Areography

from Wikipedia, the free encyclopedia
Topographic map of Mars. The blue regions are below the specified zero level , the red above

Areography or areography (from ancient Greek Άρης Ares for Mars and ancient Greek γράφειν gráfein for describe ) describes the geography of Mars , i.e. the investigation, description and mapping of the surface structures of the fourth planet of the solar system .

Its northern and southern halves differ significantly from each other, which is known as the dichotomy of Mars : the northern hemisphere consists predominantly of a single lowland , while the southern hemisphere is five kilometers above the level of the northern plains.

Geodetic and cartographic definitions

Coordinates

Three images from Airy-0 (the larger crater in the upper center of the image), A from Mariner 9 , B from Viking 1 and C from Mars Global Surveyor

As on Earth, ellipsoidal coordinates are used, but if the accuracy is higher, because of the dichotomy, they do not have to be related to a (two-axis) rotational ellipsoid , but to a three-axis ellipsoid . They are called areographic coordinates ; their latitudes are counted from the equator determined by the rotation , all longitudes go through the two poles of rotation . The prime meridian is defined by the small (diameter 500 m) Airy-0 crater , located a little south of the Martian equator . This lies within the larger Airy crater , named after the English astronomer George Biddell Airy , whose definition of the earth's prime meridian was adopted internationally by the center of the Royal Greenwich Observatory in 1884.

Airy, on the other hand, lies within the "Meridian Bay" ( Sinus Meridiani ), which appears noticeably dark from the earth. It was used by the German astronomers Wilhelm Beer and Johann Heinrich Mädler in the 1830s as an observation feature to determine the period of rotation of Mars. This feature called simply "A" by them was used in 1877 by Giovanni Schiaparelli to determine the prime meridian for his maps of Mars. The data from Mariner 9 in 1972 required a more precise definition of what the Airy-0 crater was chosen for.

height

Earlier topographic models defined the zero level above atmospheric pressure , and a pressure of 6.1 mbar was chosen as the reference pressure  . This value roughly corresponds to the average pressure in the Martian atmosphere and coincidentally also to the triple point of water. This means that water can theoretically only occur in liquid form on the surface of Mars at locations below zero.

When defining the zero level using atmospheric pressure, however, seasonal movements in the Martian atmosphere in particular lead to inconsistencies . Therefore, an areoid was defined according to the geoid on earth, which is defined by an equipotential surface of gravity . This definition was chosen in relation to the previously used definition for a pressure of 6.1 mbar.

Individual evidence

  1. ↑ Oliver Morton: Mapping Mars: Science, Imagination, and the Birth of a World. Picador USA, New York 2002, ISBN 0-312-24551-3 , page 22f
  2. ↑ Michel Capderou: Satellites: orbits and missions. Springer Verlag, Paris 2005, ISBN 2-287-21317-1 , page 406
  3. ↑ David E. Smith et al .: Mars Orbiter Laser Altimeter: Experiment summary after the first year of global mapping of Mars. In: Journal of Geophysical Research. 106: 23689-23722, 2001 ( online ; PDF file; 3.7 MB)
  4. ↑ NASA: Mars Global Surveyor: MOLA MEGDRs
  5. ^ AA Ardalan, R. Karimi, EW Grafarend: A New Reference Equipotential Surface, and Reference Ellipsoid for the Planet Mars. In: Earth Moon Planet. 106: 1–13, 2010 ( doi: 10.1007 / s11038-009-9342-7 )

literature