What did the Earth look like as it was forming?

Based on the kinds of bodies we now see in our solar system today, it is easy to imagine the infant Earth looking like an irregular clump of cratered rock when it was about a few hundred kilometers across. It might have broken up and reformed several times as impacts with larger bodies occurred. Once it was a few thousand kilometers in radius, it's surface probably looked like the Moon, but then around 4.3 billion years ago or so, a huge impact with a body nearly as big as it was, smashed the Earth, and ejected matter into space which eventually became the Moon. The Earth, meanwhile, probably became molten through-and-through by this impact as the kinetic energy was converted into heat. By 4.1 billion years, or so, the Earth had absorbed probably 99 percent of its current mass. Its surface may have been barren but covered with magmatic flows and deep basins. It was heavily cratered, and may have had a thick atmosphere, possibly rich in carbon dioxide outgassed from the interior. The water we now have may have also been outgassed from the interior and condensed on the surface by about 4.0 billion years, or it may have arrived in the form of cometary bodies. The period of intense meteoritic bombardment seems to have ended around 3.8 billion years, at about the time when the first fossil bacteria have been detected.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the