Condensation

1. Condensation • As the cloud shrunk molecules started finding each other • If cold enough, they could stick together • Proto-Sun created heat – hotter for the inner regions • Most of the region was cool enough to have rock and metal begin to form dust grains • Vaporizes only around 1000+ K • Beyond the “frost line” there were ices mixed in • Ices vaporize around 150 K • Beyond a few AU it was cool enough to form ices • Nowhere cold enough for hydrogen or helium to condense Q. 26: What Materials Condensed?

2. Collide the Pieces • Zillions of bits of dust orbiting the protostar • Collisions are inevitable • Tiny bits stick together • Bits get bigger

3. Planetismals and Early Planets Planetismals Planets • Small grains of dust stick together • Eventually get big clumps - planetismals • Gravity slowly begins pulling them together • Eventually only a few left - planets

4. What are Early Planets Like? • Inside the frost line, only metals and rocks • Planets were rocky/metallic • About ½% of total mass • Outside the frost line, rock, metals and ices • About 2% of total mass • Creates larger planets • The remaining hydrogen and helium gas was still in gaseous form

5. Primary Atmospheres • The early planets are still in a background of hydrogen and helium • Gravity will attract atmospheres • Larger planets get a lot more • Outer planets get a large primary atmosphere of hydrogen and helium • Inner planets get a thin atmosphere

6. Stellar Winds • The early Sun starts blowing out excess gas • This begins clearing away what is left of the protoplanetary disk • The disk gets cleared away

7. Atmospheres vs. Stellar Winds • The wind blows away the remaining gas and dust • Only planets and planetismals remain • It also can disperse the primary atmospheres • More likely with nearby planets • More likely with smaller planets • Later, other atmospheres may be formed from gasses inside the planets • Thin, not hydrogen/helium

8. Cratering, the Final Touch • Largest objects are now planets and minor planets • Objects that circle planets are called moons • Many small objects still survive • Small objects hit the planets and moons • At first cratering is fast - eventually slows down • When things get resurfaced, craters are wiped out • Young terrain has few craters - old terrain has many Pot Holes Q. 27: Jupiter’s moon Io

9. Differentiation Atmosphere • Planets formed from collision of ice, rock, and metal • Collision created energy – it melted • Heaviest parts fell to the bottom • Atmosphere, which is lightest, was then gathered on top IceRockMetal

10. Planets: Hot on the Inside • Collision of planetismals created a great deal of heat • But mostly, this heat has long since dissipated • Differentiation is ongoing and makes a little heat • The main source of internal heat now is radioactivity • Slow, steady, production of heat inside the planet • Heat escapes from the inside of the planet – slowly • Therefore, planets are hot on the inside • They cool over time • Big planets still (partly) liquid on the inside

11. Planets: Outside Temperature • The planets are getting heat from the Sun • The closer a planet is to the Sun, the more heat it gets • They reradiate this power as thermal energy • If this is in perfect balance, and if planet is a perfect blackbody, then the closer a planet is to the Sun, the hotter the surface is • Closer planets are hotter • Almost always

12. Keeping Your Atmosphere • Gravity holds molecules onto planet • If atoms/molecules exceed escape velocity, they leave • Lighter molecules move faster • Easier to keep heavy molecules • Heat is motion of molecules • Hot molecules leave • Planets/moons thatare massive and coldare more likely to haveatmospheres Q. 28: Titan’s Atmosphere

13. Magnetic Fields • Flow of charges (electric current) creates magnetic fields • Motion needed to regenerate the fields What you need for magnetic fields: • A layer that conducts electricity (metal?) • It must be liquid • Rotation