Areography

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.[11]

The datum for Mars was defined initially in terms of a constant atmospheric pressure. From the Mariner 9 mission up until 2001, this was chosen as 610.5 Pa (6.105 mbar), on the basis that below this pressure liquid water can never be stable (i.e., the triple point of water is at this pressure). This value is only 0.6% of the pressure at sea level on Earth. Note that the choice of this value does not mean that liquid water does exist below this elevation, just that it could were the temperature to exceed 273.16 K (0.01 degrees C, 32.018 degrees F).[12]

In 2001, Mars Orbiter Laser Altimeter data led to a new convention of zero elevation defined as the equipotential surface (gravitational plus rotational) whose average value at the equator is equal to the mean radius of the planet.[13]

Airy-0 crater (27 October 2021)

Mars's equator is defined by its rotation, but the location of its prime meridian was specified, as is Earth's, by choice of an arbitrary point which later observers accepted. The German astronomers Wilhelm Beer and Johann Heinrich Mädler selected a small circular feature in the Sinus Meridiani ('Middle Bay' or 'Meridian Bay') as a reference point when they produced the first systematic chart of Mars features in 1830–1832. In 1877, their choice was adopted as the prime meridian by the Italian astronomer Giovanni Schiaparelli when he began work on his notable maps of Mars. In 1909 ephemeris-makers decided that it was more important to maintain continuity of the ephemerides as a guide to observations and this definition was "virtually abandoned".[14][15]

After the Mariner spacecraft provided extensive imagery of Mars, in 1972 the Mariner 9 Geodesy / Cartography Group proposed that the prime meridian pass through the center of a small 500 m diameter crater (named Airy-0), located in Sinus Meridiani along the meridian line of Beer and Mädler, thus defining 0.0° longitude with a precision of 0.001°.[14] This model used the planetographic control point network developed by Merton Davies of the RAND Corporation.[16]

As radiometric techniques increased the precision with which objects could be located on the surface of Mars, the center of a 500 m circular crater was considered to be insufficiently precise for exact measurements. The IAU Working Group on Cartographic Coordinates and Rotational Elements, therefore, recommended setting the longitude of the Viking 1 lander – for which there was extensive radiometric tracking data – as marking the standard longitude of 47.95137° west. This definition maintains the position of the center of Airy-0 at 0° longitude, within the tolerance of current cartographic uncertainties.[17]

Observers of Martian topography will notice a dichotomy between the northern and southern hemispheres. Most of the northern hemisphere is flat, with few impact craters, and lies below the conventional 'zero elevation' level. In contrast, the southern hemisphere is mountains and highlands, mostly well above zero elevation. The two hemispheres differ in elevation by 1 to 3 km. The border separating the two areas is very interesting to geologists.

One distinctive feature is the fretted terrain.[18] It contains mesas, knobs, and flat-floored valleys having walls about a mile high. Around many of the mesas and knobs are lobate debris aprons that have been shown to be rock-covered glaciers.[19]

Other interesting features are the large river valleys and outflow channels that cut through the dichotomy.[20][21][22]

• 
  Fresh impact crater on Mars 3.7°N 53.4°E / 3.7; 53.4 (November 19, 2013).
• 
  Fretted terrain of Ismenius Lacus showing flat floored valleys and cliffs. Photo taken with Mars Orbiter Camera (MOC) on the Mars Global Surveyor.
• 
  Enlargement of the photo on the left showing cliff. Photo taken with high resolution camera of Mars Global Surveyor (MGS).
• 
  View of lobate debris apron along a slope. Image located in Arcadia quadrangle.
• 
  Place where a lobate debris apron begins. Note stripes which indicate movement. Image located in Ismenius Lacus quadrangle.

The northern lowlands comprise about one-third of the surface of Mars and are relatively flat, with occasional impact craters. The other two-thirds of the Martian surface are the southern highlands. The difference in elevation between the hemispheres is dramatic. Because of the density of impact craters, scientists believe the southern hemisphere to be far older than the northern plains.[23] Much of heavily cratered southern highlands date back to the period of heavy bombardment, the Noachian.

Multiple hypotheses have been proposed to explain the differences. The three most commonly accepted are a single mega-impact, multiple impacts, and endogenic processes such as mantle convection.[20] Both impact-related hypotheses involve processes that could have occurred before the end of the primordial bombardment, implying that the crustal dichotomy has its origins early in the history of Mars.

The giant impact hypothesis, originally proposed in the early 1980s, was met with skepticism due to the impact area's non-radial (elliptical) shape, where a circular pattern would be stronger support for impact by larger object(s). But a 2008 study[24] provided additional research that supports a single giant impact. Using geologic data, researchers found support for the single impact of a large object hitting Mars at approximately a 45-degree angle. Additional evidence analyzing Martian rock chemistry for post-impact upwelling of mantle material would further support the giant impact theory.

Although better remembered for mapping the Moon starting in 1830, Johann Heinrich Mädler and Wilhelm Beer were the first "areographers". They started off by establishing once and for all that most of the surface features were permanent, and pinned down Mars's rotation period. In 1840, Mädler combined ten years of observations and drew the first map of Mars ever made. Rather than giving names to the various markings they mapped, Beer and Mädler simply designated them with letters; Meridian Bay (Sinus Meridiani) was thus feature "a".

Over the next twenty years or so, as instruments improved and the number of observers also increased, various Martian features acquired a hodge-podge of names. To give a couple of examples, Solis Lacus was known as the "Oculus" (the Eye), and Syrtis Major was usually known as the "Hourglass Sea" or the "Scorpion". In 1858, it was also dubbed the "Atlantic Canale" by the Jesuit astronomer Angelo Secchi. Secchi commented that it "seems to play the role of the Atlantic which, on Earth, separates the Old Continent from the New;" this was the first time the fateful canale, which in Italian can mean either "channel" or "canal", had been applied to Mars.

In 1867, Richard Anthony Proctor drew up a map of Mars. It was based, somewhat crudely, on the Rev. William Rutter Dawes' earlier drawings of 1865, then the best ones available. Proctor explained his system of nomenclature by saying, "I have applied to the different features the names of those observers who have studied the physical peculiarities presented by Mars." Here are some of his names, paired with those later used by Schiaparelli in his Martian map created between 1877 and 1886.[25] Schiaparelli's names were generally adopted and are the names actually used today:

Proctor's nomenclature has often been criticized, mainly because so many of his names honored English astronomers, but also because he used many names more than once. In particular, Dawes appeared no fewer than six times (Dawes Ocean, Dawes Continent, Dawes Sea, Dawes Strait, Dawes Isle, and Dawes Forked Bay). Even so, Proctor's names are not without charm, and for all their shortcomings they were a foundation on which later astronomers would improve.

Planet Mars - Topographical Map (USGS; 2005)

Informal names near Curiosity's landing site in contrast with official Herschel crater.

Today, names of Martian features derive from a number of sources, but the names of the large features are derived primarily from the maps of Mars made in 1886 by the Italian astronomer Giovanni Schiaparelli. Schiaparelli named the larger features of Mars primarily using names from Greek mythology and to a lesser extent the Bible. Mars's large albedo features retain many of the older names, but are often updated to reflect new knowledge of the nature of the features. For example, 'Nix Olympica' (the snows of Olympus) has become Olympus Mons (Mount Olympus).

Large Martian craters are named after important scientists and science fiction writers; smaller ones are named after towns and villages on Earth.

Various landforms studied by the Mars Exploration Rovers are given temporary names or nicknames to identify them during exploration and investigation. However, it is hoped that the International Astronomical Union will make permanent the names of certain major features, such as the Columbia Hills, which were named after the seven astronauts who died in the Space Shuttle Columbia disaster.