Caloris Basin
From Wikipedia, the free encyclopedia
The Caloris Basin, also called Caloris Planitia, is an impact crater on Mercury about 1350km in diameter, one of the largest impact basins in the solar system. Caloris is Latin for heat and the basin is so-named because the Sun is almost directly overhead every second time Mercury passes perihelion. The crater is surrounded by a ring of mountains about two kilometers tall.
Contents |
[edit] Appearance
The Caloris Basin was discovered on images taken by the Mariner 10 probe in 1974. It was situated on the terminator - the line dividing the daytime and nighttime hemispheres - at the time the probe passed by, and so half the crater has never been imaged.
The crater is about 1300km (810 miles) in diameter, and is ringed by mountains up to 2km high. Inside the crater walls, the floor of the crater is filled by lava plains, similar to the maria of the Moon. Outside the walls, material ejected in the impact which created the basin extends for 1000 km, and concentric rings surround the crater.
[edit] Formation
Bodies in the inner solar system experienced a heavy bombardment of large rocky bodies in the first billion years or so of the solar system. The impact which created the Caloris basin must have occurred after most of the heavy bombardment had finished, because fewer impact craters are seen on its floor than exist on comparably-sized regions outside the crater. Similar impact basins on the Moon such as the Mare Imbrium and Mare Orientale are believed to have formed at about the same time, possibly indicating that there was a 'spike' of large impacts towards the end of the heavy bombardment phase of the early solar system [1]
[edit] Antipodal chaotic terrain and global effects
The giant impact believed to have formed Caloris may have had global consequences for the whole planet. At the exact antipode of the basin is a large area of hilly, grooved terrain, with few small impact craters that is known as the Chaotic Terrain (also 'Weird Terrain'). It is thought by some to have been created as seismic waves from the impact converged on the opposite side of the planet [2]. This hypothetical impact is also believed to have triggered volcanic activity on Mercury, resulting in the formation of smooth plains [3].
[edit] Emissions of gas
Mercury has a very tenuous and transient atmosphere, containing small amounts of hydrogen and helium captured from the solar wind, as well as heavier elements such as sodium and potassium. These are thought to originate within the planet, being 'out-gassed' from beneath its crust. The Caloris Basin has been found to be a significant source of sodium and potassium, indicating that the fractures created by the impact facilitate the release of gases from within the planet. The Weird Terrain is also a source of these gases. [4].
[edit] References
- ^ Gault D.E., Cassen P., Burns J.A., Strom R.G. (1977), Mercury, Annual review of astronomy and astrophysics, v. 15, p. 97-126.
- ^ Schultz P.H., Gault D.E. (1975), Seismic effects from major basin formations on the moon and Mercury, The Moon, v. 12, p. 159-177.
- ^ Kiefer W.S., Murray B.C. (1987), The formation of Mercury's smooth plains, Icarus, v. 72, p. 477-491
- ^ Sprague A.L., Kozlowski R.W.H., Hunten D.M. (1990), Caloris Basin - an enhanced source for potassium in Mercury's atmosphere, Science, v. 249, p. 1140-1142
