Howardites
Group · Stony meteorites › Achondrites › Asteroidal achondrites › HED meteorites: pieces of Vesta

Howardites

Broken rubble from the surface of asteroid Vesta, mixing crust and mantle rocks.

Howardite
Kapoeta, Howardite, South Sudan
Classification
Stony meteorites › Achondrites › Asteroidal achondrites › HED meteorites: pieces of Vesta › Howardites
Share of witnessed falls
about 1.6%
In our catalogue
241 meteorites, 16 seen to fall
Heaviest
Lohawat, 40 kg
Earliest recorded fall
Mässing, 1803
Most recent fall
Lohawat, 1994

Howardites are regolith breccias: rocks made from the broken surface layer of Vesta, where countless impacts pulverised and mixed the crust. They contain fragments of both eucrite and diogenite, and by definition at least 10% of each. Some also contain bits of carbonaceous chondrite from asteroids that crashed into Vesta, and gases implanted by the solar wind when the grains lay exposed on the surface.

They are named after the British chemist Edward Charles Howard (1774–1816), whose 1802 analyses found nickel in both stony and iron meteorites, an early clue that they share an origin beyond Earth. About 16 howardites have been seen to fall, including Kapoeta in Sudan in 1942.

Mineralogy and chemistry

A howardite is a breccia containing fragments of basaltic and gabbroic eucrite, orthopyroxene-rich diogenite, impact-melt glass and fine dust, all compacted together. The minerals are pyroxene of many compositions, calcium-rich plagioclase, some olivine and minor chromite, troilite and metal. The modern definition, proposed in the 1980s, calls a polymict breccia a howardite when it contains more than 10% of both eucritic and diogenitic material; mixtures with less diogenite are polymict eucrites, and those with less eucrite are polymict diogenites.

Their bulk composition lies between eucrite and diogenite, depending on the mix. Oxygen isotopes match all other HEDs.

Parent body and formation

Howardites formed in the regolith, the layer of broken rock that covers Vesta, churned by billions of years of impacts. Many contain gases implanted by the solar wind, glass beads and dark clasts of carbonaceous chondrite, mostly CM-like, from small asteroids that hit Vesta at low speed. These dark clasts may explain the dark patches Dawn saw on Vesta's surface. Dawn's spectrometers showed that much of Vesta's surface has a howardite-like composition, so howardites are the best match for its surface as a whole.

Because howardites mix rock from across Vesta's surface, they act as natural soil samples. Measuring the share of eucrite and diogenite in each, and comparing it with Dawn's maps, helps scientists judge where on Vesta they came from. Their solar-wind gases record how long the grains lay exposed before being buried and compacted.

Name, abundance and famous examples

The German mineralogist Gustav Rose named the group in 1863 after Edward Charles Howard, the British chemist whose analyses of 1802 helped prove that meteorites are not Earth rocks. This site takes its name from them. About 16 howardites have been seen to fall. Kapoeta, which fell in what is now South Sudan in 1942 (about 11 kg), is the best studied, rich in solar-wind gases and dark clasts. Le Teilleul (France, 1845), Pavlovka (Russia, 1882), Frankfort (Alabama, 1868), Bholghati (India, 1905) and Bununu (Nigeria, 1942) are other falls. Many more come from Antarctica and North Africa.

How to recognise one

A howardite shows a jumble of angular light and dark fragments in a fine grey matrix, sometimes with yellow-green diogenite grains and dark specks of carbonaceous material. Its fusion crust is black and often glossy. It contains little metal, so it is barely magnetic. It looks like a terrestrial breccia and needs laboratory confirmation.

Famous examples

Related groups

See all 241 Howardites

Photos shared under Creative Commons licences (Wikimedia Commons).