1、Astro 10-Lecture 6: Formation and Structure of the Solar System,What are the properties of the solar system?How are these properties explained by theories of the formation of the solar system?Why are the planets different from one another?,Scale of the Solar System A reminder of Exercise #1,Scale Mo
2、del of the Solar System,The Sun (basketball) Mercury (tiny ballbearing at 10 yds),Scale Model of the Solar System,Venus (pinhead at 18 yds),Scale Model of the Solar System,Earth (pinhead at 25 yds) Moon (tiny ball bearing 2.4 inches away).,Scale Model of the Solar System,Mars (tiny ballbearing at 39
3、 yards),Scale Model of the Solar System,Asteroids (A few thousand specks of dust between 50 and 75 yards away from the volleyball),Scale Model of the Solar System,Jupiter (1 inch diameter at 132 yards),Scale Model of the Solar System,Saturn (0.7 inch diameter at 242 yards),Scale Model of the Solar S
4、ystem,Uranus and Neptune (0.3 inch diameter at 487 and 762 yards),Scale Model of the Solar System,Pluto & Charon (tiny ballbearings at 1000 yds),Characteristics of the Solar System (formation theory must explain all),1. All of the planets orbit in the same direction and in the same plane. (Within a
5、few degrees) This plane corresponds with the equator of the sun. Exceptions: Mercury (7 degrees), Pluto (17 degrees) 2. Most of the planets rotate in the same direction and have their equators roughly aligned with the plane of the solar system. Exception: Venus rotates in the opposite direction (ret
6、rograde). Exception: Uranus and Pluto highly tilted (90 degrees) with respect to the plane of solar system.,Characteristics of the Solar System,3. Orbits of moons around planets are in the planets equatorial plane. Exception: Earths moon rotates in plane of the solar system One of Neptunes moons orb
7、its retrograde. 4. Two (three?) types of planets Small rocky planets (Mercury, Venus, Earth, Mars) Large gaseous planets (Jupiter, Saturn, Uranus, Neptune) Tiny icy planets? (Pluto?),Terrestrial Planets,Terrestrial Planets,Small and Rocky. Crust mainly composed of Silicon and Oxygen (Silicates). Atm
8、osphere ranges from none to thick. Atmosphere,Terrestrial Planets,Near to the sun. ( 5 astronomical units) Reminder of what an astronomical unit is. Small and Rocky. (From 0.1 to 1 Me) High density (volume/mass) (3-5x the density of water). How do we measure the density? Crust mainly composed of Sil
9、icon and Oxygen (Silicates) with small fraction of Sulfates. Exception: Earth has higher percentage of carbonates in its rocks. Cores of Nickel and Iron Some solid, some liquid,Terrestrial Planets,Cratering is common on the surface of terrestrial planets.From the ages of the craters, we can tell tha
10、t impacts were much more common early in the history of the solar system,Terrestrial Planets,Atmosphere ranges from none (Mercury) to thick (Venus). Typical atmospheric components are Carbon Dioxide (CO2) and Nitrogen (N2) Exception: Earth has Oxygen (O2) and Water (H2O) in its atmosphere. Typically
11、 have no moons. Exception: Earth has a very large moon. Exception: Mars has two tiny moons.,Terrestrial Planet Interiors,Jovian (Gas Giant) Planets,Jovian (Gas Giant) Planets,Primarily composed of hydrogen (H2) and Helium (He) Massive (15 to 300 Me) Thick Atmosphere H2, He, Methane (CH4), Ammonia (N
12、H3) Low density (0.7 to 1.8x the density of water) Saturn would float! Small rocky core surrounded by huge ocean of liquid hydrogen. Is the core a terrestrial planet? All are found more distant than about 5 astronomical units from the sun.,Jovian Planets,Rings! All Jovian planets have them.,Jovian P
13、lanets,Moons! Jovian planets tend to have very many,Jovian Planets,Interior,Core,Atmosphere,Liquid Hydrogen,Liquid Metallic Hydrogen,Jovian Planets,Some (Jupiter and Saturn at least) radiate more energy than they receive from the sun. They generate energy from gravitational contraction.,Tiny Icy Pla
14、nets,Pluto? Charon? Are they really planets The moons of the Jovian planets?These are like the terrestrial planets, but instead of SiO2 they have H2O Tiny rocky core underneath the ice.,Characteristics of the Solar System,5. Three types of space debris: Asteroids: Chunks of rock between 10 meters an
15、d a thousand km in size. 20,000 of them Concentrated in the plane of the solar system between Mars and Jupiter,Characteristics of the Solar System,Comets: Chunks of ice and rock between 10 meters and a thousand km in size. Billions? of them Most reside outside the orbit of Pluto in the Oort cloud. N
16、ot concentrated into the plane of the solar system Occasionally one will fall into the inner solar system (on a very elliptical orbit).,Characteristics of the Solar System,Meteoroids: Tiny bits of rock and metal Most 1 gram Heated by atmospheric friction until they glow. Most follow along the orbits
17、 of comets Debris left behind when a comet goes by.,Characteristic of the Solar System,6. Age: The objects in the solar system are all about 4.6 billion years old. How do we know this? Radioactive dating A radioactive element decays into a daughter element. We dont know what time a specific atom wil
18、l decay but we know how long it will take for half the atoms to decay. (Demo),Radioactive Dating,U238Pb206 Halflife: 4.5 billion years Oldest earth rocks 3.96 billion years Meteors and Moon rocks 4.6 billion years This is the time they solidified The solar system is older than this.,Theory of the fo
19、rmation of the Solar System,A story the fits the facts. Needs to explain, or at least be consistent with all the characteristics that we listed. Needs to also be consistent with what we know about the rest of the galaxy. Other stars and solar systems should form in the same way.,The Solar Nebula The
20、ory,The sun and solar system formed from the collapse of a cloud of gas and dust. The cloud was slowly rotating, so centrifugal force made it into a disk (accretion disk) transferring matter to the center. Conservation of angular momentum made it rotate more quickly,The Solar Nebula Theory,Instabili
21、ties in the disk may have formed smaller sub-disks where giant planets formed,The Solar Nebula Theory,Dust, rock and ice condense and stick together to make small bodies called planetesimals. Heat from the forming sun only allowed certain elements to condense nearby. Ices could only condense far awa
22、y.,The Solar Nebula Theory,Two ways of building planets Larger planetesimals attract smaller ones. They collide and merge to make a bigger planetesimals. These attract more and eventually form the planets Near the sun, the nebular hydrogen gas is too hot (moving to fast) to form an atmosphere around
23、 the planets. Distant planets begin to form hydrogen atmospheres once they get big enough. The Jovian planets captured their atmospheres. As time goes on the nebula cools, making the “frost line” move inward.,The Solar Nebula Theory,The young planets start out fairly warm (in a liquid or nearly liqu
24、id state). Heavy elements start to sink This concentrates the radioactive elements in the center (and explains why the earths core is hot).This process is still occurring in the giant planets. It releases gravitational energy.,Differentiation,The Solar Nebula Theory,On the terrestrial planets, gasse
25、s are released from the hot interior to form atmospheres. Volcanic processes release H2S, SO2, CO2, H2O, NH3, N2 Solar UV radiation breaks apart NH3, hydrogen escapes, leaving N2 Solar UV radiation also breaks apart H2O, hydrogen escapes leaving O, which reacts with rock to form solid oxides. H2O co
26、mbines with H2S, and SO2 to make sulfuric and sulfurous acid. This eats away rock to form solid sulfates. Whats left? CO2 and N2 in the atmosphere. Oxygen and Sulfur in the rocks. (Why is Earths atmosphere different?),Where did the nebula go?,Solar wind, heat, and light pressure drove the gas away.
27、What about the left over planetesimals? Most of the rocky ones in the inner solar system eventually collided with planets. (Thats why the rate of impacts was high 4 Gya, but is low now.) Theres about 20,000 left over mostly between Mars and Jupiter (Asteroids!) Jupiters gravity prevented a planet fr
28、om forming there. Encounters with the giant Jovian planets kicked most of the remaining icy ones into the outer solar system or interstellar space These are comets! The encounters would kick them in any direction. (This explains why comets arent concentrated in the plane of the solar system.),How do
29、es this theory fit the characteristics of the Solar System?,1. & 2. Collapse to a disk explains the concentration in the plane of the solar system, and why almost everything moves in the same direction. 3. The giant planets had disks of their own so their moons orbit in their equatorial plane 4a. Be
30、cause the inner solar system was hot, only rock and metal could condense which resulted in terrestrial planets 4b. The outer solar system was cold enough for ices to condense and for hydrogen gas to be captured by a massive enough body. This resulted in Jovian planets. 4c. If an object in the outer
31、solar system wasnt massive enough to capture hydrogen gas, it remained as a small icy body. (Pluto, the outer planet moons, comets),How does this theory fit the characteristics of the Solar System?,4d. The terrestrial planets released their atmospheres from their interiors. The Jovian planets captur
32、ed theirs. The icy planets werent massive enough to capture one, or hot enough to release one. 4e. The inner structure of the planets is explained by differentiation. Heavier elements sink to the core. Lighter ones float to the surface. 5. Asteroids and comets are left over planetesimals. Meteors ar
33、e bits of dust that have fallen off of comets 6. Everything is the same age because it all formed at about the same time.What about the exceptions?,For every exception there is a rule.,Tilted orbits of Mercury and Pluto. Mercury probably suffered a large impact late in its formation Pluto might be a
34、 left-over planetesimal. Retrograde rotation of Venus: Probably due to a large impact late in formation. Probability favors, but does not require, rotation in the same direction as the orbit. High axial tilt of Uranus and Pluto: Also likely to be due to a large impact Also, in the outer solar system
35、, computer models suggest the nebula was less concentrated in the plane, which could result in large tilt of sub-disks.,For every exception there is a rule.,Retrograde moon of Neptune. Probably a captured planetesimal. Oxygen in the atmosphere of earth. Earths atmosphere is highly modified by life. Earths moon orbits in the plane of the solar system. This is likely because the moon was formed from an impact with another body traveling in the plane of the solar system.,
