1 Formation of Our Solar System Image: Lunar and Planetary Laboratory: 1

2 Some data to explain: 1.Planets isolated 2.Orbits ~circular / in ~same plane 3.Planets (and moons) travel along orbits in same direction…. same direction as Sun rotates (CCW) Lunar and Planetary Institute image at Venus slowly rotates CW Uranus on its side Pluto on its side – captured asteroid? Moons go CCW around planets (few exceptions) 2

3 Solar System is highly differentiated Terrestrial planets –Slow rotators, few or no moons Gas Giants –Fast rotators, many moons Asteroids –Old –Different from rocky or gaseous planets Comets –Old, icy –Do not move on same plane as planets 3

4 Planets, most moons, and asteroids revolve around the Sun in the same direction (CCW) They all move in ~ circular orbits Pluto-special case –Orbit is highly inclined (18°) – oval shape 4

5 Some more data to explain: 4.Most planets rotate in this same direction NASA images edited by LPI Mercury 0° Venus 177° Earth 23° Mars 25° Jupiter 3° Saturn 27° Uranus 98° Neptune 30° 5

6 And some more data to explain: 5.Solar System highly differentiated: Terrestrial Planets (rocky, dense with density ~4-5 g/cm3) Jovian Planets (light, gassy, H, He, density ) Images: Lunar and Planetary Laboratory: 6

7 How Did We Get a Solar System? Huge cloud of cold, thinly dispersed interstellar gas and dust (mostly H & He) Hubble image at Image: LPI Active region of Star formation in the Large Magellanic Cloud (LMC) – satellite galaxy of Milky Way (Hubble) 7

8 Concentrations of dust and gas in the cloud; material starts to collect (gravity > magnetic forces) How Did We Get a Solar System? Hubble image at Image: LPI 8

9 How Did We Get a Solar System? Gravity concentrates most stuff near center Heat and pressure increase Collapses – central proto-sun rotates faster (probably got initial rotation from the cloud) Image: LPI 9

10 How Did We Get a Solar System? NASA artwork at Rotating, flattening, contracting disk - solar nebula! Equatorial Plane Orbit Direction 10

11 After ~10 million years, material in center of nebula hot enough to fuse Hydrogen (H) “...here comes the Sun…” How Did We Get a Solar System? NASA/JPL-Caltech Image at 11

12 How Did We Get a Solar System? Hubble photo at Metallic elements (Mg, Si, Fe) condense into solids at high temps. Combined with Oxygen to make tiny grains Lower temp (H, He, CH4, H2O, N2, ice) - outer edges Planetary Compositions 12

13 How Did We Get a Solar System? Inner Planets: Hot – Silicate minerals, metals, no light elements, ice Begin to stick together with dust  clumps Image: LPI 13

14 How Did We Get a Solar System? Outer Solar System Cold – ices, gases – 10x more particles than inner May have formed icy center, then captured lighter gases (Jupiter and Saturn first? Took H and He?) Leave C,O, and N for the others Image: LPI 14

15 Terrestrial planets –Heavier elements stable at higher temperature –Condensed in inner nebula Gas giants –Lighter elements (H, He, C, O, N) stable at lower temperature –Condensed in outer nebula 15

16 Where do Comets Originate? 16

17 Orbital paths of comets –Highly elliptical (oval-shaped) –1 complete orbit is called a period –Short-period comets Revolve around the Sun less than 200 yrs E.g. Comet Halley Paths are close to the same plane of orbit as planets Orbit is the same direction as the Sun Originate from the Kuiper belt 17

18 Long-period comets –Longer than 200 years to go around once –Orbital path is random Direction and plane of orbit –E.g. Comet Hale-Bopp –Originated in Oort cloud Spherical cloud, 20 trillion miles beyond the Sun 18

19 How Did We Get a Solar System? Accretion - particles collide and stick together … or break apart … gravity not involved if small pieces Form planetesimals, up to a few km across Image: LPI 19

20 How Did We Get a Solar System? Gravitational accretion: planetesimals attract stuff Large protoplanets dominate, grow rapidly, clean up area ( takes ~10 to 25 My) Image: LPI 20

21 How Did We Get a Solar System? Early burst of solar wind - sweeps debris out of system Gravitational accretion of gas for protoplanets in the coolest nebular parts Image: LPI 21

22 Smaller protoplanets (inner solar nebula) –Unable to accrete gas because of their higher temperature –Obtain their atmospheres from the impact of comets Largest protoplanets (outer solar nebula) –Accrete gas because of their cooler temperature –Strongly influence the orbits of the remaining comets Either send them out to the Oort cloud or Send them inward where they collide with the terrestrial planets 22

23 The Asteroid Belt ? Should have been a planet instead of a debris belt? Jupiter kept it from forming How Did We Get a Solar System? Eros image at 23

24 Beyond the Gas Giants - Pluto, Charon and the Kuiper Belt objects Chunks of ice and rock material Little time / debris available to make a planet – slower!! How Did We Get a Solar System? Taken from Hubble Telescope Charon is Pluto’s moon, only a Little smaller than Pluto Pluto’s surface temp. is as low as -400° F From the surface of Pluto, the Sun looks like a very bright star 24

25 Early in the Life of Planets Planetesimals swept up debris Accretion + Impacts = HEAT Eventually begin to melt materials Iron, silica melt at different temperatures Iron sank – density layering Image from LPI: 25

26 Mercury Average density of 5430 kg/m 3 Second highest density of all planets Like Earth, has an Iron core –2/3 to ¾ of the radius of the planet! –Iron-Nickel core 26

27 Venus Composition ~ to Earth Crust km thick Mantle Core – Iron-Nickel Average density is 5240 kg/m 3 27

28 Earth Crust, mantle, and core Crust –~ 30 km thick for land (granite) –~ 5 km for oceanic crust (basalt) Mantle Core, Iron-Nickel –Liquid outer core –Inner solid core Average density ~ 5520 kg/m 3 28

29 Mars ~ ½ the diameter of Earth Crust Mantle Core, –Iron-Nickel – and Iron sulfide Density ~ 3930 kg/m 3 29

30 Pluto Structure not very well understood Surface is covered with methane ice Surface temp ~ 400° F Frozen methane shows a bright coloration Density ~ 2060 kg/m 3 –This low of a density suggests that the planet must be a mix of rock and ice 30

31 Planetary Interiors Differentiation Separation of homogenous interior into layers of different compositions Early – hottest time – dense iron-rich material  core Releases additional heat Leaves mantle with molten ocean enriched in silica Crust eventually forms from lightest material Image from LPI: 31

32 Planetary Interiors Differentiation Continues! Radioactive decay = primary heat source Partial melting of mantle material  rising magma  volcanoes / lava flows Image from LPI: 32