Achondrites are rare rock-like bodies found in the Solar System or as meteorites on Earth, distinguished from chondritic rocks. They are thought to have formed through recent processes in space and are classified into stony, iron, and stony-iron compositions. The study of achondrite rocks comes from meteorites that have fallen to Earth and are classified into four groups: Howardite-Eucrite-Diogenite, Shergottite-Nakhlite-Chassignite, Aubrite, and Ureilite. The SNC versions of achondrite are thought to have originated from hot planets that were volcanically active, with Mars being the prime candidate. Aubrites are believed to have formed in low-oxygen environments, while Ureilites contain a large amount of carbon and microscopic diamonds. Some controversy remains about the true origin of achondrite and chondrite rocks in the Solar System.
An achondrite is a rock-like body found in the Solar System or is a type of meteorite if it gets caught in the Earth’s gravitational pull and falls to Earth. It is distinguished from a chondritic rock which contains chondrules, a set of small spherical features. Chondrites are known to have formed when the planets of the Solar System themselves were forming, but achondritic rocks are thought to have formed through processes in space at a much more recent date.
The volume of rocks in the Solar System that are classified as achondrites in nature make up only 4% of the total, which also makes achondrite meteorites rare. There is some debate about their origins, as there can be many possible causes for them. They most frequently resemble igneous rocks on Earth that were produced under intense heat and pressure, such as from deep underground or through rock melting in volcanic eruptions. The general categories for achondrites divide them into stony, iron, and stony-iron compositions.
The study of achondrite rocks comes entirely from meteorites that have fallen to Earth, which usually contain coarse-grained internal structures and often lack metal content such as iron or nickel. They are generally classified into four groups, although there are dozens of several further classifications within these groups as well. They are referred to as Howardite-Eucrite-Diogenite (HED), Shergottite-Nakhlite-Chassignite (SNC), Aubrite, or Ureilite forms of achondrite.
The HED specimens are all believed to have originated from the massive asteroid known as 4 Vesta, which itself accounts for 9 percent of the mass of the entire asteroid belt with a diameter of 329 miles (530 kilometers). Eucrites have a volcanic structure indicating that they were formed as a result of lava flows on the asteroid’s surface, while diogenite samples are believed to come from deep craters where 4 Vesta’s inner mantle is exposed to space. Howardites are a blend of both Eucrite and Diogenite characteristics.
The SNC versions of achondrite are some of the youngest meteorites ever discovered and are estimated to be only about 1,300,000,000 years old compared to chondrite rocks which are 4,500,000,000 years old. They are thought to have originated from hot planets that were volcanically active during this time period, with Mars being the prime candidate for their probable origin. The now extinct volcano of Olympus Mons on Mars, which is 16 miles (26 kilometers) high, is believed to be their main source of ejection into space.
Aubrites are another common variant of achondrite, believed to have formed in low-oxygen environments. Ureilites are the rarest form of the group comprising only 2% of all achondrite samples and contain a large amount of carbon and microscopic diamonds. Lunar meteorites are another possible source of achondritic rocks, although only 12 meteorite samples overall from the Moon have been found on Earth since the year 2000.
Some controversy remains about the true origin of achondrite and chondrite rocks in the Solar System. This is because some specimens exhibit unique chemical and crystalline characteristics, such as that of pervoskite carbonaceous chondrite, which can only occur in volcanic activity that occurred long after the formation of planets, although they are supposed to be the oldest of all asteroid compositions. Many achondrite specimens also come from Antarctica and have been implicated in controversy in recent years as they show evidence of primitive Martian microbial fossils dating back 3,600,000,000 years, nearly to the point of formation of the planets of the Solar System itself.
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