Achondrites: rocks from melted worlds

Achondrites are stony meteorites without chondrules. They come from worlds that melted, and many of them look surprisingly like volcanic rocks on Earth.

What makes an achondrite?

An achondrite is a stony meteorite that formed by melting, or by heating close to melting, on its parent body. Any chondrules it once had were destroyed. The result is a rock that has crystallised from a melt, much like the igneous rocks of Earth, such as basalt and gabbro.

When a parent body melts, its ingredients separate. Heavy metal sinks to the centre to form a core. The rest separates into layers: a mantle rich in olivine and pyroxene, and a crust of basalt-like rock. This process is called differentiation. Achondrites sample the crusts and mantles of these differentiated worlds.

Achondrites come from asteroids, but this family also includes the meteorites from the Moon and from Mars, because those bodies are differentiated too.

Vesta and the HED meteorites

The largest group of asteroidal achondrites is called the HED meteorites. HED stands for three related types: howardites, eucrites and diogenites.

Eucrites are basalts, the lava rocks of an asteroid's crust. Diogenites are made mostly of the mineral pyroxene and probably formed deeper down. Howardites are breccias, rocks made of broken fragments cemented together, containing pieces of both eucrites and diogenites mixed by impacts on the surface.

The parent body of the HEDs is very probably the asteroid 4 Vesta, about 525 kilometres across. Telescopes showed that light reflected from Vesta matches HED meteorites. NASA's Dawn spacecraft, which orbited Vesta in 2011 and 2012, found a huge impact basin near its south pole, called Rheasilvia, which may have launched many of the fragments.

Other achondrites

Not all achondrites come from Vesta. A few other groups are worth knowing.

Angrites are basaltic achondrites that are among the oldest igneous rocks known, crystallising only a few million years after CAIs formed.

Aubrites are pale rocks made mostly of a magnesium-rich pyroxene called enstatite. They formed in conditions with very little oxygen. The large Norton County meteorite, which fell in Kansas and Nebraska in 1948, is an aubrite.

Ureilites are unusual dark achondrites containing carbon, sometimes as graphite and tiny diamonds. The first meteorite tracked from space to the ground as an asteroid, 2008 TC3, produced the Almahata Sitta meteorites in Sudan, many of them ureilites.

There are also primitive achondrites, rocks that were heated enough to partly melt but still keep traces of their chondrite origins.

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