Solar System - Lecture notes 1 PDF

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Dynamic Solid Earth L1 - Introduction to the Solar System There are five celestial bodies in the solar system (SS): 1. 2. 3. 4. 5.

The Sun (small star) + other stars Intergalactic nebulae + interstellar gas Planets + moons Asteroids + meteorites Comets

We know the abundance of elements in our SS by conducting spectroscopic analysis of the light from the sun, and asteroids landing on Earth. Most asteroids come from a belt between Mars + Jupiter. Spectroscopic analysis reveals that most of the interstellar gas in Earth’s system is hydrogen. Now there is a mix but historically, it was only hydrogen and helium. Stars are nebulae collapsing on themselves. Sometimes, stars become so hot that hydrogen undergoes fusion and can form helium. Our sun is a small star composed of hydrogen, but its composition is dependent upon the material that made it and its present stage of evolution. Spectroscopic analysis was conducted on the outer surface of the sun, and it is assumed to be the same inside. The sun is 73% hydrogen and 25% helium. Meteorites Geochemistry indicated that they were once a part of larger planetary bodies that were destroyed. There are three types: Stones, irons and stoney irons. Stones are mostly composed of silicate materials (silicon and oxygen). Irons are mostly iron-nickel alloys and stoney irons are a mix of the two.

90% of meteorites are stone. Chondrites (a type of stone) have millimetre sized rounded masses called chondrules and anchondrites do not. The four types of chondrite are called:  Enstatite (E)  High Fe (H)  Low Fe (L)  Very low Fe (LL) Carbonaceous chondrites have three types:  Type 1 – C1  Type 2 – C2  Type 3 – C3 Chondrules consist of glass and various other materials. It is believed they were molten. Carbonaceous chondrites were formed of low temperature hydrous materials, whereas ordinary chondrites are made of high temperature minerals. Chondrites’ compositions are similar to that of the sun and thus are similar to other planetary bodies. The most primitive chondrites are the carbonaceous chondrites and they formed at the same time as the sun. The compositions of these primitive meteorites indicate that elements/compounds separated from a gaseous phase over a range of temperatures. Almost all meteorites ages show that they were formed at 4.56Ga (the age of the Earth). Oddo-Harkins effect: elements with even atomic numbers and neutrons are more abundant in the solar system that those with odd atomic numbers. The composition of the solar system is determined from carbonaceous chondrites and analysis of the solar system. A timeline of the formation of the solar system:

1. In the year 4.56 Ga, condensation began from a solar nebula. Solid material eventually condensed from the gaseous mass as it cooled. 2. Chondrules formed from this complex condensation process due to repeated melting, evaporation and re-condensation. 3. All subjects (the Sun + planets) formed by the successive accretion of small masses into one larger mass. This was also a highly complex process that involved collisions and fractioning. Seven elements account for Earth’s mass – oxygen, sulphur, aluminium, calcium, iron, magnesium and silicon. The compositional structure of the Earth The crust only forms 1% of the volume of Earth and can be separated into oceanic and continental crust. Oceanic crust is thin (10km) and information of its structure comes from:    

Seismic velocities Deep sea drilling programmes Dredging of fracture zone scarps Ophiolite (subducted and exposed crust)

Continental crust is thicker; reaching 40km under stable areas like England but up to 90km in mountainous areas. It is comprised of igneous, metamorphic and sedimentary rock and can be up to 4 Ga old. The mantle lies below the crust, extends 3000km and makes up 83% of the Earth’s volume. It is made up of peridotite composition (igneous rock) and this is mostly Fe and Mg rich silicate materials. The boundary between the crust and the mantle is known as the Mohorovicic discontinuity (Moho). How do we know about the composition of peridotites?

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Obducted sections through oceanic crust Mantle nodules High pressure and temperature tests Dredged samples and kimberlite pipes Meteorites have the same make-up as planets

The low-velocity zone occurs to the boundary between the lithosphere and the asthenosphere in the upper mantle. It is characterised bu unusually low seismic shear wave velocity compared to the surrounding depth intervals. This is due to 1-5% of partial melt. The core-mantle boundary lies between the mantle and the iron-nickel outer core. The 200km thick layer below the upper mantle is called the D’’ layer. The silicate mantle changes to the denser Fe-rich metallic alloy of the core. Rheology: how material reacts to deformation The crust and uppermost mantle above the LVL acts as the lithosphere (50250km thick). The ductile mantle below the lithosphere is called the asthenosphere. This is what the tectonic plates of the lithosphere move upon. The remaining mantle below belongs to the mesosphere....