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Geologic History “Big Bang” about 13.7 billion years ago All matter in the universe created made entirely of Hydrogen and Helium ~13 b.y.a. oldest galaxies form and are moving apart ~ 5 b.y.a our sun begins to form in the milky way galaxy Question you should be asking… If the universe was entirely H, and He…. Where did all the other elements come from that we know and love?! (O, C, Fe, Ag, etc.) That leaves 8 – 9 billion years between the big bang and our sun. We know that stars, in life and death create new elements. We can also see stars forming from the nebula of stars that have died before them. (think recycling!) In our solar system, and on Earth, we have 92 naturally occurring elements. The abundance of these elements indicates that our sun is definitely not the first, or likely even the second start that has lived here. Since in recycling you never regain 100% of the ingredients each star that has come since has been smaller and longer lived. After a while our sun would have had enough gravity and heat to begin explosive fusion, which blew away most of the gas and ice away from the inner planets, leaving only rock and dust. Farther away from the sun large gas giants were forming that had the gravity to keep and pull in more gas. Farther still, there was mainly gas and ice left to form planets. This was referred to as the nebular hypothesis. In this model the sun and planets form in a process called accretion. About 4.6 b.y.a all the planets in our solar system were forming. But, the solar system was full of asteroids from the size of a grain of sand to the size of Kentucky. The gravity of the early planets pulled in most of the left over asteroids over the next 2 billion years. But the question remained: “Where did our big beautiful moon come from?” Theories: Fission Theory: Moon is a chunk of Earth that broke off on its own. Capture Theory: Moon was snagged by Earths gravity while strolling by. Co-creation Theory: We are just really close neighbors that formed at the same time Fission: Wrong! It would go against gravity for a blob of molten rock to go away from the Earth on its own. Capture: Wrong! The moon is too big to be pulled into such a nice orbit if it were passing by. Its geochemistry is too similar for it to be made somewhere else and drift here. Co-creation theory: Wrong! It would be made out of exactly the same material as Earth and it isn’t. Where did it come from?! 1985: Bill Kaufmann, Jay Melosh and others, proposed the Giant Impact Theory. About 4.4 b.y.a. a mars sized planet, Theia, smashed into Earth. Presence of Oxygen isotopes is too close to Earth’s composition to have come from somewhere else. The rocks of the moon are nearly chemically identical to the rocks of Earth’s mantle, which would have been ejected during impact. Moon has a small iron core. The impact would not have been able to eject much of the iron at Earth’s core. Universe began about 13.7 b.y.a. Over the following 8-9 billion years stars have created the elements that make-up Earth and all the planets. Our sun and solar system formed about 5 b.y.a. Earth started forming around 4.6 b.y.a. Theia struck early earth about 4.4 b.y.a. Creating our moon.