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Lecture 7. The Hadean Earth, heavy bombardment, origin of the atmosphere and oceans

Lecture 7. The Hadean Earth, heavy bombardment, origin of the atmosphere and oceans. announcement: schedule for next week. reading: Chapter 4. The Hadean Eon. rise in oxygen. oldest rocks on Earth - end of heavy bombardment. plate tectonics?. origin of the solar system. first multi-

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Lecture 7. The Hadean Earth, heavy bombardment, origin of the atmosphere and oceans

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  1. Lecture 7. The Hadean Earth, heavy bombardment, origin of the atmosphere and oceans announcement: schedule for next week reading: Chapter 4

  2. The Hadean Eon rise in oxygen oldest rocks on Earth - end of heavy bombardment plate tectonics? origin of the solar system first multi- cellular fossils Cambrian Explosion Phanerozoic Hadean Archean Proterozoic billions of years ago: 2.5 3.8 0.55 4.56 present

  3. Distribution of Hadean Rocks on the Earth Slave Province, northern Canada 3.96 - 4.0 Ga gneiss

  4. Porpoise Cove, Canada 3.825 Ga volcanic and sedimentary rocks

  5. Western Greenland

  6. 3.8 Ga Isua Greenstone Belt, southern West Greenland 2.991 Ga felsic (Si-rich) gneiss calc-silicate rocks, once thought to be sedimentary, now thought to be metamorphosed basalt

  7. Greenland, cont. felsic dikes in gneisses pillow basalt - formed when basalt erupts underwater (water 3.8 Ga)

  8. Pilbara, Western Australia komatiite, Mg-rich form of basalt very hot eruptions 3.5 Ga stromatolites - may be the oldest microbial mats

  9. Jack Hills, Western Australia rock contains 4.4 Ga zircon grains - oldest dated material on Earth banded iron formation

  10. What about Earlier Hadean? Little or no direct evidence. Moon formed by 4.4 Ga. Know the Earth was differentiated by then. Isotopic evidence suggests Earth’s core formed very early - 45 Ma after accretion began. Moon age 4.3-4.4 Ga. Zircon grains have oxygen isotopes that indicate the presence of liquid water - oceans.

  11. Vigorous Volcanic Eruptions Higher (~5x) heat flux due to abundant heat sources. Abundant magma, recycling and remelting of crust. Earliest rocks are either volcanic rocks or sedimentary rocks that are heavily metamorphosed. How does heat escape from the Earth? Hydrothermal vents Mid-ocean ridges Volcanoes and hot springs

  12. Impacts are Common Interplanetary Dust 10 µm diameter Small impacts - 20,000 tons of interplanetary dust every year Giant impacts - rare Over 100 craters identified. Chicxulub Crater Yucatan Peninsula ~180 km diameter 65 Ma Manicouagan Crater, Quebec 70 km diameter 206-214 Ma Meteor Crater, Arizona 1 km diameter ~50,000 years old Aorounga Impact Crater, Chad 17 km diameter several hundred Ma

  13. Heavy Bombardment Cratering rate drops with time. Cratering rate determined from the Moon. But Earth is larger - so it makes a bigger target. Earth has 6x gravity field of Moon - attracts more impactors. Cratering rate of the Earth ~ 10x the Moon. Older surfaces of the Moon - Highlands saturated with craters. Cratering rate actually difficult to measure. Younger surfaces - Mare/maria have few craters. Radiometric dating of maria basalts indicate they are 3-3.9 Ga.

  14. Size Distribution of Craters microscopic Moon craters # of events N greater than mass m per year modern Earth

  15. Size Really Does Matter Meteor Crater impactor 50-60 m. Chicxulub Crater impactor 10-15 km. 350-400 km diameter: - could vaporize entire oceans - raise surface temperatures by 2000˚C - sterilize Earth’s surface - could have been a couple Source of temperature increase: - heat of impact - vaporization of rock (radiates heat into the atmosphere) - vaporized water absorbs sunlight and heats the atmosphere 150-190 km diameter: - vaporized top few hundred meters of the oceans - killed most things except those in deep oceans or deep subsurface - probably several/many

  16. Large Impactors Through Time High energy impactors likely hit during the Hadean era.

  17. Early Atmosphere and Oceans If there was an early atmosphere, it was likely lost from the large number of early impacts. Planet too small to capture H and He gas from the nebula. Formed in a region with few volatiles. Planetesimals and comets from farther out of the solar system also impacted Earth. Some gases mixed in with the mantle.

  18. Impact Delivery of Volatiles Most comets originate from the Oort cloud. Highly elliptical orbits. Composition: water, dust, methane, ammonia, CO2. Small percent of organic carbon. Stardust mission: caught comet dust from Wild2 in aerogel Comet West, 1975

  19. Outgassing of Volatiles Magmas nearing the surface undergo degassing. Gases: water, CO2, CH4, N2, NH3, H2S, SO2. Hydrothermal vents and hot springs also contain magmatic gases. Water vapor cooled, then formed the oceans. Difficult to determine how much came from comets, how much from outgassing. Also don’t know what early atmosphere was like - no oxygen.

  20. Faint Young Sun Sun has been getting brighter with time. Sun today 30% brighter than when solar system formed. Early Earth received less sunlight. Atmosphere not warmed as much as it is today. Faint Young Sun Problem: Faint Sun yet had early liquid oceans. Have some form of crust. Thought plate tectonics were much different.

  21. Lecture 8. The Archean Earth, the Greenhouse Effect reading: Chapter 4

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