bb How and when did the Earth and Solar System Form?

NEBULAR HYPOTHESIS b. Condensation of primordial dust. Forms disk-shaped nubular cloud rotating counter- clockwise. a. Supernova and formation of primordial dust cloud. c. Proto sun and planets begin to form. d. Accretion of planetesimals and differentiation of planets and moons (4.6 billion years ago. e. Existing solar system takes shape.

1.Planets and moons revolve in a counter-clockwise direction (not random). 2.Almost all planets and moons rotate on their axis in a counter-clockwise direction. 3.Planetary orbits are aligned along the sun’s equatorial plane (not randomly organized). 4.Observations from Hubble telescope and radio astronomy indicate that other planetary systems are forming from condensed nebular dust. Evidence to support the nebular hypotheses:

Terrestrial Planets Close to the sun, dense Small rocky (silicate minerals, metallic cores) Jovian Planets Far from the sun, low density Large, gaseous (hydrogen, methane) Possible Exam Question: Can you explain why the earth and terrestrial planets have so little molecular hydrogen comprising their respective atmospheres; yet the primoridal dust cloud was mostly comprised of hydrogen gas? Note: Twelve earth’s would fit across the diameter of Jupiter.

Differentiated Earth 1. Iron-Nickel Core (outer core liquid) (inner core solid) 2.Fe-Mg Silicate Mantle 3.Fe-Mg-Al Silicate Crust (ocean and continental) 4.Oceans 5.Atmosphere How is the earth compositionally zoned? Along a density gradient lighter denser

How did the earth become compositionally zoned? 1.Accretion of planetesimals. 2.Initial heating due kinetic energy of colliding planetesimals and compressional heating. 3.Additional heating from radiocactive decay. 4.Iron catastrophe (melting temperarure of iron reached and dense iron-nickel sink to core and lighter materials are displaced outwards (including silicate rock of mantle and crust, ocean waters and atmospheric gases. 5.Earth become compositionally zoned based on density (Densest iron-nickel in core-least dense materials comprise the atmosphere). 6.Convective overturn in asthenosphere, mantle and outer core still occur today. You should understand this sequence of evolutionary events for the midtem exam.

Iron catastrophe and differentiation of the earth. Why did the earth heat up and then rapidly cool during the differentiation process? Think about transfer of heat when the earth was solid versus when it became completely molten following the iron catastrophe. Which heat transfer process is more efficient (conduction versus convection)?

Cratering on the moon is indicative of early accretion process. Why does the earth possess little evidence of its early accretion history?

Degassing of the earth occurred following “iron catastrophe” and differentiation. The oceans and atmosphere formed during the differentiation process.

Emissions from degassing of the Earth during its differentiation. Note that molecular H and He escape to space and that oxygenation of the atmosphere occurred later following evolution of marine algae and plants that use photosynthesis to convert CO 2 to O 2 as a part of their life processes.

NEBULAR HYPOTHESIS b. Condensation of primordial dust. Forms disk-shaped nubular cloud rotating counter- clockwise. a. Supernova and formation of primordial dust cloud. c. Proto sun and planets begin to form. d. Accretion of planetesimals and differentiation of planets and moons (4.6 billion years ago. e. Existing solar system takes shape.

Differntiated Earth 1. Iron-Nickel Core (outer core liquid) (inner core solid) 2.Fe-Mg Silicate Mantle 3.Fe-Mg-Al Silicate Crust (ocean and continental) 4.Oceans 5.Atmosphere What evidence do scientists use to support the above inferred compositional zonation? Note that only the crust and uppermost mantle can be directly observed.

Evidence of initial composition of the solid earth based on meteorite studies. Two major compositional classes of meteorites dominate collected samples. Carbon based meteorites are much rarer, but indicate that the precursor of life was present early on. Metallic meteorites Iron-Nickel,density gm/cm 3 ) Chondritic meteorites Fe-Mg silicate (rocky, density gm/cm 3 ) Carbonaceous chondrites (rare)

Density Properties of the Earth 1. The earth has an average density of 5.5 gm/cm 3. The average density of the earth can be inferred based on gravitational properties of the earth and its effect on known masses such as orbiting satellites). 2. The earth’s crust has a density between 2.6 and 3.0 gm/cm 3 (directly measured). 3. The density of the uppermost mantle is 3.0 and 3.3 gm/cm 3 (directly measured). Based on the above density information what can you infer about the density of the lower mantle and the earth’s core? Is this observation consistent with the data collected from meteorite samples?

The presence of the Earth’s magnetic field provides evidence that the Earth likely possesses a metallic core and that a component of this core must be liquid and convecting around the solid metallic portion of the core (think about the principles behind a simple electro-magnet).

Seismic wave evidence. Compression (P-waves) Waves (Velocity: 6-7 km/sec within lithosphere). Propagate through all phases of matter.

Seismic wave evidence: Shear (S-waves) waves. (3-4 km/sec). Only propagate through solid phases of matter.

Seismic waves refract because of velocity changes related to density changes within the earth. Seismic wave accelerate with increasing density.

P-wave shadow zone. Note “bagel-shaped” shadow zonee exist between 105°-140° from the epicenter due refraction at outer core mantle boundary.

S-wave shadow zone. Note only one large shadow zones at an angle greater than 105° of the epicenter, due refraction at outer core mantle boundary and because S-waves are absorbed by the liquid outer core.

Note the change in seismic wave velocity as the seismic waves propagate through the earth. Note the decrease in seismic wave velocity at a depth of km and at the mantle-core boundary. Note that S-waves are only absorbed at the mantle-core boundary. What does that tell you about the physical property of the upper mantle (i.e., is it a complete liquid)?

P-wave velocity profiles within the lithosphere (continental and ocean crust and uppermost solid mantle) and asthenosphere (upper ductile mantle). Low velocity zone ( km) in the upper mantle is due to decreasing density. This low velocity zone defines the asthenosphere. Why does the density decrease in this region of the upper mantle?

Lithosphere “floats” on a partially melted asthenosphere, similar to a raft floating on water. The lithosphere is in isostatic equilibriium with the asthenosphere.