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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.
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Alice Pevyhouse (MS), SJSUHamadi McIntosh (BS), SJSU Xander Tielens, Leiden Univ.Michael Frenklach, UC BerkeleyVikki Meadows, U. WashingtonSean Raymond, U. Colorado
Collaborators:
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http://www.spaceflight.esa.int/users/images/commonpic/ISM.jpe
Origins & Astrobiology: Interstellar medium --> planets --> life
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• PAHs in space and in meteorites • Destruction of PAHs in planet-forming
disks• Delivery of organics to Earth via
micrometeorites
pyrene
Outline
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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
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PAHs in astrophysical environmentsPAHs in astrophysical environments
Ames Astrochemistry Lab
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“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
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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)
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Cody & Alexander 2005
Carbon in primitive meteorites is mostly aromatic
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PAHs are the most abundant form of condensible carbon in terrestrial planet-forming region of disks:
H2 + COCondensible carbon + OH, H
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• 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
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Cyclopropene Cyclopropene
BenzeneBenzene
NaphthaleneNaphthalene
Acenaphthene Acenaphthene
PhenanthrenePhenanthrene
PyrenePyrene
PAH and related compoundsPAH and related compounds
QuickTime™ and a decompressor
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A3-C2H
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Pathways to destroying PAH
€
A4 +H → A3- 4 +C2H2
€
A4 +H → A3- C2H +H€
A4 +OH → A3- 4 +CH2CO
€
A4 +O → A3- 4 +HCCO
€
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
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Model results: 1200 K, starting with HCN:PAHs destroyed ~103 yr
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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)
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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)
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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
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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
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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
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(c) Tezel 2001
Micrometeorites are very strongly heated as they enter the atmosphere
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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
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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
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What happened to the carbon in these strongly-heated micrometeorites?
~100 m in diameter; olivine, magnetite, glass... metal sulfide
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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
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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
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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
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• 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)
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• 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)
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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
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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!
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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
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• 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
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Data/ Constraints/ Tests of models:
Numerical experimentsobservations of diskslaboratory experiments
Disks are complex regions
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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
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