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 h as “gaps” in which asteroids are “missing”. •The Kirkw ood Gaps ar e “locations” where resonances with Jupiter’s orbit occur; i.e. where gravitational disturbances by Jupiter are the strongest. •Mayyp explain wh y 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 disrupti on o f l arger 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 ast eroid s b eli eve d to b e in or bits s tabl e f or 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 most 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

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)π σ the net force is forward. 8 ⎛ T 4 ⎞⎛ ΔT ⎞ 2 ⎜ ⎟ FY = s ⎜ ⎟⎜ ⎟cosζ 3 ⎝ c ⎠⎝ T ⎠ δ 1/ 2 γ ⎛ P ⎞ Burns et al (1979) ΔT ≈ (1− ) S⎜ ⎟ ⎝ 2π ⎠ 21 Yarkovsky Effect

Diurnal Yarkovsky: net force opposite the Seasonal Yarkovsky: net force opposite afternoon di rec tion. P rograd e ro tat or 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. • Light 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. • Troj an ast eroid s tend to sh ow 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 teroid s are too l arge – 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 . 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 Chambers and Wetherill (2001) Same dynamical processes which remove embryos also remove MB asteroids 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 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 Tagish Lake, measured LL

L Densities for most

H asteroids much lower

CV than meteorite-analogs

CR CO Suggests lots of porosity CM

CI

Phobos 3782 Celle V 4 Vesta V 87 Sylvia X 22 Kalliope M 16 Psyche M 1999 KW4 S 2000 DP107 S 433 Eros S 243 Ida S 20 Massalia S 15 Eunomia S 11 Parthenope S 762 Pulcova C 253 Mathilde C 121Hermione C 45 Eugenia C 10 Hygiea C 2 Pallas B 1 Ceres G

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Bulk Density (gcm-3) Britt et al. 2001 43 Discovery image (top) of the asteroid satellite of 45 Eugenia Orbit of the system shown to thihhe right Inferred density (assuming 45 Eugenia is spherical) is 1.2 g cm-3 1991 VH binary asteroid detected based on li gh t curve variations. Primary rotates every 2.6 hours; eclipse/ occultations occur every 33 hours 1.2 km diameter primary and05kd 0.5 km secon dary separated by 3 km Most NEA binaries found to be in circular orbits; primary very round and slow rotation Origgyin of Binary Asteroids • Binary NEAs show small separations, circular orbits, round ppprimaries with near critical rotation periods – Suggests these systems are formed due to tidal spin- up/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.

2000 DP107 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 bodyyp does not spin about one of its “ ppprincipal” axes then the wobble is unstable. Wobble damping can be very slow if the rotation is slow. • Bodies are most stable if theyyg 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 damppging timescale de pends on the densit y, radius and ri gidit y of the body. 47 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 sppyy(in 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 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 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…), esppyecially 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 • 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 • 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 1/1/r4 whithditfEthhere r is the distance from Earth

52 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 Tagish Lake Typical asteroid spectra lab spectra of minerals expected

Iron-nickel

Olivine

Pyroxene

Spinel Unfortunatelyyy many of the sp ectral 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 Asteroid classes The most widely used scheme is that of Dave Tholen proposed in 1984. 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)& 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

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 priftit?Ifthti?imary source of meteorites? If not, what is?

59 Chapman 2004 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 oflf low dose ra ditidiation can be rep lica te did in a reasonable length lab experiment)

61 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 theyyp formed this represents signature of original nebular temperature gradient • Examination of meteorites – different heating

histories for different classes 62 Present Main Belt Features Stratification in MB E HOT S • Asteroid taxonomies appear tbto be ord ere dbd by solar distance AU) (( M – Reflect original formation locales?

stance – Clue to nature of asteroid ii types? – Proxy for primordial solar ntric D C ee nebula temperature gradient Helioc

P COLD 63 Structure of the Asteroid Belt: Variation of Taxonomic Types... …with Distance from the Sun Despite voluminous data acquisition, almost no bias- corrected statistical studies have been published since the 1980s… Difficulty in bias corrections and uncertain assumptions mean this gradient may not be so clear…

Gradie, et al. (1988) Structure of the Asteroid Belt: Variation of Taxonomic Types...

Mothe, et al. (2003) Asteroid Heat Sources

• Thermal processing – two main sources: 1. Collisional (primordial/formation and recent) 2. Radioactive

Also (possibly): Magnetic Induction from early sun?

66 • 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.shthort-live d ra dioac tiv ity (Al26 ?) 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 Collisional Heating

Add shell of thickness dr contributing 4 mass dM to existing body of mass M dm = πr 2 ρ dr 3 Gravit ittiational pot enti tilal energy conver tdted to KE in moving mass dm from infinity to surface of body of mass M is: GMdm 16 dW = = π 2 ρ 2Gr 4dr r 3 4 M = πr 3ρ 3 Starting from a small planetesimal with r ~ 0 to asteroid of radius R liberates potentilial energy o f:

2 R 16 R 3 GM W = dW = π 2 ρ 2G r 4dr = ∫0 3 ∫0 5 R 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 glbllobal c hange is not more t han a few degrees even for big collisions

69 Collisional Heating - III

Absolute upper limit to global thermal change can be estimated by assuming ALL gravitational potential energy chdtKEtitl(thlhanged to KE goes to internal (thermal energy ).

Recall Cv = dE / dT, where Cv = R (km) dT (degrees) specific heat at constant volume, 1 0.0007 E is internal energy and T is 2 0.0028 40.011 temperature. Thus dT = dE / Cv and if we allow dE ~ W we have 8 0.045 2 16 0.183 dT=0.6GM /(/ (Cv R) 32 0.735 64 2.94 128 11.76 256 47.05 512 188.23 70 Shock Pressure % (N) T Increase Stage GPa S1 < 4 - 5 11.6% (257) 10 - 20 K

S2 5 - 10 34.0% (753) 20 - 50 K

S3 15 - 20 34.8% (770) 100 - 150 K

S4 30-35 12. 9% (286) 250 - 350 K

S5 45 - 55 4.2% (94) 600 - 850 K 1500 - 1750 S6 70 - 90 2.5% (55) K

Stöffler, Keil, and Scott, GCA 55, 3845 & Grady (2000) Radioactive Heating

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

72 The Complete List

• Shortest lifetime sets time scale of CAI formation relative to nucleosynthesis events. • If 106 yr is “first enough”, CAIs were the first. • 244Pu, 129I, 182Hf, 60Fe require supernova. 60 • Fe requires supernova within 5(?) myr. 73 •We did form in an Orion-like cloud (?) Which asteroids melt? How do they melt?

Thermal model for the asteroid belt assuming heating from decay of short-lived radioactive isotopp,e, aluminum-26. Asteroids farther from the sun accreted later, and incorporated less “live” 26Al. 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 Time-temperature curves plotted at various depths in the (a) uncompacted and (b) compacted H-chondrite parent bodies (Bennett and McSween, 1996). 75 The End

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