In the beginning…

1. In the beginning… Geology 311 : • Formation of the elements • Formation of the solar system • Formation of the Earth !

2. The Big Bang… • 13.7 billion years ago (13,700,000,000 yrs) • Explosion so powerful that space itself was propelled outwards almost instantaneously • 200 Million years later, the first stars formed • Universe is still expanding today

3. Formation of the Elements • Cosmological Nucleosynthesis: • Most H and He formed shortly (few seconds) after the Big Bang, when T cooled to 1 billion degrees • (H and He make up most of the universe!) • Stellar Nucleosynthesis: • Most heavier elements (metals) form either in Red Giants or Supernovae by nuclear fusion • (but without these there would be no life!)

4. Stellar Nucleosynthesis • Red Giants • Large stars that have exhausted hydrogen fuel in their cores and go on to ‘burn’ (i.e., fuse) He, C, O, etc. • Creation of elements up through Fe • Extra neutrons can be captured to produce heavier elements • Supernovae • Once core is converted to iron, large stars collapse and then explode • Enormous numbers of neutrons produced are captured to produce heavy elements

5. Solar System Formation Solar System Formation

6. Formation of the Solar System: Slowly rotating nebula begins to contract, condense, and spin faster

7. The Orion Nebula

8. Gas and dust particles collide and coalesce into planetesimals Multiple collisions and accretion into terrestrial planets

9. Our Solar System

10. Terrestrial Planets: Mercury, Venus, Earth, and Mars • Rocky (silicate) outer parts (crust and mantle) and inner cores of Fe-Ni metal. • Silicates are compounds of silica (SiO2) and the oxides of other metals. • Common silicates: • Olivine: (Mg,Fe)SiO4 • Pyroxene: (Mg,Ca,Fe)2Si2O6 • feldspar (e.g., (Na,K)AlSi3O8) • mica (e.g., biotite K(Mg,Fe)3AlSi3O10(OH)).

11. The Giant Planets: Jupiter and Saturn • Like the Sun, they consist primarily of H and He. Thus they are approximately “solar” in composition (though not exactly). • Small rocky core overlain by metallic H layer, overlain by molecular H, He layer (gradual transitions from liquid to gas).

12. The Icy Planets: Uranus and Neptune • These also consist of largely of H and He, but they are enriched in C and N compared to Jupiter and Saturn. • Their solid parts consist of ices of methane and nitrogen.

13. Summary of Planet Formation • The Sun constitutes over 99% of the mass of the Solar System. Therefore the solar nebula had a “solar composition”. • The giant planets must have formed early, before the gas of the nebula dissipated, because they are rich in volatiles (H, He). • Icy planets form more slowly because of lower densities in outer part of nebula • The inner planets form after the gas had largely dissipated from the inner solar system. • The terrestrial planets are depleted not only in gaseous elements such as H, He, C, and N, but in “moderately volatile” elements as well. These include the alkalis (Na, K, Rb, Cs) and elements such as sulfur, lead, and indium. • High temperatures in the inner solar system delayed planet formation.

14. Much of the evidence for how the solar system formed comes from meteorites. Most come from asteroids of the asteroid belt (e.g., Eros). A few from Mars and the Moon. Most meteorites give ages close to 4.56 Ga by most dating methods. (Ga = billion years) Small group of achondrites give much younger ages (e.g., 1.3 Ga). These meteorites are thought to be from Mars. Observations from Meteorites

15. Lunar constraints on planet formation • Moon is the only other planetary body we have directly sampled and explored • Moon and Earth and closely related, both physically and chemically • ‘Hadean’ record preserved on the Moon • Oldest rocks from the Moon are 4.45 Ga • Record of the ‘early Earth’ (Hadean period) is missing, destroyed by subsequent events

16. The Moon: some key observations • No other planet has such a large moon relative to its size (except Pluto). • Moon has only a very small iron core • Moon has a bulk density about the same as the Earth’s mantle (suggests compositional similarity). • Highly depleted in highly volatile elements (gaseous elements); depleted in moderately volatile elements. • Has identical oxygen isotope composition to the Earth • Bottom line: Earth and Moon probably have a shared history

17. Giant Impact Hypothesis • Idea in a nutshell: • Body about 1/10 the mass of the Earth (Mars-sized) struck the Earth after it is half or more accreted, and after the Earth’s core had at least partially formed. • Material, mainly from silicate mantle, is blasted into orbit around the Earth, eventually accreting to form the Moon.

18. The “late heavy bombardment” • Many impact craters on Moon date to around 3.9Ga. • If the Moon was bombarded, wouldn’t the Earth be as well since they are very close to each other? • Why doesn’t the Earth’s surface look like the Moon?

19. After all we are still being bombarded… (Dots show location of known large impact craters)

20. Chicxulub, Yucatan Peninsula, 65 Ma Digital shaded relief image of crater thought to have caused extinction of the dinosaurs

21. Meteor Crater, AZ 49,000 yrs ago

22. Earth is a dynamic planet! Earth is constantly being resurfaced due to plate tectonics and the rock cycle.

23. ..a really dynamic planet!

24. Differentiation of the Earth • Early homogenous Earth • Lighter matter “floats” toward surface • Modern structure of the Earth

25. Earth has a layered structure • Atmosphere and Hydrosphere • Low Density Crust (6-35 km thick) • Intermediate Density Mantle (~3000 km thick) • High density core (~3000 km thick) • Liquid outer core • Solid inner core

26. Earth’s Layers

28. When did the continental crust begin to form? • Oldest known rocks are from the Great Slave Province in Canada and are approximately 4 Ga old. • Oldest known mineral is a zircon is from Australian sediments whose metamorphic age is 3.5 Ga. • Inherited zircons are as old as 4.4 Ga!

29. Earth’s Atmosphere and Hydrosphere • When did it form? • How did it form? • Has it evolved with time?

30. Hypotheses • (1) The atmosphere (& hydrosphere) formed immediately by degassing of the Earth’s interior. • (i.e., about the same time the Earth formed). • (2) The atmosphere (& hydrosphere) formed slowly over geological time by degassing of the Earth’s interior.

31. Which is right? • Based on isotopic data, >85% of atmosphere was produced by “early catastrophic degassing”; the rest through “continual” degassing. Volcanic Degassing contributed large amounts of H2O, CO2, and other gases to the atmosphere

32. Our unique atmosphere • Why does our atmosphere have so much O2 when Venus and Mars have hardly any? • Why is our atmosphere so poor in CO2 compared to that of our neighbors? • Answers to these questions are related.

33. Where has all the carbon gone? • Amount of carbon in sediments exceeds the amount of carbon in the atmosphere (as CO2) by a factor of 200. • How did the carbon get there? • This carbon represents the remains of once living organisms (almost entirely plants). • In other words, life, through photosynthesis, is partly responsible for the low levels of CO2 in the atmosphere. • Corollary: life is entirely responsible for the presence of free oxygen in the atmosphere. • Far more carbon than this is stored in sediments as carbonate rocks.

34. When did the oceans form? • H2O would have been degassed from the Earth’s interior simultaneously with gases of atmosphere. • But, when was the Earth’s surface cool enough for oceans to form? • The 4.4 Ga zircon has d18OSMOW up to 9‰. • This suggests the magma from which the zircon crystallized from reacted with or contained material that had reacted with liquid water. • Earth’s surface was apparently cool enough for oceans to form at 4.4 Ga!

35. Our unique planet • Earth’s uniqueness is a consequence of its formation • Formation in the gas-depleted inner part of solar nebula leads to moderate size and depletion in volatile elements • Its violent early history probably further contributes to volatile depletion and atmospheric composition uniquely suited for life