Chapter 9 - Asteroids Chapter 9 - All 1

Transcription

1 Chapter 9 - Asteroids Chapter 9 - All 1

2 THE INNER SOLAR SYSTEM 2

3 WHO CARES? SOLAR SYSTEM FORMATION CONTINUING EVOLUTION ASTEROID STRENGTHS EARTH IMPACT HAZARD

4 MAIN BELT e-a &i-a ν 6 Eos 4

5 Asteroids: Gaps & Resonances Astronomer Daniel Kirkwood (1886) noticed that t the Main Belt has gaps in which asteroids are missing. The Kirkwood Gaps are locations where resonances with Jupiter s orbit occur; i.e. where gravitational disturbances by Jupiter are the strongest. May explain why there is no planet there: Jupiter only allowed small bodies to coalesce and prevented a larger planet from forming. 5

6 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) Collisional i l disruption of larger bodies long ago has left 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 populated p with Trojan 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!

7 Formation and History of the Asteroid Belt Main-belt asteroids believed to be in orbits stable for long 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 ejected 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

8 Asteroids: Size Distribution The number of asteroids of a given diameter D is proportional to 1/D 2 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

9 N ( R ) Size Distribution N R = 0 R0 ς R is the radius, N 0 dr, 0 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 t most of the mass is in the large bodies, though most of the surface area is in the small ones. 9

10 DAVIS SIZE DISTRIBUTIONS MAIN BELT

11 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

12 Radar Observations of 4179 Toutatis Radar observations of Toutatis

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14 Ceres as observed by Hubble,

15 Spacecraft measurements 15

16 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

17 NEA TYPES Amor AMOR 1017AU<q<13AU < 1.3 EARTH

18 NEA TYPES Apollo APOLLO q < AU a > 1.0 AU EARTH

19 NEA TYPES Aten ATEN Q > AU a < 1.0 AU EARTH

20 Origin of NEAs NEAs are continually injected into NEA region from main-belt Collisions between larger asteroids produce fragments which evolve due to radiation forces and weak resonances until reaching a major resonance escape hatch (takes 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

21 Yarkovsky Effect The evening hemisphere radiates extra energy and momentum because it is hotter than the morning hemisphere. For prograde rotation, the net force is forward. Burns et al (1979) Asymmetric re-radiation of thermal energy for a rotating body Uncertainties: Thermal properties, albedo Spin rate (changes in spin rate) F Y ΔT 8 3 σt c ΔT T 4 2 = π cosζ s P ( 1 δ ) γs 2π 1/ 2 21

22 Yarkovsky Effect Diurnal Yarkovsky: net force opposite the afternoon direction. Prograde rotator t spirals out; retrograde rotator spirals in. Bottke et al. (2001) Seasonal Yarkovsky: net force opposite the summer hemisphere. h Makes the object spiral inwards. 22

23 Meteorite parents Largest NEAs Tunguska-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 Yarkovsky: k high h K by seasonal. Bottke et al. (2001)

24 Light Curves Shape changes tend to produce double peaked curves, whereas albedo variations tend to produce single peaked curves. Light curves at different position angles can be used 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. Trojan asteroids tend to show larger amplitudes, suggesting that they are more elongated than main belt asteroids. 24

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27 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. Craters on some asteroids are too large should break apart a solid body, but would be possible if body is rubble-pile 27

28 Asteroids: Geology ~12 asteroids visited up close by spacecraft: 951 Gaspra: Galileo flyby in Ida: Galileo flyby in Mathilde: NEAR flyby in Eros: NEAR orbital mission in Itokawa: Hyabusa sample-return (?) mission in Annefrank: Stardust in Vesta by Dawn in 2011 etc Also: Spacecraft images of Martian moons Phobos and Deimos: captured asteroids? Abundant evidence for impacts, and surprising evidence for erosion and tectonism on these small bodies.

29 NEAR at Eros 29

30 Itokawa

31 Phobos and Deimos, Mars two moons as seen by the Mars Reconnaissance Orbiter. Captured asteroids? 31

32 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 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. Binaries i are useful since they provide mass estimates. t Some craters on Earth are pairs (10%). 32

33 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

34 Main Belt Evolution Shortly after formation (~of order 10 Ma) most of MB experienced large amount of collisional fragmentation followed by mass removal Event likely related to formation/migration of Jupiter Models (cf. Petit 2002) suggest that the MB was heavily influenced by the final stages of planet formation/nebular clearing 34

35 Present Main Belt Features I Dynamical Excitation of MB Petit et al (2002) Most of the belt is dynamically (not thermally!) warm to hot Not consistent with relative velocities needed for original accretion Present planetary perturbations ti even over 4.5 Ga not sufficient to explain excitation (Duncan et al 1989) 35

36 Main belt Present Main Belt Features II Massive Main Belt Mass Loss Weidenschilling, 1977 Surface density of material in solar system Current mass of main belt M e 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 requires primordial mass reservoir at least 100 times current value 36

37 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 sweep through the belt and how long they remain in the belt Drag from residual gas also helps mass removal small bodies decay, depopulating p the outer belt

38 Removing MB Mass via Planetary Embryos: A different approach Formation of numerous moon Mars Earth sized bodies in or near MB Numerous models (eg. Chambers and Wetherill 2001) Some portion (or all) controlled by Jupiter which leads to dynamical excitation of the population This scattering produces gravitational effects on entire MB population through close encounters with small MB asteroids (exciting them dynamically) Critically depends on how long the embryo can remain in MB before dynamical removal and how big Jupiter is as a function of time during this process 38

39 Chambers and Wetherill (2001) Same dynamical processes which remove embryos also remove MB asteroids

40 Mass in asteroid belt (2.1 < a < 4.0 AU) versus time for simulations with modern Jupiter and Saturn (upper curve), and slightly eccentric (e=0.1) Jupiter and Saturn (lower curve). Chambers and Wetherill (2001) 40

41 Asteroid Masses and Densities Asteroid masses found From their gravitational effects on spacecraft paths From their gravitational effects on each other Size of orbit and period of revolution of satellite moons If volume is computed then density can be found Density provides insight into composition and structure Densities generally found to be lower than meteorites 41

42 LL L H CV CR CO CM CI Tagish Lake, measured Densities for most asteroids much lower than meteorite-analogs Suggests lots of porosity Phobos 3782 Celle 4 Vesta 87 Sylvia 22 Kalliope 16 Psyche 1999 KW DP Eros 243 Ida 20 Massalia 15 Eunomia 11 Parthenope 762 Pulcova 253 Mathilde 121Hermione 45 Eugenia 10 Hygiea 2 Pallas 1 Ceres S X S C C C C M C G V S S M S S S B V Bulk Density (gcm )

43 Britt et al

44 Discovery image (top) of the asteroid satellite of 45 Eugenia Orbit of the system shown to the right Inferred density (assuming 45 Eugenia is spherical) is 1.2 g cm -3

45 1991 VH binary asteroid detected based on light curve variations. Primary rotates every 2.6 hours; eclipse/occultations occur every 33 hours 1.2 km diameter primary and d05k 0.5 km secondary separated by 3 km Most NEA binaries found to be in circular orbits; primary very round and slow rotation

46 Origin of Binary Asteroids Binary NEAs show small separations, circular orbits, round primaries with near critical rotation periods Suggests these systems are formed due to tidal spinup/distortion at time of planetary close approaches Main-belt asteroids show larger separations and larger primary/secondary diameter ratios; possible origins: 1) creation of mutually co-orbiting fragments from a catastrophic disruption of a parent body 2) reaccretion of ejecta from a major, oblique, sub-catastrophic collision between an impactor and the primary 3) bifurcation of a (rubble-pile) parent body after rapid spin-up by a large impactor DP 107

47 Rotation 80% of planetary bodies rotate with a period between 4 and 16 hours. There is a correlation between size and period. Smaller asteroids spin faster. Spin rates for large asteroids (>60km) are probably determined by their collision history since the distribution is not smooth. Some asteroids can be tidally despun by small satellites. If a body does not spin about one of its principal p axes then the wobble is unstable. Wobble damping can be very slow if the rotation is slow. Bodies are most stable if they are rotating about their short axis. Inertial stresses caused by motions in the body will slowly damp the rotation until the asteroid is rotating about its shortest principal axis. The damping timescale depends on the density, radius and rigidity of the body. 47

48 Asteroid Spins and Shapes Larger asteroids are spherical due to high gravity Smaller (~km) sized asteroids vary greatly in shape Elongated shapes of some NEAs interpreted as tidal distortion of weakly held together objects Asteroids spin relatively slowly (hours months) Fast spinning small bodies (<2.2h) in the NEA population may be monolithic rocks Spins determined by variations in light curves 48

49 Only the smallest bodies rotate with periods less than ~2 hours. This is taken to support the rubble pile model for asteroids, that is, larger asteroids cannot rotate faster than this limit or they will rotationally disrupt 49

50 Sources of Information about Asteroid Surfaces Telescopes. Hemispherically averaged, ambiguous info on particle size, mineralogy, and not much else Radar. Roughness on a human-scale and reflectivity (ice, metal, rock ), especially for NEAs that pass close Inferences from Meteorites. Better than nothing, but meteorites are not regolith samples Spacecraft. Only a few studied to date; best data by far for Eros (NEAR) 50

51 Example of an asteroid detection with the 3.6m CFHT. The asteroid is not resolved, the apparent size is the size of the point-spread function 51

52 Radar observation of 1999 JM8 taken on August 3, 1999 with the Arecibo radar. The asteroid is resolved and we can see features on it, but this technique can only be applied to asteroids which pass quite close to the Earth. The radar power required drops like 1/r 4 where r is the distance from Earth 52

53 Asteroid Physical Classification Different asteroid classes established based on reflectance spectra (colors) Shapes of spectra indicate major minerals present on the surface ONLY No direct means to determine the mineral composition of the interior of an asteroid Issues of space weathering make compositional links with meteorites uncertain Do not get highly defined spectral absorption lines (like in a gas) due to reflectance from powdered surface 16 Major defined classes and many more sub-divisions As asteroid spectra become more refined and applied to smaller bodies, more asteroid groups are defined

54 Tagish Lake

55 Typical asteroid spectra lab spectra of minerals expected Iron-nickel Olivine Pyroxene Spinel

56 Unfortunately many of the spectral features that tell you the most about the minerals present are in the infrared, which is very difficult to observe from the ground. As a result, many current asteroid classifications rest on a sparse number of observations in the IR

57 Asteroid classes The most widely used scheme is that of Dave Tholen proposed in There are 14 categories, but 3 main ones C-type (dark carbonaceous objects). This group contains about 75% of asteroids in general (includes subtypes B, F &G) S-type (silicaceous (or "stony") objects). This class contains about 17% of asteroids in general. M-type metallic objects, the third most populous group, include subtypes E and P and small classes that include just a few asteroids that don t fit in the scheme above. A-type (446 Aeternitas) D-type (624 Hektor) T-type (96 Aegle) Q-type (1862 Apollo) R-type (349 Dembowska) V-type (4 Vesta) 57

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59 The puzzle of meteorite sources No class of asteroids have reflectance spectra that resemble the reflectance spectra of ordinary chondrites. And yet OC meteorites are the most common meteorites. Does that mean that asteroids are NOT the primary source of meteorites? If not, what is? 59

60 Chapman 2004

61 The puzzle of meteorite sources Space weathering is the leading theory here. The idea is that constant bombardment by energetic particles in space has modified a veneer on the asteroid s surface so the colours of asteroids are different from those of meteorites (which have had any such veneer burned off) Experiments attempting to reproduce this modification in the lab have had some success but its not always clear that the processes are comparable (e.g. that a million years of flow dose radiation can be replicated din a reasonable length lab experiment) 61

62 Evidence related to formation of Asteroids Significant gradient in asteroid types through the MB Silicate/Metallic rich in the inner portion of the belt, more primitive, organic rich, water-rich(?) in outer belt If asteroids are still sorted approximately in the same locations as they formed this represents signature of original nebular temperature gradient Examination of meteorites different heating histories for different classes 62

63 Present Main Belt Features Stratification in MB Helioce entric Di istance (AU) E S M C HOT Asteroid taxonomies appear to be ordered d by solar distance Reflect original formation locales? Clue to nature of asteroid types? Proxy for primordial solar nebula temperature gradient P COLD 63

64 Structure of the Asteroid Belt: Variation of Taxonomic Types... with Distance from the Sun Gradie, et al. (1988) Despite voluminous data acquisition, almost no biascorrected statistical studies have been published since the 1980s Difficulty in bias corrections and uncertain assumptions mean this gradient may not be so clear

65 Structure of the Asteroid Belt: Variation of Taxonomic Types... Mothe, et al. (2003)

66 Asteroid Heat Sources Thermal processing two main sources: 1. Collisional (primordial/formation and recent) 2. Radioactive Also (possibly): Magnetic Induction from early sun? 66

67 Asteroids never exceeded the size of planetesimals and (in a few cases) small proto-planets (Ceres) The larger (>100 km) bodies underwent heating which led to differentiation due to: 1. short-lived radioactivity it (Al 26?) 2. Collisions These differentiated asteroids were mini-planets, with iron cores and silicate mantles Some of these objects were broken apart by collisions (note: differentiated meteorites testify to this) 67

68 Collisional Heating dm = 4 M = π r 3 ρ 3 Add shell of thickness dr contributing 4 mass dm to existing body of mass M πr 2 ρ dr 3 Gravitational ti potential ti energy converted to KE in moving mass dm from infinity to surface of body of mass M is: dw = GMdm r = π ρ Gr 3 4 dr Starting from a small planetesimal with r ~ 0 to asteroid of radius R liberates potential energy of: W R 16 2 = dw = π ρ G 3 R r dr = GM R 2

69 Collisional Heating - II For collisional heating to be the main source for thermal metamorphism, it has to occur very fast and the main body has to be big (large gravitational potential well) Neither condition is likely met for asteroids which are parent bodies of meteorites Collisional energy raises temperature locally, but global l change is not more than a few degrees even for big collisions 69

70 Collisional Heating - III Absolute upper limit to global thermal change can be estimated by assuming ALL gravitational potential energy changed to KE goes to internal (thermal energy). Recall C v = de / dt, where Cv = R (km) dt (degrees) specific heat at constant volume, E is internal energy and T is temperature. Thus dt = de / C v and if we allow de ~ W we have dt=0.6gm 2 /(C v R)

71 Shock Stage Pressure GPa % (N) T Increase S1 < % (257) K S % (753) K S % (770) K S % (286) K S % (94) K S % (55) K Stöffler, Keil, and Scott, GCA 55, 3845 & Grady (2000)

72 Radioactive Heating Short-lived nuclides abundant in early SS Most important for thermal alteration in early SS is 26 Al 72

73 The Complete List Shortest lifetime sets time scale of CAI formation relative to nucleosynthesis events. If 10 6 yr is first enough, CAIs were the first. 244 Pu, 129 I, 182 Hf, 60 Fe require supernova. 60 Fe requires supernova within 5(?) myr. We did form in an Orion-like cloud (?) 73

74 Which asteroids melt? How do they melt? Thermal model for the asteroid belt assuming heating from decay of short-lived radioactive isotope, aluminum-26. Asteroids farther from the sun accreted later, and incorporated less live 26 Al. Those closer to the sun were heated to higher temperatures. Asteroids with diameters of 100 km within 2.7 AU of the Sun produced achondrites. Ice melted in bodies between 2.7 and 3.4 AU, allowing aqueous alteration of chondrites. At great solar distances, asteroids never warmed above the melting point of ice. 74

75 Time-temperature curves plotted at various depths in the (a) uncompacted and (b) compacted H-chondrite parent bodies (Bennett and McSween, 1996). 75

76 The End 76