What happens when you have no atmosphere
The Caloris Basin
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.
Caloris Planitia is a plain within a large impact basin on Mercury, informally named Caloris, about 1,550 km (960 mi) in diameter. It is one of the largest impact basins in the Solar System. “Calor” 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, discovered in 1974, is surrounded by a ring of mountains approximately 2 km (1.2 mi) tall.
The impacting body is estimated to have been at least 100 km (62 miles) in diameter.
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 Caloris 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. Based on MESSENGER’s photographs, Caloris’ age has been determined to be between 3.8 and 3.9 billion years.
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