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On Earth, the zero elevation datum is based on sea level (the geoid). Since Mars has no oceans and hence no 'sea level', it is convenient to define an arbitrary zero-elevation level or "vertical datum" for mapping the surface, called areoid. [9] The datum for Mars was defined initially in terms of a constant atmospheric pressure.
Airy-0 is a crater inside the larger Airy Crater on Mars, whose location historically defined the Martian prime meridian.It is about 0.5 km (0.3 mile) across and lies within the dark region Sinus Meridiani, one of the early albedo features to be identified on Mars.
Listed are the elevations of the peaks (the vertical position relative to the areoid, which is the Martian vertical datum — the surface defined as zero elevation by average martian atmospheric pressure and planet radius), which is not the height above the surrounding terrain (topographic prominence). Listed mons elevation is the highest point ...
Because Mars has no oceans, and hence no "sea level", a zero-elevation surface had to be selected as a reference level; this is called the areoid [84] of Mars, analogous to the terrestrial geoid. [85] Zero altitude was defined by the height at which there is 610.5 Pa (6.105 mbar) of atmospheric pressure. [86]
In some situations sea level does not apply at all – for instance for mapping Mars' surface – forcing the use of a different "zero elevation", such as mean radius. A geodetic vertical datum takes some specific zero point, and computes elevations based on the geodetic model being used, without further reference to sea levels.
The crater's depth of 7,152 m (23,465 ft) [1] below the topographic datum of Mars explains the atmospheric pressure at the bottom: 12.4 mbar (1240 Pa or 0.18 psi) during winter, when the air is coldest and reaches its highest density.
A planetary datum is a generalization of geodetic datums for other planetary bodies, such as the Mars datum; it requires the specification of physical reference points or surfaces with fixed coordinates, such as a specific crater for the reference meridian or the best-fitting equigeopotential as zero-level surface. [9]
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]