chemical evolution of presolar compounds: from disks to earth- like planets. monika kress department...

Chemical evolution of presolar compounds:

from disks to earth-like planets.

Monika Kress

Department of Physics & Astronomy,San Jose State University

Virtual Planetary Laboratory,NASA Astrobiology Institute

ASU: Nov 9, 2009.

Alice Pevyhouse (MS), SJSUHamadi McIntosh (BS), SJSU Xander Tielens, Leiden Univ.Michael Frenklach, UC BerkeleyVikki Meadows, U. WashingtonSean Raymond, U. Colorado

Collaborators:

http://www.spaceflight.esa.int/users/images/commonpic/ISM.jpe

Origins & Astrobiology: Interstellar medium --> planets --> life

• PAHs in space and in meteorites • Destruction of PAHs in planet-forming

disks• Delivery of organics to Earth via

micrometeorites

pyrene

Outline

Polycyclic Aromatic Hydrocarbons (PAHs) Polycyclic Aromatic Hydrocarbons (PAHs) Strongly bound pi-bonded cyclic

hydrocarbons (‘aromatic’)Prominent nonthermal emission

featuresForm in carbon starsReaction mechanism is very well

studied experimentally

Extremely stable: • oxidizing/reducing conditions • high temperatures • UV radiation

In ISM: ~10% of C is in PAHs

PAHs in astrophysical environmentsPAHs in astrophysical environments

Ames Astrochemistry Lab

“protoplanetary disks”

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Geers et al, A&A 2008

Observations of Observations of disks around disks around young stars:young stars:

PAHs are modified in disk environments

PAHs are at lower abundance in disks than in diffuse ISM

http://www.gl.ciw.edu/~cody/meteorite_files/IMAGE006.JPG

Carbonaceous chondrites contain abundant aromatic carbon Carbonaceous chondrites contain abundant aromatic carbon (G. Cody, Carnegie)(G. Cody, Carnegie)

Cody & Alexander 2005

Carbon in primitive meteorites is mostly aromatic

PAHs are the most abundant form of condensible carbon in terrestrial planet-forming region of disks:

H2 + COCondensible carbon + OH, H

• PAHs are well-studied under combustion conditions: P ~ 1atm, T ~ 1000 - 2500 K

• Combustion kinetics model developed by M. Frenklach (UC Berkeley) for sooting flames

• Considers only thermally-driven reactions between H, C, O and N

• Largest PAH in model is pyrene (A4), the smallest ‘stabilomer’

Modeling the destruction of PAHs

Cyclopropene Cyclopropene

BenzeneBenzene

NaphthaleneNaphthalene

Acenaphthene Acenaphthene

PhenanthrenePhenanthrene

PyrenePyrene

PAH and related compoundsPAH and related compounds

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A3-C2H

Pathways to destroying PAH

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A4 +H → A3- 4 +C2H2

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A4 +H → A3- C2H +H€

A4 +OH → A3- 4 +CH2CO

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A4 +O → A3- 4 +HCCO

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log(

rate

s(m

oles

/cm

3/s

ec))

(started with A2 initially)

T = 1000 K

€

rate = k A[ ] B[ ]

€

k = AT ne−E /RT

Model results: 1200 K, starting with HCN:PAHs destroyed ~103 yr

Model results: condensible carbon (PAH) is destroyed in the inner disk

• Reactions driven by H and OH• Highly T-dependent:

• T > 1100 K: destruction < ~ few kyr• T < 1000 K: survive over disk timescales

• Small organics form in great abundance, can persist for ~ disk timescales

• HCN forms when NH3 is initially present, & vice versa

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Carr & Najita (Science 2008)

H2O 1.3OH 0.18HCN 0.13C2H2 0.016CO2 0.004-0.26

Abundances relative to CO

T = ~500-1000 K

High abundances of simple organics exist in the inner regions of planet-forming disks

(Unlabeled features are H2O)

Model results for T = 1100 K, P = 10Model results for T = 1100 K, P = 10-6-6 atm. atm. Input: Pyrene, water, CO and HInput: Pyrene, water, CO and H22 only. only.

Abundances, relative to CO:

observed model (peak value) (Carr & Najita 2008)

H2O 1.3 1OH 0.18 3 x 10-6 (shocks, UV, x-rays?) C2H2 0.016 0.1 CO2 0.004-0.26 0.002HCN 0.13 ~0.1 (highly dep. on t and NH3)

1010

105

log(

e-fo

ldin

g tim

e fo

r P

AH

des

truc

tion,

sec

)

1000 1500 2000 2500

Temperature (K)

300 years

~1 day

~106 years

Midplane temperature profile for disks from Bell et al 1997.

time

Interpretation:PAHs should survive in the gas phase; may or may not condense

disk timescale = PAH destruction timescale

Terrestrial planets Terrestrial planets form from solids not form from solids not gasgasSolids agglomerate Solids agglomerate for ~1 Myrfor ~1 Myr

ConclusionsConclusions• PAHs are the most abundant condensible form of carbon in

the terrestrial-planet forming region of disks

• Inner disk conditions destroy rather than form PAHs via thermally-driven reactions

=> PAHs must have presolar heritage

=> high abundances of CO2, C2H2, CH4 and HCN can persist for > 105 yr

=> abundances consistent with observations of disks

• Earth got (most of?) its carbon from asteroid belt (same place as water)

• A “soot line” occurs where T ~ 1000 K:

=> consistent w/ bulk compositions of primitive meteorites

(c) Tezel 2001

Micrometeorites are very strongly heated as they enter the atmosphere

30,000,000 kg of meteorites fall to Earth every year

mountaindust sand rock bouldersmoke

increasing particle size

0.1 mm

shooting stars fireballs

Anders 1989

Exogenous influx at 4 Ga would have been >> than today:

Most stars have debris disks for 300 Myr

timescale ~Late heavy bombardment

Flux ~ 106 x today

Beuzit et al, ESO/Obs. Grenoble

-Pictoris

What happened to the carbon in these strongly-heated micrometeorites?

~100 m in diameter; olivine, magnetite, glass... metal sulfide

Don Brownlee

unmelted~10m50%wt C

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Experiment: Simulate atmospheric entry

1. Grind up bulk Murchison matrix into ~300 m particles

2. Flash-heat in pyroprobe: 500 K/sec to ~900-1000 K

3. Volatile products analyzed with GC

Products released during Murchison flash-heating experiments

Major products: • CO, CO2, H2O (as expected)

• CH4, SO2 and H2S (interesting!)

Other products (very interesting!):• Hydrocarbons • Numerous functionalized polycyclics (PAHs)• Various heterocycles

OH

S

CH3

S

S

CN

CH3

H3C

CN

S

OH

S

S

710 °C @ 500 °C/sec

610 °C @ 500 °C/sec

Flash heating of Murchison Meteorite Powder

Organics DetectedAlkylbenzenesPhenolAlkylthiophenesBenzonitrileBenzothiopheneHydrocarbonsNaphthaleneStyreneContaminant...

GC retention timeG. Cody, Carnegie

• CH4 - an important greenhouse gas in Archean and Proterozoic (and Hadean?)

What are the implications for early Earth?

Assume that Murchison is representative, and that 10% of the C --> CH4:

modern CH4 formation rate from micrometeorites ~108 g yr-1

compare to modern abiotic CH4 formation rate ~1013 g yr-1

At 4 Ga, CH4 form. rate ~ 1014 g yr-1 (~ total modern rate)

• Hydrocarbons (e.g. CH4, C2H6) play key role in smog/haze formation

• PAHs provide pre-O3 UV protection?

• Disequilibrium chemistry : false positive biosignature in exoplanet atmosphere?

...More implications ....

... more than just prebiotic organics!

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Entry angle = 80o from vertical

At what altitude are organics released in Earth’s atmosphere?(Alice Pevyhouse, MS Thesis)

Altitude of Release Affects Fate• Consider methane (CH4)

at 100 km: CH4 destroyed by photochemistry before it can be mixed by atmospheric motions

at 70 km: CH4 lives long enough to mix zonally and vertically

• Survival of organics is favored by delivery deeper in atmosphere

• Compounds that are more photochemically stable than methane, such as naphthalene and other PAHs, may live long enough to mix down into the atmosphere, even if deposited as high as 100 km

if released at 100 km if released at 70 km

photochemical lifetime ~3-4 days ~ 8 months

vertical mixing ~1 month ~ 1 month

zonal mixing ~ 3-4 days < 1 day

Biggest challenge to delivering organics to Habitable planets:

Getting below as much of the atmosphere as possible!

Conclusion…

Don’t write off micrometeorites just yet!

Use PAH model and new generation of disk models & observations

• to constrain the extent of mixing in the disk

• to isolate which meteoritic constituents are presolar and which are likely due to processing in the disk or parent body

• to further define the link between the ISM and the compounds arriving on early Earth

• Molecular abundances in disks: clocks, thermometers?

Further studiesFurther studies

• Given variations in disk evolution (i.e. how fast does it cool and disperse) and the luminosity of the star, exoterrestrial planets may have >> earth abundance of C and water, or much less?

• What is the primordial composition (before heat and aqueous alteration) of planet-building materials? What fell when, and what was it made of?

VPL scienceVPL science

Data/ Constraints/ Tests of models:

Numerical experimentsobservations of diskslaboratory experiments

Disks are complex regions

New disk models (e.g.Gail 2001,2002) consider initial chemical composition (ISM) and conditions in disk

Hot material transported out, cool material falling in