Download Formation of the Solar System The Solar System

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 abundances
and volatility of
elements.
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Relative Abundance of Elements
Hydrogen
Helium
Note that each
vertical bar
represents a
factor of 100
10
(number of protons)
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Condensation Sequence
T(K)
1500
1300
1200
1000
680
175
Condensate
Metal oxides
Mercury
Fe (iron), Ni (nickel)
Silicates
Aluminum oxides
Venus
FeS (iron sulfates)
Earth,
H2O (water)
Mars
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6
Condensation Sequence
T(K)
175
150
120
65
Condensate
H2O (water)
Jovian
NH3 (ammonia)
CH4 (methane)
Pluto
Ar (argon), Ne (neon)
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Inner system: rocky material
Outer 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
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-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.
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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 Evolution
2 Cratering
– Crust modification,
persisted until about
3.3 billion years ago
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Stages of Planetary Evolution
3 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 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.
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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)
Enceladus
(Saturn)
Europa
(Jupiter)
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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)
Titan
(Saturn)
Titania
(Uranus)
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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
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