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Solar System Formation Also: measuring ages using Radioactive Decay Chapter 8 Goals and objectives • Develop a sense of what scientists know about the overall universe, its constituents, and our location • Understand the link between the composition and location of the constituents in the solar system • Sketch how the planets were formed. • Compare and contrast the terrestrial, jovian, and uranian planets. • Estimate the age of the solar system, given data on the isotopic composition of meteorites. Major criteria necessary in a formation model • Motions – counterclockwise, almost circles, and in same disk (flat) • Two types of planets: – Terrestrial – small, rocky & metallic, near the Sun & close together – Jovian – large, gaseous, far away & far apart • Asteroids & comets – asteroid belt, and the comet places: Kuiper belt, Oort cloud • Must also allow for exceptions to major patterns Age of Solar System – Universe is 13.7 Gyr old – Solar system is “only” 4.6 Gyr old • What took so long? – Universe began as H, He – Earth-like planets need heavy elements – Heavy elements made in stars • “Galactic recycling” • Takes a few billion years to make enough heavy metals inside stars that later explode. Evolution • Solar nebula began as a BIG low density gas cloud and it was: (page 237) – Cold (no sunlight) – Rotating counterclockwise slowly – Spherical (almost) • The gas cloud shrank. (Why?) This results in: – Faster rotation. Demo. Sports analogy. – Sun formed. – Leftovers: • Planets give off mostly which kind of light? • Excess IR Light is detected in other star-forming systems • Suggests ____________ are forming, also. Collisions in early solar system • Flattens disk – Collisions average out differences (up & down cancel) • Removes “retrograde” orbits due to MANY collisions • circularizes orbits • Described on page 238. Sun Top view Collide & stick! More on collisions • Results: objects orbit the same way as rotation of the original cloud. – Orbits will tend to be counterclockwise. – Collisions will tend to make rotations counterclockwise. • Why would there be more collisions in the early solar system than there are today? • Other collision systems look similar (flat, circular orbits) – Spiral galaxies, planet rings • See also page 238. Forming planets • Gas cloud shrank – Most gas went into _____________ – Leftovers can’t easily pull themselves together – So they collide – Solid parts form “seeds” – have larger than average gravity. They are planet precursor. – Seeds need to be solid for things to stick & stay • See table 8.1 page 240. NOTES: – Hydrogen gas = pure hydrogen. – Hydrogen compounds are molecules such as: Water (H2O), methane (CH4), ammonia (NH3) – Demo: what happens to liquid if no atmosphere… – Work on the “Forming Planets” worksheet. Terrestrial planets • “Seeds” are: – Metallic & rocks – Small. Why? • Less stuff for seeds. No solid H-compounds (1.4%). Only 0.6% metals/rocks. • MANY collisions/encounters: – Nearby stuff sticks forms “planetesimals” (pre-planets) – Collisions can destroy small planetesimals. – What’s left at the end? – What’s the temperature like near the Sun? • Gases won’t stay on the planets – See figure 8.6 (page 241) to summarize Frost Line • “Frost line” = where ices (H compounds) freeze (condense in book) – Guess where. – What would happen to an ice ball that got closer than the frost line? – What are ice balls called? – What happens when they get closer than Jupiter? – Evaporated ices surround the comet, this region is called the comet’s “coma” Jovian Planet formation • Same idea as Terrestrial planets except: – Seeds are bigger. Why? – ALSO: Gas is colder • Moves slower • Doesn’t escape as easily • Planets can hold onto the gas – Is there a little gas or a lot of gas in solar nebula? • Which gases? • Another possibility: Jovian planets formed without seeds. New theory – needs more testing & details Planetary rotation • Planets rotate faster when things hit them in the direction of their spin. • Terrestrials form from collisions of big rocks. – Collision directions are random – Usually leads to slow rotation unless BIG collision. • Jovian planets get most mass from gas falling (accreting*) onto them from orbit – Gas accretes all in the same direction (dir. of orbit) – Lots of gas falls down (BIG planets) & is moving fast! – Usually leads to fast rotation – *For more information about how accretion works, read about “accretion” and “accretion disks”, which is also important for star formation & black holes. See pages 241-242, 554, 589-590, 600-601, 667, 671-672. Which planets had more collisions? 1. Terrestrial 2. Jovian 3. Same 0 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 Planet forming questions • Which planet should be mostly metal? • Which planets should be mostly rock? • What is there more of: metal/rock or hydrogen ice-forming compounds? • Which planets should be bigger (and more massive): inside or outside the frost line? Why? Exceptions to planet patterns • Unusual tilts & rotations collisions • Backwards orbiting moons captured objects • Earth’s unusually large Moon BIG collision – Mars-sized rock hit Earth early on – Outer layers come off Earth • Moon forms from debris. • Looks like Earth’s outside – Core of impactor stays on Earth • Moon lacks metal core density should be __________. Summary • See page 247 – excellent summary • One last topic from chapter 8 – calculating ages of rocks & solar system stuff. Calif. Elementary School Science Standards for solar system • From California Science Standards, high school – Students know how the differences and similarities among the sun, the terrestrial planets, and the gas planets may have been established during the formation of the solar system Astronomers would be surprised to find a Jupiter-like planet at Mercury’s location around another star. 1. True 2. False 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 Radiometric Dating using radioactive decays P Parent Element D Daughter Element Half-life – time it takes for ½ of Parent to decay into Daughter. Examples of radioactive isotopes - 238U half-life = 4.5 Gyr 232Th half-life = 3.5 Gyr 14C half-life = 6,000 yrs (Carbon dating) See also pages 248-250 for more on radiometric dating! Fill this in based on the next slide. Number of Time Number of half lives (years) Parents 0 1 2 3 4 5 Fraction parents still left Number of Daughters # Daughter/ # Parent Assume we’re doing Iron-60 dating. It has a half-life of 1.5 million years Radioactive Decay, cont. At time = 0, the rock formed 1 half-life later… 32 Parent Atoms 0 Daughter (P) (D) ______ yrs total 16 Parent 16 Daughter 16 units of heat energy ______ yrs total ______ yrs total After 2 half-lives 8 24 8 more units of energy, 24 total ______ yrs total 4 half lives Clicker question now After 3 half-lives 4 28 1 31 ______ yrs total 2 30 5 half-lives After 6 half lives … How many parents are left after 2 half lives? 1. 8 2. zero 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 Do you think Carbon dating is effective for a 1 million year old fossil? 1. Yes 2. No 0 0 Why or why not? Then how do we measure old things’ ages? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 Element P decays into element D with a half-life of 10 million years. You find 3 times as many daughters as parents. (D/P = 3) How old is the rock? 1. 2. 3. 4. 5. 6. 0 0 0 0 0 0 5 million years 10 million years 20 million years 30 million years 40 million years I have no clue 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 You find an animal skeleton that has 1/8th as much carbon-14 in it as living samples have. How old is the skeleton? 1. 2. 3. 4. 5. 6. 0 0 0 0 0 0 3,000 years 6,000 years 9,000 years 12,000 years 18,000 years 24,000 years 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 Radiometric dating • Any questions? • Radiometric dating also called: – Carbon dating (Carbon-14 dating) if using C. – Radioactive dating – Radioisotope dating. – I won’t call it by these names. These names won’t be on your test.