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TITLE: Achondrites

[Photo: Volcanism on Mars]

From the crusts of differentiated asteroids and planets come the meteorites we call achondrites. These igneous rocks formed when large, rocky bodies accumulated heat faster than they were able to lose it. Their interiors partially melted, and the molten rock rose buoyantly to the surface to create volcanoes the source of achondrite meteorites. Volcanism ceased billions of years ago on the asteroids, perhaps only 180 million years ago on Mars, and still occurs on Earth.

See caption at right.

Meteorites From Mars
Most meteorites are nearly the age of the Solar System. But a handful of igneous specimens are much younger. They must have originated on a body with fairly recent volcanic activity. With its relatively uncratered hence, young lava plains, Mars is thought to be the likely source. A direct link was established in 1976 when the Viking spacecraft analyzed the Martian atmosphere. Its composition closely matches the trapped gasses within some of the young achondrites.


See caption at right.

Meteorites From Vesta
Sunlight reflected from Vesta, the third largest asteroid, yields an unusual spectrum of light. In laboratory analysis, one of the main groups of igneous meteorites—the eucrites—shows virtually the same distinctive spectrum. This similarity suggests that eucrites formed on the surface of Vesta.



See caption at left.Diogenites are chemically related to eucrites and probably came from Vesta, too. They formed underground when pyroxene crystals solidified and sank in magma. Impacts excavated and broke up layers of the crystals, ultimately blasting these fragments into space.



See caption at left.Howardites are mixtures of eucrite and diogenite fragments. Continuous bombardment of Vesta's surface excavated and reassembled pieces of those rocks into the hybrid howardites.

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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits