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Belt Asteroids THE INNER SOLAR SYSTEM Chapter 9 - Asteroids Chapter 9 - All 1 2 WHO CARES? MAIN BELT e-a & i-a 9SOLAR SYSTEM FORMATION 9CONTINU ING EVOLUTION ν 9ASTEROID 6 STRENGTHS 9EARTH IMPACT HAZARD Eos 4 Asteroids: Gaps & Resonances Distribution and Orbits of Main- •Astronomer Daniel Belt Asteroids Kirkwood (1886) noticed • Most asteroids found between Mars and Jupiter that the Main Belt has “gaps” in which asteroids • Distribution in the main-belt not uniform are “missing”. – strongly influenced by resonances with Jupiter (Kirkwood Gaps) • The Kirkwood Gaps are “locations” where • Collisional disruption of larger bodies long ago has left resonances with Jupiter’s physical and dynamical asteroid “families” orbit occur; i.e. where – Eos, Hirayama, Themis and Koronis are major groups gravitational disturbances – Asteroids in each family show similar spectra by Jupiter are the strongest. • Protective zones near Lagrange points of Jupiter heavily • May explain why there is populated with Trojan asteroids no planet there: Jupiter – Mars also has five known Trojan asteroids (Neptune has six) only allowed small bodies to coalesce and prevented • Typical main-belt orbit stable on timescales of Ga a larger planet from • Separation of asteroids 1 km and larger ~5 million km! forming. 5 1 Formation and History of the Asteroids: Size Distribution Asteroid Belt • The number of asteroids of a given diameter D is • Main-belt asteroids believed to be in orbits stable for long 2 periods, but higher relative energies than original primordial proportional to 1/D orbits – Collisional distribution • For example: • Very little mass today (fraction of Earth’s mass), but likely – 3 > 500 km much more originally (comparable to Earth) – 13 > 250 km 1. Most material ejected from the solar system by Jupiter or driven into – Hundreds > 100 km the Sun (Iron overabundance observed in other stars?) – 10,000+ > 10 km (?) 2. Average mass density in SS shows discontinuity at MB Total: 1,000,000 > 1 km (?) • Asteroids are plantesimals which never were able to grow – Most of the mass is in the larger via collisional accretion because Jupiter increased largest few asteroids their average impact velocity early in the SS, turning accretional events into disruption events. – Total mass of all asteroids is only ~5-10% mass of the Moon 7 DAVIS SIZE DISTRIBUTIONS – MAIN Size Distribution BELT −ς ⎛ R ⎞ ⎜ ⎟ R is the radius, N0dR N(R) = N0 ⎜ ⎟ ⎝ R0 ⎠ is the number of asteroids between R, R + dR The power law form of the size distribution is explained by collisional evolution. Theory suggests that populations will evolve to ζ~3.5 when disruption is self-similar. The steep slope implies that most of the mass is in the large bodies, though most of the surface area is in the small ones. 9 Radar Observations of 4179 Toutatis Radar observations of Toutatis Asteroid Sizes I Determination of asteroid sizes done via: 1. Assume albedo; measure brightness and distance and get geometric surface area 2. Measure visible and IR flux at the same time; ratio gives albedo since visible light depends on albedo and IR on (1-A). Also need thermal model of asteroid 3. Occultations by Stars 4. Direct measurement by Spacecraft (NEAR, Galileo) 5. Direct imaging with Adaptive optics 6. Radar observations 11 2 13 14 Ceres as observed by Hubble, 2004 Spacecraft measurements Near-Earth Asteroids (NEAs) • Smaller asteroids “escape” main-belt from resonances • Classes of NEAs based on orbits – Amor (()outside Earth’s orbit) – Apollo (cross Earth’s orbit, a>1 AU) – Aten (cross Earth’s orbit, a<1 AU) – Atira (inside Earth’s orbit) • Potentially Hazardous Asteroids (PHAs) : minimum orbital distance with Earth’s orbit < 0.05 A.U. 15 16 NEA TYPES NEA TYPES AMOR APOLLO 1.017 AU < q < 1.3 AU q < 1.017 AU •Amor • Apollo a > 1.0 AU EARTH EARTH 3 NEA TYPES Origin of NEAs ATEN • NEAs are continually injected into NEA region Q > 0.983 AU from main-belt • Aten a < 1.0 AU • Collisions between larger asteroids produce fragments which evolve due to radiation forces and weak resonances until reaching a maj“”hth(tk10jor resonance “escape” hatch (takes 107- 8 EARTH 10 years depending on size of daughter asteroids) • Orbit then rapidly evolves – Some are ejected from solar system before becoming NEAs – Some become NEAs • May eventually hit a planet, get ejected or be driven into the sun 20 Yarkovsky Effect Yarkovsky Effect • Asymmetric re-radiation of thermal energy for a rotating body • Uncertainties: – Thermal properties, The evening hemisphere radiates albedo extra energy and momentum because it is hotter than the morning – Spin rate (changes in spin hemisphere. For prograde rotation, rate) the net force is forward. 8 ⎛σT 4 ⎞⎛ ΔT ⎞ 2 ⎜ ⎟ FY = πs ⎜ ⎟⎜ ⎟cosζ 3 ⎝ c ⎠⎝ T ⎠ Diurnal Yarkovsky: net force opposite the Seasonal Yarkovsky: net force opposite 1/ 2 afternoon direction. Prograde rotator the “summer” hemisphere. Makes the ⎛ P ⎞ Burns et al (1979) ΔT ≈ (1−δ )γS⎜ ⎟ spirals out; retrograde rotator spirals in. object spiral inwards. ⎝ 2π ⎠ 21 22 Bottke et al. (2001) Meteorite parents Light Curves Largest NEAs • Shape changes tend to produce double peaked curves, Tunguska-like objects whereas albedo variations tend to produce single peaked curves. • Light curves at different position angles can be used to 1. K=0.002 W/mK 2.K=0020.02 W /mK determine the pole position and sense of rotation (which 3. K=0.2 W/mK may have two components, like a badly thrown football). 4. K=2 W/mK 5. K=40 W/mk • Comets tend to have higher amplitude variations and slower rotation. • Trojan asteroids tend to show larger amplitudes, Mean change in a of inner main belt asteroids over their collisional lifetimes suggesting that they are more elongated than main belt versus radius, for 5 thermal conductivities. Low K is dominated by diurnal asteroids. Yarkovsky: high K by seasonal. 24 Bottke et al. (2001) 4 25 Asteroid Physical Structure Asteroids: Geology • Many asteroids believed to be collections of • ~12 asteroids visited up close by spacecraft: a few large fragments and smaller debris – 951 Gaspra: Galileo flyby in 1991 bound together as a “rubble-pile” – 243 Ida: Galileo flyby in 1993 Evidence: – 253 Mathilde: NEAR flyby in 1997 1. Rotation rates of most asteroids sharply cut off at critical value – 433 Eros: NEAR orbital mission in 2000-2001 (2. 2 h) imp ly ing no tens ile s treng th to o bjec ts – 25143 Itokawa: Hyabusa sample-return (?) mission in 2005 2. Densities lower than meteorites suggesting large amounts of – 5535 Annefrank: Stardust in 2002 empty space – 4 Vesta by Dawn in 2011 3. Models of early solar system formation suggest most (or all) –etc moderate-sized (<100 km) asteroids catastrophically disrupted over lifetime of the solar system • Also: Spacecraft images of Martian moons Phobos and 4. Simulations of catastrophic disruption show debris reforming Deimos: captured asteroids? into piles of flying debris under gravitational attraction. • Abundant evidence for impacts, and surprising evidence 5. Craters on some asteroids are too large – should break apart a solid body, but would be possible if body is rubble-pile for erosion and tectonism on these small bodies. 27 Itokawa NEAR at Eros 29 5 Collisions • Escape velocities are very small for most bodies. • Therefore most collisions are explosive. • Orbital families are collections of smaller asteroids which are apparently fragments of a larger parent – namedftthild after their larges t ex itiisting mem ber. • Collisions also produce dust which is seen in infrared observations (zodiacal dust). • Some collision fragments can re-coalesce into a “rubble pile”. Phobos and Deimos, Mars’ two moons as seen by the Mars • Binary asteroids can also be formed. Reconnaissance Orbiter. Captured asteroids? • Binaries are useful since they provide mass estimates. 31 • Some craters on Earth are pairs (10%). 32 Collisions Main Belt Evolution • Statistics of binaries, shapes, rotation periods, and even albedo variations are related to the • Shortly after formation (~of order 10 Ma) collisional history of the population. most of MB experienced large amount of • Size distributions of individual asteroid classes collisional fragmentation followed by mass or reggpions can have bumps. This could be a removal result of a remnant large body or because of – Event likely related to formation/migration of differences of material strength in different Jupiter populations. • Models (cf. Petit 2002) suggest that the • The current asteroid belt is likely only a remnant MB was heavily influenced by the final of an earlier population. However, it is difficult to stages of planet formation/nebular clearing figure out what the original population of objects was like (mass, composition, distribution). 33 34 Present Main Belt Features I Present Main Belt Features II Dynamical Excitation of MB Massive Main Belt Mass Loss • Current mass of main belt 10-4 • Most of the belt is -3 –10 Me dynamically (not • Discontinuity in surface thermally!) warm to hot density of SS suggests mass – Not consistent with loss in this region relative velocities needed • Accretion of largest asteroids for original accretion (Ceres, Vesta) over timescale comparable to range of – Present planetary meteoritic solidification ages perturbations even over Main belt requires primordial mass 4.5 Ga not sufficient to reservoir at least 100 times explain excitation current value (Duncan et al 1989) Weidenschilling, 1977 35 Surface density of material in solar system 36 Petit et al (2002) 6 Removing Mass & Mixing things Removing MB Mass via Planetary up
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