On Earth, the longitude of the Royal Observatory in Greenwich, England, is defined as the "prime meridian", or zero degrees longitude. Locations on Earth are measured in degrees east or west from this position, but where is the equivalent position on Mars? Earth's prime meridian was defined by international agreement in 1884 as the position of the large "transit circle", a telescope in the Royal Observatory's Meridian Building. The transit circle was built by Sir George Biddell Airy, the 7th Astronomer Royal, in 1850.
For Mars, the prime meridian was first defined by the German astronomers W. Beer and J. H. Mädler in 1830-32. They used a small circular feature on the surface, which they called "A", as a reference point to determine the rotation period of the planet. The Italian astronomer G. Schiaparelli used this feature as the zero point of longitude in his 1877 map of Mars. It was subsequently named Sinus Meridiani ("Middle Bay") by French astronomer Camille Flammarion. A crater in the Sinus Meridiani was later called Airy, named to commemorate the builder of the Greenwich transit. When the US Mariner 9 spacecraft mapped the planet at about 1 kilometer resolution in 1972, a more precise definition was needed.
The 0.5-kilometer-wide crater, subsequently named 'Airy-0' (within the larger crater Airy), chosen as the zero degrees longitude point. Here the outlines of the Mariner 9, Viking and Mars Global Surveyor images are shown on a wide-angle context image taken by the US Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC). Image credit: NASA. Merton Davies of the RAND Corporation was analyzing surface features and designated a 0.5-kilometer-wide crater, subsequently named "Airy-0" (within the larger crater Airy) as the zero point. This crater was imaged once by Mariner 9 and once by the Viking 1 orbiter in 1978, and these two images were the basis of the martian longitude system for the rest of the 20th century. The US Mars Global Surveyor (MGS) attempted to take a picture of Airy-0 on every close overflight it made since the beginning of its mapping mission. This shows how difficult it is to hit such a small target: nine attempts were required, and the spacecraft did not pass directly over Airy-0 until almost the end of the MGS primary mission in January 2001.
Originally, a system with "planetographic" latitude and longitude increasing to the west was developed to be used with the Viking observations. The US Geological Survey and other organizations then adopted a system with "planetocentric" latitude and longitude increasing to the east for making future Mars maps and imagery. Both systems were approved for use on Mars by the International Astronomical Union in 2000. (The "planetocentric" system uses co-ordinates derived from the angle measured from the equator to a point on the surface at the center of the planet, whereas the "planetographic" system uses co-ordinates which are mapped on the surface.)
Most maps produced before 2002 use the earlier coordinates system, but now the majority of Mars missions and instrument teams have now adopted the latter system defined for Mars, namely the planetocentric latitude and east longitude system. These definitions have been widely adopted by NASA and ESA missions and other users of planetary data and are likely to remain in use for a decade or more.
The articles featuring the latest Mars images published on the ESA Mars Express web site quote positions given in this latest system, with longitude ranging from 0-360 degrees East. This is different to Earth, where we give longitudes as 0-180 degrees, East or West.
Read the original news release at http://www.esa.int/SPECIALS/Mars_Express/SEM0VQV4QWD_0.html.
Mars on Your Desktop—New Screensaver
20 August 2004
Download more astonishing images of Mars in a fascinating new screensaver from ESA. This is a collection of the best 3D images to date produced by the High Resolution Stereo Camera on board ESA's Mars Express. Fifteen fantastic images of the surface of Mars are presented here for you to download and enjoy for yourself on your PC. Download the screensaver at http://esamultimedia.esa.int/images/marsexpress/screensaver/Mars3D.zip.
Read the original news release at http://www.esa.int/SPECIALS/Mars_Express/SEMK6XW4QWD_0.html.
Crater Dunes in Argyre Planitia
23 August 2004
These images, taken by the High Resolution Stereo Camera (HRSC) on board ESA's Mars Express spacecraft, show a martian crater with a dune field on its floor. The images were taken during orbit 427 in May 2004, and show the crater with a dune field located in the north-western part of the Argyre Planitia crater basin. The images are centered at Mars longitude 303° East and latitude 43° South. The image resolution is approximately 16.2 meters per pixel. The crater is about 45 kilometers wide and 2 kilometers deep. In the north-eastern part of this crater, the complex dune field is 7 kilometers wide by 12 kilometers long.
In arid zones on Earth, these features are called "barchans", which are dunes having an asymmetrical profile, with a gentle slope on the wind-facing side and a steep slope on the lee-side. The dune field shown here suggests an easterly wind direction with its steeper western part. The composition of the dune material is not certain, but the dark sands could be of basaltic origin.
The color image has been created from the HRSC’s nadir (vertical view) and three color channels. The 3D anaglyph image has been created using the nadir and stereo channels. The perspective views have been calculated from the digital terrain model derived from the stereo channels.
Read the original news release at http://www.esa.int/SPECIALS/Mars_Express/SEMYRSW4QWD_0.html.