Steve Desch

Arizona State University

School of Earth and Space Exploration

Meteoritic Constraints on our Protoplanetary Disk

NEW!

Meteorites severely constrain models of how our protoplanetary disk formed and evolved.

Short-Lived Radionuclides in CAIs (Ca,Al-rich inclusions)

Chondrule Formation

Meteorites: under most astronomers’ radar?

Daylight fireball, Grand Tetons National Park, August 10, 1972

Antoine Lavoisier (1743-1797)

Thomas Jefferson (1743-1826)

“...it is easier for me to believe that two Yankee professors would lie than that stones would fall from heaven.”

In response to the fireball / meteorite of Luce, France, 1772...

“There are no rocks in the sky. Therefore rocks do not fall from the sky.”

Chondrites are rocks from asteroids (2-3 AU).

They are mostly unaltered since the birth of the Solar System.

rocks

sky

Cross section of Carraweena (L3.9) MATRIX GRAINS CHONDRULES CAIs

Chondrites: Leftover crumbs from solar system formation

Chondrule Formation: Key Constraints

•Peak temperatures > 1800 K

•Cooled from peak temperatures to liquidus (1800 K) in minutes

•Cooled from liquidus to solidus (1400 K) in hours

•During formation, chondrule densities > 10 m-3

•Scale of chondrule-forming events were > 1000 km

•Chondrule formation took place in presence of gas and of matrix dust, probably in present-day asteroid-belt region

•Chondrule formation took place repeatedly over many Myr.

Chondrule Thermal Histories

Chondrules crystallized from a nearly complete melt: peak temperatures exceeded liquidus temperature, ~ 1575C

Chondrules did not lose significant sulfur: chondrules were completely molten for only minutes (Yu & Hewins 1998)

Chondrule textures require cooling rates ~ 100 K/hr (porphyritic) to ~ 1000 K/hr (radial, barred): chondrules took hours to crystallize.

radial pyroxene texture porphyritic texture

Chondrule Densities

About 5% of all chondrules are compound chondrules, stuck together while molten (Ciesla et al. 2004)

Given likely relative velocities, chondrule densities were ~ 10 m-3 in zone where chondrules melted (Gooding & Keil 1981)

Chondrules lost K, Fe, Mg, Si through evaporation, but did not experience measureable isotopic fractionation.

K, Fe, Mg and Si vapor didn’t leave chondrule formation region: demands chondrule density ~ 10 m-3

(Cuzzi & Alexander 2006)

Chondrule Densities

Chondrule densities nc ~ 10 m-3 are high!

nc = (c / gas) gas

4/3 s ac3

= C (0.01) (10-9 g cm-3) (gas / 10-9 g cm-3)

4/3 (2.5 g cm-3) (300 m)3

C (gas / 10-9 g cm-3) ~ 300

At midplane at 2 AU in MMSN, gas = 2 x 10-10 g cm-3

Hard to concentrate solids to C > 100

nc = 0.03 C (gas / 10-9 g cm-3) m-3Gas was very dense: •formation at midplane •massive disk •gas compressed?

Chondrules concentrated locally

Chondrule Formation Region

In order for chondrules to collide, region must be >> 10 km

In order for K, Fe, Mg, Si vapor to not diffuse away from chondrules, size of chondrule formation region must exceed 1000 km (Cuzzi & Alexander 2006)

Chemical kinetic models show evaporation took place in presence of gas, P > 10-3 atm (Alexander 2004)

Chondrules and matrix grains have complementary chemical compositions (Wood 1985; Palme et al. 1993): chondrules formed in the presence of dust, in the asteroid belt region

Chondrule Formation Epoch

Relict chondrules are found within other chondrules: chondrule formation occurred repeatedly

Chondrule formation was still taking place 2 Myr after CAIs formed (Amelin et al. 2002)

Some chondrules formed at same as CAI formation (Bizarro et al. 2004)

Chondrules formed over a 2 Myr span

Chondrule Formation: Models •Asteroid impacts (Urey & Craig 1953; Urey 1967; Sanders 1996)

•Asteroid magmatic processes (Merill 1920; Chen et al. 1998; Lugmair & Shukolyukov 2001)

•Formation by ablation in bipolar outflows (Skinner 1990; Liffman 1995)

•Flares near early Sun / X-wind model (Shu et al. 1996,1997, 2001)

•Magnetic flares within the solar nebula (Levy & Araki 1989)

•Lightning (Morfill et al. 1993; Pilipp et al. 1998; Desch & Cuzzi 2000)

•Shock waves in solar nebula gas (Wood 1963; Iida et al. 2001; Desch & Connolly 2002; Ciesla & Hood 2002)

No gas, matrix dust. Thermal histories? Scale?

No gas, matrix dust. Thermal histories?

Scales too small. Thermal histories?

Thermal histories just right!

Chondrule Formation: Shock Models

Desch & Connolly (2002) Iida et al. (2001) Ciesla & Hood (2002) Desch et al. (2005)

gas ~ 0.3 - 3 x 10-9 g cm-3

C < 10 (porphyritic) to C ~ 100 (radial, barred)

Vs ~ 6 - 9 km s-1

Reproduce chondrule thermal histories

Chondrule Formation: Shock Models •X-ray flares impinging on top of disk (Nakamoto et al. 2005)

•Clumpy accretion (Boss and Graham 1993)

•Accretion shock on top of disk (Ruzmaikina & Ip 1994)

•Accretion shocks in Jovian subnebula (Ruffert & Nelson 2005)

•Bow shocks of eccentric planetesimals (Hood 1998; Weidenschilling et al. 1998; Ciesla et al. 2004)

•Large-scale shocks from gravitational instabilities (Wood 1984; Wood 1996; Desch & Connolly 2002; Boss & Durisen 2005)

Densities too low

Scales too small? Timing?

Satisfies all constraints!

Chondrule Formation: Shock Models

Boss & Durisen (2005)

spiral-density waves

shock front at 2-3 AU, propagating at about 5-10 km s-1 relative to surrounding gas

Short-Lived Radionuclides CAIs & chondrules held radionuclides like 26Al, (t1/2 = 0.7 Myr)

26Mg

24Mg

27Al

24Mg26Mg 26Mg 26Al 27Al

24Mg 24Mg 27Al 24Mg= + x

0 0

26Al/27Al ~ 5 x 10-5

Short-Lived Radionuclides WARNING! 26Al/27Al = 5 x 10-5 is NOT the value at the beginning of the Solar System, nor when the first CAIs formed. It is the value when most CAIs stopped being isotopically disturbed (melted).

Bulk isotopic analyses show that 26Al/27Al was as high as 6 x 10-5 (Young et al. 2005) at some point. Most CAIs experienced isotopic closure (stopped melting) 0.4 Myr after that.

Solar System existed for an unknown period of time before the time when 26Al/27Al = 6 x 10-5. Was it many Myr? Was it effectively zero (i.e., the Solar System formed already containing 26Al)?

Short-Lived Radionuclides FUN inclusions

Some CAIs are FUN inclusions (Fractionation and Unknown Nuclear effects), and are thermally processed more than other CAIs, have very odd stable isotope anomalies, and show no evidence at all for 26Al or other radionuclides (26Al/27Al < 10-8)

Most likely explanation: these CAIs formed first, before Solar System acquired 26Al, etc. Other CAIs formed later. “Late injection” (Sahijpal & Goswami 1998)

Solar System formed

Solar System acquired 26Al/27Al = 6 x 10-5

CAIs stopped melting, 26Al/27Al = 4.5 x 10-5

Chondrules

t=0t ~ - 1 Myr? t=+0.4 Myr t=+2.4 Myr

FUN CAIs

CAIs

Short-Lived Radionuclides

And also 10Be, t 1/2 = 1.5 Myr (McKeegan et al. 2000; Sugiura et al. 2001;

MacPherson & Huss 2001; Chaussidon et al. 2001; Srinivasan 2002)

and 60Fe, t 1/2 = 1.5 Myr (Tachibana & Huss 2003; Huss & Tachibana

2004; Mostefaoui et al. 2005; Quitte et al. 2005; Tachibana et al. 2006)

McKeegan et al. (2000)

10Be/9Be ~ 10-3

Tachibana et al. (2006)

60Fe/56Fe ~ 10-6

Short-Lived Radionuclides

41Ca (t1/2 = 0.1 Myr) (Srinivasan et al. 1994, 1996) 36Cl (t1/2 = 0.3 Myr) (Murty et al. 1997; Lin et al. 2004) 26Al (t1/2 = 0.7 Myr) (Lee et al. 1976; MacPherson et al. 1995) 60Fe (t1/2 = 1.5 Myr) (Tachibana & Huss 2003; Mostefaoui et al. 2004) 10Be (t1/2 = 1.5 Myr) (McKeegan et al. 2000; Sugiura et al. 2001) 53Mn (t1/2 = 3.7 Myr) (Birck & Allegre 1985) 107Pd (t1/2 = 6.5 Myr) (Kelly & Wasserburg 1978)

182Hf (t1/2 = 9 Myr) (Harper & Jacobsen 1994)

129I (t1/2 = 15.7 Myr) (Jeffery & Reynolds 1961)

In fact, 9 radionuclides with t1/2 < 16 Myr have been confirmed from meteorites...

Where did they come from?! Especially 60Fe!!

Short-Lived Radionuclides Irradiation / Spallation within the Solar System (a la X-wind)?

Shu et al. (1996)

Irradiation of rocky material at 0.1 AU in principle can produce radionuclides like 60Fe

Only 64Ni(p,p)60Fe reaction can happen, but 64Ni is rare and cross section is < 0.1 mbarn

Predicted yields:

60Fe/56Fe ~ 10-11

(Lee et al 1998; Leya et al 2003)

Short-Lived Radionuclides

Inherited from the Interstellar Medium?

Steady-state “average” abundance of 60Fe in Galaxy is 60Fe/56Fe ~ 3 x 10-8 (Wasserburg et al 1996) to 60Fe/56Fe ~ 3 x 10-7 (Harper 1996), even with prompt delivery. The ratio decreases the longer molecular clouds take to form stars.

AGB star would have to be < 1 pc away, < 1 Myr before Solar System formed. Observed probability of this event is < 3 x 10-6 (Kastner & Myers 1994)

Same arguments would hold for Type Ia supernovae, novae, etc.

Injected by AGB star just before Solar System formation?

Short-Lived Radionuclides Injected by supernova during Solar System formation?

Type II supernovae are the only source of 60Fe naturally associated with star-forming regions.

70 - 90% of all low-mass stars within 2 kpc form in rich embedded clusters, with > 100 members (Lada & Lada 2003).

Probability of a such clusters containing a massive (> 25 M) star are ~ 70% (Adams & Laughlin 2001). Confirmed by census of such clusters.

Therefore > 50% of all low-mass stars are associated with massive stars that will go supernova within a few Myr.

Short-Lived Radionuclides Injected by supernova during Solar System formation?

Type II supernovae are the only source of 60Fe naturally associated with star-forming regions.

protoplanetary disks

~ 0.2 pc

1 Ori C: 40 M star will supernova in 1-4 Myr

HST image, Orion Nebula

Short-Lived Radionuclides G353.2+0.9 H II region in NGC 6357 (Healy et al. 2004; Hester & Desch 2005)

~ 0.4 pc

Pismis 24-1 (O3 If*), Pismis 24-17 (O3 IIIf*) and Wolf-Rayet stars (Massey et al. 2001)

These stars will supernova in < 1 Myr

Short-Lived Radionuclides

Iron likely in form of dust grains: gas-phase Fe disappeared from SN 1987A ejecta at same time (2 years post-explosion) that 10-3 M of dust formed (Colgan et al 1994)

Mass of 60Fe ejected by 25 M supernova ~ 8 x 10-6 M (Woosley & Weaver 1995)

Fraction intercepted by 30 AU radius disk at 0.3 pc away ~ (30 AU)2 / 4 (0.3 pc)2 ~ 6 x 10-8

Mixed with 0.01 M of solar composition material, 60Fe / 56Fe ~ 1 x 10-6

Injection of radioactive grains directly into protoplanetary disk supplies just enough 60Fe!

One supernova can also inject the other short-lived radionuclides with observed abundances (Ellinger et al. 2007, in preparation)

Short-Lived Radionuclides Protoplanetary disks ~ 0.3 pc from a supernova (1051 erg) are not destroyed! (Chevalier 2000; Ouellette et al. 2005; Ouellette et al. 2006 in prep.)

Ouellette et al. 2006 (in preparation)

Radioactive grains aren’t deflected around disk

Conclusions CHONDRULE FORMATION:

Disk was likely massive (10 x minimum-mass solar nebula), and experienced gravitational instabilities and shocks.

Other solids would have been heated by shocks (crystalline silicates)

SHORT-LIVED RADIONUCLIDES:

Disk was too hot (at least transiently) for first 1-2 Myr for most solids to survive.

60Fe shows Solar System formed near a supernova in an H II region, definitely not in a quiescent molecular cloud like Taurus.

High UV fluxes would have photoevaporated our disk down to 30 AU, and stellar encounters would disrupt orbits, consistent with observations of Kuiper Belt.