Shock Processing of Chondritic Material

Steve Desch (Arizona State University)Fred Ciesla (NASA Ames Research Center)

Lon Hood (University of Arizona)Taishi Nakamoto (University of Tsukuba)

Chondrites and the Protoplanetary Disk

Kauai, Hawaii

November 11, 2004

MotivationThree groups calculating thermal histories of chondrules in shocks:•Nakamoto & collaborators (Iida et al. 2001 = INSN)

•Desch & collaborators (Desch & Connolly 2002 = DC02)

•Ciesla, Hood and collaborators (Ciesla & Hood 2002 = CH02)

Where do they agree? Where do they disagree?

Do solar nebula shock models melt chondrules in ways consistent with their petrography?

Do shocks affect other solar nebula solids?

Shocks in the Solar NebulaPlanetesimal Bow ShocksWeidenschilling et al 1998; Hood et al 1998; Hood et al [poster, this conference]

At disk midplane, but small scale (< 1000 km)

Bow shock

~ 1000 km

Gravitational Instabilities (Spiral Shocks)Boss 2001; Nelson [poster this conference]; Moley & Durisen [poster this conference], etc.

Act over entire nebula.

Shocks Induced by X-ray Flares Nakamoto et al 2004; Miura & Nakamoto [poster, this conference]

Act only in the low-density regions high above disk

flares accelerate gas, which flows into disk

shock here

How do Shocks Work?

Shocks are a “Hydrodynamic Surprise!”

Vs

There is a discontinuity in gas properties

Effects of discontinuity on gas are communicated upstream at sound speed

C

If gas is supersonic, it reaches discontinuity before hearing about it

When gas hits discontinuity, it must rapidly readjust by collisions between molecules

A few mean-free-paths: < 1 m

“SHOCK FRONT”

Supersonic w.r.t. shock front

Subsonic w.r.t shock front

Gas is slowed by shock

1 2

V1 V2

d

d x( V) = 0 1 V1 = 2 V2

X

Conservation of Mass

(Frame of reference of Shock Front)

Gas is slowed and compressed by shock

d

d x( P + V2) = 0

Conservation of Momentum:

Conservation of Energy:

d

d x [ ]( V)7 P 1

2 2+ V2 + FRAD = -

If not for radiation terms FRAD and , we could find V, and P (and T!) behind shock right now

Gas is slowed and compressed and heated by shock

Line Cooling () = rate at which gas cools by emitting “line radiation”: infrared photons emitted by water or CO molecules

INSN assume ALL line photons escape, = Neufeld & Kaufman (1993), gas cools in ~ 10 minutes

DC02, CH02 assume NO line photons can escape (gas optically thick), = 0

Truth is in between; more important at low densities

The Difficulty of Including SolidsFRAD = Flux of (infrared) radiation emitted by solids: chondrules and dust grains -- must include to get TFINAL!

Hood & Horanyi (1993), DC02 assumed FRAD = T4

FINAL ... but that’s not right

FRAD must come from radiative transfer calculation of JRAD

FRAD -d JRAD

d

“mean intensity of radiation field”

“optical depth”

JRAD() = TINIT4 E2 ( - INIT)

+ TFINAL4 E2 ( - + FINAL)

+ T4(t) E1 | t - | dt

1

21

2

JRAD depends on chondrule temperatures everywhere

JRAD depends on TFINAL

DC02 and CH02 solved for JRAD

INIT

FINAL

Chondrule Temperatures

4a2 T4 = 4a2 JRAD + “Heating by Gas”

Chondrules emit infrared and cool

Chondrules absorb radiation and are heated

Chondrules exchange thermal energy with gas

Chondrules heated by friction immediately after passing through shock

1 2

V1 V2

gas slowed in shock front in < 1 ms

V1 V1

V2

V2

chondrule takes about 1 minute to slow down

drag heating stage short-lived

Considerable Number of Feedbacks!JRAD and FRAD depend on T everywhere and TFINAL

T everywhere depends strongly on JRAD

TFINAL depends on FRAD

TFINAL will NOT equal TAMBIENT in 1-D calculation!

CH02 underestimate final T, effects of radiation

DC02 overestimate final T, effects of radiation

INSN do not calculate radiative transfer from solids, but do include gas radiative losses ()

Application to Chondrules

CH02TAMBIENT

TAMBIENT

Radiation heats chondrule before it reaches shock

Radiation heats chondrule before it reaches shock

Friction adds spike to heating while chondrule slows down (lasts about 1 minute)

Friction adds spike to heating while chondrule slows down (lasts about 1 minute)

Chondrule heated by radiation, hot gas, for hours

Chondrule heated by radiation, hot gas, for hours

TFINAL TAMBIENT

(CH02)

CH02

Cooling very rapid (~ 104 K/hr) in drag heating stage

After drag heating stage, cooling takes hours because gas is hot

crystallization temperatures

1 hr

INSN

In INSN model, line emission cools gas, chondrules in ~ minutes

Consistency with Meteoritic RecordCooling Rates

Crystallization textures constrain cooling rates in crystallization temperature range 1400 - 1800 K

Porphyritic chondrules: 10 - 1000 K/hr (Hewins 1996; reviewed in DC02)

Barred olivine chondrules: 500 - 3000 K/hr (Connolly et al 1998; reviewed DC02)

Cooling rates above liquidus (1800K) constrained to be > 5000 K/hr by retention of volatiles (Yu & Hewins 1996)

Cooling rates correlate with chondrule density

Compound chondrules preferentially form where chondrule densities are higher

Compound chondrules are ~ 70% barred olivines, cooled > 1000 K/hr

Regular chondrules are ~ 85% porphyritic chondrules, cooled < 1000 K/hr

After drag stage, cooling rates proportional to chondrule density in shock models (DC02, CH02)

Heating rate

Lack of isotopic fractionation (e.g., K) constrains heating rate > 104 K/hr in temperature range 1300 - 1600 K (Tachibana et al 2004)

Consistent with shocks (if TFINAL < 1100 K: not Hood & Horanyi 1993 / DC02 jump conditions)

High Pressures

High pressures (~ 10-3 atm) needed to suppress evaporation of Fe, etc. (e.g., Miura et al 2002)

Maximum Size of Chondrules

Chondrules > 1 mm very rare

Consistent with shocks: as melted chondrule droplets decelerate, large ones breaks apart (Susa & Nakamoto 2002)

Chondrule - Matrix Complementarity

Compositions of chondrules and surrounding matrix grains in OCs, CVs, CMs strongly suggest they came from same region (Wood 1985; Palme et al 1993)

Thicknesses of fine-grained rims correlate with chondrule size, also suggesting chondrules and matrix grains came from same region (Morfill et al 1999)

Shocks & Other Solar Nebula SolidsAnnealing of Silicate Grains

Crystalline silicate grains like those observed in comets (e.g., Wooden et al 1999) can be produced if shocks anneal amorphous grains (e.g., Harker & Desch 2002)

Chemical Reactions

Behind shocks, water vapor pressures elevated and kinetic rates increased, allowing formation of fine-grained phyllosilicates (Ciesla et al 2003)

nitrogen processed into NO, HCN, etc.

Kress et al 2002

Summaryo Differences remain among three groups:

o Do line photons escape? How to implement proper jump condition for TFINAL?

o INSN model tends to be more appropriate to lower densities, DC02 and CH02 to higher densities

o Nonetheless, good convergence among groups

o All shock models consistent with wide variety of meteoritic constraints on chondrules

o Shocks very likely thermally processed chondrules, and other solar nebula solids, too