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1 Chapter 9 - Asteroids 1 Chapter 9 - All THE INNER SOLAR SYSTEM 2 WHO CARES? 9SOLAR SYSTEM FORMATION 9CONTINUING EVOLUTION 9ASTEROID STRENGTHS 9EARTH IMPACT HAZARD

Transcript of Chapter 9 - Asteroids - Meteor Physicsmeteor.uwo.ca/~mcampbell/A9601/Chapter 9 -...

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Chapter 9 - Asteroids

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Chapter 9 - All

THE INNER SOLAR SYSTEM

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WHO CARES?

SOLAR SYSTEMFORMATION

CONTINUINGCON NU NGEVOLUTION

ASTEROIDSTRENGTHS

EARTH IMPACTHAZARD

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MAIN BELT e-a & i-a

4

ν6

Eos

Asteroids: Gaps & Resonances•Astronomer Daniel Kirkwood (1886) noticed that the Main Belt has “gaps” in which asteroids are “missing”.• The Kirkwood Gaps are “locations” where resonances with Jupiter’s

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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.

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 disruption of larger bodies long ago has left

physical and dynamical asteroid “families”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 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!

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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)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.

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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 Distributionς−

⎟⎟⎠

⎞⎜⎜⎝

⎛=

00)(

RRNRN R is the radius, N0dR

is the number of asteroids between R, R + dRThe power law form of the size distribution is

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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.

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DAVIS SIZE DISTRIBUTIONS – MAIN BELT

Asteroid Sizes IDetermination of asteroid sizes done via:1. Assume albedo; measure brightness and

distance and get geometric surface area2. 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 Stars4. Direct measurement by Spacecraft (NEAR,

Galileo)5. Direct imaging with Adaptive optics6. Radar observations 11

Radar Observations of 4179 ToutatisRadar observations of Toutatis

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14Ceres as observed by Hubble, 2004

Spacecraft measurements

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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.

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NEA TYPES

• Amor

AMOR1.017 AU < q < 1.3 AU

EARTH

NEA TYPES

• Apollo

APOLLOq < 1.017 AUa > 1.0 AU

EARTH

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NEA TYPES

• Aten

ATENQ > 0.983 AUa < 1.0 AU

EARTH

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

j “ ” h t h (t k 107major resonance “escape” hatch (takes 107-108 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

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Yarkovsky Effect• Asymmetric re-radiation

of thermal energy for a rotating body

• Uncertainties:– Thermal properties,

albedo– Spin rate (changes in spin

rate)

21Burns et al (1979)

The evening hemisphere radiates extra energy and momentum because it is hotter than the morning hemisphere. For prograde rotation, the net force is forward.

2/1

42

2)1(

cos38

⎟⎠⎞

⎜⎝⎛−≈Δ

⎟⎠⎞

⎜⎝⎛ Δ⎟⎟⎠

⎞⎜⎜⎝

⎛=

πγδ

ζσπ

PST

TT

cTsFY

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Yarkovsky Effect

22Bottke et al. (2001)

Diurnal Yarkovsky: net force opposite the afternoon direction. Prograde rotator spirals out; retrograde rotator spirals in.

Seasonal Yarkovsky: net force opposite the “summer” hemisphere. Makes the object spiral inwards.

Meteorite parents

Tunguska-like objects

Largest NEAs

1. K=0.002 W/mK2 0 02 /

Bottke et al. (2001)

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: high K by seasonal.

2. K=0.02 W/mK3. K=0.2 W/mK4. K=2 W/mK5. K=40 W/mk

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 (whichdetermine 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.

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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) i l i t il t th t bj t(2.2 h) implying no tensile strength to objects2. Densities lower than meteorites suggesting large amounts of

empty space3. 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

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Asteroids: Geology• ~12 asteroids visited up close by spacecraft:

– 951 Gaspra: Galileo flyby 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– 25143 Itokawa: Hyabusa sample-return (?) mission in 2005– 5535 Annefrank: Stardust in 2002– 4 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.

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NEAR at Eros

Itokawa

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

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 parentd ft th i l t i ti b– 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 are useful since they provide mass estimates.• 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 g presult 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

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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

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Present Main Belt Features IDynamical Excitation of MB

• Most of the belt is dynamically (not thermally!) warm to hot

Not consistent with– Not consistent with relative velocities needed for original accretion

– Present planetary perturbations even over 4.5 Ga not sufficient to explain excitation (Duncan et al 1989)

35Petit et al (2002)

Present Main Belt Features IIMassive Main Belt Mass Loss

• Current mass of main belt 10-4

– 10-3 Me• Discontinuity in surface

density of SS suggests mass loss in this regionloss 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

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Weidenschilling, 1977Surface density of material in solar system

Main belt

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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 MBMB– 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 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)S ( )• 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

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

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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).

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Chambers and Wetherill (2001)

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• If volume is computed then density can be found– Density provides insight into composition and structure– Densities generally found to be lower than meteorites

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Densities for most asteroids much lowerthan meteorite-analogs

Suggests lots of porosity

CI

CM

CO

CR

CV

H

L

LLTagish Lake, measured

Bulk Density (gcm-3)0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

1 Ceres2 Pallas

10 Hygiea45 Eugenia

121Hermione253 Mathilde

762 Pulcova11 Parthenope

15 Eunomia20 Massalia

243 Ida433 Eros

2000 DP1071999 KW416 Psyche

22 Kalliope87 Sylvia

4 Vesta3782 Celle

Phobos

GB

CCC

CC

SS

SSS

SS

MM

XV

V

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43Britt et al. 2001

Discovery image (top) ofDiscovery 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

1991 VH binary asteroid detected based on light curve variations.

Primary rotates every 2.6 hours; eclipse/occultationsoccur every 33 hours

1.2 km diameter primary and 0.5 km secondary separated by 3 kmp y

Most NEA binaries found to be in circular orbits; primary very round and slow rotation

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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 spin-

up/distortion at time of planetary close approaches

• Main-belt asteroids show larger separations and larger primary/secondary diameter ratios; possible g p y y ; porigins:– 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• Some asteroids can be tidally despun by small satellites.• If a body does not spin about one of its “principal” 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.

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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 objectsd s o o o ea y e d oge e objec s• 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

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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

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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 )

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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)

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

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• 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/r4 where r is the distance from Earth

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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

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Typical asteroid spectra lab spectra of minerals expected

Olivine

Pyroxene

Iron-nickel

Spinel

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

The most widely used scheme is that of Dave Tholenproposed in 1984. There are 14 categories, but 3 main onesC-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

Asteroid classes

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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 Pand 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)

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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?

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Chapman 2004

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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)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 low dose radiation can be replicated in a reasonable length lab experiment)

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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 outermore 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

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Present Main Belt Features Stratification in MB

• Asteroid taxonomies appear to be ordered by solar distance– Reflect original formation

locales?nce

(AU

)

HOT

SE

Mlocales?

– Clue to nature of asteroid types?

– Proxy for primordial solar nebula temperature gradient

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Hel

ioce

ntric

Dis

tan

COLD

C

P

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Structure of the Asteroid Belt:Variation of Taxonomic Types...

…with Distance from the SunDespite voluminous data acquisition, almost no bias-corrected statistical studies have been published since the 1980s…

Gradie, et al. (1988)

Difficulty in bias corrections and uncertain assumptions mean this gradient may not be so clear…

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?

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• 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 (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)

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Add shell of thickness dr contributingmass dM to existing body of mass M

Collisional Heating

drrdm ρπ 2

34

=

drGrGMdmdW 42216 ρπ==

Gravitational potential energy convertedto KE in moving mass dm from infinity to surface of body of mass M is:

ρπ 3

34 rM =

drGrr

dW3

ρπ==

RGMdrrGdWW

RR 24

0

22

0 53

316

=== ∫∫ ρπ

Starting from a small planetesimal withr ~ 0 to asteroid of radius R liberatespotential energy of:

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 change is not more than a few degrees even for big collisions

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Collisional Heating - III

R (km) dT (degrees)1 0 0007

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 Cv = dE / dT, where Cv = specific heat at constant volume, 1 0.0007

2 0.00284 0.0118 0.045

16 0.18332 0.73564 2.94

128 11.76256 47.05512 188.23 70

specific heat at constant volume,E is internal energy and T is temperature. Thus dT = dE / Cvand if we allow dE ~ W we have dT=0.6GM2/ (Cv R)

Shock Stage

Pressure GPa % (N) T Increase

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

S6 70 - 90 2.5% (55) 1500 - 1750 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

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The Complete List

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

Which asteroids melt?

How do they melt?

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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” 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.

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

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The End

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