What is a howardite?
A howardite is a type of achondrite meteorite made of broken fragments of two kinds of rock, eucrite and diogenite, fused together. Howardites are thought to be pieces of the surface rubble of the asteroid Vesta.
Howardites are a kind of breccia, a rock made of angular fragments cemented together. If you look at a cut face, you will see a patchwork of pale and dark pieces, rather like a fruit cake. Those fragments come from two kinds of rock: eucrites, which are basalts that crystallised from lava, and diogenites, which are made mostly of the mineral orthopyroxene and formed more slowly, deeper underground. To be called a howardite, a meteorite must contain a substantial share of both, with the diogenite component making up more than about a tenth.
Together, howardites, eucrites and diogenites form the HED group, and the best evidence points to the asteroid Vesta as their main source. Howardites tell a particularly vivid story. They seem to come from Vesta's regolith, the layer of loose, impact-churned rubble at its surface. Some contain gases implanted by the solar wind, showing that their grains once lay exposed at the surface, and many contain small dark fragments of carbonaceous chondrite, debris from other asteroids that crashed into Vesta. When NASA's Dawn spacecraft orbited Vesta in 2011 and 2012, its instruments found that much of the surface has a howardite-like composition.
The group is named after the British chemist Edward Howard, whose analyses of meteorites in 1802 helped show that they were unlike Earth rocks. Well-known howardites include Kapoeta, which fell in what is now South Sudan in 1942, and many finds from the Sahara and Antarctica.
For this website, howardites are the perfect emblem: each one is a sample of another world's surface, mixed by billions of years of collisions and delivered to Earth for us to study.