.png) Mosaic of the Caloris basin based on photographs by theMESSENGER orbiter. | |
| Location | Raditladi quadrangle,Mercury |
|---|---|
| Coordinates | 30°30′N189°48′W / 30.5°N 189.8°W /30.5; -189.8 |
| Diameter | 1,550 km (963 mi) |
| Eponym | Latin for "heat" |
Caloris Planitia/kəˈlɔːrɪspləˈnɪʃ(i)ə/ is aplain within a largeimpact basin onMercury, informally namedCaloris, about 1,550 km (960 mi) indiameter.[1] It is one of thelargest impact basins in the Solar System. "Calor" isLatin for "heat" and the basin is so-named because theSun is almost directly overhead every second time Mercury passesperihelion. The crater, discovered in 1974, is surrounded by theCaloris Montes, a ring ofmountains approximately 2 km (1.2 mi) tall.
Caloris was discovered on images taken by theMariner 10 probe in 1974. Its name was suggested byBrian O'Leary, astronaut and member of theMariner 10 imagery team.[2] It was situated on theterminator—the line dividing the daytime and nighttime hemispheres—at the time the probe passed by, and so half of the crater could not be imaged. Later, on January 15, 2008, one of the first photos of the planet taken by theMESSENGER probe revealed the crater in its entirety.
The basin was initially estimated to be about 810 mi (1,300 km) in diameter, though this was increased to 960 mi (1,540 km) based on subsequent images taken byMESSENGER.[1] It is ringed by mountains up to 2 km (1.2 mi) high. Inside the crater walls, the floor of the crater is filled by lava plains,[3] similar to themaria of theMoon. These plains are superposed by explosive vents associated with pyroclastic material.[3] Outside the walls, material ejected in the impact which created the basin extends for 1,000 km (620 mi), and concentric rings surround the crater.
In the center of the basin is a region containing numerous radial troughs that appear to beextensional faults, with an unrelated 40 km (25 mi) crater,Apollodorus, located near the center of the pattern. The exact cause of this pattern of troughs is not currently known.[1] The feature is namedPantheon Fossae.[4]
The impacting body is estimated to have been at least 100 km (62 miles) in diameter.[5]
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 that created Caloris must have occurred after most of the heavy bombardment had finished, because fewerimpact craters are seen on its floor than exist on comparably-sized regions outside the crater. Similar impact basins on the Moon such as theMare Imbrium andMare 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.[6] Based onMESSENGER's photographs, Caloris' age has been determined to be between 3.8 and 3.9 billion years.[1]
A gravitational high, also known as amascon, is centered on Caloris Planitia.[7] Most large impact basins on the moon, such asMare Imbrium andMare Crisium, are also the site of mascons.
The giant impact believed to have formed Caloris may have had global consequences for the planet. At the exactantipode of the basin is a large area of hilly, grooved terrain, with few small impact craters that are known as chaotic terrain (also "weird terrain").[8] It is thought by some to have been created asseismic waves from the impact converged on the opposite side of the planet.[9] Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin's antipode.[10] This hypothetical impact is also believed to have triggeredvolcanic activity on Mercury, resulting in the formation of smooth plains.[11] Surrounding Caloris is a series of geologic formations thought to have been produced by the basin's ejecta, collectively called theCaloris Group.
Mercury has a very tenuous and transient atmosphere, containing small amounts ofhydrogen andhelium captured from thesolar wind, as well as heavier elements such assodium andpotassium. 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 unusual terrain is also a source of these gases.[12]
