Astro Review Notes for Unit 2

Astro Review Notes for Unit 2 Information on the test itself ("Test 1" covers Units 1 & 2) • 50 multiple-choice questions (5 answer choices each) • Half of the questions will relate to material from each of major topics in Unit 2 — Kepler’s Laws; Jupiter’s Moons; Mercury; Venus; Earth; Earth's Moon (Luna); Mars, the Jovian planets (Jupiter, Saturn, Uranus, & Neptune); dwarf planets; Kuiper Belt & Oort cloud; comets, asteroids & meteors; the formation of the Solar System. (the other half of the questions will relate to topics in Unit 1; see Unit 1's review notes) • Some questions will be identical to those given in the game material — selected from "Copper’s" quiz questions and those in with the lessons' dialogue • Some questions will be drawn directly from the "Sample Test-1 Questions" packet • Additional questions similar in nature to questions you have seen before • A few questions will present pictures of planets or major moons that you will be asked to identify. This document contains an overview of major concepts and facts that you should take away from Unit 1 to prepare for the first test (which also covers Unit 1). These are the major points, but of course the game gives more detail and explanation. One thing to note is that you are not expected to remember exact dates and numbers. Try for understanding rather than memorization, so that you can possibly pick the number out from very-different values. For example, you might be asked if Jupiter is 10x less massive, roughly equal mass, 3x more massive, 300x more massive, or 3 billion times as massive as the Earth. The answer: 300x as massive — i.e., without remembering a more-exact 318x. You are expected to know the names of, and be able to visually identify, the 8 planets and several of their major moons (as noted, below), and recall a few of their basic properties. 1 1. Kepler's Laws Copernicus proposed the heliocentric (Sun-centered) model of the Universe, which replaced the geocentric (Earth-centered) model. Distance between Earth and Sun = 1 "Astronomical Unit" (1 AU) Time for Earth to orbit the Sun = 1 Earth year (365 days) Kepler used Tycho Brahe’s data to determine these laws: • 1st law: The planets orbit the Sun in elliptical orbits with the Sun at one focus. These ellipses tend to be nearly circular for the planets in our Solar System. • 2nd law: "A line joining a planet and its star sweeps out equal areas during equal intervals of time." Or, a planet's speed is fastest when the planet is closest to the Sun (at a point called perihelion), and slowest when it is farthest away (aphelion). • 3rd law: P 2 = a 3, where P is the orbital period (a planet's year, given as a mulitple of Earth years) and a is the semimajor axis (average distance from Sun, given in AU). example of a problem using Kepler's 3rd law: A hypothetical planet takes 3 years to orbit the Sun. How far from the Sun does it orbit? 3 years is the period ("P"). Calculate P 2 : P 2 = 3 × 3 = 9. So, from P 2 = a 3 : a 3 = 9. Well, 2 3 = 2 × 2 × 2 = 8, and 3 3 = 3 × 3 × 3 = 27. So a must be something between 2 and 3 — and much closer to 2 than to 3. In a test question, you would be given answer choices that make it pretty obvious which answer is correct, like "1 AU," "2.08 AU," "3.08 AU," "9 AU," and "9.7 AU." or, to work the equation in the other direction: A hypothetical planet orbits the Sun at a distance of 5 AU. How long is its orbital period (i.e., its year)? 5 AU is the semimajor axis ("a"). Calculate a 3 : a 3 = 5 × 5 × 5 = 125. So, from P 2 = a 3 : P 2 = 125. Well, 10 2 = 10 × 10 = 100, 11 2 = 11 × 11 = 121, 12 2 = 12 × 12 = 144. So P must be something between 11 and 12 — and closer to 11 than to 12. Again, you would be given answer choices that make it pretty obvious which is correct, like "5 years," "11.2 years," "15.2 years," "25 years," and "125 years." 2 2. Mercury • Can only see close to Sun — and hard to do with the naked eye. Displays phases. • Mercury is much smaller than Earth. It is similar to Mars in mass and radius, and only a bit bigger than Earth’s Moon; surface gravity less than on Earth • Temperature is extremely high on the day side and extremely low on the night side, due to lack of an atmosphere to distribute heat evenly around the planet (or to retain that heat on the night side) • Many craters, like the Moon, including a few permanently-shadowed craters near the poles that are cold enough all the time as to hold persistent ice deposits; shadows in other craters may be cold, even on the hot/daytime side • Rotates very slowly — 59 Earth days to rotate; has a short year —" 88 Earth days to orbit the Sun • Mercury has no significant atmosphere; the sky appears black (can see in to deep space), even in daytime —"except for the exact position of the Sun • Density = mass / volume density is related to what substance(s) something is made of • Mercury's average density is 5 grams per cubic centimeter, similar to metals • and rocks 3. Venus • Closest planet to Earth; visible in western sky in evenings near the Sun, and in the eastern sky just before sunrise; looks like a very bright star. Also shows phases, like the Moon. • Just slightly smaller than Earth in mass (82%) and in radius (95%). • Venus has a longer rotational period (243 days) than its year (225 Earth days) Venus' rotation is in the opposite direction to its orbit around the Sun — i.e., it • rotates clockwise when viewed from above (Earth & most other planets rotate counter-clockwise). The combination of these factors leads to a day/night cycle on Venus of about • 117 Earth days; also, the Sun travels "west to east" ("backwards") across the sky. 3 • Dense atmosphere — mostly carbon dioxide — of gases produced by volcanic outgassing, but not dissolved in ocean like on Earth, also sulfuric acid in clouds • Venus' surface is very hot (> 700 K, hotter than Mercury), because of the presence of large amounts of greenhouse gases (mostly CO2) in its atmosphere to contain heat. • Density of Venus is about 5.2 grams per cubic centimeter — similar to Mercury — and made of metals/rock • Active volcanoes, huge lava flows and channels, some large craters, but small meteors burn up in the thick atmosphere before they can impact to leave small craters. 4. Earth • fairly thick atmosphere, mostly composed of nitrogen and oxygen molecules; atmosphere causes sky to appear bright during the daytime; this is the scattered light from the Sun — otherwise the sky would appear dark • aurora caused by solar wind particles that hit atmospheric gas and lead to emission of different colors; they are prominent near the north and south poles • plate tectonics = rocky plates on the surface of Earth float on denser but more-fluid rocky material, and move around —"leads to continental drift, mountains caused by collisions of plates; plates sliding past each other cause faults, quakes • ozone (three oxygen atoms bonded together) layer (in stratosphere at 6 – 30 miles above the surface) protects Earth from solar UV radiation; ozone can be destroyed by certain gases produced by human activity • "Greenhouse Effect": Energy from Sun heats Earth. Earth's surface radiates this heat back towards space by giving off infrared radiation. Particular gases (including water vapor and CO2) in the Earth’s atmosphere • recapture and redirect this heat back towards the surface, preventing it from escaping to space. This trapped heat is redistributed back on Earth. The naturally abundant • greenhouse gases in the Earth’s atmosphere act like a thermal blanket keeping Earth warm. Without greenhouse gases Earth’s surface would be about 33 degrees • Centigrade cooler. When humans produce large amounts of greenhouse gases, it leads to extra heating, potential extra reinforcing feedback loops — i.e., global warming —"beyond stabilized longer-term temperatures. 4 Earth's Moon (AKA "Luna") • terra (Latin for “land”): light-colored features, heavily-cratered, high peaks; geologically older; highlands uplifted from impacts in early solar system • maria (Latin for “seas”): dark-colored, less cratered, valleys; geologically younger; filled by lava 1 to 4 billion years ago • regolith = powdery dust and rocky debris that covers the Moon; broken apart by small meteorites hitting moon continuously • impact craters = gouged out from explosions, asteroids or comets that hit the moon • The Moon rotates on its own axis with exactly the same period that it takes to travel once around the Earth; consequentially, the same side of the Moon always faces Earth —"this is known as "tidal locking" • Moon lacks as much high-density material such as iron — no large iron core • 1/4 diameter of Earth • Moon has no atmosphere; daytime sky is dark • Surface gravity is 6 times stronger on the Earth than on the Moon. • ∝ M/R 2 • g where g is the acceleration due to gravity, M is the mass, and • R is the radius of the planet. Large-mass planets that are compact have larger gravity. Formed later than Earth, from giant impact of a large object (Mars-sized) with Earth. 5. Mars • Mars' days and nights are similar in length to those on Earth • About half Earth’s diameter; 1/10th Earth’s mass • > 12 spacecraft have visited Mars; rovers explored surface • No thick atmosphere, but does have thin one that causes orangish sky; mostly carbon dioxide; only modest Greenhouse effect because the atmosphere is so thin 5 • Average surface temperature about –65 ˚C, but can be warmer at its equator • Iron oxide ("rust") in surface rocks gives Mars its red color • Has two moons, Phobos and Deimos, but they are tiny"— only 0.3 % of Mars' size • Olympus Mons (large volcano on Mars; do not need to remember name) is 3 times higher than Earth’s highest mountain; Martian volcanos appear dormant • Valles Marineris (do not need to remember name) is the deepest canyon in Solar System — about 300 times size of the "Grand Canyon" on Earth • Water once existed on the surface of Mars, but it is not flowing regularly there at present; permafrosts = water ice locked beneath Martian soil • Dust storms are common; occasionally make it hard to see surface features. 6. Comets, Meteors, & Asteroids Comets • A few kilometers in size • Made of ice (both water ice and "dry ice") and dust —" remnants of Solar System formation • Tails pointing away from the Sun develop due to the solar wind • Shine by reflected sunlight — i.e., comets do not produce light of their own • Highly-elliptical orbits that take them far from the Sun; as would be predicted by Kepler's 2nd Law, comets spend most of their time in the outer reaches of the Solar System — they zip by the inner Solar System and travel back out very quickly • Comets move slowly across the sky from our perspective on Earth • • Short-period comets have periods < 200 years long-period comets have much longer periods (can be 1,000s of years) and come from the Oort cloud, as much as 50,000 AU from Sun • Halley’s Comet is a famous short-period comet that returns every 76 years — its last visit to the inner Solar System was in 1986; the next return will be in 2061 6 Asteroids • remnants of the Solar System's formation —"rocks left over when the planets form • many are in the "Asteroid Belt" between Mars and Jupiter • asteroid orbits are typically slightly elliptical • a few major types exist: carbon-rich, metallic, and silicate (stony) • we see asteroids by reflected sunlight; they do not shine visibly on their own • the orbits of some asteroids intersect Earth’s orbit, and lead to meteors • most are less than 1 kilometer in size, but some are bigger • like comets, asteroids tend to move very slowly across the sky to a viewer on Earth Meteors • streak across the sky very quickly —"they are also called "shooting / falling stars" (they can travel up to a hundred thousand kilometers per hour) • most meteors are tiny dust particles or grains — less than a centimeter in size —"that rapidly burn up in Earth’s atmosphere; pieces of Solar System debris • meteors occur when an asteroid crosses Earth’s orbit, or when Earth passes through a cloud of debris left behind by a comet passing through the inner Solar System (the famous Halley's Comet is reponsible for the "Orionids" annual meteor shower) • a "fireball" = very bright meteor due to larger-than-usual chunk of debris • a "meteorite" = piece of meteor that survives atmospheric entry, hits Earth's surface • asteroid collisions were more common in the younger Solar System • 65 million years ago a large meteor collision with Earth contributed to the extinction of dinosaurs (the "K/T event"); dust and smoke thrown into the atmosphere was greater cause of extinction — not so much the initial impact itself 7 7. Jupiter • Jupiter is about 300 times the mass of the Earth, but is on average far less dense — overall about 1.3 grams per cubic centimeter (about the density of milk); it has a radius of 11 times that of the Earth. • Jupiter has a small ring system. • Jupiter is made mostly of hydrogen and helium gas. • Gaseous molecules of ammonia, methane, and water vapor also present; these lead to different colors in Jupiter's stripey, swirling appearance. • Clouds of different colors are also at different altitudes. • Windspeeds can reach 360 km/hr; the "Great Red Spot" is a giant storm larger than Earth — like an anticyclone on Earth — and has lasted for at least 300 years • Jupiter rotates rapidly (once every 10 hours), which stretches the clouds into long bands. • It takes Jupiter 12 Earth years to orbit the Sun. • Interior temperatures and pressures get very high, deep inside Jupiter's gaseous body • Hydrogen gets compressed into a liquid-metallic form. • In its very center, Jupiter is likely to have a rocky/metallic core. • Magnetic fields are 10 times stronger than Earth's due to the rotating liquid-metallic region; interaction between this magnetic field, the solar wind, and Jupiter's atmosphere leads to aurora like we see on Earth • Methane can convert to carbon soot, and high pressures inside Jupiter can compress that into diamonds (leading to "diamond rain" in Jupiter's atmosphere). • Life forms in Jupiter’s atmosphere were proposed to exist by astronomers Carl Sagan and Edwin Salpeter; these speculative creatures were referred to as "sinkers," "floaters," and "hunters." 8 Jupiter’s moons • Jupiter has more than 50 moons; the four largest were discovered by Galileo in the early 17th century CE;"they • looked like stars, but were found to orbit Jupiter — important because it helped demonstrate that Earth is not the center of Universe Using Kepler’s laws, the mass of Jupiter can be measured using the periods • and semimajor axes of the orbits of Jupiter’s moons; periods range from 1.8 to 17 Earth days, and Jupiter’s mass is about 317 Earth masses. You should remember the names and basic information (below) for these four • largest "Galilean" moons of Jupiter... 1. Io : the closest; mostly coppery-yellow with black dots, which are active volcanic sites; overall very volcanically active, due to "tidal heating" from Jupiter; low crater density; yellow color from sulfur 2. Europa : water-ice surface; no craters —" ice movement wiped them out; liquid water ocean below icy crust 3. Ganymede : largest moon in the Solar System (larger than the planet Mercury); icy crust; has craters so not active now, but cracks show it once was more active 4. Callisto : the farthest out of the four large Galilean moons; very heavily cratered — many young craters; does not get heated much, and has not changed much since its formation; about the size of the planet Mercury 8. Saturn • Galileo discovered its rings (although he did not know what they were, at the time). the rings are incredibly thin; made of trillions of individual orbiting objects • composed of dusty water-ice crystals, of various sizes • Saturn's mass is about 100 x Earth's (about 1/3 Jupiter); diameter about 10 x Earth's. overall average density is 0.7 grams per cubic centimeter, which is less than • that of water —""Saturn could float!" the spin axis of Saturn is tilted about the same as Earth’s • like Jupiter, Saturn is mostly made of Hydrogen and Helium; clouds arranged • into belts and zones; temperatures cooler than on Jupiter. • More than 50 moons; by far the largest is Titan (remember this one) —" secondlargest moon in Solar System; very thick nitrogen-rich atmosphere like the young Earth, but perhaps too cold for life. 9 9. Uranus • Third-largest planet, after Jupiter (1st) and Saturn (2nd). • Orbital period is about 84 Earth years. • Uranus' thick atmosphere is made of gas, mostly hydrogen and helium, but some methane too that gives it the blue-green color atmosphere does not have as continually prominent belts or zones, or bright • clouds, like Jupiter has liquid, icy core surrounding a smaller rocky core • • Spin axis is nearly in plane of orbit around Sun (it is kind of "rolling on its side"); probably caused by collision early in its history; leads to continual near-darkness or light for 42 Earth years in a row, at its poles • Uranus has many thin, faint, dark rings made of carbon ("soot"). • 27 known moons (you do not need to know the exact number, or names, of these) Miranda (one of the larger moons): heavily-cratered, with weird valleys and • cliffs; appearance caused by upwelling of ices 10. Neptune • Discovered in 1846 (do not need to memorize exact date), based on the orbit of Uranus not looking quite right • Similar to Uranus, but a deeper blue color; atmosphere of hydrogen, helium, and methane (causes blue color); a solid core the size of earth; surrounded by a mantle of semi-fluid ices • Rapid winds and large storms/vortices; include Great Dark Spots, big storms that can come and go over the timescale of a decade • Ring structure, faint and dark; made of clumps of dust • 14 known moons Triton (largest): thin atmosphere made of nitrogen and methane; rocky • composition; reflective icy surface; ice volcanoes with liquid nitrogen, dust, or methane coming out of them; unusual orbit suggests Triton may be a captured Kuiper Belt object • 10 11. Pluto and Other Dwarf Planets, & the Kuiper Belt • Pluto was discovered in 1930; NASA's New Horizons probe passed it in July 2015 248 Earth-year period of orbit • orbit more highly-elliptical than any of the 8 planets, and not in the same • plane; briefly intersects orbit of Neptune icy ball of rock • • Pluto is very small compared to the planets; Earth’s Moon is 7 times as massive • Despite its size, Pluto has 5 moons; largest is Charon, over half diameter of Pluto • Pluto not considered a "planet" anymore, because ... 1. other similar objects were found farther from the Sun — Quaoar, Sedna, Eris 2. not like other outer planets / small; referred to as an “ice dwarf”, which are very abundant beyond the orbit of Neptune • Pluto is now considered a dwarf planet, in the Kuiper belt The Kuiper belt is a disk-shaped region beyond Neptune, ranges 30 – 55 AU. • The Oort cloud (houses long-period comets) is out beyond the Kuiper belt. • • In order to be considered a planet, now, an object must: 1. orbit the Sun (not be a moon around another object); 2. be massive enough to be nearly round; 3. clear the region around its orbit (Pluto does not; many other objects there). 12. Formation of the Solar System 1. the Solar System formed 4.5 billion years ago 2. starting with cloud of gas and dust, it collapses due to its own gravity 3. as it collapses, the spinning cloud flattens and forms a disk 4. the Sun forms at center 5. "lumps" in the disk become planets 6. the regions around these proto-planetary clumps clear out (or material is absorbed) 7. only rocks and metals survive in inner region; outside, ices and gases also survive 8. takes a few tens of millions of years to form 11 13. Planet Comparison Focus on understanding the general differences between the major groups of planets — • the inner four, Terrestrial ("Earth-like" / rocky) planets: 1) Mercury, 2) Venus , 3) Earth , 4) Mars • the outer four, Jovian ("Jupiter-like" / gas giant) planets: 5) Jupiter, 6) Saturn , 7) Uranus , 8) Neptune —"and of what characteristics are common within each group. "Study the following table: Terrestrial Planets (rocky, "Earth-like"): (1) Mercury, (2) Venus, (3) Earth, (4) Mars " " general size (radius) length of day (rotation) length of year (orbital period) distance from the Sun distance from each other density Earth-sized, or smaller much larger than Earth-sized long day (often much longer than an Earth day); slow rotation short day (much less than an Earth day); fast rotation short (due to being close to Sun) long close very far pretty close spaced very far apart high (made of rocks and metals) low (primarily composed of gaseous or light elements) rings variable, but generally warmer for being closer to the Sun no (none of them) moons few or none temperatures Jovian Planets" (AKA "Gas Giants"):" (5) Jupiter, (6) Saturn, (7) Uranus, (8) Neptune very cold yes (all of them) many Often"for test questions that compare planet or moon properties, the "shortcut" to finding the best answer is to identify which family of planets four of the answer choices belong to, and then choose the one answer that is different. Here are some other questions you should be able to answer: Which planets have dense atmospheres, thin atmospheres, no atmospheres? • Which planets have strong and weak surface gravities? • Which planets have the Greenhouse Effect? • What are the colors of the planets? (see section, below, for pictures) • The surface temperatures of planets generally decrease with their distance from the Sun; however, Venus is an exception with a much higher temperature than expected. This is due to an especially strong Greenhouse effect with Venus' dense atmosphere. 12 Remember the order of the planets! Going outward from the Sun ... " 1. !Mercury 2. !Venus 3. !Earth 4. !Mars ... then the main asteroid belts ... 5. !Jupiter 6. !Saturn 7. !Uranus 8. !Neptune ... then the Kuiper Belt"—"containing several dwarf planets — objects like Pluto — and numerous short-period comets ..." ... and outside all that, the diffuse Oort cloud, with a large number of long-period comets and a large number of other small, icy objects Planets & Moons images " The test will include a few"image-identification questions (e.g., "Which object is this?"). For those questions, the images of Solar System objects will look like they did in the game when you were orbiting those objects. The following pages show example images, with hints for distinguishing between them ... 13 Mercury" —" Note the grayish color, and numerous craters." Overall it looks a lot like Earth's moon, but does not have prominent"maria, the dark "seas," like the Moon does. " " Venus"—"Bright, yellowish, and cloud-covered (no sharp surface features visible)." A clue that this is"not" a gas giant is that it does not have rings, which all gas giants (aka" "Jovian planets") in our Solar System have. " 14 Earth"— Blue seas, green and tan landmasses, bright patchy white clouds. " " Earth's moon," Luna" — Grayish and cratered, and with distinctive large dark" maria, the remains of old lava flows that covered large patches of its surface. " 15 Mars" — "The Red Planet," Mars is the color of rust." Has whitish ice caps at its poles, many craters, some prominent old volcanoes, some large valleys, but no significant surface water (lakes, oceans,"etc.). " " an asteroid,"Vesta"— Distinctly non-spherical, unlike the planets." Dull gray and pitted with craters. " 16 Jupiter" — Distinctive stormy stripes, cloud bands in different colors (red or brown, to white or even blue-ish)." Also has the Great Red Spot, an Earth-sized circular storm in its southern hemisphere. " " 17 Jupiter's moon,"Io"—" "I've seen better looking pizzas!" Sulfur volcanoes lend to its yellow color with black specks. " " Jupiter's moon,"Europa"— Pale tan/white and covered in fine cracks." Very smooth-looking compared to the more cratered moons. " " 18 Jupiter's moon,"Ganymede"— An odd marbled look, alternating dark and light areas, and some bright white spots highlighting recent cratering events. " " Jupiter's moon," Callisto" — Generally darker and more cratered than the other Galilean (largest) moons of Jupiter." The largest crater is surrounded by what resemble frozen water ripples (icy body melted from large impact, then re-froze). " 19 Saturn" —" Large" ring system." Central planet tan-colored and has long-stretched cloud bands, but less color contrast between them as on Jupiter. " " Saturn's moon," Titan" — Saturated orange color (as opposed to Venus' pale yellow)." Complete cloud cover; cannot see sharp surface features." Like with Venus, the lack of rings is a hint to help not mistake this for a gas giant. " 20 Uranus" — Distinctive deep blue-green color." Generally few visible cloud features." A wide ring system, but the individual rings are very dark and thin, with clear gaps between them (compare to Saturn's)." Also, it is tipped nearly on its side, compared to other planets. " Uranus' moon," Miranda" — Dull and gray like many moons, but with few large craters and instead an oddly-jumbled striped look, as if it had broken up and been sloppily reassembled at some point —"kind of a wrinkled look. " 21 Neptune"— Dark and deep blue in color." A few visible dark storm and cloud features." A smaller ring system than Uranus, with just a few thin dark rings. " " Neptune's moon," Triton"— Similar in some ways to Miranda, another moon with an odd mix of different colors and textures over patches of its surface." Looks more "bumpy" instead of cratered like other moons. " 22 Pluto"(on the left, with"its largest moon Charon on the right) —"a tiny "icy dwarf," it orbits the center of gravity of its system with the proportionally-large moon, Charon. "Pluto's and Charon's appearances were"revealed in high detail only"very recently,"in July 2015. " 23