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Planet Building Part 1

Solar Nebula’s Chemical Composition

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Chemical Composition• Everything astronomers know about the solar system

indicates that it began as an interstellar gas cloud.– Cloud would have been mostly hydrogen with some

helium – small amounts of the heavier elements.– One piece of evidence is that we see the composition of

the gas cloud of the composition of the sun.

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Chemical Composition

• The suns elemental composition revealed by spectrum analysis indicates it is mostly hydrogen, helium (about 25%), and 2% other heavier elements. – Some hydrogen has been converted to helium due

to fusion reactions. – Astronomers draw the conclusion, looking at other

stars as well as the sun, that the solar nebula must have had the same composition.

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Chemical Composition

• We see evidence reflected in the planets – as we have discussed.– Inner planets are rock and metal while outer planets are

rich in low-density gases such as hydrogen and helium. – The chemical composition of Jupiter resembles that of the

sun.

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Chemical Composition• If the low-density gases were allowed to escape from a body

such as the sun or Jupiter, the remaining heavier elements would resemble Earth’s chemical composition.

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Conversion of Matter• Important evidence for SNT comes from how

nebular gas converted into solid matter.

From Orion Nebula, what appears to be disks of dust and gas surrounding newly formed stars.

From: http://atropos.as.arizona.edu/aiz/teaching/nats102/mario/solar_system.html

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Conversion of Matter

• The density-variations originated when the Solar System (SS) first formed solid grains. – The kind of matter that could condense in a

particular region depended on temperature and the gas that was present.

– In the inner regions the temperature was around 2,240 degrees Fahrenheit.

– Only compounds with high melting points could condense (e.g. metal oxides and pure metals).

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Conversion of Matter

• A little farther out in the nebula – it was cooler allowing silicates (rocky material) to condense (along with metal). – Mercury, Venus, Earth, and Mars are composed of

a mix of metals, metal oxides, and silicates.– Mercury is composed of more metals (closer to

sun) with less metal as you proceed outbound from the sun.

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Ice/Frost Line

• Beyond Mars, there is the ice line, a boundary beyond which water vapor freezes to form ice particles.

• Farther still from the sun, compounds like methane and ammonia condense to form other types of ice.

• Water vapor, ammonia, and methane were abundant in the nebula.

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Ice/Frost Line

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Ice/Frost Line

• Due to the abundance of water, methane, and ammonia, the nebula would have been a “blizzard” of ice particles.– Small amounts of silicates and metal (far, far less

than in the inner SS) would also have been found there (having condensed).

– We know this because, among other evidence, the Jovian planets are a mix of ices and small amounts of silicates and metal.

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Condensation Sequence

• Is the sequence in which different materials condense from the gas as you move away from the sun – toward lower temperatures.– The condensation sequence implies that different

kinds of materials will condense in predictable ways – which is exactly what we see.

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Condensation Sequence

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Common Misconceptions

• It is a common misconception that the solar nebula was sorted by density.– Heavier metals and rock “sinking” toward the sun

and low-density gases blown outward. – This is not the case, the chemical composition of

the solar nebula was originally roughly the same throughout the disk.

– The important factor was temperature. The inner nebula was hot and the outer cold.

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Common Misconceptions

– Only rocks and metals could condense in the inner nebula.

– The outer nebula, beyond the ice line, formed lots of ices along with metals and rock.

– Ice line is the boundary between high-density planets and low-density planets.