• Our solar system consists of the sun, 9 planets, 61 moons and a multitude of asteroids, comets and meteoroids.
• The orbits of the planets are elliptical around the sun.
• The planets generally revolve in the same direction around the sun and within the plane of the ecliptic, except for Pluto which is tilted at 17^o to the ecliptic.
• Most of the moons revolve around the planets in the same direction as the planets revolve around the sun.
• Meteoroids, asteroids and comets also follow orbits around the sun.
• The rotations of the planets, moons and other bodies are inherited from the rotation of the ancient gas cloud from which they formed.

• Closest to the sun and consist of Mercury, Venus, Earth and Mars
• Generally small, rocky bodies with densities greater than 3gm/cm³
• Composed mainly of silicates, Fe and Ni
• Volcanism mostly basaltic

• Consist of Jupiter, Saturn, Uranus, Neptune and Pluto
• Generally larger than terrestrial planets
• Have densities less than 3 gm/cm³
• Each (except for Pluto) consist of solid, rocky core surrounded by thick atmosphere of hydrogen, helium, methane, ammonia, and other gasses
• Pluto lacks a thick atmosphere and instead consists of a solid core with a thick outer layer of ice
• Most Jovian planets have multiple moons
• Most Jovian planets also have impressive ring systems composed of dust- to boulder-sized particles of mostly ice

• Early theories (19th century) envisioned nebula as a giant cloud of hot, gaseous material.
• Later theories (early 20th century) considered nebula to include dust and small grains in addition to gas.
• Some theories therefore envisioned a hot gaseous nebula, others a cold cloud of dust, grains and gasses.

• Developed early in the 20th century byChamberlin (a geologist) and Moulton (an astronomer).
• Proposed that the sun was present before the planets.
• Gravitational pull from passing star extracted gaseous materials from the sun.
• Gaseous materials surrounded the sun and began condensing small, solid bodies about the size of asteroids (tens to hundreds of km in diameter) called planetesimals.
• These multitudes of planetesimals then aggregated (clumped together) to form the early planets
• These early protoplanets continued to grow by attracting still more particles.
• Growing protoplanets eventually became massive enough that gravitational attraction took over.
• Protoplanets contracted into denser, more compact spheres through gravitational collapse to form the planets.

• Sun and planets formed at the same time.
• Five or six billion years ago, slow rotation caused the solar nebula to flatten into a giant, rotating disk of gases and dust.
• As rotation continued, much of the material migrated inward and became concentrated near the center of the disc.
• This centralized mass underwent further compression, reaching temperatures of several million degrees that set off thermonuclear reactions to form the early sun.
• The embryonic sun was surrounded by an envelope of gas and dust.
• Turbulance within the surrounding gas and dust first caused condensation of planetesimals.
• The cold planetesimals then aggregated to form 9 or 10 protoplanets.
• The protoplanets underwent gravitational collapse to form the planets.

• Protoplanets grew by sweeping up material from the nebula until they eventually became large, compositionally homogenous planetary bodies.
• Growing protoplanets eventually became massive enough that internal gravitational forces took over, resulting in the protoplanets contracting and becoming more dense.
• This gravitational collapse caused internal heating and softening of planet, followed by separation of different materials into layers through differentiation.
• Dense iron sunk into the planet interior while lighter elements (K, Na, Al, Si, etc.) and gasses migrated or floated to the surface.
• Light gases H and He were lost to space.

• Internal zonation of planets developed during, not after, accretion of solar material.
• Accretion may have begun within the solar nebula when the gases were still very hot (>1000 ^oC).
• As nebula began to cool, primitive Fe and Ni accumulated first to form metallic core
• Silicates accreted later around earlier-formed core as temperatures dropped.
• Finally, planet differentiated to form crust, leaving behind residual mantle.

• Around 4.5 b.y. ago, the earth had as much as five times the amount of radioactive heat production as now.
• Early heating was so intense as to completely melt the earth for a short while.
• A distinct core and mantle formed by 4.5 b.y. ago while the earth’s surface was covered by an immense ocean of magma.

Intense Heating of the Early Earth 4.5 - 4.2 Billion Years ago due to:

• Gravitational contraction which raised temperature of the earth’s center by 1000 ^oC
• Radioactive heat production which raised the temperature by an additional 2,000 ^oC
• Intense meteorite bombardment until about 3.9 billion years ago

• Earth’s crust differentiated (separated) from the underlying mantle on the basis of chemical composition.
• During mantle differentiation, relatively light elements such as Si, O, Al, K, Na, Ca, C, N, H, and He rose to the surface to form the crust, seawater and atmosphere.

• Early magma ocean prior to 4 billion years ago solidified into an ultramafic rock called komatiite.
• Recycling of komatiite over time gradually transformed oceanic crust to basalt.
• Continental crust did not become stable until about 3.9 - 4.1 billion years ago, almost half a billion years after formation of the core and mantle.

Oceanic Crust:

• ranges from 0 - 10 km thick
• average composition of basalt

Continental Crust:

• ranges from 33 - 70 km thick
• average composition of granodiorite

• Base of the lithosphere marked by the upper boundary of the asthenosphere (low-velocity zone).
• The asthenosphere consists of partly molten (~3% partial melt) mantle material (peridotite).

• Compose of peridotite, a dark-green rock composed primarily of olivine (>70%) and pyroxene.
• Below the asthenosphere, the mantle is solid down to a depth of ~2,900 km where it meets the outer core.

• The earth’s mantle extends down to a depth of 2,900 km where it meets the outer core.
• Outer core extends from 2,900 km to 5,100 km depth.
• Outer core consists mainly of molten Fe and is the source of earth’s magnetic field.

• The inner core occurs below 5,100 km depth.
• Composed mostly of solid Fe and Ni.

• Considerable hydrogen and helium escaped into space during early differentiation of the earth.
• Most of the remaining hydrogen was locked up in water.
• The earth’s early atmosphere consisted mainly of methane, ammonia and water vapor.
• The early atmosphere of the Earth had practically no molecular oxygen (O₂).
• Oxygen accumulated in the atmosphere much later, after photosynthetic life forms appeared around 3.5 billion years ago.

• In 1951, W.W. Rubey proposed that most gases in the earth’s early atmosphere were derived from interior of the planet.
• Gases brought to surface by way of volcanoes, geysers and hot springs.
• This process is known as outgassing.
• Continuous outgassing throughout earth history can account for all the N, He, Ar and water vapor in the atmosphere today.
• Oxygen had a separate origin as a product of photosynthesis beginning 3.5 billion years ago.
• Emergence of cyanobacteria in ancient oceans 3.5 billion years ago.

• Early atmosphere composed primarily of methane, ammonia and water vapor.
• This early atmosphere was exposed to ultraviolet light from the sun.
• No ozone layer back then, so ultraviolet light reached the surface of the early Earth.
• 1. Dissociation of water vapor to hydrogen and oxygen with hydrogen escaping into space:
• 2H₂O + uv light = 2H₂ + O₂
• 2. Newly formed oxygen reacted with methane to form carbon dioxide and water:
• CH₄ + 2O₂ = CO₂ + 2H₂O
• 3. Oxygen also reacted with ammonia to form nitrogen and water:
• 4NH₃ + 3O₂ = 2N₂ + 6H₂O

• After all the methane and ammonia were converted to carbon dioxide and nitrogen, then excess oxygen (O₂) started to accumulate as more water vapor dissociated.
• Over time, our present atmosphere of nitrogen, carbon dioxide and oxygen formed.

• The rate of seawater accumulation is directly tied to atmospheric production of water vapor following chemical differentiation of the earth.
• Did the atmosphere and oceans accumulate slowly at a uniform rate or rapidly during the early stages of earth history?
• Intense early bombardment of the earth by icy comets may have contributed to the planet’s supply of water and gasses, implying that the atmosphere and seawater formed early and rapidly.
• On the other hand, seawater may have accumulated slowly through outgassing of planet.
• Either way, the earth’s water supply was probably established by 2.5 billion years ago.