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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.

Classification: Stony meteorites › Achondrites › Asteroidal achondrites › HED meteorites: pieces of Vesta
Share of witnessed falls: about 6.1%
In our catalogue: 1,187 meteorites, 61 seen to fall
Heaviest: Millbillillie, 330 kg
Earliest recorded fall: Mässing, 1803
Most recent fall: Bunburra Rockhole, 2007

HED meteorites are the most common achondrites, about 6% of all falls, and they come from the asteroid 4 Vesta, about 525 km across and the second most massive body in the asteroid belt. The match was first made from telescope spectra and confirmed by NASA's Dawn spacecraft, which orbited Vesta from 2011 to 2012 and found that its surface minerals match these meteorites.

A giant impact near Vesta's south pole, about a billion years ago, blasted out the 500 km Rheasilvia basin and threw off a family of small asteroids, the 'Vestoids'. Fragments of these reach Earth as HED meteorites.

Eucrites are basalts and gabbros from Vesta's crust. Diogenites are pyroxene-rich rocks from deeper down. Howardites are surface rubble, mixtures of the two cemented together by impacts.

Mineralogy and chemistry

HED meteorites are made of pyroxene and plagioclase, with smaller amounts of olivine, chromite, ilmenite, silica, troilite and a little iron-nickel metal. Eucrites are basalts and gabbros rich in calcium plagioclase and pigeonite. Diogenites are almost pure orthopyroxene, sometimes with olivine. Howardites mix the two. All three share the same oxygen isotope signature, a Δ17O of about −0.24 per mil, and the same iron-to-manganese ratio in their pyroxene, around 30, which shows they come from a single body.

Parent body: Vesta

In 1970 Thomas McCord and colleagues showed that the reflected spectrum of Vesta matched that of eucrites, the first link between a meteorite group and a particular asteroid. Later, small asteroids with the same spectrum were found spreading from Vesta towards the 3:1 resonance with Jupiter, a gravitational escape route into Earth-crossing orbits. Dawn, which orbited Vesta from July 2011 to September 2012, mapped a surface covered in howardite-like material with eucritic and diogenitic areas, and confirmed a metal core about 220 km across, just as the meteorites predicted.

Vesta formed and melted within the first few million years. Its crust of eucrite basalt and its pyroxene-rich lower crust formed by 4,550 million years ago. Two great impact basins near its south pole, Veneneia and the younger Rheasilvia, about 500 km wide, excavated deep material and launched the Vesta family of asteroids, the source of most HEDs.

Abundance and famous examples

HEDs are about 6% of witnessed falls and the most abundant achondrites. Juvinas (eucrite, France, 1821), Stannern (eucrite, Moravia, 1808), Millbillillie (eucrite, Western Australia, 1960), Johnstown (diogenite, Colorado, 1924) and Kapoeta (howardite, South Sudan, 1942) are famous falls. Thousands of fragments have been found in Antarctica and the hot deserts.

Vesta is the only intact, differentiated protoplanet from which we confidently hold samples. Not every eucrite-like rock is Vestan, though: Bunburra Rockhole, which fell in Australia in 2007 and whose orbit was measured by fireball cameras, has slightly different oxygen isotopes and must come from another body.

How to recognise one

Fresh HEDs, especially eucrites, have a strikingly shiny black fusion crust, because their calcium-rich feldspar melts to a glossy glass, over a pale grey interior. Many are breccias of light and dark fragments. They contain very little metal and barely react to a magnet. Diogenites are pale green to yellowish inside. Any suspected HED should be reported to a museum, and searches need the landowner's permission.

Groups

Famous examples

Kapoeta Howardite, South Sudan, 1942 · 11.36 kg
Millbillillie Eucrite-mmict, Australia, 1960 · 330 kg
Johnstown Diogenite, United States, 1924 · 40.3 kg
Juvinas Eucrite-mmict, France, 1821 · 91 kg
Stannern Eucrite-mmict, Czechia, 1808 · 52 kg

Related groups

Howardites Broken rubble from the surface of asteroid Vesta, mixing crust and mantle rocks.
Eucrites Volcanic basalt from the crust of Vesta.
Diogenites Pyroxene-rich rocks from deep inside Vesta.
Mesosiderites Jumbled breccias of metal and basalt, made in a violent collision.
Main-group pallasites Nine in ten pallasites: magnesium-rich olivine in metal related to the IIIAB irons.

Photos shared under Creative Commons licences (Wikimedia Commons).