fairly active with marsquakes trembling underneath the ground, dust devils sweeping across the landscape, and cirrus clouds .
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A detailed discussion of layering with many Martian examples can be found in Sedimentary Geology of Mars. [9] Layers can be hardened by the action of groundwater. Martian ground water probably moved hundreds of kilometers, and in the process it dissolved many minerals from the rock it passed through.
The most conspicuous feature of Mars is a sharp contrast, known as the Martian dichotomy, between the Southern and the Northern hemispheres. The two hemispheres' geography differ in elevation by 1 to 3 km. The average thickness of the Martian crust is 45 km, with 32 km in the northern lowlands region, and 58 km in the southern highlands.
Mars comes closer to Earth more than any other planet save Venus at its nearest—56 million km is the closest distance between Mars and Earth, whereas the closest Venus comes to Earth is 40 million km. Mars comes closest to Earth every other year, around the time of its opposition, when Earth is sweeping between the Sun and Mars. Extra-close ...
Olympus Mons stands 24 km tall and is nearly 600 km in diameter. The adjoining Tharsis Montes consists of Ascraeus, Pavonis, and Arsia. Alba Mons, at the northern end of the Tharsis plateau, is 1500 km in diameter, and stands 6 km above the surrounding plains. In comparison, Mauna Loa is merely 120 km wide but stands 9 km above the sea floor. [4]