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Structure of the Solar System
Where and why it is what it is
Laws of motion
 Planets move around Sun
 Not always a given,
 Anthropic Earth-centered Ptolomaic cosmology
 Copernicus published his seminal work on his
deathbed (1543)
 A case of publish and perish
 De revolutionibus orbium celestium
 Conservation of angular momentum
 v1r1 = v2r2 = constant (for constant mass)
 The two body problem
Kepler’s Laws
 Planets move around the Sun in elliptical
orbits, with Sun as one of the foci
 A radius vector sweeps out equal area in
equal time
 Squares of the periods of the revolutionof
the planets are proportional to the cubes of
their distance from the Sun
Titius-Bode Law
 Distances of planets from Sun
0.4, 0.7, 1.0, 1.6, 2.8, 5.2, …
Can be formulated
R = 0.4 + 0.3k
K = 0, 1, 2, 4, 8, 16, 32
0.4, 0.7, 1.0, 1.6, 2.8, 5.2, …
 Titius 1729-1776, Bode 1747-1826
Titius-Bode Law
Planet missing between Mars and Jupiter
At 2.8 au
 Ceres discovered in 1801 at 2.77 au
Pallas, Juno, Vesta by 1804
 Exploded planet
No common origin point
 Failed planet
Titius-Bode Law
 Okay for Uranus, not so good for
Neptune (38 predicted vs 30 actual au)
 No other correlation with planetary
properties
Secondary effect after formation
Related to stable resonances of orbital
periods
Planets have moved
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Asteroids
Vesta, Ceres, Moon
 Total mass less than 5% of Moon
1-2 Million asteroids with size > 1km
 Asteroid belt
Gaps/concentrations due to resonances
with Jupiter (Kirkwood Gaps)
Gaps at 2:1 (3.28 au) and 3:1 (2.50 au)
Concs at 1:1 3:2 (3.97 au) 4:3 (4.2 au)
Orbital resonances
 Fractional orbital periods have greater
orbital stability to perturbation
Constructive or destructive interference
Gaps or concentrations
1:1
2:1
3:2
Asteroids
 Resonances and gaps
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 Trojan Asteroids
Lagrange points
Gravitation = centripetal
L4 and L5 ± 60°
Equal gravity to Jup & Sol
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L1, L2, L3 unstable; L4,L5 stable
Asteroids
Asteroids
 Several hundred thousand discovered
 26 > 200 km
 Solid rock bodies
 Rubble piles
 Visits by NEAR, Hayabusa
 NEAR landed on Eros
 Hayabusa landed on Itokawa
 Plus flybys of other missions on way to Jupiter
Asteroid Spectral Classes
 Definition
 Based on light reflectance (Albedo)
 Spectral features
 Spectral shape
 Mineralogical features
 e.g. olivine, pyroxene, water, …
 Chapman 1975
 3 types
(C-carbonaceous, S-stony, and U)
 Tholen 1984
 used spectra 0.31-1.06 µm
 Types A-X (23)
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Mathilde
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Spectral Class
 C-type (Most abundant 75 %)
 Low albedo (0.03-0.10)
 Strong UV absorption below 0.4 µm
 Longer wavelengths featureless
 Reddish
 Water feature at 3 µm
 Type 10-Hygeia
 4th largest asteroid
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Spectral Class
Ida + Dactyl
 S Class (17%)
Moderately bright
Albedo 0.10-0.22
Metallic Fe-Ni + magnesium silicate
Spectrum has steep slope < 0.7µm
Absorption features around 1 and 2 µm
Largest is 15 Eunomia (330 km diam)
Spectral Class
 M class (3rd abundant)
Metallic Fe-Ni
Moderately bright (0.10-0.18)
Spectrum is flat to reddish
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16 Psyche
Absorption features at 0.55 and 0.75 µm
16 Psyche (330 km)
Asteroids
 Compositional trends?
Igneous inside 2.8 au (S class)
Metamorphic around 3.2 au (M class)
Primitive outside 3.4 au (C class)
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Origin of asteroid belt
 Failed planet
Meteorites
Iron meteorites from core
Pallasites show mantle olivine
Igneous achondrites
Crustal carbonaceous chondrites
But not from single body
Oxygen isotopes, chemistry
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Origin of asteroid belt
 Planetoids form in early SS
Coalesce to form planets
 Presence of Jupiter
Pumped up the eccentricities
Limits growth
Many small bodies
 No planet at 2.8 au
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Near-Earth asteroids
 Apollos, Atens and Armors
 Few thousand > 1km
107 10-100m
 1036 Ganymed, 433 Eros
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 Source of meteorites?
Eros could survive 50-100 Myr
5% chance of hitting Earth
Spectrophotometric Paradox
 Most common meteorites are chondrites
Parent body apparently absent
 3628 Boznemcová
8km body with Ord-chondrite spectrum
 Of 35 NEA, 6 have Ord-chondrite spectra
Plus 10% of Main Belt asteroids of size ≈1km
 Chondrites dominate meteorites,
But not asteroids
Asteroids to Meteorites
 Relative frequency of meteorites
depends on efficiency of delivery
Meteorites unlikely to be sourced from
deep within asteroid belt
Asteroids must be close to resonances to
supply meteorites into Earth-crossing orbit
 6 Hebe near 3:1(2.50 au)
Source of H-Chondrites + IIE Irons
Missing Olivine Meteorites
 Iron Meteorites
 Cores
 Pallasites
 Core-mantle
 Achondrites, Chondrites
 Crust
 Where’s the mantle olivine?
Individual asteroids
 1 Ceres
Largest 933 km diameter
2.7 g/cm3
2.77 au
C class
9/13 largest asteroids similar
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Individual asteroids
 4 Vesta
Irregular shape (460 km across)
3.7 g/cm3
Intact differentiated crust (basalt)
Source of HED meteorites (4.560 Gyr)
460 km crater, 13 km deep
Two more
large craters (100 km+)
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Individual asteroids
 433 Eros
S class
2nd largest NEA
33x13x13 km
Density 2.5 ± 0.8 km
Coherent rather than
rubble pile
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Individual asteroids
 NEAR Lands on Eros - 2001
Boulders on surface from 250 m
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5m
Individual asteroids
 25143 Itokawa (1998)
S class
Hayabusa (Muses-C)
500 m long
2.0 g/cm3
Rubble pile
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Individual asteroids
 Visits to Mathilde, Gaspra, Ida
Ida has satellite (Dactyl)
NEAR Mission
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Interplanetary dust
 Sources
Asteroids (5 km/s)
Comets (20-60 km/s)
Interstellar grains?
 10,000 tons/year to Earth
Fluffy grains can survive
atmospheric entry
Many carbonaceous
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Moving Giant Planets
 Jupiter moved sunwards depleting
asteroid belt beyond 4 au
Saturn, Uranus, Neptune move out
Saturn now in 2:1 resonance with Jupiter
Produced by bombardment of centaurs
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Centaurs
 Between Saturn and Uranus
 2060 Chiron - 1977
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 182 km
 Dark-grey-black object (albedo 0.1)
 Similar in size and colour to Phoebe (Sat Moon)
 Orbit 8.5 - 19 au
 Fits definition of comet
 5145 Pholus - 1992
 185 km, red
 Nessus, Asbolus, Chariklo
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Moving Giant Planets
 Neptune plows into and depletes inner
zone of Kuiper Belt (30-35 au)
Pluto swept into a 3:2 orbital resonance at
high eccentricity and inclination
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Moving Giant Planets
 can throw KBO out to the Oort Cloud
 Only few % retained from Jupiter
 Rest lost
 5-10% from Saturn
 10-40% from Uranus
 40% from Neptune
 Can throw out Rocky and Icy bodies
 Oort cloud primitive?
 Throws objects in
 The late heavy bombardment for inner SS
Solar System
 Dynamic
Many time scales
4 Vesta has survived 4.56 Gyr
But Exposure ages of HED meteorites 5-80 Myr
Survival time of some asteroids
50,000 years
Near Earth Asteroid Orbits
 http://neo.jpl.nasa.gov/orbits/