Geography of Mars
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The geography of Mars (Areography) entails the delineation and characterization of regions on Mars. The term areography was sometimes used in the early 20th century to refer to this subject but this term is rarely used today[1]. Martian geography is mainly focused on what is called physical geography on Earth; that is the distribution of physical features across Mars and their cartographic representations.
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[edit] History
The first observations of Mars were, of course, from land-based telescopes. The history of these observations are marked by the oppositions of Mars, when the planet is closest to Earth and hence is most easily visible, which occur every couple of years. Even more notable are the perihelic oppositions of Mars which occur approximately every 16 years, and are distiguished because Mars is close to perihelion making it even closer to Earth.
In September 1877, (a perihelic opposion of Mars occurred on September 5), Italian astronomer Giovanni Schiaparelli published the first detailed map of Mars. These maps notably contained features he called canali ("channels"), that were later shown to be an optical illusion. These canali were supposedly long straight lines on the surface of Mars to which he gave names of famous rivers on Earth. His term was popularly mistranslated as canals, and so started the Martian canal controversy.
Following these observations, it was a long held belief that Mars contained vast seas and vegetation. It was not until spacecraft visited the planet during NASA's Mariner missions in the 1960s that these myths were dispelled. Some maps of Mars were made using the data from these missions, but it wasn't until the Mars Global Surveyor mission, launched in 1996 and still operational as of 2006, that complete, extremely detailed maps were obtained. These maps are now available online at http://www.google.com/mars/
[edit] Topography
The dichotomy of Martian topography is striking: northern plains flattened by lava flows contrast with the southern highlands, pitted and cratered by ancient impacts. The surface of Mars as seen from Earth is consequently divided into two kinds of areas, with differing albedo. The paler plains covered with dust and sand rich in reddish iron oxides were once thought of as Martian 'continents' and given names like Arabia Terra (land of Arabia) or Amazonis Planitia (Amazonian plain). The dark features were thought to be seas, hence their names Mare Erythraeum, Mare Sirenum and Aurorae Sinus. The largest dark feature seen from Earth is Syrtis Major.
The shield volcano, Olympus Mons (Mount Olympus), is at 26 km the highest known mountain in the solar system. It is in a vast upland region called Tharsis, which contains several large volcanos. See list of mountains on Mars. The Tharsis region of Mars also has the solar system's largest canyon system, Valles Marineris or the Mariner Valley, which is 4000 km long and 7 km deep. Mars is also scarred by a number of impact craters. The largest of these is the Hellas impact basin, covered with light red sand. See list of craters on Mars.
Mars has two permanent polar ice caps, the northern one located at Planum Boreum and the southern one at Planum Australe.
The difference between Mars' highest and lowest points is nearly 31 km (from the top of Olympus Mons at an altitude of 26 km to the bottom of the Hellas impact basin at an altitude of 4 km below the datum). In comparison, the difference between Earth's highest and lowest points (Mount Everest and the Mariana Trench) is only 19.7 km. Combined with the planets' different radii, this means Mars is nearly three times "rougher" than Earth.
The International Astronomical Union's Working Group for Planetary System Nomenclature is responsible for naming Martian surface features.
[edit] Zero elevation
Since Mars has no oceans and hence no 'sea level', it is convenient to define an arbitrary zero-elevation level or "datum" for mapping the surface. The datum for Mars is defined in terms of the height at which the air has a particular pressure at about the freezing point of water: a pressure of 610.5 Pa (6.105 mbar), approximately 0.6% of Earth's, at a temperature of 273.16 K. This pressure and temperature correspond to the triple point of water. Conditions on Mars are so different from those on Earth that Martian altitude readings should not be directly compared to Earthly ones for purposes of finding life, potential colony sites, etc..
[edit] Zero meridian
Mars' equator is defined by its rotation, but the location of its Prime Meridian was specified, as was Earth's, by choice of an arbitrary point which was accepted by later observers. The German astronomers Wilhelm Beer and Johann Heinrich Mädler selected a small circular feature as a reference point when they produced the first systematic chart of Mars features in 1830-32. 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. After the spacecraft Mariner 9 provided extensive imagery of Mars in 1972, a small crater (later called Airy-0), located in the Sinus Meridiani ('Middle Bay' or 'Meridian Bay') along the line of Beer and Mädler, was chosen by Merton Davies of the RAND Corporation to provide a more precise definition of 0.0° longitude when he established a planetographic control point network.
[edit] Nomenclature
[edit] Early nomenclature
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' 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 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 proposed by Schiaparelli:
- Kaiser Sea = Syrtis Major
- Lockyer Land = Hellas Planitia
- Main Sea = Lacus Moeris
- Herschel II Strait = Sinus Sabaeus
- Dawes Continent = Aeria and Arabia
- De La Rue Ocean = Mare Erythraeum
- Lockyer Sea = Solis Lacus
- Dawes Sea = Tithonius Lacus
- Madler Continent = Chryse Planitia, Ophir, Tharsis
- Maraldi Sea = Maria Sirenum and Cimmerium
- Secchi Continent = Memnonia
- Hooke Sea = Mare Tyrrhenum
- Cassini Land = Ausonia
- Herschel I Continent = Zephyria, Aeolis, Aethiopis
- Hind Land = Libya
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.
[edit] Modern nomenclature
Today, features on Mars derive from a number of sources. 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 Rover are given temporary names or nicknames to identify and marker them, however certain major features such as the Columbia Hills were named after the seven astronauts of the Space Shuttle Columbia disaster, and the hope is that the names are made permanent by the International Astronomical Union.
[edit] See also
- Geology of Mars
- Category:Surface features of Mars - Category of various surface features on Mars
[edit] References
- ^ Cartographic Inscription and Exploration Narrative in Late Victorian Representations of the Red Planet
- Sheehan, William, "The Planet Mars: A History of Observation and Discovery" (Full text online) The University of Arizona Press, Tucson. 1996.
[edit] External links
- Google Mars - Google Maps for Mars, with various surface features and interesting places pointed out
