Ast101: The Sun And Its Neighbours - Terrestrial Planets Part 1 PDF

AST101: The Sun and its Neighbours Class 14: Teresstrial Planets Part 1 Professor C. Barth Netterfield https://www.solarsystemscope.com/ Class 2: Solar System Tour | Prof. Netterfield https://www.solarsystemscope.com/...

AST101: The Sun and its Neighbours Class 14: Teresstrial Planets Part 1 Professor C. Barth Netterfield https://www.solarsystemscope.com/ Class 2: Solar System Tour | Prof. Netterfield https://www.solarsystemscope.com/ Class 2: Solar System Tour | Prof. Netterfield https://www.solarsystemscope.com/ Class 2: Solar System Tour | Prof. Netterfield The force of gravity pulls a molecular cloud (made of gas and dust) together. As it collapses, it begins to spin faster (conservation of momentum) Collisions between particles flatten the orbit Forming a spinning disk of gas and dust. Class 14: Teresstrial Planets Part 1 | Prof. Netterfield  Next Step Tiny small objects stick together to form planetesimals These are the seeds (cores) of what will become planets Credit: The Cosmic Perspective Class 14: Teresstrial Planets Part 1 | 6 Prof. Netterfield Frostline! Protoplanetary Disk Inside the frostline: 0.2%: Metal (Solid) 0.4%: Rock (Solid) 1.4%: Hydrogen compounds (Gas) 98%: Helium and Hydrogen (Gas) Inside the frost line only Metal e r: and Rock can form planetesimals! l os C e r t t Outside the frost line: Ho 0.2%: Metal (Solid) Fu : rt 0.4%: Rock (Solid) he r 1.4%: Hydrogen compounds (Solid Ice) Co 98%: Helium and Hydrogen (Gas) ld er Outside the frost line, Metal, Rock, and Hydrogen compounds can form planetesimals! Class 14: Teresstrial Planets Part 1 | Prof. Netterfield  Next Step Tiny small objects stick together to form planetesimals These are the seeds (cores) of what will become planets Credit: The Cosmic Perspective Class 14: Teresstrial Planets Part 1 | 8 Prof. Netterfield Instant Quiz: PollEv.com/uoftastro Rhea is one of Saturn’s moons. Based on where it was formed, its composition is probably: A) Mostly rock, with a metal core. B) Mostly ice, but also rock and metal. C) Mostly metal, with some ice on the surface. D) The composition doesn’t depend on where it is formed. Class 14: Teresstrial Planets Part 1 | Prof. Netterfield Terrestrial Planets (and the Earth’s Moon) Earth Mercury Small Rocky Relatively thin or no Atmosphere Few moons Made from Heavy Elements (Rock & Mars Metal) Earth’s moon Venus [NASA] Class 14: Teresstrial Planets Part 1 | Prof. Netterfield  Mercury Craters Mercury and the Moon are covered in craters. Various Sizes Overlapping What are Craters? [NASA] Earth’s Moon Class 14: Teresstrial Planets Part 1 | Prof. Netterfield  Creating a Crater An planetesimal strikes the surface Typical speed is 100,000 km/hr The collision vaporises the rock and creates an enormous explosion A crater is left But: There are not enough planetisimals around for this many craters. Why are there so few on Earth, Mars, and Venus? [Cosmic Perspective, Fig 9.7] Class 14: Teresstrial Planets Part 1 | Prof. Netterfield  Planet Formation Tiny small objects stick together to form planetesimals These are the seeds (cores) of what will become planets [The Cosmic Perspective] Class 14: Teresstrial Planets Part 1 | 13 Prof. Netterfield  Planet Formation There were far more planetesimals right after the planets formed. This period is called the Heavy Bombardment. It ended about 4 billion years ago. [The Cosmic Perspective] Class 14: Teresstrial Planets Part 1 | 14 Prof. Netterfield  Mercury Craters Mercury and the Moon are covered in craters. But not everywhere. These areas are called Mares. Apparently something happened after the heavy bombardment. [NASA] Earth’s Moon Class 14: Teresstrial Planets Part 1 | Prof. Netterfield Can we look under the surface? Drill holes? No. The deepest hole ever is 12 km. The Earth’s radius is 6371 km. X-rays? No they can’t penetrate rock. Sound waves (like how an ultrasound works)? Yes! Class 14: Teresstrial Planets Part 1 | Prof. Netterfield  A seismometer detects it here. Earthquake happens here. Using direction and timing, the interior of the planet can be probed! [The Cosmic Perspective] Class 14: Teresstrial Planets Part 1 | Prof. Netterfield  Earthquakes cause vibrations to travel through the Earth There are 2 types: P Waves – compression S Waves – Side to Side [The Cosmic Perspective] AST 101 Class | Teresstrial 14: UofT | Dr.Planets Netterfield Part 1 | Prof. Netterfield Timing for siesmometers at different places depends on how far and how fast the wave traveled. Seismometers placed here don’t see S-waves from the earthquake: Liquids stop S waves. There must be a liquid layer in the core! Class 14: Teresstrial Planets Part 1 | Prof. Netterfield Timing for siesmometers at different places depends on how far and how fast the wave traveled. Seismometers placed here don’t see P waves from the earthquake: But they do here! There must be layers! Class 14: Teresstrial Planets Part 1 | Prof. Netterfield [Karla Panchuk, Steven Earle - CC BY-SA 4.0] [https://pressbooks.bccampus.ca/physicalgeologyh5p/chapter/3-2-understanding-earth-through-seismology-2/] The direction of the vibrations tells the Combining data from many difference between S earthquakes and many waves and P waves. seismometers all over the earth gives a complete picture. Class 14: Teresstrial Planets Part 1 | Prof. Netterfield Planetary Interiors Low Density Rock Medium Density Rock Highest Density – Iron and Nickle Estimated from Esitimated density and from density Measured with seismometers spin rate variations Partially liquid core [NASA] Class 14: Teresstrial Planets Part 1 | Prof. Netterfield Why do the planets have Cores? Differentiation: Lighter materials (eg, lower density rock) float to the surface While heavier materials (eg, Iron, nickel) sink to the core. [The Cosmic Perspective] Class 14: Teresstrial Planets Part 1 | Prof. Netterfield Planetary Interiors Low Density Rock Medium Density Rock Highest Density – Iron and Nickle Estimated from Esitimated density and from density Measured with seismometers spin rate variations Partially liquid core [NASA] Class 14: Teresstrial Planets Part 1 | Prof. Netterfield Why are planet interiors hot? Accretion: Dominent during planet formation. Differentiation: More significant earlier. Radioactive Decay: The dominent source of heat today. Class 14: Teresstrial Planets Part 1 | Prof. Netterfield How do planets cool? Step 1: Convection A planet’s mantle is not completely rigid. Hot rock weighs less than cooler rock. The hot rock rises and cooler rock sinks. This brings heat up from the core. Class 14: Teresstrial Planets Part 1 | Prof. Netterfield How do planets cool? Step 1: Convection A planet’s mantle is not completely rigid. Hot rock weighs less than cooler rock. The hot rock rises and cooler rock sinks. This brings heat up from the core. Class 14: Teresstrial Planets Part 1 | Prof. Netterfield How do planets cool? Step 2: Conduction A planet’s cool crust is ridgid, so there is no convection there. Heat is conducted through the rock (slower than convection) Crust is very thin. Class 14: Teresstrial Planets Part 1 | Prof. Netterfield How do planets cool? Step 3: Radiation Infrared light carries energy away from the surface of the Planet If there is more light leaving the planet than coming from the Sun, then the planet cools. Class 14: Teresstrial Planets Part 1 | Prof. Netterfield Cooling: does planet size matter? For Spheres: Volume = 4/3πr3 Area = 4πr2 If you double the radius: Volume is 8 times bigger. Area is 4 times bigger. Smal planets have less mass compared to it’s surface area than a large planet. Small planets cool faster. Class 14: Teresstrial Planets Part 1 | Prof. Netterfield Instant Quiz: PollEv.com/uoftastro Imagine that planet and its much smaller moon start at the same temperature. After 1 billion years, which will be cooler? A)The Planet, because it has larger surface area to radiate from. B)The Moon, because it has a greater surface area to volume ratio. C)They will be the same because they both radiate the same way. D)It depends on whether they were formed outside the frost line. Class 14: Teresstrial Planets Part 1 | Prof. Netterfield Planetary Interiors Low Density Rock Medium Density Rock Highest Density – Iron and Nickle Estimated from Esitimated density and from density Measured with seismometers spin rate variations Partially liquid core [NASA] Class 14: Teresstrial Planets Part 1 | Prof. Netterfield  Mercury Craters Mercury and the Moon are covered in craters. But not everywhere. These areas are called Mares. Apparently something happened after the heavy bombardment. [NASA] Earth’s Moon Class 14: Teresstrial Planets Part 1 | Prof. Netterfield Start with a A large impact Molten lava leaks up, cratered surface damages the flooding the crater lithosphere Leaving a This happened after smooth the heavy surface. bombardment, but Future impacts before the moon create (a few) cooled enough. new craters. Class 14: Teresstrial Planets Part 1 | Prof. Netterfield This also happened on Mercury Note that the moon and Mercury have both cooled to the point that this no longer happens. Class 14: Teresstrial Planets Part 1 | Prof. Netterfield  Instant Quiz: PollEv.com/uoftastro On this icy moon of A Saturn (Enceladus) where has the surface changed more recently? ie, where is the surface younger? B Class 14: Teresstrial Planets Part 1 | Prof. Netterfield