An achondrite is a rock-like body found in the Solar System or it is a type of meteorite if one gets captured by the Earth's gravitational pull and falls to Earth. It is distinguished from a chondrite rock which contains chondrules, a collection of small, spherical features. Chondrites are known to have formed when the planets of the Solar System themselves were forming, but achondrite rocks are believed to have formed through processes in space at a much more recent date.
The volume of rocks in the Solar System that are categorized as achondrites in nature make up only 4% of the total, which also makes achondrite meteorites themselves rare. There is some debate about their origins, as many possible causes for them can exist. They most frequently resemble igneous rocks on Earth that were produced under intense heat and pressure, such as from deep underground or through the melting of rock in volcanic eruptions. General categories for achondrites break them down into stone, iron, and stony-iron compositions.
The study of achondrite rocks comes entirely from meteorites that have fallen to Earth, all of which usually contain coarse-grained internal structures and often lack metallic content such as iron or nickel. They are generally classified into four groups, though there are dozens of different further classifications within these groups as well. They are referred to as Howardite-Eucrite-Diogenite (HED), Shergottite-Nakhlite-Chassignite (SNC), Aubrite, or Ureilite achondrite forms.
HED specimens are believed to have all originated from the massive asteroid known as 4 Vesta, which itself accounts for 9% of the mass of the entire asteroid belt at a diameter of 329 miles (530 kilometers) in size. Eucrites have a volcanic structure that indicates that they were formed due to lava flows on the surface of the asteroid, while Diogenite samples are believed to originate from deep craters where the interior mantle of 4 Vesta is exposed to space. Howardites are a mixture of both Eucrite and Diogenite features.
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 versus chondrite rocks that stretch back 4,500,000,000 years. They are thought to have originated from hot planets that were volcanically active during this period of time, with Mars being a chief candidate for their likely origin. The now-extinct volcano of Olympus Mons on Mars, which is 16 miles (26 kilometers) in height, is believed to be their primary source of ejection into space.
Aubrites are another common variation of achondrite, believed to have formed in low-oxygen environments. Ureilites are the rarest form of the group comprising only 2% of all achondrite specimens, and containing a large amount of carbon as well as microscopic diamonds. Lunar meteorites are another possible source for achondrite rocks, though only 12 samples of meteorites in general from the Moon have been found on Earth as of the year 2000.
Some controversy remains as to the true origin of achondrite and chondrite rocks in the Solar System. This is because certain specimens display unique chemical and crystalline features, such as that of carbonaceous chondrite pervoskite, which can only occur in volcanic activity that took place well after the formation of the planets, though they are supposed to be the oldest of all asteroid compositions. Many achondrite specimens also come from Antarctica, and they have been implicated in controversies in recent years as displaying evidence of primitive microbial Martian fossils dating back 3,600,000,000 years, nearly to the point of formation of the planets of the Solar System itself.