Planet Building Part 1

1. Planet Building Part 1 Solar Nebula’s Chemical Composition

2. 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.

3. 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.

4. 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.

5. 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.

6. 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

7. 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).

8. 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.

9. 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.

10. Ice/Frost Line

11. 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.

12. 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.

13. Condensation Sequence

14. 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.

15. 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.