VESTA & HED Meteorites
Full-frame view of Vesta including the "Snowman" craters (left side of the image) from NASA's Dawn probe, August 2011
Vesta: Very nearly a Planet
Vesta is the second largest asteroid in the Solar System with a diameter of around 525 km (362 miles), and orbits between Mars and Jupiter in the main asteroid belt.
Discovered in 1807 by H.W.M. Olbers, Vesta was named after the Roman goddess of hearth and home and was visited by NASA's Dawn mission in 2011-2012 on its way to Ceres. Dawn produced beautiful high resolution images of Vesta alongside a range of spectral data that scientists have used to reconstruct Vesta's 4.5 billion year long history, as well as confirming the link between Vesta and the HED meteorites.
Asteroids are often thought of as lumps of rock whirling through space, but Vesta is actually very complicated. It has a differentiated structure, meaning that it has a rocky crust and mantle with a metal core much like Earth and other planets do, and Vesta's surface is also made (mostly) out of basalt which is the same type of lava that erupts in Hawaii and Iceland. Vesta had a very active igneous history - full of diverse magmas and eruptions - and was well on its way to becoming a proper protoplanet which makes it a fascinating asteroid to study.
Like all rocky planets and asteroids, Vesta's surface records a violent collisional history and is covered in craters. These are the result of impact events between Vesta and smaller pieces of rock in the early Solar System, and some of them left enormous scars. Vesta's south pole was almost entirely obliterated by two massive impacts that created the Veneneia (395 km/245 miles) and Rheasilvia (505 km/314 miles) impact basins. These two huge craters overlap and are almost as wide as Vesta's entire diameter, and the central peak in the Rheasilvia basin formed by the rocks rebounding is 25 km (16 miles) high from base to peak, which makes it the tallest mountain in the solar system!
The HED Meteorites: Magma from another world
HED stands for howardite-eucrite-diogenite. These are an abundant and diverse group of achondrites which are largely accepted to come from Vesta but probably only represent a small portion of the types of rock that exist there.
Eucrites are Vesta's basalts and gabbros and are rich in pyroxene and plagioclase minerals. Almost all eucrites record thermal metamorphism, likely the result of repeated eruptions burying older rocks and reheating them, and once again shows that Vesta is much more than a lump of cold rock floating in space. Eucrites are divided into a number of different subgroups or "trends" based on subtle differences in their chemical compositions, each reflecting different igneous processes that happened billions of years ago.
Diogenites are orthopyroxene-rich cumulate rocks with large crystals that were formed during slow cooling. Although scientists are happy that eucrites represent surface lavas, diogenites are a bit more controversial. Some scientists think that diogenites make up a layer in Vesta's lower crust or the upper mantle, and some think that they were formed in magma chambers that were emplaced inside the eucrite-rich crust. Unfortunately there's no way to know for sure without sending someone with a shovel and a hammer to Vesta, so we have to make do with lab-based experiments and chemical models.
Howardites are a mixture of eucrite and diogenite material that has been broken up and mixed together through repeated impacts on Vesta's surface (we call this process "impact gardening"). In some howardites you can find evidence of the impactor is preserved as well, and so far scientists have identified chunks of non-Vestan achondrites and chondrites in these meteorites, and we've even found traces of carbonaceous chondrites on Vesta itself!
Combined element maps of (from top to bottom) diogenite Dhofar 700, eucrite Northwest Africa 3141, and howardite NWA 8595 showing differences in mineral abundance and texture.
Red = low calcium pyroxene; yellow = high calcium pyroxene; turquoise = plagioclase; dark blue = silica glass; black = voids; bright green = weathering products
Produced by Dr Jen Mitchell.