Planetesimal building blocks

We have already seen that the various chondrite groups have distinct compositions, and we might expect that these compositions would have varied systematically with solar distance. The asteroids show a rough radial gradient in composition, with E and S asteroids concentrated nearer the Sun, C asteroids farther outboard, and P and D asteroids at still greater 

Relative masses Mantle plus crust Core 

Data sources Mercury (Fegley and Cameron, 1987 Goettel, 1988), Venus (Basaltic Volcanism Study Project, 1981), Earth (Taylor, 1992), Mars (Taylor, 1992). 

High-resolution chronometers (Wadhwa et al., 2006), especially that based on the 

Major-element compositions (weight ratios of Mg/Si and Al/Si) for mantle rocks (peridotites) and estimates of the primitive mantle composition of the Earth compared with various groups of chondrites and the Sun. No mixture of chondrite types provides an exact match to the primitive mantle composition, although some carbonaceous chondrites provide the closest match. Modified from Righter et al. (2006). 

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Tiny microparticles came together to form microagglomerates, and these in turn formed larger clots, which then formed larger bodies, the diameter of which was initially measured in centimetres but later increased to metres such planetary building blocks are known as planetesimals . Computer simulations indicate that these existed around four and a half billion years ago (Wetherhill, 1981). Planetesimals grew to form bodies which were several kilometres across, and there were often collisions in which larger bodies were swallowed up by smaller ones a process which is not unknown in modern economics ... 

Anhydrous planetesimals formed within the inner solar system, unlike the ice-bearing bodies discussed in the next chapter. These objects, composed of rock and metal, were the primary building blocks of the terrestrial planets. Relics of that population may survive today as asteroids that dominate the inner portions of the main belt. 

Many of the observations described in the previous section can be explained by the planetesimal theory, in which rocky planets form by the aggregation of huge numbers of asteroid-sized building blocks called planetesimals (Safronov 1969). This... 

Both comets and asteroids are surviving planetesimals from the solar nebula, preserved building blocks of planets. Comets are generally considered to be relic materials from the outer regions of the solar nebula, while asteroids are considered to be materials that formed or ended up near the asteroid Belt, mostly between 1.9 AU and... 

Planetesimals The building blocks of the planets. Small rocky or icy bodies formed in the primordial solar nebula. They range greatly in size from one kilometer up to the size of small planets. 

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