Comparative Planetology
Magma oceans and mantles of rocky planets
Hilary Downes
Birkbeck University of London
Terrestrial planetary bodies have internal layered structures with an iron-rich core, silicate mantle and silicate crust. Although their mantles are all dominantly composed of magnesium silicate minerals such as olivine and pyroxene, differences are evident between the mantles of different planets, as a result of minor differences in bulk planetary composition and oxidation state. Terrestrial bodies for which we have samples (Earth, Mars, plus the Moon and asteroid 4Vesta) show evidence of having well-mixed silicate mantles. For Earth this can be seen both in mantle-derived magmas such as basalts, and also in mantle fragments brought to the surface by volcanic or tectonic activity. Evidence from the other terrestrial bodies is in the form of basaltic rocks (including meteorites from Mars, 4Vesta and the Moon, and returned lunar samples) which have been formed by partial melting of planetary mantles and eruption of magma to form a silicate crust. The well-mixed mantles are considered to be a result of each planet experiencing a “magma ocean” early in its history. However, some silicate meteorites (ureilites) show evidence for being derived from the mantle of an unknown asteroid. Their ages suggest that their parent body was formed and destroyed within the first 5 Ma of solar system history. These meteorites show strong chemical and isotopic heterogeneity which suggests that the mantle of their parent body must have avoided the magma ocean stage. Thus they allow us to see evidence for some of the earliest processes of planetary accretion and differentiation.
16:30 - 18:00