Lecture 8.5

Asteroids and Meteorites and the Birth of Rocky Planets, Take 2

About 4.6 billion years ago....

Composition of the solar nebula.

Cooling and condensation of the nebula to form chondrules. These are incorporated into rocks across the solar system. Some of these rocks are preserved today in the asteroid belt and some are delivered to Earth as the carbonaceous chondrites - samples of the nebula! We date these rocks at 4.6 billion years.

(In the inner solar system, temperatures were high enough to allow rocks and metals to condense. In the outer solar system ices also condensed.)

The rocks of the solar system are dominated by silicate minerals containing SiO2 in the mineral structure. The metals of the solar system are dominated by Fe.

The rocks of the infant solar system continued to accrete into larger bodies. As bodies grow larger, they undergo heating from a variety of sources.

Heated bodies can undergo differentiation into a core (dense Fe, Ni), mantle (silicates of moderate density) and crust (silicates of lower density). This happened in the larger bodies of the asteroid belt and in all the planets. Iron and stony-iron meteorites are samples of differentiated asteroids.

The process of accretion continues until the large planets sweep everything up. This is the


 Just look at the Moon! or Mercury! Rotation of Venus and Uranus.

They are covered with impact craters from this era ~3.8-4.5 billion years ago. After this time most large objects were swept up.


The Earth was also bombarded during this time and its crust was broken and heated. The atmosphere was forming from gases released by volcanoes and impact. Despite all of this there is evidence for life on Earth at 3.8 billion years.

Why isn't the Earth covered with impact craters? (There are some however, here's one in Canada, here's one in Arizona, here's one in Mexico that killed the dinosaurs).


Not all heating sources are active today and the planets are cooling down losing heat (by conduction, convection and radiation) to space.

Smaller rocky planets cool faster than larger rocky planets - explain Io.