Category · Stony meteorites › Achondrites

Asteroidal achondrites

Fully melted, differentiated rocks from the crusts and mantles of asteroids, Vesta above all.

Classification: Stony meteorites › Achondrites › Asteroidal achondrites
In our catalogue: 1,283 meteorites, 71 seen to fall
Heaviest: Al Haggounia 001, 3 t
Earliest recorded fall: Mässing, 1803
Most recent fall: Bunburra Rockhole, 2007

These 'evolved' achondrites come from asteroids that melted completely and separated into a metal core, a rocky mantle and a basaltic crust, a smaller version of how Earth formed.

The largest family is the HED group (howardites, eucrites and diogenites) from the asteroid 4 Vesta. Angrites and aubrites come from other, unidentified asteroids.

Building a small planet

An asteroid that melts completely becomes a miniature planet. Within a few million years of forming, bodies tens to hundreds of kilometres across heated by aluminium-26 developed oceans of magma. Iron-nickel metal sank to make a core, crystals of olivine and pyroxene gathered into a mantle, and lighter basaltic melt rose and erupted or froze into a crust. Tungsten isotopes from iron meteorites show that some cores formed within about 1 to 3 million years of the first solids.

The asteroidal achondrites come from the outer layers. Eucrites are crust; diogenites are lower crust or upper mantle; howardites are surface rubble mixing the two. Angrites are basalts from a different, very volatile-poor body, and aubrites are crust or mantle rocks from a body formed from enstatite-chondrite-like material.

The missing mantles

If many asteroids melted, many olivine-rich mantles must have existed. Yet olivine-dominated achondrites, apart from a few pallasites and brachinites, are rare in collections, and olivine-rich asteroids are rare in space. This 'missing mantle' problem suggests that most differentiated bodies were destroyed early in collisions, and that their mantles were ground to dust or swept up by growing planets, while tougher metal cores survived as iron meteorites.

Ages and fingerprints

These are some of the oldest igneous rocks known. Quickly cooled angrites crystallised about 4,563 to 4,564 million years ago, and eucrites mostly between 4,550 and 4,560 million years ago, giving precise dates for the start of volcanism in the Solar System. Each body has its own oxygen isotope value: about −0.24 per mil for Vesta's HED meteorites, close to zero for aubrites, and about −0.07 per mil for angrites.

Recognising one

These rocks look like terrestrial basalt, gabbro or pyroxenite. A fresh black fusion crust, often glossy on eucrites, is the best clue, along with brecciated textures of light and dark fragments. Most contain little metal and attract a magnet only weakly, so magnetic tests that work for chondrites fail here.

Related stones help fill the gaps. Mesosiderites contain silicates very like eucrites and diogenites, and main-group pallasites have oxygen isotopes close to those of the HEDs, leading some scientists to suggest that they too sample Vesta or a closely related body. Iron meteorites sample the cores. Between them, these meteorites let researchers build a picture of complete differentiated asteroids from crust to core.

Groups

Related groups

HED meteorites: pieces of Vesta The howardites, eucrites and diogenites: crust and mantle rocks from the giant asteroid 4 Vesta. The meteorites this site is named after.
Angrites The oldest volcanic rocks known, from a vanished early planetesimal.
Aubrites Pale, almost iron-free rocks made of enstatite, from an extremely oxygen-poor world.
Mesosiderites Jumbled breccias of metal and basalt, made in a violent collision.
Iron meteorites Almost pure iron-nickel metal, mostly from the cores of asteroids that melted and were later shattered.

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