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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.

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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.

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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)

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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!

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Star Formation

• Star formation must occur in dark (dusty) cold (T < 10 K) regions– Gas cloud can collapse

only if self-gravity exceeds internal pressure

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Star Formation

• Any net angular momentum leads to disk formation– Central region

collapses to form a star

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Star Forming Region in M 16

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Proplyds: Disk-Shaped Protostars

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Planet Formation

• As disk cools, solids condense (snowflakes)– inner disk is richer in

less-volatile elements.

• We need to understand relative abundancesand volatility of elements.

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Relative Abundance of Elements

Note that eachvertical bar

represents a factor of 100

(number of protons)

Hydrogen

Helium

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Condensation Sequence

T(K) Condensate1500 Metal oxides1300 Fe (iron), Ni (nickel)1200 Silicates1000 Aluminum oxides680 FeS (iron sulfates)175 H2O (water)

Mercury

Venus

Earth,Mars

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Condensation Sequence

T(K) Condensate175 H2O (water)150 NH3 (ammonia)120 CH4 (methane)65 Ar (argon), Ne (neon)

Jovian

Pluto

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Inner system: rocky materialOuter system: rocky/icy material

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Planet Formation• As central regions contract to form the Sun, the

“snowflakes” begin to stick together and grow by accretion.

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Planet Formation• Planetesimals form by accretion (snowballs).

– gentle collisions, since in similar orbits– planetesimals have sizes up to about 1 km

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Planet Formation• Planetesimals coalesce to form protoplanets

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Uncompressed Densities of Terrestrial Planets

PlanetDensity(gm cm-3)

Mercury 5.4Venus 4.2Earth 4.2Mars 3.3Moon 3.35

Note: density of water is 1 gm cm-3

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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

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The Final Stage: Giant Impacts

• Coalescence of large bodies can have dramatic effects– retrograde rotation of Venus, Uranus, and Pluto– origin of the Moon

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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.

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.

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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

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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

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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

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Stages of Planetary Evolution2 Cratering

– Crust modification, persisted until about 3.3 billion years ago

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Stages of Planetary Evolution3 Flooding

– Overlaps in time with cratering– Fracturing of crust leads to

flooding by lava and water

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Stages of Planetary Evolution

4 Slow surface evolution– Plate tectonics builds features– Wind and water erode them

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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)

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The surfaces of the Moon and Mercury are old surfaces with very little modification since the heavy bombardment phase.

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• 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.

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• Mars is an intermediate case– evidence of both heavy

cratering and volcanism– evidence of past water flows– clear examples of wind

erosion

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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.

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In order of increasing mass, consider:• Pluto: cold, but too small to retain anything but a thin CH4

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 CO2

atmosphere.• Venus: hot, but massive enough to retain a dense CO2

atmosphere.• Earth: warm, but massive enough to retain CO2 atmosphere,

which evolved into a thinner N2 + O2 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.

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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

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Mare Orientale, formed 3.8 billion years ago, is the youngest of thelarge lunar impactbasins. The outer ring is about 1000 km indiameter. The centralpart of the basin subsequently floodedwith lava, forming themare.

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Caloris Basin on Mercury

• Formed as a result of an enormous direct impact.

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Callisto • Jupiter’s satellite Callisto also shows the result of a large direct impact.

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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

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Low density, geologically active satellites are near their parent planets.

Io(Jupiter)

Europa(Jupiter)

Enceladus(Saturn)

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– Largest objects form in middle of system• Like Jupiter and Saturn in Solar System• Examples: Ganymede, Titan, Titania

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The largest satellites form near the middle of their respective systems.

Ganymede(Jupiter)

Titania(Uranus)

Titan(Saturn)

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Basic Facts that Must Be Explained by a Theory of Origin

• Planetary orbits:– Orbits are all in a single plane.ü– The Sun’s equator lies in this plane. ü– Planetary orbits are nearly circular. ü– Planets all revolve in the same direction. ü– Most planets and the Sun rotate in the same direction

that the planets revolve. ü– Planets have almost all of the angular momentum of the

Solar System. ü– Spacing between planets follows a regular pattern

(Bode’s Law). ?

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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. ü– Satellite systems show similar patterns: inner

satellites are dominated by less-volatile elements. ü

– Composition of meteorites differs from composition of planets. ü