Origin of the Solar System Nebular hypothesis Proposed by Immanuel Kant and Pierre- Simon Laplace in the 18 th century Nebula = cloud of gas, dust (+ energy) Step 1: cloud collapses when gravitational energy > expansion due to gas pressure (what would trigger the initial collapse?). Step 2: system rotates faster as it collapses to conserve its original angular momentum.

The dance continues… Step 3: interaction of the effects of gravity, gas pressure and rotation flattens the contracting nebula (pancake-like) with a bulge in the center. Step 4: instabilities in the collapsing, rotating cloud cause local regions to begin to contract gravitationally. These local regions of condensation will become the Sun and the planets, as well as their moons and other debris in the Solar System.

Theory compares to Reality Some consistencies…  The orbits of the planets lie nearly in a plane with the sun at the center (let's neglect the slight eccentricity of the planetary orbits to simplify the discussion)  Planets all revolve in the same direction.  Planets mostly rotate in the same direction with rotation axes nearly perpendicular to the orbital plane. The nebular hypothesis explains many of the basic features of the Solar System, but we still do not understand fully how all the details are accounted for by this hypothesis.

Evidence from the Hubble Telescope? Horsehead, also known as Barnard 33, is a cold, dark cloud of gas and dust, silhouetted against the bright nebula, IC 434. The bright area at the top left edge is a young star still embedded in its nursery of gas and dust. But radiation from this hot star is eroding the stellar nursery. The top of the nebula also is being sculpted by radiation from a massive star located out of Hubble's field of view. This and other nebulae are known today as cocoon of stars or stellar nurseries.

Let there be Earth… Stages of planetary accretion: 1. Dust grains stuck to each other until objects were large enough to begin to attract material with their gravity fields, producing planetesimals (up to a few hundred kilometers in diameter). 2. A period of runaway growth took place, leading to tens of objects much larger than the Moon. Most of the mass of the inner Solar System was contained within these planetary embryos. It may have taken only about a million years from the end of stage 1 to the end of stage 2.

Wham, there goes the moon… 3. During the final stage, these huge objects collide creating larger planets, but a smaller number of them. The entire process was dominated by large impacts, making the formation of the Moon by a giant impact a natural consequence of planet formation. Estimated duration of planet formation: million years

Evolution of the Earth Age of dated oldest solids in the Solar System = billion years Giant impact events = heat = magma oceans Differentiation = internal layering of the Earth Techniques to image the interior: 1.Geomagnetic and gravity studies 2.Mantle and lower crust xenoliths; ophiolites 3.Mines and oil wells 4.Seismic studies

Component 1: Lithosphere Majority of the Earth’s crust is a product of volcanic activity. The continental crust might have grown mainly because of the accretion of volcanic archipelagos like the Philippines.

Component 2: Atmosphere Earth’s atmosphere through time… First Atmosphere: nebular components Composition - probably H2, He; rare in the atmosphere today because (1) Earth's gravity was not strong enough to hold them, and (2) Earth's magnetic field (magnetosphere = Van Allen Belt), which deflects solar winds, was non-existent yet. Second Atmosphere: produced by volcanic out gassing. Composition - probably similar to those created by modern volcanoes (H 2 O, CO 2, SO 2, CO, S 2, Cl 2, N 2, H 2, NH 3 and CH 4 ). No free O2 at this time. Third Atmosphere: today’s atmosphere; build-up of N 2 and O 2 Composition – N 2 =78%; O 2 =21%; Ar=0.9%; CO 2 =0.03%; etc.

The oxygen crisis Where did the oxygen come from? According to the oxygen cycle, O 2 produced by: (1)Photochemical dissociation: breakup of water molecules by ultraviolet radiation (e.g., production of O 3 ) (2)Photosynthesis: CO 2 +H 2 O+sunlight = organic compounds+O 2 (produced by cyanobacteria, and eventually higher plants) Oxygen Consumers (1)Chemical weathering - through oxidation of surface materials (2)Animal respiration (3)Burning of fossil fuels

Evidence from the rock record >2.8 billion years: occurrence of minerals that only form in non- oxidizing environments in Archaean sediments (e.g., Pyrite (Fools gold; FeS 2 ), Uraninite (UO 2 ) B.y.: occurrence of the Banded Iron Formation (BIF) - Deep water deposits in which layers of iron-rich minerals (iron oxide, iron carbonate, iron silicate, iron sulfide) alternate with iron-poor layers, primarily chert. BIFs are a major source of iron ore (magnetite; Fe 3 O 4 ). <2.3 B.y.: formation of Red beds - continental siliciclastic deposits. They consist of the highly oxidized mineral hematite (Fe 2 O 3 ), that probably formed by oxidation of other Fe minerals that have accumulated in the sediment.

Evidence from the biological record (1)Chemical building blocks of life could not have formed in the presence of atmospheric oxygen. Chemical reactions that yield amino acids are inhibited by presence of very small amounts of oxygen. (2)Oxygen prevents growth of the most primitive living bacteria such as photosynthetic bacteria, methane-producing bacteria and bacteria that derive energy from fermentation. (3)Since today's most primitive life forms are anaerobic, the first forms of cellular life probably had similar metabolisms. Today these anaerobic life forms are restricted to anoxic (low oxygen) habitats such as swamps, ponds, and lagoons.

Oxygen in the Earth’s atmosphere through time

Component 3: Hydrosphere Potential origin: 1. Excess Volatile Theory Excess Volatiles (10 20 ) grams Present Atmosphere, Hydrosphere, Biosphere = 14,600 Buried in Sedimentary Rocks = 2,100 16,700 Supplied by weathering of rocks = 130 Excess volatile unaccounted for = 16,570 Solution: Volatiles from volcanoes, fumaroles, hot springs

Not again! The extra-terrestrial connection 2. Small Comet Bombardment Theory  Present estimated rate: in the order 10 per minute  Each event deposits tons (!) of water vapor in the upper atmosphere  Influx of 2.54 cm of water to the surface of the Earth every 20,000 years (enough to provide the world’s oceans in 4.6 billion years)

Evidence from the rock record (again!) Zircon from Western Australia yields an age ca. 4.4 billion years Simon Wilde (Curtin University of Technology,Perth, Western Australia), William Peck (Colgate University) and Colin Graham (University of Edinburgh, Scotland) Implications:  Hydrosphere formed by 4.4 b.y. ago  Temperature at a range of 100 o C  Development of continental-type crust early in the Earth’s geologic history  Questions on the origin of the moon Copyrighted by University of Wisconsin-Madison

A possible timeline of the Earth

References/Sources of materials Nebular hypothesis: Planetary accretion: Origin of the Atmosphere: and the University of Michigan's Introduction to Global Change web site. Origin of Water: Oldest rock on Earth: Photos Horsehead nebula: Oldest zircon: