Lecture 10: Formation of the Earth

EPS 106

Nov. 15^th, 2000

Review. Show cross section of Solar System.

Origin of the Solar System

1) The nebula rotated about a central mass, which, under gravitational force, slowly started to collapse on itself

2) The majority of material swarmed to the center, the cloud taking on a disk-shape.

3) Ice crystals (H₂O, methane, CO₂, ammonia, nitrogen) formed in the outer regions.

4) Heating occurred due to potential energy increase. Volatilization occurred in the central hotter regions, casting the volatiles into the outer regions. Only more refractory elements, silicon, iron, aluminum, etc. remained in the inner regions

5) The small particles coalesced under the force of gravity, forming planetesimals. These swept up the remaining gas in their orbits.

6) The sun formed as the mass in the center coalesced. Under the tremendous pressure, nuclear reactions started to take place, causing the sun to give off heat.

7) Venus and Mercury are too hot for liquid water. (Venus has some as gas). Mars is too cold, although there is ice. The Earth is 'just right', which allows for life to occur here. Perhaps in the distant past, life existed on Mars, when it was hotter. Now it is inhospitable.

8) Life is key. As we will see when we discuss Daisyworld, life can change the surface conditions of a planet. Today, we have an O₂-rich atmosphere, and little CO₂. This atmospere has been modified to these conditions by living organisms. Had Venus developed life, it might have evolved to a life-friendly planet.

· Most of the planets revolve around the sun in a orbit with an axis parallel to each other, as do their rotations. Venus rotates slowly the other way, due to an impact?

· The Moon was formed by a large impactor.

· Meteorites and asteroids are from the asteroid belt, between the Earth and Jupiter. Pertubations cause them to fall into an Earth-crossing orbit. Apollo objects are large meteorites. Comets are early-formed 'ice balls', with eccentric orbits.

Formation of Earth.

· Coalescence of silicate material. Cool. Then heating by:

1) meteorite impact

2) gravitational compression (potential energy)

3) decay of radioactive elements (not a lot of material, but packs a whollop).

One problem is the increase in spin during accretion. The cosmic wind break can slow things down!

The heating caused the segregation of the Earth’s core, which concentrated iron. Caused melting or iron-nickel and formation of core. The ‘falling’ of this iron core would cause a heating increase of over 2000°C. Planetary differentiation is the most significant event in the history of the Earth. Caused ‘resetting’ of the planet.

· This caused further heating, which drove off lighter elements. Show cross section

· Early atmospheres would be blown off by solar winds or by early large impacts **explain why large impacts would be early on and then end later.

· Finally, the Earth starts cooling, and gases such as nitrogen (N), sulfur (S), carbon dioxide (CO₂) and water (H₂O) were degassed from mantle via volcanoes and hot springs. In addition, material may be added by late, cool planetesimals. These may also have been the source for the building blocks of life?

· Early on, the atmosphere would be crushing. Later on, the Earth cools, water precipitates, forming Oceans, rivers, lakes. Acids react with the rocks, reducing them to harmless compounds. **demonstrate with HCl and carbonate**

· The Earth is ready for life.

The structure of the Earth

Today, the Earth is relatively stable. It can be divided into three parts, the core, mantle and crust.

The core: Can be divided into two parts, inner and outer core. The inner core is 1200 km thick, the outer core is 2270 km thick and is molten. Composed principally of iron and nickel. The core gives us our magnetic field, which may shield us from harmful radiation.

The mantle: 2885 km thick. Composed primarily of olivine (Mg₂SiO₄). This can be divided into three parts; the mesosphere (hot, but rigid, due to high pressure), the 100-350 km asthenosphere (hot, weak and plastic, like butter or tar) and the lithosphere (cooler and rigid; brittle).

The lithosphere: it is brittle, consists of an upper mantle region and all of the crust. It is only about 1/100 the thickness of the Earth. The lithosphere floats on the asthenosphere, which is the cause of earthquakes, mountain building, etc. We will discuss this when we get to plate tectonics.

The composition of the crust is quite different from the whole earth. It is much more siliceous, consisting of silicon, aluminum and lesser amounts of iron, magnesium, calcium, potassium and sodium, balanced by oxygen. The whole Earth contains much more iron.

The crust can further be divided into several parts: continental crust and oceanic crust. The later is denser and rides lower on the asthenosphere. Hence, the placement of the oceans.