1.2 How Did the Planets Form? Planetary Geology

1.2 How Did the Planets Form? Planetary Geology & the Solar Nebula

The Sun and planets formed at the same time from a large rotating cloud of interstellar dust and gas called the solar nebula. As the solar nebula contracted, the vast majority of material collected in the center to form the hot “protosun”. The remainder formed a flattened, spinning disk. Within this spinning disk, matter gradually formed clumps of material that collided, stuck together and grew into asteroid size objects called “planetesimals”. The composition of these planetesimals depended largely on their location with respect to the protosun. Temperatures were greater near the protosun and much lower in the outer reaches of the disc. This was critical since only those materials that could condense, (form solid or liquid clumps), in a particular location would be available to form planetesimals.

Near the present orbit of Mercury only metallic grains condensed. It was simply too hot for anything else to exist. Further out, near Earth’s orbit, metallic as well as rocky substances condensed. Beyond Mars, ices of water, carbon dioxide, ammonia, and methane formed. The planetesimals formed and through repeated collisions and accretion (sticking together), grew into eight proto-planets and their moons. It took roughly a billion years after the protoplanets formed to gravitationally sweep the solar system clear of interplanetary debris. This was a period of intense bombardment that is clearly visible on the moon and elsewhere in the solar system. Only a small amount of the interplanetary matter escaped capture by a planet or Moon and became the asteroids, comets and meteoroids.


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