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  v 1 r 1 = v 2 r 2 = 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 = k  K = 0, 1, 2, 4, 8, 16, 32  0.4, 0.7, 1.0, 1.6, 2.8, 5.2, …  Titius , Bode

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

Asteroids  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) Vesta, Ceres, Moon

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

Asteroids  Trojan Asteroids  Lagrange points  Gravitation = centripetal  L4 and L5 ± 60°  Equal gravity to Jup & Sol L1, L2, L3 unstable; L4,L5 stable

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 µm  Types A-X (23)

 C-type (Most abundant 75 %)  Low albedo ( )  Strong UV absorption below 0.4 µm  Longer wavelengths featureless  Reddish  Water feature at 3 µm  Type 10-Hygeia  4th largest asteroid Mathilde Spectral Class

 S Class (17%)  Moderately bright  Albedo  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) Ida + Dactyl Spectral Class

 M class (3rd abundant)  Metallic Fe-Ni  Moderately bright ( )  Spectrum is flat to reddish  Absorption features at 0.55 and 0.75 µm  16 Psyche (330 km) 16 Psyche

Asteroids  Compositional trends?  Igneous inside 2.8 au (S class)  Metamorphic around 3.2 au (M class)  Primitive outside 3.4 au (C class)

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

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

Near-Earth asteroids  Apollos, Atens and Armors  Few thousand > 1km  m  1036 Ganymed, 433 Eros  Source of meteorites?  Eros could survive 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

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+)

Individual asteroids  433 Eros  S class  2nd largest NEA  33x13x13 km  Density 2.5 ± 0.8 km  Coherent rather than rubble pile

Individual asteroids  NEAR Lands on Eros  Boulders on surface from 250 m 5 m

Individual asteroids  Itokawa (1998)  S class  500 m long  2.0 g/cm 3  Rubble pile Hayabusa (Muses-C)

Individual asteroids  Visits to Mathilde, Gaspra, Ida  Ida has satellite (Dactyl) NEAR Mission

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

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

Centaurs  Between Saturn and Uranus  2060 Chiron  182 km  Dark-grey-black object (albedo 0.1)  Similar in size and colour to Phoebe (Sat Moon)  Orbit au  Fits definition of comet  5145 Pholus  185 km, red  Nessus, Asbolus, Chariklo

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

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 