An achondrite is a stony

meteorite A meteorite is a rock (geology), rock that originated in outer space and has fallen to the surface of a planet or Natural satellite, moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical ...

that does not contain

chondrule A chondrule (from Ancient Greek χόνδρος ''chondros'', grain) is a round grain found in a chondrite. Chondrules form as molten or partially molten droplets in space before being Accretion (astrophysics), accreted to their parent asteroids ...

s. It consists of material similar to terrestrial

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

s or plutonic rocks and has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystallization on or within meteorite parent bodies. As a result, achondrites have distinct textures and mineralogies indicative of igneous processes. Achondrites account for about 8% of meteorites overall, and the majority (about

2/3

) of them belong to the HED clan, possibly originating from the crust of asteroid Vesta. Other types include

Martian Mars, the fourth planet from the Sun, has appeared as a setting in works of fiction since at least the mid-1600s. Trends in the planet's portrayal have largely been influenced by advances in planetary science. It became the most popular celes ...

, Lunar, and several types thought to originate from as-yet unidentified

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

s. These groups have been determined on the basis of e.g. the Fe/ Mn chemical ratio and the ¹⁷O/¹⁸O

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

ratios, thought to be characteristic "fingerprints" for each parent body.

Classification

Achondrites are classified into the following groups:O. Richard Norton. The Cambridge encyclopedia of meteorites. UK, Cambridge University Press, 2002. . * Primitive achondrites * Asteroidal achondrites * Lunar meteorites * Martian meteorites

Primitive achondrites

Primitive achondrites, also called PAC group, are so-called because their chemical composition is '' primitive'' in the sense that it is similar to the composition of

chondrites A chondrite is a stony (non-metallic) meteorite that has not been modified by either melting or planetary differentiation, differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar Syste ...

, but their texture is igneous, indicative of melting processes. To this group belong: * Acapulcoites (after the meteorite Acapulco, Mexico) * Lodranites (after the meteorite Lodran) * Winonaites (after the meteorite Winona) * Ureilites (after the meteorite Novy Ureii, Russia) * Brachinites (after the meteorite Brachina)

Asteroidal achondrites

Asteroidal achondrites, also called evolved achondrites, are so-called because they have been ''differentiated'' on a parent body. This means that their mineralogical and chemical composition was changed by melting and crystallization processes. They are divided into several groups: * HED meteorites (Vesta). They may have originated on the asteroid

4 Vesta Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta (mytho ...

, because their reflection spectra are very similar. They are named after the initial letters of the three subgroups: ** Howardites ** Eucrites ** Diogenites * Angrites * Aubrites

Lunar meteorites

Lunar meteorites are meteorites that originated from the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

Martian meteorite

Martian meteorites are meteorites that originated from

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

. They are divided into three main groups, with two exceptions (see last two entries): * Shergottites * Nakhlites * Chassignites * OPX martian meteorites ( ALH 84001) * Regolith/Soil samples ( NWA 7034 and pairings)

References

External links

Achondrite Images
from Meteorites Australia {{Authority control Planetary science Meteorite types