Howardite

Abstract: The Dawn spacecraft targeted 4 Vesta, believed to be a remnant intact protoplanet from the earliest epoch of solar system formation, based on analyses of howardite-eucrite-diogenite (HED) meteorites that indicate a differentiated parent body. Dawn observations reveal a giant basin at Vesta's south pole, whose excavation was sufficient to produce Vesta-family asteroids (Vestoids) and HED meteorites. The spatially resolved mineralogy of the surface reflects the composition of the HED meteorites, confirming the formation of Vesta's crust by melting of a chondritic parent body. Vesta's mass, volume, and gravitational field are consistent with a core having an average radius of 107 to 113 kilometers, indicating sufficient internal melting to segregate iron. Dawn's results confirm predictions that Vesta differentiated and support its identification as the parent body of the HEDs.

Abstract: Our Solar System formed about 4.6 billion years ago, and within 4 million years small planetary bodies had formed, some melting to form volcanic and related rocks. Two families of meteorites (the HEDs and angrites) are thought to have originated from asteroids that melted at this time. New oxygen isotope measurements confirm that these meteorites are from two distinct asteroids that underwent large-scale melting in the early Solar System. These new results show that early, global-scale melting was a feature of all the differentiated planets (Earth, Moon and Mars) and asteroids so far sampled. Immediately following the formation of the Solar System, small planetary bodies accreted1, some of which melted to produce igneous rocks2,3. Over a longer timescale (15–33 Myr), the inner planets grew by incorporation of these smaller objects4,5 through collisions. Processes operating on such asteroids strongly influenced the final composition of these planets4, including Earth5. Currently there is little agreement about the nature of asteroidal igneous activity: proposals range from small-scale melting, to near total fusion and the formation of deep magma oceans2. Here we report a study of oxygen isotopes in two basaltic meteorite suites, the HEDs (howardites, eucrites and diogenites, which are thought to sample the asteroid 4 Vesta6) and the angrites (from an unidentified asteroidal source). Our results demonstrate that these meteorite suites formed during early, global-scale melting (≥ 50 per cent) events. We show that magma oceans were present on all the differentiated Solar System bodies so far sampled. Magma oceans produced compositionally layered planetesimals; the modification of such bodies before incorporation into larger objects can explain some anomalous planetary features, such as Earth's high Mg/Si ratio.