Unit 06 - Our Solar System PDF

ASTR 1205 Unit 6 Dr. Bryan Rowsell Unit 6: Our Solar System 6.1 A Brief Tour of the Solar System The Solar System consists of a central star (the Sun) and everything that orbits the Sun, including planets, dwarf planets, moons, asteroids, and comets....

ASTR 1205 Unit 6 Dr. Bryan Rowsell Unit 6: Our Solar System 6.1 A Brief Tour of the Solar System The Solar System consists of a central star (the Sun) and everything that orbits the Sun, including planets, dwarf planets, moons, asteroids, and comets. How many of the planets orbit the Sun in the same direction as Earth does? (a) A few (b) Most (c) All Planetary orbits in our Solar System are: (a) very eccentric ellipses oriented in every direction (b) very eccentric ellipses oriented in the same plane (c) Fairly circular, oriented in every direction (d) Fairly circular, oriented in the same plane Our Solar System ·6−1· 6.1: A Brief Tour of the Solar System ASTR 1205 Unit 6 Dr. Bryan Rowsell The International Astronomical Union defined a planet in 2006 as something that satisfies these three conditions: 1. it must orbit a star; no interstellar wandering 2. it must be big enough that its gravity has forced it into a spherical shape; no lumpy potatoes! 3. it must be big enough that is gravity has cleared its orbit of other objects of a similar size; owns its orbit What is a dwarf planet? Satisfies 1 and 2 above, but not 3. Several objects in the Solar System are smaller than dwarf planets. − They aren’t big enough to have enough gravity to become spherical. − They stay as lumpy potatoes. Comet: Icy object smaller than a dwarf planet. Asteroid: Rocky object smaller than a dwarf planet. Meteoroid: A small chunk that may have broken off either a comet or an asteroid. Meteor: Shooting Star! A meteoroid that leaves a glowing streak in the sky as it passes through Earth’s atmosphere at super high speed. Meteorite: A meteoroid that survives the trip through the atmosphere to crash into the ground. Let’s do a quick tour of the solar system and make some highlights. Our Solar System ·6−2· 6.1: A Brief Tour of the Solar System ASTR 1205 Unit 6 Dr. Bryan Rowsell The Sun Mass: 333,000 M⨁, that’s 99.8% of the mass of the Solar System! Radius: 108 R⨁ Composition: mostly hydrogen (H) and helium (He). #1 Mercury Length of Year Avg. Distance from Sun 88 Days 0.4 AU Mass Radius 0.06 M⨁ 0.38 R⨁ Moons Rings 0 No Composition Avg. Surface Temp Rocks, Metals 100−700 K (night−day) #2 Venus Length of Year Avg. Distance from Sun 225 Days 0.7 AU Mass Radius 0.8 M⨁ 0.95 R⨁ Moons Rings 0 No Composition Avg. Surface Temp Rocks, Metals 740 K Our Solar System ·6−3· 6.1: A Brief Tour of the Solar System ASTR 1205 Unit 6 Dr. Bryan Rowsell #3 Earth Length of Year Avg. Distance from Sun 365 Days 1.0 AU Mass Radius 1.0 M⨁ 1.0 R⨁ Moons Rings 1 No Composition Avg. Surface Temp Rocks, Metals 290 K #4 Mars Length of Year Avg. Distance from Sun 1.9 Years 1.5 AU Mass Radius 0.1 M⨁ 0.5 R⨁ Moons Rings 2 No Composition Avg. Surface Temp Rocks, Metals 220 K [all images courtesy of NASA] The first four planets are often called terrestrial planets as their composition is similar to Earth. After the terrestrial planets, there is a rather large asteroid belt. See image on 6−1. Most asteroids orbit in a belt (doughnut) between the orbits of Mars and Jupiter, while the belt is big, it’s not very dense. Most asteroids range in diameter from 10 m to 500 km but most are on the small side. All the asteroids in this belt add up to about the mass of Earth’s Moon. Our Solar System ·6−4· 6.1: A Brief Tour of the Solar System ASTR 1205 Unit 6 Dr. Bryan Rowsell #5 Jupiter Length of Year Avg. Distance from Sun 12 years 5.2 AU Mass Radius 318 M⨁ 11 R⨁ Moons Rings > 90 Yes Composition Avg. Surface Temp Mostly H, He 125 K #6 Saturn Length of Year Avg. Distance from Sun 29 years 9.5 AU Mass Radius 95 M⨁ 9.4 R⨁ Moons Rings > 60 Yes Composition Avg. Surface Temp Mostly H, He 95 K #7 Uranus Length of Year Avg. Distance from Sun 84 years 19 AU Mass Radius 15 M⨁ 4.0 R⨁ Moons Rings > 25 Yes Composition Avg. Surface Temp Mostly H, He 60 K Our Solar System ·6−5· 6.1: A Brief Tour of the Solar System ASTR 1205 Unit 6 Dr. Bryan Rowsell #8 Neptune Length of Year Avg. Distance from Sun 165 years 30 AU Mass Radius 17 M⨁ 3.9 R⨁ Moons Rings > 15 Yes Composition Avg. Surface Temp Mostly H, He 60 K The collection of planets beyond the asteroid belts are often referred to as gas giants or jovian planets, though technically Uranus and Neptune have solid hydrogen compounds such as water (H2O), methane (CH4) and/or ammonia (NH3), and thus are often called ice giants. Kuiper Belt Hundreds of thousands—if not millions—of small icy objects orbit in a belt (doughnut) past the orbit of Neptune (with orbital periods less than 200 years), collectively called the Kuiper Belt Objects (KBO). Pluto is the most famous of the KBOs [NASA] Our Solar System ·6−6· 6.1: A Brief Tour of the Solar System ASTR 1205 Unit 6 Dr. Bryan Rowsell Oort Cloud Hundreds of millions—if not trillions—of small icy objects orbit in a spherical shell (thick bubble) waaaay past the Kuiper Belt. Comets in the Oort cloud take > 200 years to orbit the Sun. More of a prediction: We predict it’s there partly because we know there are some comets that have only ever been seen once in all of recorded history. Our Solar System ·6−7· 6.1: A Brief Tour of the Solar System ASTR 1205 Unit 6 Dr. Bryan Rowsell The biggest rocky planet and biggest gas planet are, respectively: (a) Venus and Jupiter (b) Venus and Saturn (c) Earth and Jupiter (d) Earth and Saturn Is there a pattern to the distance between the orbits of the planets in our Solar System? (a) all planet orbits are separated by approximately the same distance (b) the orbits of the terrestrial planets are relatively close together, while the gas/ice giants are more spread out (c) the orbits of the gas/ice planets are relatively close together, while the terrestrial planets are more spread out (d) There is no pattern to the distance between planet orbits Where are most asteroids located? (a) inside the orbit of Mercury (b) between the orbits of Mars and Jupiter (c) past the orbit of Neptune (d) they are scattered evenly throughout the solar system Where are most comets located? (a) inside the orbit of Mercury (b) between the orbits of Mars and Jupiter (c) past the orbit of Neptune (d) they are scattered evenly throughout the solar system 6.2 The Nebular Theory of Solar System Formation Before we get into the formation of the solar system, lets look at a few patterns that may arise from the planets and their motion. Any successful theory of Solar System formation must explain the patterns and allow for the exceptions. Our Solar System ·6−8· 6.2: The Nebular Theory ASTR 1205 Unit 6 Dr. Bryan Rowsell 1. Orderly Motions − all planetary orbits are nearly circular and lie in the same plane − all planets orbit the Sun counter−clockwise1 − most large moons orbit their planet counter−clockwise − the Sun, most planets and most large moons spin on their axis counter−clockwise 2. Two Major Planet Types 1 In the above statements, “counterclockwise” refers to the direction as viewed from high above Earth’s North Pole. Our Solar System ·6−9· 6.2: The Nebular Theory ASTR 1205 Unit 6 Dr. Bryan Rowsell Based on the data in the table, this planet should be classified as: (a) Venusian (b) Terrestrial (c) Jovian (d) Saturnarian (e) none of the above 3. Asteroids and Comets Vast numbers of smaller objects orbit the Sun in three distinct regions: 1. Asteroid Belt (asteroids) 2. Kuiper Belt (comets) 3. Oort Cloud (comets) Some notable exceptions to these three patterns: − Mercury has an exceptionally large metallic core. − Venus spins on its rotation axis backwards (clockwise). − Earth’s Moon is unusually large. − Uranus rotates nearly on its side (as it orbits the Sun, it’s more like a rolling ball than a spinning top). − Neptune’s largest moon, Triton, orbits Neptune backwards (clockwise). Our Solar System · 6 − 10 · 6.2: The Nebular Theory ASTR 1205 Unit 6 Dr. Bryan Rowsell Imagine the Earth rotated in the opposite direction (clockwise when looking down from the North Pole). According to convention, what would be its spin axis tilt? (a) −23.5° (b) 23.5° (c) 156.5° (d) 203.5° (e) 336.5° Looking at the diagram on 6−10, Of these three planets, which do you expect to have the most and least dramatic seasons, respectively? (a) Uranus and Venus (b) Venus and Uranus (c) Venus and Earth (d) Uranus and Earth Many hypotheses have tried to explain how the Solar System formed, but only one has stood the test of time and become a theory: The Nebular Theory of Solar System Formation. nebula a cloud of dust and gas [NASA/JPL] The nebula from which our Solar System formed is called the Solar Nebula. Our Solar System · 6 − 11 · 6.2: The Nebular Theory ASTR 1205 Unit 6 Dr. Bryan Rowsell 6.3 Explaining the Major Features of the Solar System Nebular Theory Step 1: Gravitational Collapse As the solar nebula collapses… 1. It heats up Conservation of energy. Gravitational potential energy gets converted into kinetic energy. 2. It spins faster Conservation of angular momentum. Recall the figure skater, angular velocity increases as radius decreases. 3. It flattens to a disk Conservation of angular momentum. The short answer is that collisions of particles cancel any up/down motion. Our Solar System · 6 − 12 · 6.3: Explaining the Solar System ASTR 1205 Unit 6 Dr. Bryan Rowsell This nebular theory explains Pattern #1 on page 6−9! Nebular Theory Step 2: Condensation Step #2 of the Nebular Theory of Solar System Formation involves condensation. What is condensation? In astronomy, condensation is when particles of a “condensed” state (solid or liquid) form in a gas. Different substances condense at different temperatures. Hydrogen compounds (H2O, NH3, CH4) need very cold temperatures to condense (< 150 K). Rocks/metals condense at higher temperatures (> 500 K). The Solar Nebula had the same composition throughout (in other words, it was thoroughly mixed, i.e. homogeneous), but there was a temperature gradient—warmer close to the Sun, colder far from the Sun. Think about this…how does our Solar System possibly reflect this? Pattern #2 on page 6−9 is now mostly explained! Our Solar System · 6 − 13 · 6.3: Explaining the Solar System ASTR 1205 Unit 6 Dr. Bryan Rowsell The Sun’s surface temperature is 5777 K and the frost line of our Solar System lies roughly at 5 AU. In another star system, the central star has a surface temperature of 8900 K. Approximately where would you expect the frost line to lie in that system? (a) 1 AU (b) 5 AU (c) 10 AU (d) 500 AU Our Solar System · 6 − 14 · 6.3: Explaining the Solar System ASTR 1205 Unit 6 Dr. Bryan Rowsell Why do we think the inner (terrestrial) planets became more dense than the outer planets? (a) In the collapsing solar nebula, denser materials sank toward the center due to the Sun’s gravity (b) The inner nebula was so hot that only metals and rocks were able to condense (c) The rotating disk in which the planets formed flung lighter elements outward due to the centrifugal force Nebular Theory Step 3: Accretion The microscopic particles that condensed in the Solar Nebula “stuck together” and grew larger with time, eventually becoming planets. The process by which small things gather together to make large things is called accretion. What do we think first caused the microscopic solid particles in the Solar Nebula to “stick together”? (a) gravity (b) electrostatic forces (i.e. static electricity) (c) pressure Our Solar System · 6 − 15 · 6.3: Explaining the Solar System ASTR 1205 Unit 6 Dr. Bryan Rowsell Once enough microscopic solid particles stuck together, what force allowed them to grow to the size of boulders even faster? (a) gravity (b) electrostatic forces (i.e. static electricity) (c) pressure These boulder-sized objects that are not yet big enough to be called planets are called planetesimals, which means “pieces of planets.” But why are Jovian planets so much bigger than terrestrial planets? 1. They formed beyond the frost line, where there was more condensed material available for accretion (metals, rocks, and hydrogen compounds). 2. Their gravity became strong enough to attract hydrogen and helium gas from the Solar Nebula. Moons of Jovian planets formed from miniature disks due to the size of these planets/planetesimals. Recall, the Jovian planets are often regarded as “failed stars”. Not all planetesimals become planets! The young solar system is a very chaotic place with lots of collisions (by modern standards). Our Solar System · 6 − 16 · 6.3: Explaining the Solar System ASTR 1205 Unit 6 Dr. Bryan Rowsell Smaller planetesimals would shatter during high−speed collisions. Only the most massive planetesimals survived these collisions without shattering, eventually becoming large enough to accrete into a planet. These failed planetesimals became: asteroids and comets It makes sense that rocky asteroids would be closer to the Sun and icy comets farther from the Sun because of what we’ve already learned about the: (a) conservation of energy (b) conservation of angular momentum (c) frost line (d) we can’t explain this phenomenon yet Pattern #3 on page 6−10 is now explained! What about the exceptions? This model explains those too! Amidst the chaos of the early Solar System, some small planetesimals could have been captured by planets during a close approach. Due to the random nature of the capture process, captured moons wouldn’t necessarily exhibit the orderly motions seen elsewhere in the Solar System. Moons with unusual orbits (like Neptune’s moon, Triton) are probably captured planetesimals. Earth is too small and our moon too large for it to be a captured moon… Based on samples of meteorites, we know the earliest planetesimals formed 4.55 billion years ago. Based on samples brought back from the Apollo missions to the Moon, we know many lunar craters formed ∼4 billion years ago. Therefore, scientists hypothesize that the inner Solar System was bombarded by comets and asteroids near the end of the planetary Our Solar System · 6 − 17 · 6.3: Explaining the Solar System ASTR 1205 Unit 6 Dr. Bryan Rowsell accretion time, during a period we call the Late Heavy Bombardment, where a disproportionately large number of asteroids and comets collided into the terrestrial planets and their moons. The Late Heavy Bombardment likely occurred due to the migration of the Jovian planets causing disruption in the orbits of the asteroids and comets of (what would become) the asteroid and Kuiper belts. This likely means that the water on Earth arrived from frequent, early collisions with comets! One of these impacts split the early Earth into the planet we know today and the moon. The exceptions to the patterns in the Solar System are generally explained by: (a) luck (b) the Frost Line (c) collisions and gravitational encounters (d) not explained yet 6.4 The Age of the Solar System How do we know the solar system formed 4.6 billion years ago? Radioactive Decay Isotopes of a given element: same # protons, different # neutrons. Not all isotopes are stable; unstable isotopes decay into more stable daughter isotopes in a random and spontaneous way. We can’t predict when an atom will decay, but we can predict how long it will take for half the atoms in a sample to decay. One important isotope is potassium−40, or 40K: 40 40 K→ Ar + 𝑒 + + 𝑣𝑒 Our Solar System · 6 − 18 · 6.4: Age of the Solar System ASTR 1205 Unit 6 Dr. Bryan Rowsell The time required for half of the amount of an isotope to decay is called the half−life; for 40K, that half−life is not far off from the age of the solar system. Imagine we start with 16 atoms of the unstable 40K. We know the half−life of 40K to be 1.25 billion years, so after 1.25 billion years, how many 40K atoms will we have left? 8! The 8 that decayed turned into 40Ar (argon) After 2.5 billion years, how many 40K atoms remain? (a) 16 (d) 4 (b) 12 (e) 2 (c) 8 The precise proportion of the two isotopes tells us the age of the rock. This is called radiometric dating. There are problems with radiometric dating however: Our Solar System · 6 − 19 · 6.4: Age of the Solar System ASTR 1205 Unit 6 Dr. Bryan Rowsell − Need to know isotope proportions at t = 0 (usually assume 100% unstable parent and 0% stable daughter). − There can be no other way for either the parent or daughter isotope to enter/leave the sample. − It’s best to use a parent whose half-life is a similar timescale to the age of the sample. Looking at the graph on 6−19… A sample of rock is found with 81% argon-40 and 19% potassium- 40. How many years have elapsed since the rock formed? (a) 0.33 billion years (d) 2.5 billion years (b) 1.5 billion years (e) 3.05 billion years (c) 2 billion years Uranium-235 has a half-life of about 700 million years. After 2.8 billion years, 4 half-lives have elapsed. What fraction of the original uranium-235 will be left after 2.8 billion years? 1 1 (a) (d) 2 16 1 1 (b) (e) 4 32 1 (c) 8 If a rock started with 41.7 g of uranium-235, how many grams of 235 U will it contain after 2.8 billion years? (a) 2.07 g (d) 14.9 g (b) 2.61 g (e) 16.0 g (c) 10.4 g Our Solar System · 6 − 20 · 6.4: Age of the Solar System ASTR 1205 Unit 6 Dr. Bryan Rowsell Chapter 6: The Essential Cosmic Perspective End−of−Chapter Questions: 1−34, 38. Solutions found in Bb. Extra Resources: Interact with these websites about our Solar System: https://solarsystem.nasa.gov/planets/overview/ https://artsandculture.google.com/story/iAUR-B3izSZiVA Watch the following Crash Course Astronomy videos: Introduction to the Solar System [10 min]: https://www.youtube.com/watch?v=TKM0P3XlMNA Explore the Solar System 360 Degree Interactive Tour [5 min]: https://www.youtube.com/watch?v=0ytyMKa8aps Watch this simulation of the collapse of the Solar Nebula into a rotating disk [1 min]:https://www.youtube.com/watch?v=j4YD550ig0U Watch this video about the formation of the Solar System and why all the planets are on the same orbital plane [8 min]: https://www.youtube.com/watch?v=ceFl7NlpykQ Watch this video for a quick review of several ideas taught in class and to see a simulation of the formation of the Solar System [4 min]: https://www.youtube.com/watch?v=yXq1i3HlumA Watch this short animated video to learn more about the characteristics and formation of the jovian planets' ring systems [3 min]: https://www.youtube.com/watch?v=0SwphjTYjcA Watch this video if you're feeling confused about radiometric dating [5 min]: https://www.youtube.com/watch?v=oe45GegJUvM Read this site and watch the embedded videos for a more detailed explanation of radiometric dating: https://earthsky.org/earth/age-of-earth-how-old-is-planet-earth/ Practice radioactive decay math problems by looking at the example questions on this website and trying to solve them without looking at the posted solutions: https://www.chemteam.info/Radioactivity/Radioactivity-Half-Life-probs1-10.html Fun visualization: https://www.instagram.com/reel/Com0O1wIeuZ/?igshid=YmMyMTA2M2Y% 3D Our Solar System · 6 − 21 · Problems and Resources