PHYSICAL PROPERTIES OF PHYSICAL PROPERTIES OF PRIMITIVE ASTEROIDS: PRIMITIVE ASTEROIDS:

Clues from pClues from porosity and strengthorosity and strengthJosep M. Trigo-Rodríguez (ICE-CSIC, IEEC, Barcelona)

Catastrophic Disruption Workshop 2007, Alicante

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Aqueous flow in CM chondrite MET 01070 (Trigo-Rodríguez and Rubin, 2006)

MINOR BODIES: DIRECT INFORMATION MINOR BODIES: DIRECT INFORMATION ON DISK COMPOSITION AND PROCESSESON DISK COMPOSITION AND PROCESSES

Violent processes in the SS proplyd– Fragile starting materials– Depletion of volatile and

moderately volatile elements in inner regions

– Outer SS enrichment in Na, K, etc...Chondritic meteorites are coming from undifferentiated bodies:

– Relatively pristine, but they suffered collisions and, some of them, aqueous alteration

– Every minor body followed a particular evolutionary pattern

Diversity of formation conditions depending of heliocentric distance:Gravitational shock waves and turbulence mixed the forming materials and altered the simple scenario (Boss, 2006; Brownlee et al., 2006)

Ice-rich cometesimal

Rocky planetesimal

PRIMORDIAL PROPERTIES OF PLANETESIMALSPRIMORDIAL PROPERTIES OF PLANETESIMALSPresence of debris disks around 10 to 20 Myr old stars:– High spatial densities implying a

continous dust replenishment by planetesimals’ fragmentation

– Falling Evaporating Bodies model (FEB)

– Lifetime of a dust grain is very short: τ < 10.000-100.000 yr

Protoplanetary disk around HD142557, Subaru

PRIMITIVE CHONDRITESPRIMITIVE CHONDRITESWe have been focusing in the texture, mineralogy and internal structure of chondrites to get clues on their particular evolutionary tracks

Carbonaceous chondrites are typically fragile and porous materials

Some CCs are hydrated (e.g. CI and CM) and other not (CO, CV)

I’ll focus here mainly in the CM CCs

– Clues on their parent body

SEM image of CM-like ungrouped Acfer 094

CM CHONDRITESCM CHONDRITESCM CCs are water-rich aggregates containing chondrules, inclusions and fine dust materials:

– Until 10% H2O and 4% C in mass

CMs experienced aqueous alteration to different degrees (McSween, 1979)

– Many mineral phases are alteration products(Zolensky & McSween, 1988):

• phyllosilicates, sulfides, carbonates, oxides, and poorly characterized phases or “PCP”

CM chondrites suffered secondary processes after accretion (Brearley & Jones, 1998):

– Aqueous alteration – Brecciation (consequence of impacts)

Two main scenarios for aqueous alteration:– Parent body (Zolensky & McSween, 1988)– Preaccretionary, occurred in uncompacted

precursors (Metzler et al. 1992; Bischoff, 1998)

Recent results support parent-body alteration (Trigo-Rodríguez et al., 2006; Rubin et al., 2007)

QUE93005 CM2

Y791198 CM2

EVIDENCE OF IMPACT COMPACTIONEVIDENCE OF IMPACT COMPACTIONMetzler et al. (1992) suggested that some CMs remained essentially intact since their accretion:– “Primary accretionary rocks like

Y791198”But the texture and mineralogy of Y791198 and other CMs were caused by compaction + aqueous alteration (Trigo-Rodríguez et al., 2006, GCA)– Water released from hydrated minerals

likely during impact compactionMany CMs are breccias (very friable):– Many CMs contain clasts with different

degrees of aqueous alteration – Different alteration stages in similar rocks

suggest that aqueous alteration was dependent on local conditions, e.g., depth of burial

Lineation patterns in Y791198 (Trigo-Rodriguez et al., 2006)

PROGRESSIVE AQUEOUS PROGRESSIVE AQUEOUS ALTERATION IN ALTERATION IN CMsCMs

Trigo-Rodríguez et al. (2006) studied 10 CMs that span the range of moderate to extreme aqueous alteration

Their structure suggest highly porous precursors

Many CMs are breccias containing clasts of different alteration stages (e.g., Cold Bokkeveld, Murray)– Important collisional history– They were altered to different

degrees, possibly due to their burial depth

– The samples were later fragmented and compacted by impacts

(Trigo-Rodriguez et al., GCA 4581 , 2006).

QUE 97990QUE 97990

QUE 99355QUE 99355

Cold Cold BokkeveldBokkeveld

LAP 02277LAP 02277

MurrayMurray QUE 93005QUE 93005

FeFe--RICH LENSES IN MET 01070RICH LENSES IN MET 01070Trigo-Rodríguez & Rubin (2006) and likely produced by mobilization of chemical elements dissolved in an aqueous fluid

MET 01070 exhibits extensive aqueous alteration, but in particular we found several lenses rich in Fe,Ni sulfides and carbonates in MET 01070

– The lenses are oriented in approx. the same direction as the lineation in the meteorite

Parent-body processing produced aqueous alteration:– Impact compaction released water from

hydrated minerals producing aqueous alteration

– Ice may have been melted or water may have been released from phyllosilicates by sporadic impacts. Water flows may have formed the veins.

POROSITY vs. PRIMITIVINESSPOROSITY vs. PRIMITIVINESSMany asteroids have very high bulk porosity (Britt et al., 2002)– P<15% Differentiated Bodies: e.g. 1

Ceres, 2 Pallas, 4 Vesta, 20 Massalia, etc…– 15%<P<50%: Like e.g. 243 Ida, 433 Eros– P>50%. Extreme cases like e.g. 22

Kalliope or 16 Psyshe

Main effect in impacts:– Porosity attenuates the stress wave

generated in an impact.– All material is shacked and redistributed

• Shear evidence in carbonaceous chondritesPrimitive (undifferentiated) bodies should have higher degree of porosity.– Most of the present MB bodies are

compactedWhat empirical evidence in meteorites?– Clues from laboratory simulations…Don Dixon

POROSITY AND DENSITY OF METEORITESPOROSITY AND DENSITY OF METEORITES

All meteorites arriving the Earth are compacted samples:– Not surprising because materials were biased during atmospheric entry towards the tougher

objects (!)

Blum et al. (2006)

LABORATORY LABORATORY EXPERIMENTSEXPERIMENTS

In Jürgen Blum’s laboratory we have been trying toreproduce the first stages of accretion

– Low relative velocity sticking of dust to form first aggregatesMain goal:

– To learn about the primordial physical properties of primevalbodies by building them (a, b)

– To study bulk density and porosity versum tensile strength

Blum et al. (2006)

Tensile strength determination

Study of macroscopic aggregates based in different types of grains:

– c) Spherical monodisperse SiO2 grains– d) Irregular diamonds– e) Irregular polydisperse SiO2

MORE PRISTINE MORE PRISTINE OBJECTS: COMETS!OBJECTS: COMETS!

Porosity, bulk density and strength can be key properties to decipher how pristine is a body!

Blum et al. (2006)

Comet SW3, HST (NASA)

COMETARY METEOROIDS STRENGTHCOMETARY METEOROIDS STRENGTHObserved velocity versus fragmentation strengths deduced for cometary meteoroids for different showers and sporadic meteors.

– Sporadic (SPO) particles with high entry velocities (suggesting cometary origin) have typical disruption strengths of 104

dyne/cm2.

– Below this value are probably young fluffy meteoroids as for the case of CAP, LEO and ORI.

– Finally GIA are exhibiting extremely low strengths.

– It is remarkable that particles coming from old streams (TAU, GEM and QUA) associated with parent bodies exhibiting low or null cometary activity have the highest strengths.

• A question remains: are these bodies releasing tougher meteoroids from inner layers or is it an aging effect?

Trigo-Rodriguez & Llorca (2006, 2007), MNRAS.

Accurate meteor trajectory data can provide insight into physical properties like e.g. the dynamic strength (S)

S vatm= ⋅ρ 2

Compacted interiors

2 P/Encke 3200 Phaeton

- - - - - - - - - - - - - - - 1 kPa - - - - - - - - - - - - - - - - - - - - - - -

---------------------------- 253 Mathilde’s central pressure -----------------------------

SEARCH FOR COMETARY METEORITESSEARCH FOR COMETARY METEORITESExist cometary meteorites in our collections?– Probably yes, but only a few!

How to identify them? – Peculiar mineralogy,

components and properties (Campins & Swindle, 1998)

– Fine grain materials of CI CCs are coming from (compacted) comets?

Tagish Lake (ungrouped but CI-CM related):– High macroporosity of

impacting meteoroid (ReVelleet al., 2002)

Gounelle et al. (2006) obtained a likely orbit for CI1 chondrite Orgueil.– Similar to JFCsCM-CI Tagish Lake (Greshake et al., MAPS 40-9/10 , 2005)

CONCLUSIONSCONCLUSIONSThe petrographic features of CCs provide valuable information about parent-body aqueous alteration:– Aqueous-alteration products are changing the fragmentation products– Impacts and aqueous alteration contributed to compaction (at microscopic scale!)– High porosities of C- and D-types asteroids suggest important macroporosity: rubble

piles– Flynn et al.(2005) studied the dust production from impact disruption of hydrated

targets• Impact physics: macro and micro-porosity should be incorporated in models!

Study of unprocessed cometary materials can provide additional information on the role of compaction in minor bodies:– New clues on non-preserved volatiles originally present in chondritesSome cometary materials have the lowest strengths, but it depends:– Some (periodic) comets have been processed:

• Collisional compaction and/or aqueous alteration.– Density and strength are good indicators of the degree of primitiveness.

FUTURE WORK– Numerical models can establish pre-compaction and pre-aqueous properties– Laboratory work is required to better understand the physical properties of the rock

prior to aqueous alteration / compaction

FeFe--RICH LENS COMPOSITIONRICH LENS COMPOSITIONLENS COMPOSITION (n = 23 points)

SiO2 27.0 ± 4.6 TiO2 0.10 ± 0.06 Al2O3 2.5 ±0.38 FeO 35.8 ±3.5 Cr2O3 0.42 ±0.18 MnO 0.25 ±0.04 MgO 16.4 ±2.2 CaO 0.15 ±0.19 Na2O 0.20 ±0.03 K2O <0.04 ±0.01 P2O5 <0.04 ±0.07 NiO 0.72 ±0.44 S 1.5 ±0.97 total 85.0

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2 MURCHISON AUREOLES

(Hanowski & Brearley, 2000)

SiO2 5.61TiO2 0.16Al2O3 2.0 FeO 43.6Cr2O3 6.1 MnO 0.23 MgO 6.7 CaO 0.3 Na2O 0.23 NiO 7.6 S 8.3 total 80.9

Low SiO2 and MgO in aureoles are consistent with production from altered metallic FeHigh SiO2 and MgO in lenses close to PCP compositionNo recognizable weathered metal blob suggests that lenses formed by PCP precipitation through parent-body aqueous flow