Chapter 7: Our Planetary System.docx

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Astronomy & Astrophysics
Ian Shelton

Chapter 7: Our Planetary System 7.1 Studying the Solar System - Comparing the worlds to one another, seeking to understand their similarities and differences = comparative planetology. Astronomers use the term planetology broadly to include moon’s asteroids, and comets as well as planets What can we learn by comparing the planets to one another? Cosmic context –the solar system - 1. Large bodies in the solar system have orderly motions. All planets have nearly circular orbits going in the same direction in nary the same plane. Most large moons orbit their planets in this same direction, which is also the direction of the sun’s rotation - 2. Planets fall into two major categories. Small, rocky terrestrial planets and large, hydrogen- rich jovian planets o Terrestrial planets:  Small in mass and size  Close to the sun  Made of metal and rock  Few moons and no rings o Jovian planets  Large mass and size  Far from the sun  Made of H, He, and hydrogen compounds  Rings and many moons - 3. Swarms of asteroids and comets populate the solar system. vast numbers of rocky asteroids and icy comets are found throughout the solar system, but are concentrated in three distinct regions o Asteroids are made of metal and rock and most orbit the asteroid belt between mars and Jupiter o Comets are ice rick and many are found in the Kuiper belt beyond neptune’s orbit o Even more comets orbit the sun in the distant, spherical region called the Oort cloud, and only a rare few ever plunge into the inner solar system - 4. Several notable exceptions to these trends stand out. Some planets have unusual axis tilts, unusually large moons or moons with unusual orbits o Uranus rotates nearly on its side compared to its orbit and its rings and major moons share this sideways orientation o Our own moon is much closer in size to earth than most other moons in comparison to their planets The Sun - 99.8% of the solar system’s total mass, making it more than a thousand times as massive as everything else in the solar system combined - Its gravity governs the orbits of the planets - Its heat is the primary influence on the temperatures of planetary surfaces and atmospheres. It is the source of virtually all the visible light in our solar system –the moon and planets shine only by virtue of the sunlight they reflect. Charged particles flowing outward from the sun (the solar wind) help shape planetary magnetic fields and can influence planetary atmospheres. Mercury - Composition: rocks, metals - Average temp: 700 K (day) and 100 K (night) - Innermost planet of our solar system, and the smallest of the eight official planets - Desolate, cratered world with no active volcanoes, no wind, no rain, and no life - Both hot and cold extremes - High density indicates that it has a very large iron core, perhaps because it once suffered a huge impact that blasted its outer layers away Venus - Composition: rocks, metals - Average temp: 740 K and no moons - Nearly identical in size to Earth - Strange rotation: it rotates on its axis very slowly and in the opposite direction of earth, so days and nights are very long and the sun rises in the west and sets in the east instead of rising in the east and setting in the west - “sister planet” to earth - Venus has mountains, valleys and craters and shows many of past or present volcanic activity Earth - Composition: rocks, metals - Average surface temp: 290 K - Earth’s atmosphere contains just enough CO2 and water vapor to maintain a moderate greenhouse effect - The moon is surprisingly large compared with Earth, it isn’t the largest moon in the solar system Mars - Composition: rocks, metals - Average surface temp: 220 K - Moons: 2 (very small) Phobos and Deimos - Although mars is frozen today, presence of dried up riverbeds, rock strewn floodplains and minerals that form in water offers clear evidence that mars had at least some warm and wet periods in the past - Major flows of liquid water probably ceased at least 3 billon years ago, but some liquid water could persist underground perhaps flowing to the surface on occasion - Surface looks almost earth like Jupiter - Composition: mostly hydrogen and helium - Cloud top temperature: 125 K - Moons: at least 63 - Its mass is more than 300 times that of earth, and its volume is more than 1000 times that of earth. - Over dozens of moons and a thin set of rings - Most of moons are very small, but four are large enough that we’d probably consider them planets if they orbited the sun independently - These four moons –Io, Europa, Ganymede and Callisto are often called the Galilean moons because galileo discovered them shortly after he first turned his telescope toward the heavens Saturn - Composition: mostly hydrogen and helium - Cloud-top temperature: 95 K - Moons: at least 60 - Spectacular rings; only saturn’s rings can be seen easily through a small telescope - 2 geologically active moons: Enceladus and Titan Uranus - Composition: hydrogen, helium, hydrogen compounds (H20, NH3, and CH4) - Cloud top temperature: 60 K - Moons: at least 27 - Methane gas gives Uranus its pale blue-green colour. - More than two dozen moons orbit Uranus, along with a set of rings somewhat similar to those of Saturn but much darker and more difficult to see - The entire Uranus system is tipped on its side compared to the rest of the planets, which gives it the most extreme seasonal variations of any planet in our solar system Neptune - Composition: hydrogen, helium, hydrogen compounds (H20, NH3, and CH4) - Cloud top temperature: 60 K - Moons: at least 13 - Rings and numerous moons; largest moon is Triton, is larger than pluto, and its backward orbit makes it a near certainty that Triton once orbited the sun independently before somehow being captured into Neptune’s orbit Pluto - Composition: ices, rock - Average surface temperature: 40 K - Moons: 3 - Pluto’s largest moon, Charon, is locked together with it in synchronous rotation, so Charon would dominate the sky on one side of pluto but never be seen from the other side 7.2 Patterns in the Solar System What features of our solar system provide clues to how it formed? - 1. Patterns of motion among large bodies. The sun, planets, and large moons generally orbit and rotate in a very organized way - 2. Two major types of planets. The eight official planets divide clearly into groups: the small, rocky planets that are close together and close to the sun and the large, gas-rich planets that are farther apart and farther from the sun - 3. Asteroids and comets. Between and beyond the planets, huge numbers of asteroids and comets orbit the sun. The locations, orbits and compositions of these asteroids and c
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