Lec18-Mar26 - Discussion Ques7on The planets are...

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Unformatted text preview: 3/26/10 Discussion Ques7on The planets are thought to have formed from a disk of gas and dust around the Sun. The chemical composi7on of the planets varies systema7cally with distance from the Sun because Astro 109 Lecture 18: Forma7on of the Solar System March 26 A.  the original gas cloud was not uniform in composi7on. B.  temperature varia7ons in the disk determined which materials condensed at each radius. C.  all the hydrogen in the inner disk combined with oxygen to form water. D.  there was no iron or silicon in the outer parts of the disk. E.  much of the hydrogen in the inner disk caught fire and burned away. Mar. 26 Key Concepts Features of the Solar System •  Radioac7ve da7ng, age of Solar System •  Condensa7on model for planet forma7on •  Atmospheric evapora7on –  gas temperature and mo7on –  escape velocity size and composi7on orderly mo7on Mar. 26 Mar. 26 How old is the Solar System? How old is the Solar System? •  Earth is geologically ac7ve  ­ ­> few features are truly old •  But meteorites can be much older Mar. 26 Mar. 26 http://www.solarviews.com/eng/meteor.htm" 1 3/26/10 Radioac7ve da7ng Discussion Ques7on Parent element Daughter element Half ­life (years) Rubidium (87Rb) Stron7um (87Sr) 47.0 billion Uranium (238U) Lead (206Pb) 4.5 billion Potassium (40K) Argon (40Ar) 1.3 billion Carbon (14C) Nitrogen (14N) Three ­quarters of the radioac7ve potassium (40K) originally contained in a certain volcanic rock has decayed into argon (40Ar). How long ago did this rock form? 5730 Solar System is about 4.56 billion years old Mar. 26 Mar. 26 Discussion Ques7on Discussion Ques7on In radioac7ve decay, how much of the original element is leb aber one half ­life has elapsed? A.  B.  C.  D.  E.  100% 75% 50% 25% none In radioac7ve decay, how much of the original element is leb aber two half ­lives have elapsed? A.  B.  C.  D.  E.  100% 75% 50% 25% none Radioac:ve decay 120.00% 100.00% 80.00% 60.00% 40.00% 20.00% 0.00% 0 Mar. 26 2 4 6 8 10 Mar. 26 Discussion Ques7on Three ­quarters of the radioac7ve potassium (40K) originally contained in a certain volcanic rock has decayed into argon (40Ar). How long ago did this rock form? A.  B.  C.  D.  E.  Mar. 26 1.3 billion years 2 billion years 2.6 billion years 4.5 billion years 9 billion years Parent element Daughter element Half ­life (years) Rubidium (87Rb) Stron7um (87Sr) 47.0 billion Uranium (238U) Lead (206Pb) 4.5 billion Potassium (40K) Argon (40Ar) 1.3 billion Carbon (14C) Nitrogen (14N) 5730 Mar. 26 2 3/26/10 Why why why? Cosmic abundances small, rocky, dense created in the Big Bang forged in stars Mar. 26 large, gaseous, diffuse Theory: •  Solar system formed from a spinning cloud of gas and dust. •  Planetesimals formed when par7cles stuck together. •  Planets grew by collec7ng more planetesimals and par7cles. Mar. 26 Condensa7on – water Warm Colder gas (vapor) Mar. 26 Cooler liquid solid Mar. 26 Condensa7on model Discussion Ques7on The planets are thought to have formed from a disk of gas and dust around the Sun. The chemical composi7on of the planets varies systema7cally with distance from the Sun because A.  the original gas cloud was not uniform in composi7on. B.  temperature varia7ons in the disk determined which materials condensed at each radius. C.  all the hydrogen in the inner disk combined with oxygen to form water. D.  there was no iron or silicon in the outer parts of the disk. E.  much of the hydrogen in the inner disk caught fire and burned away. Mar. 26 Mar. 26 3 3/26/10 Atmospheric evapora7on Par7cles in a gas •  Why do some planets/moons have atmospheres while others don’t? •  A gas contains a myriad of atoms/ molecules flying around. •  Is it just forma&on, or also evolu&on? •  Consider par7cle mass m and temperature T: •  How long can a planet/moon retain an atmosphere? typical speed: vgas = ￿ 3kT m k = 1.38 × 10−23 kg m2 s−2 K−1 Mar. 26 Mar. 26 Example Discussion Ques7on Earth’s atmosphere contains a lot of molecular nitrogen, N2, which has a par7cle mass of 4.7×10 ­26 kg. Room temperature is about 300 K. How fast are the nitrogen molecules moving? We just calculated the typical speed of nitrogen (N2) molecules in Earth’s atmosphere. What about hydrogen (H2, less massive) and oxygen (O2, more massive) molecules? vgas = = ￿ ￿ 3kT m 3 × (1.38 × 10−23 kg m2 s−2 K−1 ) × (300 K) 4.7 × 10−26 kg = 515 m/s Mar. 26 A.  B.  C.  D.  E.  H2 molecules move faster and O2 molecules move slower H2 molecules move slower and O2 molecules move faster both H2 and O2 molecules move slower both H2 and O2 molecules move faster all molecules move at the same speed. Mar. 26 Example Escape velocity •  If you throw something fast, you can throw it into orbit. Earth’s mass is 5.97×1024 kg and its radius is 6.38×106 m. What is its escape velocity? •  If you throw it fast enough, you can throw it out of orbit! vesc ￿ •  To escape from planet with mass M and radius R: ￿ 2GM vesc = R Mar. 26 ￿ 2GM R 2 × (6.67 × 10−11 m3 kg−1 s−2 ) × (5.97 × 1024 kg) = 6.38 × 106 m = 1.12 × 104 m/s = 11.2 km/s = Mar. 26 4 3/26/10 Discussion Ques7on Atmospheric evapora7on Compared with planets closer to the Sun, planets farther from the Sun retain more hydrogen in their atmospheres because •  Roughly speaking, a planet can retain its atmosphere only if vesc > 6 vgas T (K) vesc vesc/6 Mercury 452 4.2 0.7 vgas (H2) vgas (He) 2.4 1.7 Venus 726 10.4 1.7 3.0 2.1 Earth 281 11.2 1.9 1.9 150 5.0 0.8 1.4 1.0 Jupiter 120 59.5 9.9 1.2 0.9 Saturn 88 35.5 5.9 1.0 0.7 Mar. 26 Mar. 26 HW ques7ons Chapter 7 8.  Why are hydrogen and helium abundant in the atmospheres of the Jovian planets but present in only small amounts in the Earth’s atmosphere? Chapter 8 2.  The graphite in your pencil is a form of carbon. Where were those carbon atoms formed? 5. Why are terrestrial planets smaller than Jovian planets? Mar. 26 they move more slowly in the orbits. they feel less gravity from the Sun. they have more moons. their atmospheres are cooler. they receive less infrared light from the Sun. 1.3 Mars A.  B.  C.  D.  E.  HW ques7ons 6. How do radioac7ve elements make it possible to determine the age of the solar system? What are the oldest objects that have been found in the solar system? 15. If hydrogen and helium account for 98% of the mass of all the atoms in the universe, why aren’t the Earth and Moon composed primarily of these two gases? 17. Why did the terrestrial planets form close to the Sun while the Jovian planets formed far from the Sun? 31. Three ­quarters of the radioac7ve potassium (40K) originally contained in a certain volcanic rock has decayed into argon (40Ar). How long ago did this rock form? Mar. 26 5 ...
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