WHAT IS AN ACHONDRITE
Meteorite Science
Achondrites are stony meteorites that lack chondrules, the defining feature of primitive chondrites. Their absence tells a story: achondrites come from parent bodies that melted and differentiated, erasing the original chondritic texture and producing igneous rocks similar to those found on Earth, but from other worlds.
What Differentiation Means
When a planetary body grows large enough, the decay of radioactive isotopes in its interior generates enough heat to melt the rock. In a molten state, denser iron and nickel sink to form a metallic core while lighter silicate minerals rise to form a mantle and eventually a crust. This process, differentiation, is what happened on Earth, Mars, the Moon, and several asteroids.
Achondrites are the geological products of differentiation on asteroid-scale bodies. They are the meteoritic equivalent of Earth's igneous rocks, basalts, gabbros, dunites, and pyroxenites, but from worlds that no longer exist.
The contrast with chondrites is fundamental. Chondrites are primitive: they preserve the original composition of the early solar nebula because their parent bodies never melted. Achondrites are processed: their parent bodies went through the same geological transformation Earth did, and the original chondritic material was destroyed in the process.
Achondrite Groups
HED Meteorites
The largest and best-studied achondrite family is the HED group, which originates from asteroid 4 Vesta. NASA's Dawn spacecraft orbited Vesta from 2011 to 2012 and confirmed the connection: the surface composition of Vesta matches the geochemical signatures of HED meteorites precisely. The HED group is the only achondrite family with a confirmed, spacecraft-verified parent body.
Angrites
Angrites are one of the rarest and most scientifically significant achondrite groups. They come from a parent body that was strongly depleted in volatile elements and highly oxidized, a unique composition not matched by any other meteorite group. Radiometric dating places angrite crystallization among the earliest in the solar system, within the first few million years after solar system formation.
Because angrites crystallized so early, they provide a precise timestamp for the beginning of igneous activity on differentiated bodies. Their ages constrain the timing of planetary differentiation itself, making them invaluable reference points in the chronology of early solar system history.
Aubrites
Aubrites, also called enstatite achondrites, come from a parent body that differentiated under highly reducing conditions, the same unusual chemistry that produced enstatite chondrites. They are pale, almost white in color, and dominated by nearly iron-free enstatite. Most aubrites are brecciated, having been disrupted by impacts on their parent body after differentiation was complete. Their pale appearance makes them visually distinctive among achondrites.
Browse AubritesUreilites
Ureilites are an unusual achondrite group that contains significant amounts of carbon, including nanoscale diamonds formed by shock compression during an impact event. Unlike most achondrites, ureilites still contain some primitive material, suggesting their parent body may have been disrupted before differentiation was fully complete. Their parent body and exact formation mechanism remain subjects of active research. Ureilites are among the more enigmatic groups in meteorite science.
Planetary Achondrites: Lunar and Martian Meteorites
A small but scientifically critical subset of achondrites comes from the Moon and Mars. Lunar meteorites represent basaltic mare material and ancient highland crust. Martian meteorites, the SNC group (shergottites, nakhlites, and chassignites), are basaltic and ultramafic rocks from the Martian crust and mantle. Both types are identified through isotopic and geochemical signatures that definitively distinguish them from all other meteorite types and from terrestrial rocks.
Achondrites vs Chondrites: The Key Distinction
Chondrites are primitive: they preserve the original composition of the early solar nebula in near-original form. Achondrites are processed: their parent bodies underwent the same planetary-scale differentiation as Earth, Mars, and the Moon, and the original chondritic material was replaced by igneous rocks produced by melting and crystallization. The two groups are fundamentally different in what they tell us. Chondrites tell us about the early solar nebula. Achondrites tell us about the geological evolution of planetary bodies.
Browse Achondrite Specimens
Frequently Asked Questions
What is the difference between a chondrite and an achondrite?
Chondrites are primitive meteorites from parent bodies that never melted. They contain chondrules and preserve the original composition of the early solar nebula. Achondrites come from parent bodies that melted and differentiated, producing igneous rocks. The chondritic texture was destroyed in the process, which is why achondrites lack chondrules.
Are lunar and Martian meteorites considered achondrites?
Yes. Lunar and Martian meteorites are achondrites because the Moon and Mars are both differentiated bodies that produced igneous rocks without chondrules. They are classified separately within the achondrite group due to their distinct planetary origins.
Why are achondrites rarer than chondrites?
Chondrites are the most common meteorite type because they come from the most abundant type of asteroid in the belt. Differentiated parent bodies capable of producing achondrites required specific conditions of size and heat to melt internally. Achondrites represent the products of those less common, larger bodies and make up only about 6% of recovered meteorites.
What is the HED group?
The HED group is the largest achondrite family, consisting of howardites, eucrites, and diogenites. All three types originate from asteroid 4 Vesta, confirmed by NASA's Dawn mission. They represent different layers of Vesta's differentiated interior: eucrites from the basaltic crust, diogenites from deeper crustal or mantle material, and howardites as impact-mixed breccias of the two.
Are achondrites scientifically valuable?
Extremely. Achondrites provide direct samples of differentiated planetary bodies that would otherwise be inaccessible. HED meteorites give us a window into Vesta's interior. Angrites constrain the timing of early planetary differentiation. Lunar and Martian achondrites are the only physical samples of those bodies available for study outside of the Apollo collection. Each group records a different chapter in the geological history of the solar system.
