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Chapter 9 - Asteroids

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Chapter 9 - Asteroids Chapter 9 - Asteroids Chapter 9 - All 1 THE INNER SOLAR SYSTEM 2 WHO CARES? 9SOLAR SYSTEM FORMATION 9CONTINUING EVOLUTION 9ASTEROID STRENGTHS 9EARTH IMPACT HAZARD MAIN BELT e -a&ia & i-a ν 6 Eos 4 Asteroids: Gaps & Resonances •Astronomer Daniel Kirkwood (1886) noticed that th e M ai n B elt has “gaps” in which asteroids are “missing”. •The Kirkwood Gaps a r e “locations” where resonances with Jupiter’s orbit occur; i.e. where gravitational disturbances by Jupiter are the strongest. •Mayyp explain why there is no planet there: Jupiter only allowed small bodies to coalesce and prevented a larger planet from forming. 5 Distribution and Orbits of Main- Belt Asteroids • Most asteroids found between Mars and Jupiter • Distribution in the main-belt not uniform – strongly influenced by resonances with Jupiter (Kirkwood Gaps) • Co llis iona l disrup tion o f larger bo dies long ago has le ft physical and dynamical asteroid “families” – Eos, Hirayama, Themis and Koronis are major groups – Asteroids in each family show similar spectra • Protective zones near Lagrange points of Jupiter heavily pppopulated with Tro jan asteroids – Mars also has five known Trojan asteroids (Neptune has six) • Typical main-belt orbit stable on timescales of Ga • Separation of asteroids 1 km and larger ~5 million km! Formation and History of the Asteroid Belt • MiMain-be lt as teroid s be lieve d to be in or bits s tabl e for l ong periods, but higher relative energies than original primordial orbits • Very little mass today (fraction of Earth’s mass), but likely much more originally (comparable to Earth) 1. Most material ejjyypected from the solar system by Jupiter or driven into the Sun (Iron overabundance observed in other stars?) 2. Average mass density in SS shows discontinuity at MB • Asteroids are plantesimals which never were able to grow larger via collisional accretion because Jupiter increased their average impact velocity early in the SS, turning accretional events into disruption events. 7 Asteroids: Size Distribution • The number of asteroids of a given diameter D is proportional to 1/D2 – Collisional distribution • For example: – 3 > 500 km – 13 > 250 km – Hundreds > 100 km – 10,000+ > 10 km (?) Total: 1,000,000 > 1 km (?) – Most of the mass is in the largest few asteroids – Total mass of all asteroids is only ~ 5-10% mass of the Moon Size Distribution −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 st eep sl ope i mpli es tha t mos t of th e mass is in the large bodies, though most of the surface area is in the small ones. 9 DAVIS SIZE DISTRIBUTIONS – MAIN BELT 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 modldel o f as tero id 3. Occultations by Stars 4. Direct measurement by Spacecraft (NEAR, Galileo) 5. Direct imaging with Adaptive optics 6. Radar observations 11 Radar Observations of 4179 Toutatis Radar observations of Toutatis 13 14 Ceres as observed by Hubble, 2004 Spacecraft measurements 15 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. 16 NEA TYPES AMOR 1017AU<q<13AU1.017 AU < q < 1.3 AU • Amor EARTH NEA TYPES APOLLO q < 1. 017 AU • Apollo a > 1.0 AU EARTH NEA TYPES ATEN Q > 0 .983 AU • Aten a < 1.0 AU EARTH Origin of NEAs • NEAs are continually injected into NEA region fifrom main-bltbelt • Collisions between larger asteroids produce fragments which evolve due to radiation forces and weak resonances until reaching a major resonance “escape” hatch (takes 107- 108 years depending on size of daughter asteroids) • Orbit then rapidly evolves – SjdflbfSome 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 • 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 ⎠ 1/ 2 ⎛ P ⎞ Burns et al (1979) ΔT ≈ (1−δ )γS⎜ ⎟ ⎝ 2π ⎠ 21 Yarkovsky Effect Diurnal Yarkovsky: net force opposite the Seasonal Yarkovsky: net force opposite afternoon di rec tion. Progra de ro ta tor the “summer ” hem isp here. Ma kes the spirals out; retrograde rotator spirals in. object spiral inwards. 22 Bottke et al. (2001) Meteorite parents Largest NEAs TkTunguska-like objects 1. K=0.002 W/mK 2. K=0.02 W/mK 3. K=0.2 W/mK 4. K=2 W/mK 5. K=40 W/mk Mean change in a of inner main belt asteroids over their collisional lifetimes versus radius, for 5 thermal conductivities. Low K is dominated by diurnal YkYarkovs ky: hig h K by seasona l. Bottke et al. (2001) Light Curves •Shappge changes tend to produce double pp,eaked curves, whereas albedo variations tend to produce single peaked curves. • Lig ht curves at diff eren t pos ition ang les can be use d to determine the pole position and sense of rotation (which may have two components, like a badly thrown football). • Comets tend to have higher amplitude variations and slower rotation. • Tro jan as tero ids ten d to s how larger amp litu des, suggesting that they are more elongated than main belt asteroids. 24 25 Asteroid Physical Structure • Many asteroids believed to be collections of a few large fragments and smaller debris bound together as a “rubble-pile” Evidence: 1. Rotation rates of most asteroids sharply cut off at critical value (2.2 h) implying no tensile strength to objects 2. Densities lower than meteorites suggesting large amounts of empty space 3. Models of early solar system formation suggest most (or all) moderate-sized (<100 km) asteroids catastrophically disrupted over lifetime of the solar system 4. Simulations of catastrophic disruption show debris reforming into piles of flying debris under gravitational attraction. 5. CtCraters on some as tero ids are too large – shldbkhould break apar t a solid body, but would be possible if body is rubble-pile 27 Asteroids: Geology • ~12 asteroids visited up close by spacecraft: – 951 Gaspra : Galileo fly by in 1991 – 243 Ida: Galileo flyby in 1993 – 253 Mathilde: NEAR flyby in 1997 – 433 Eros: NEAR orbital mission in 2000-2001 – 25143 Itokawa: Hyabusa sample-return (?) mission in 2005 – 5535 Annefrank : Stardust in 2002 – 4 Vesta by Dawn in 2011 –etc • Also: Spacecra ft images o f Mar tian moons Ph o bos an d Deimos: captured asteroids? • Abundant evidence for impacts, and surprising evidence for erosion and tectonism on these small bodies. NEAR at Eros 29 Itokawa Phobos and Deimos, Mars’ two moons as seen by the Mars Reconnaissance Orbiter. Captured asteroids? 31 Collisions • Escappye 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 – named after their largest existing member. • Collisions also produce dust which is seen in infrared observations (zodiacal dust). • Some collision fragments can re-coalesce into a “rubble pile”. • Binary asteroids can also be formed. • Binar ies are use fu l s ince they prov ide mass es tima tes. • Some craters on Earth are pairs (10%). 32 Collisions • Statistics of binaries, shapes, rotation periods, and even albedo variations are related to the collisional history of the population. • Size distributions of individual asteroid classes or regions can have bumps. This could be a result of a remnant large body or because of differences of material strength in different populations. • The current asteroid belt is likely only a remnant of an earlier population. However, it is difficult to figure out what the original population of objects was like (mass, composition, distribution). 33 Main Belt Evolution • Shortly after formation ((of~of order 10 Ma) most of MB experienced large amount of collisional fraggymentation followed by mass removal – Event likely related to formation/migration of Jupiter • Models (cf. Petit 2002) suggest that the MB was heav ily in fluence d by the fina l stages of planet formation/nebular clearing 34 Present Main Belt Features I Dynamical Excitation of MB • Most of the belt is dynamically (not thermally!) warm to hot – Not consistent with relative velocities needed for original accretion – Present planetary pertbtiturbations even over 4.5 Ga not sufficient to explain excitation (Duncan et al 1989) 35 Petit et al (2002) Present Main Belt Features II Massive Main Belt Mass Loss • Current mass of main belt 10-4 -3 –10 Me • Discontinuity in surface density of SS suggests mass loss in this region • Accretion of largest asteroids (Ceres, Vesta) over timescale comparable to range of meteoritic solidification ages Main belt requires primordial mass reservoir at least 100 times current value Weidenschilling, 1977 Surface density of material in solar system 36 Removing Mass & Mixing things up Model of Lecar and Franklin (1997); Franklin and Lecar (2000): • As solar nebula decays, change in mass distribution in the nebula forces secular resonances to “sweep” through the primordial MB – Forces asteroid e and i to larger values and “pumps” the belt – Will remove mass – amount depends on how many resonances sweeppg through the belt and how lon gyg they remain in the belt – Drag from residual gas also helps mass removal – small bodies decayyppg, depopulating the outer belt Removing MB Mass via Planetary Embryos: A different approach • Formation of numerous moon – Mars – Earth – sized bodies in or near MB – Numerous models (eg .
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