Formation of the Solar System

Transcription

1 Formation of the Solar System Any theory of formation of the Solar System must explain all of the basic facts that we have learned so far. 1 The Solar System The Sun contains 99.9% of the mass. The Solar System is mostly empty space. The Solar System is a flattened disk. All planets revolve in the same direction Most planets also rotate in the same direction All objects have similar ages (about 4.6 billion years, when measurable). Planets belong in one of two families. 2 1

2 Two Types of Planets Terrestrial planets low mass ( 1 M ) high density (rocky, metallic) slow rotators (P 24 hours) few satellites close to Sun (a 1.6 AU) Jovian planets high mass ( 15 M ) low density (gaseous) rapid rotators (P 18 hours) many satellites far from Sun (a 5 AU) 3 Star Formation Current belief is that planets are formed as part of the star formation process. star formation itself is not understood (in a mathematical predictive sense) if this scenario is correct, planets should be common throughout the Universe Number of known exoplanets now exceeds 100! 4 2

3 Star formation must occur in dark (dusty) cold (T < 10 K) regions Gas cloud can collapse only if self-gravity exceeds internal pressure Star Formation 5 Any net angular momentum leads to disk formation Central region collapses to form a star Star Formation 6 3

4 Star Forming Region in M 16 7 Proplyds: Disk-Shaped Protostars 8 4

5 Planet Formation As disk cools, solids condense (snowflakes) inner disk is richer in less-volatile elements. We need to understand relative abundances and volatility of elements. 9 Hydrogen Relative Abundance of Elements Helium Note that each vertical bar represents a factor of (number of protons) 5

6 11 Condensation Sequence T(K) Condensate 1500 Metal oxides 1300 Fe (iron), Ni (nickel) 1200 Silicates 1000 Aluminum oxides 680 FeS (iron sulfates) 175 H 2 O (water) Mercury Venus Earth, Mars 12 6

7 Condensation Sequence T(K) Condensate 175 H 2 O (water) Jovian 150 NH 3 (ammonia) 120 CH 4 (methane) Pluto 65 Ar (argon), Ne (neon) 13 Inner system: rocky material Outer system: rocky/icy material 14 7

8 Planet Formation As central regions contract to form the Sun, the snowflakes begin to stick together and grow by accretion. 15 Planet Formation Planetesimals form by accretion (snowballs). gentle collisions, since in similar orbits planetesimals have sizes up to about 1 km 16 8

9 Planet Formation Planetesimals coalesce to form protoplanets 17 Uncompressed Densities of Terrestrial Planets Density Planet (gm cm -3 ) Mercury 5.4 Venus 4.2 Earth 4.2 Mars 3.3 Moon 3.35 Note: density of water is 1 gm cm

10 Accounting for Jovian Planets In the outer disk, ices form larger masses can develop since more solids In disk, if M > 15 M, gravitational accretion works Jovian planets accrete H, He get big Perturbations by biggest planets prevents coalescence nearby asteroids 19 The Final Stage: Giant Impacts Coalescence of large bodies can have dramatic effects retrograde rotation of Venus, Uranus, and Pluto origin of the Moon 20 10

11 Probable Origin of the Moon Theory that best explains properties of Earth and Moon is giant impact between early Earth and a Mars-sized object in a similar orbit. 21 The Final Stage: Giant Impacts Coalescence of large bodies can have dramatic effects violent geological history of some planetary satellites Miranda, the fifth largest satellite of Uranus, shows evidence of violent collisions late in its formation history. 11

12 Cleaning Up the Debris In later stages, many smaller particles are removed from the Solar System swept up by protoplanets heavy bombardment phase, about 4 billion years ago radiation pressure dust particles smaller than about 0.1 µm (10-7 m) are pushed out of the Solar System by photons from the Sun 23 Cleaning Up the Debris Solar wind gas and dust are entrained by the solar wind Gravitational scattering Icy bodies in the outer Solar System (10-30 AU) have their orbits altered by Jovian planets Smaller orbits evaporate Larger orbits become comets 24 12

13 Stages of Planetary Evolution 1 Differentiation Radioactivity heats the interior Heating causes iron to melt Iron sinks to center, releasing more heat Interior separates by density 25 Stages of Planetary Evolution 2 Cratering Crust modification, persisted until about 3.3 billion years ago 26 13

14 Stages of Planetary Evolution 3 Flooding Overlaps in time with cratering Fracturing of crust leads to flooding by lava and water 27 Stages of Planetary Evolution 4 Slow surface evolution Plate tectonics builds features Wind and water erode them 28 14

15 Surfaces of planets are a result of competing internal mechanisms (volcanism, plate tectonics) and external mechanisms (cratering due to bombardment). Internal mechanisms dominate for larger bodies (e.g., Earth, Venus) External mechanisms dominate for smaller bodies (e.g., Moon, Mercury) 29 The surfaces of the Moon and Mercury are old surfaces with very little modification since the heavy bombardment phase

16 The surfaces of Venus and the Earth have been significantly modified by volcanism. Plate tectonics and water erosion are important on Earth. Venus is a little too small and rotates to slowly for plate tectonics to be very important, and too hot for liquid water to exist. 31 Mars is an intermediate case evidence of both heavy cratering and volcanism evidence of past water flows clear examples of wind erosion 32 16

17 More massive, colder bodies will better retain atmospheres. More massive bodies have higher escape velocities. At higher temperatures, atoms and molecules are moving faster. Lightest particles are moving the fastest and are thus hardest to retain. 33 In order of increasing mass, consider: Pluto: cold, but too small to retain anything but a thin CH 4 atmosphere. Moon: too small and (periodically) hot to retain anything. Mercury: too small and hot to retain anything. Mars: cold, but too small to retain anything but a weak CO 2 atmosphere. Venus: hot, but massive enough to retain a dense CO 2 atmosphere. Earth: warm, but massive enough to retain CO 2 atmosphere, which evolved into a thinner N 2 + O 2 atmosphere on account of liquid water and plant life. Jovian planets: cold and massive enough to retain H and He atmospheres, which is why they are gaseous, massive bodies

18 The most unusual features of the Solar System are likely attributable to giant impacts. Relatively large satellites of small planets Earth-Moon: non-equatorial orbit of Moon, gross differences in surface compositions. Pluto-Charon: retrograde motion of system Neptune-Triton: retrograde orbit of Triton Retrograde rotation of large bodies Uranus, Venus Other evidence: impact basins on Moon, Mercury, Callisto 35 Mare Orientale, formed 3.8 billion years ago, is the youngest of the large lunar impact basins. The outer ring is about 1000 km in diameter. The central part of the basin subsequently flooded with lava, forming the mare

19 Caloris Basin on Mercury Formed as a result of an enormous direct impact. 37 Callisto Jupiter s satellite Callisto also shows the result of a large direct impact

20 Satellites of Jovian planets show patterns consistent with our ideas about formation of the planets. Inner satellites form under warmer conditions Densities imply lower ice content Show evidence of volcanism (in this case driven by tides) and resurfacing Examples: Io, Europa, Enceladus 39 Low density, geologically active satellites are near their parent planets. Io (Jupiter) Enceladus (Saturn) Europa (Jupiter) 40 20

21 Largest objects form in middle of system Like Jupiter and Saturn in Solar System Examples: Ganymede, Titan, Titania 41 The largest satellites form near the middle of their respective systems. Ganymede (Jupiter) Titan (Saturn) Titania (Uranus) 42 21

22 Basic Facts that Must Be Explained by a Theory of Origin Planetary orbits: Orbits are all in a single plane. The Sun s equator lies in this plane. Planetary orbits are nearly circular. Planets all revolve in the same direction. Most planets and the Sun rotate in the same direction that the planets revolve. Planets have almost all of the angular momentum of the Solar System. Spacing between planets follows a regular pattern (Bode s Law).? 43 Basic Facts that Must be Explained by a Theory of Origin Planetary composition Varies with distance from Sun; metal-rich near the Sun, Hydrogen-rich farther away. Satellite systems show similar patterns: inner satellites are dominated by less-volatile elements. Composition of meteorites differs from composition of planets