The Martian craters are - like the craters on the Moon and Mercury - the result of the impact of massive small bodies such as asteroids and large meteoroids . Most of these celestial bodies collided with Mars during the " Great Bombardment " around 4 billion years ago, soon after the planets were formed .
To discover the Martian crater
The existence of Mars craters was only detected in 1965 by the US Mars probe Mariner 4 and was a certain surprise because even the largest of them could not be made out from Earth . One reason for this is the relatively low inclinations and flat lighting in a Mars opposition . However, in the years that followed, astronomers overestimated the number of craters because Mariner's trajectory led mainly over highlands and hardly any other large landscapes . It was not until about 10 years later that the recordings of newer Mars probes made it clear that the whole “Red Planet” is not covered by them and that it is in no way - as initially reported - similar to the Earth's moon.
The interpretation of such findings is tedious. It depends on the tectonics of the planet, its cooling, the type of rock and its average erosion (stronger than on the moon), the long-term development of the atmosphere , the statistical distribution of the asteroid orbits and other effects. On Venus are z. B. the craters are less common than on Mars, but more regularly distributed and more strongly changed by volcanic activity.
The morphology and size distribution of the Martian craters also differ from those on other terrestrial celestial bodies. By measuring their number and size on the great volcanoes of Mars, such as Olympus Mons or Mons Astraeus, the result was that the oldest Martian volcanoes had their main activity around 3.8 to 3.9 billion years ago. However, the count in other areas showed a different correlation between number, size and age than is the case with the other celestial bodies.
In infrared images, even in the vicinity of smaller craters about one kilometer in size, elongated, radial patterns can be seen - an indication that more rock was sprayed than previously assumed when a meteorite hit. Such “rock rains” can lead to thousands of secondary craters and falsify statistics and age determination .
Another special feature are the relatively frequent rows of craters ; some of them encompass four to five craters in an almost straight line. Whether these line structures were created accidentally or through special impacts is z. B. for the interpretation of the " Martian canals " discovered in 1877 not insignificant. Some heavily eroded " ghost craters " also form such rows.
In contrast to the moon there are z. B. hardly any grooves with small craters at their kinks. The canyons and net-like gullies , which look like dry river beds , are traced back to huge tidal waves through melted ground ice , which dried up again very quickly. At some crater edges , fan-shaped delta structures were discovered that end at the crater floor and are reminiscent of earthly meandering rivers . In order to pave the way for such changeable paths, the water must have flowed in them for a long time.
Although the Mariner 4 space probe was largely able to rule out the existence of wider Mars channels in 1965, it was not until Mariner 9 and later the Viking orbiters of the 1970s that the previous pass-by flights had mainly led over old, heavily cratered highlands in the south. Since these occupy almost 2/3 of the planet's surface, the impression of a dreary crater landscape initially solidified. The missing third in the north, on the other hand, is occupied by young, extensive plains - presumably of volcanic origin - with only a few craters. The lowlands are interrupted by two huge volcanic plateaus called Tharsis and Elysium . In addition to five large volcanoes with a diameter of up to 500 km, there is also a huge, 4000 km long rift valley , the Valles Marineris .
After decades of believing that there could be low life on Mars , the discovery of the craters initially fell silent. But when Voyager probes discovered the first channels in a dry state, the public interest in Mars shifted to the proof of the existence of water , which is concerned with space agencies like ESA and NASA to this day.
- P.Moore et al .: Atlas of the Solar System (especially chapter on the individual celestial bodies). 465 p., Herder-Verlag Freiburg-Basel-Vienna, approx. 1990
- K.Beatty et al .: The sun and its planets (especially chapter 4, collision of solid bodies ). Physik-Verlag, Weinheim 1981
- A.Rükl: Moon, Mars, Venus. Pocket atlas of the closest celestial bodies . Artaria Publishing House, Prague 1977
- Why crater counting on Mars is misleading astronomers . Stefanie Offermann, www.wissenschaft.de, article from March 24, 2005.
- comparing the bodies of the solar system ( Memento from March 20, 2005 in the Internet Archive )
- The Nereus Crater on Mars - Astronomy Picture of the Day from October 19, 2009.