Earth. Terrestrial Planet 1 AU from the Sun. Equilibrium temperature: 247 K (-26C) Actual mean temperature: 287 K

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1 Life on Earth. I.

2 Earth Terrestrial Planet 1 AU from the Sun Equilibrium temperature: 247 K (-26C) Actual mean temperature: 287 K Differentiated into Inner core Outer core Mantle Crust

3 Composition Crust: Oxygen 47% Silicon: 28% Aluminum: 8% Iron% 5% Calcium: 4% Atmosphere: N 2 : 77% O 2 : 21% Ar: 1% H 2 O: <1% Overall: Iron: 35% Oxygen: 30% Silicon: 15% Magnesium: 13% Nickel: 2%

4 Surface Water: 71% Land: 29%

5 Structure Inner core: 1.6% of mass km Outer core: 31% km Mantle: 67% km Crust: 0.4% upper 40 km Oceans: 0.02% Atmosphere: 9x10-5 %

6 The Earth is 4.5 billion years old. How do we know?

7 Radiometric Dating How old is the Earth? K 40 half-life: 1.25 billion years

8 Radioactive Decay The change of an atomic nucleus into a lower energy state decay: proton neutron + positron Atomic number decreases Inverse decay: neutron proton + electron Atomic number increases Fission: breakup into 2 smaller nuclei Inherently probablistic

9 Radioactive Dating. II. Useful radioisotoptes C 14 N 14 : t 1/2 = 5730 years Al 26 Mg 26 : t 1/2 = 717,000 years K 40 Ar 40 : t 1/2 =1.25 billion years U 238 Pb 206 : t 1/2 = 4.47 billion years Rb 87 Sr 87 : t 1/2 = 49.4 billion years

10 Radioactive Dating. III. How it works: Measure abundance of parent atom Measure abundance of daughter atom Correct for original abundance of daughter Ratio of parent to daughter (corrected) gives number of half lives Age of the Solar System: 4.57 Gyr Age of the Earth: Oldest rocks: 4 Gyr Zircon grains: 4.4 Gyr

11 Parent Original abundance: A 0 Half life: Current abundance: A = A 0 (1/2) Daughter Original abundance: B 0 Half life: Current abundance: B = B 0 + A 0 (1-(1/2) ) 1 half life: down by a factor of 2 2 half lives: down by a factor of 2x2 = 4 n half lives: down by a factor of 2 n

12 Birth of a Planet

13 Protoplanetary Disks Formation of a flattened disk is a natural consequence of the conservation of angular momentum in a collapsing, rotating system. Such disks are observed.

14 Planet Formation

15 Protoplanetary Disks

16 Birth of the Earth Inner protoplanetary disk is hot Terrestrial planets lack volatiles Temperature (K) Condensate Fe 2 O 3, FeO, Al 2 O 3 Fe, Ni Silicates MgSiO 3 FeS H 2 O NH 3 CH 4 Noble gases

17 Birth of the Earth Small dust grains collide and stick Once grain becomes large enough, gravity takes over Runaway accretion ensues.

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19 I Feel the Earth Move Under My Feet

20 Plate Tectonics Driven by internal heat: Radioactive decay Differentiation Accretion Internal heat drives convection Convection currents in mantle drive continental drift

21 Plate Tectonics Continental crust (lithosphere) floats on the denser mantle Crust is broken into about a dozen plates The continental plates move independently

22 Continental Plates

23 Plates in Motion

24 Plates Tectonics and Habitability Subduction removes carbonates into mantle Otherwise greenhouse CO 2 accumulates Vulcanism ejects gas, including H 2 O Reconstitutes atmosphere Vulcanism provides new land and raises mountains Counters weathering/erosion

25 What Planets are Tectonically Active? All planets have internal heat. Convection depends on the heat gradient (T core ) The heat content is proportional to the volume (R 3 ) Heat loss is proportional to the surface area (R 2 ) Planets stay warm for a time R 3 /R 2, so R Big rocky planets (like Earth)

26 An Advantage of a Tectonically Active Planet Molten iron cores are convective, and generate a magnetic field through dynamo action (much like the Sun). Magnetic Fields divert charged particles.

27 The Crust Low density - floats on the mantle Is eroded and weathered Records impact history Preserves the fossil record

28 The Early Earth

29 The Early Earth Formed from circumstellar disk at its present radius. Surface remelted in collision that formed Luna. Differentiated about 4.5 Gyr. Zircon grains dated to 4.4 Gyr Oldest rocks: 4 Gyr Oldest continental crust 3.9 Gyr (in Greenland) (Lunar rocks are 4.5 Gyr old)

30 Gyr Impacts melt the surface. Volatiles escape to space The Hadean Earth Source of atmosphere, oceans: outgassing and impacts Early atmosphere: CO 2, H 2 O, N 2, H 2 S, SO 2, H 2 Oceans exist by 4.4 Gyr Impacts: 4.5 Gyr Late Heavy Bombardment at 3.9 Gyr Lunar crater counts give this dating

31 Life and the Hadean Earth There is no fossil record. There is no chemical record. Impacts might have kept the surface in a molten/sterile state.