The Hadean & Archean

1. The Hadean & Archean “It’s the Earth Jim, but not as we know it.”

2. Hadean & Archean: 2500-4600 Myrs

3. Archean Divided into:

4. Hadean/Archean “Evidence” • Problems • Very few rocks/minerals exist from this period • Most have been metamorphosed, or buried • No fossils exist in Hadean, Minimal in Archean • Rely on: • Few rocks that we have • Moon, Meteorites& other planets • Models 4.4 Gyr Zircon Crystal

5. Rocks Older than 2.5 Gyrs

6. The Neighborhood: “Dim” Sun

7. The Neighborhood: “Near” Moon = Shorter Day About 10 hrs

8. Origin of the Moon: 4.5 Gyrs

9. Lunar Evidence • Moon does not orbit around the earth’s equator • The moon is retreating from the Earth (but could never have been closer than 24,000 km) • Moon is depleted in iron and “iron-loving” elements (it has a low density) • Moon is dry (no water in the minerals) • Oxygen isotope ratio in moon rocks is the same as the Earth • Lunar highlands have been dates at about 4.5 Byrs

10. Lunar Hypothesis • Capture: Formed somewhere else and was captured • Twin Planet: Formed from material left over from the Earth • Fission: “Budded” off the Earth Problem: It has the same Oxygen Chemistry as the Earth indicating it formed in the same part of the Solar System

11. Lunar Hypothesis • Capture: Formed somewhere else and was captured • Twin Planet: Formed from material left over from the Earth • Fission: “Budded” off the Earth Problem: Moon has much less iron than the Earth and is no volatiles (water & gases)

12. Lunar Hypothesis Problem: Earth was not spinning fast enough for this to be realistic. • Capture: Formed somewhere else and was captured • Twin Planet: Formed from material left over from the Earth • Fission: “Budded”off the Earth

13. Lunar Hypothesis • Capture: Formed somewhere else and was captured • Twin Planet: Formed from material left over from the Earth • Fission: “Budded” off the Earth • Collision: Formed when a Mars-sized body collided with the Earth Explains (1) isotopes, (2) lack of iron and volatiles, and (3) consistent with Earth’s spin rate.

14. Animation Link

15. Lunar Evidence / Impact  • Moon does not orbit around the earth’s equator • The moon is retreating from the Earth (but could never have been closer than 24,000 km) • Moon is depleted in iron and “iron-loving” elements (it has a low density) • Moon is dry (no water in the minerals) • Oxygen isotope ratio in moon rocks is the same as the Earth • Lunar highlands have been dates at about 4.5 Byrs     

16. Hadean Earth 2004 - Fred Sulahria

17. Hadean: What We Do Know… Earth was a very different place! • Major features of the Earth developed: • atmosphere (throughout Hadean) • oceans (water by 4.4 Gyr, stable oceans by 3.8 Gyr) • crust and continents (by 4.4-4.0 Gyr) • plate tectonics starts (about 4.0 Gyr) • magnetic field (about 4.5 Gyr) • life? (no evidence yet…)

18. Early Earth Driven by Heat From: Accretion Differentiation Radioactive Decay

19. Heat Escapes by: • Convection • Conduction • Radiation Resulting in Convection of Interior and Volcanism

20. Hadean Surface Dominated By: • Meteorite Bombardment • Volcanism

21. Heavy Bombardment Period

22. Heavy Bombardment on the Moon

23. Bombardment may have led to a Temporary “Magma Ocean”

24. Komatiites Magma Solidified into Early Crust Ultramafic, Dense, Uniform

25. Atmosphere

26. Atmospheric Formation • Hypothesis: Captured from Solar Nebula • Mostly Hydrogen and Helium • Would have been quickly lost If initially lost - where did the present atmosphere come from? Accreted as hydrous minerals and outgassed from volcanoes Returned to Earth from comets

27. Secondary Atmosphere - Outgassing • Initially Composed of: • Ammonia (NH3) • Hydrogen (H2) • Carbon dioxide (CO2) • Methane (CH4) • Water vapor (H2O) • Nitrogen (N2)

28. Similar to Saturn’s moon: Titan

29. One Reconstruction(Others have much more Carbon Dioxide in Hadean)

30. Note: No Significant Oxygen! • Evidence: • No Rust Minerals • Pyrite and Uraninite Conglomerates(normally oxidize) • Extensive carbon-rich rocks Witwatersrand conglomerate, South Africa (Middle Archean)

31. Atmospheric Evolution Primary - Captured from Solar Nebula - Mostly Hydrogen and Helium - Quickly Lost Secondary - Formed by Outgassing by Volcanoes Tertiary - Modified by Life (addition of Oxygen)

32. Oceans

33. “Raining” of the Oceans

34. “Vaporization” of the Oceans

35. Hadean Oceans • Characteristics • Liquid Water existed at 4.4 Gyrs • “Ephemeral” (periodically vaporized) • Acidic (water + carbon dioxide = carbonic acid) • Probably “salty” • Stabilized by end of Hadean (about 3.8 Gyrs) • Evidence • Jack Hills Formation zircon crystals with water inclusions • solar system evidence for heavy bombardment and models • no limestones, atmospheric models • acidic water (dissolves ions) • first limestones & sedimentary rocks

36. One Reconstruction(Other hypothesize much less water from comets)

37. Return to the Crust

38. Continents FormationRequires Plate Tectonics

39. Intermission:See Plate Tectonics ppt

40. Set the “Wayback Machine” to return to the Hadean…

41. Heat Generated by Radioactive Decay Hadean Earth was much hotter! Convection was more intense

42. Rapid Convection Created “Mini” Plates that were Quickly Subducted & Destroyed

43. Subduction Createsmore Felsic Volcanic Islands

44. As Plates Move - Islands “Accrete” to Form Larger Islands

45. Greenstones & Tonalites (Granites)

46. Continental Crust Forms 3.96 Billion Year Old Gneiss

47. Archean Crust Oldest Rocks Oldest Minerals

48. Continental Evolution:One Reconstruction Up to 60% by the Proterozoic

49. Hadean Summary