ASU: Nov 9, 2009.
Chemical evolution of
presolar compounds:
from disks to earthlike planets.
Monika Kress
Department of Physics & Astronomy,
San Jose State University
Virtual Planetary Laboratory,
NASA Astrobiology Institute
Collaborators:
Alice Pevyhouse (MS), SJSU
Hamadi McIntosh (BS), SJSU
Xander Tielens, Leiden Univ.
Michael Frenklach, UC Berkeley
Vikki Meadows, U. Washington
Sean Raymond, U. Colorado
Origins & Astrobiology: Interstellar medium --> planets --> life
http://www.spaceflight.esa.int/users/images/commonpic/ISM.jpe
Outline
• PAHs in space and in meteorites
• Destruction of PAHs in planet-forming
disks
• Delivery of organics to Earth via
micrometeorites
pyrene
Polycyclic Aromatic Hydrocarbons (PAHs)
Strongly bound pi-bonded cyclic
hydrocarbons (‘aromatic’)
Prominent nonthermal emission
features
Form in carbon stars
Reaction 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 environments
Ames Astrochemistry Lab
“protoplanetary disks”
Geers et al, A&A 2008
Observations of
disks around
young stars:
PAHs are
modified in disk
environments
PAHs are at
lower
abundance in
disks than in
diffuse ISM
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Carbonaceous chondrites contain abundant aromatic carbon
(G. Cody, Carnegie)
http://www.gl.ciw.edu/~cody/meteorite_files/IMAGE006.JPG
Carbon in primitive
meteorites is
mostly aromatic
Cody &
Alexander 2005
PAHs are the most
abundant form of
condensible carbon in
terrestrial planetforming region of
disks:
Condensible carbon + OH, H
H2 + CO
Modeling the destruction of PAHs
• 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’
PAH and related compounds
Cyclopropene
Benzene
Naphthalene
Phenanthrene
QuickTi me™ a nd a
de com press or
are need ed to se e th is p icture.
Acenaphthene
A3-C2H
Pyrene
Pathways to destroying PAH
log(rates(moles / cm3 / sec))
T = 1000 K
(started with A2 initially)
A4 +OH  A3 - 4 +CH2CO
A4 +H  A3 - 4 +C2H2

A4 +H  A3 - C2H +H

A4 +O  A3 - 4 +HCCO


rate = k AB

n E /RT
k  AT e
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
High abundances of simple organics exist in
the inner regions of planet-forming disks
Abundances
relative to CO
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
H2O
1.3
OH
0.18
HCN 0.13
C2H2 0.016
CO2 0.004-0.26
T = ~500-1000 K
(Unlabeled features are H2O)
Carr & Najita (Science 2008)
Model results for T = 1100 K, P = 10-6 atm.
Input: Pyrene, water, CO and H2 only.
Abundances, relative to CO:
observed
model (peak value)
(Carr & Najita 2008)
H2O
1.3
OH
0.18
C2H2
0.016
CO2 0.004-0.26
HCN 0.13
1
3 x 10-6 (shocks, UV, x-rays?)
0.1
0.002
~0.1 (highly dep. on t and NH3)
log(e-folding time for PAH destruction, sec)
~106 years
1010
300 years
105
~1 day
1000
1500
2000
Temperature (K)
2500
disk timescale =
PAH destruction
timescale
Interpretation:
PAHs should survive
in the gas phase;
may or may not
condense
Midplane temperature profile for disks from Bell et al 1997.
time
Terrestrial planets
form from solids not
gas
Solids agglomerate
for ~1 Myr
Conclusions
•
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
Micrometeorites are very strongly heated as they enter the atmosphere
(c) Tezel 2001
30,000,000 kg of meteorites fall to Earth every year
increasing particle size
0.1 mm
Anders
1989
shooting stars
fireballs
smoke dust sand rock boulder
mountain
-Pictoris
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
What happened to the carbon in these
strongly-heated micrometeorites?
~100 m in diameter; olivine, magnetite, glass... metal sulfide
unmelted
~10m
50%wt C
Don Brownlee
QuickTime™ and a
decompressor
are needed to see this picture.
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
Flash heating of Murchison Meteorite Powder
CH3
OH
Organics Detected
Alkylbenzenes
Phenol
Alkylthiophenes
Benzonitrile
Benzothiophene
Hydrocarbons
Naphthalene
Styrene
Contaminant
...
CN
S
S
S
710 °C @ 500 °C/sec
OH
CH3
H3 C
CN
S
S
S
G. Cody, Carnegie
GC retention time
610 °C @ 500 °C/sec
What are the implications for early Earth?
• CH4 - an important greenhouse gas in
Archean and Proterozoic (and Hadean?)
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)
...More implications ....
• 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 than just prebiotic organics!
At what altitude are organics released in Earth’s atmosphere?
(Alice Pevyhouse, MS Thesis)
QuickTime™ and a
decompressor
are needed to see this picture.
Entry angle = 80o from vertical
Altitude of Release Affects Fate
• Consider methane (CH4)
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
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
Conclusion…
Don’t write off micrometeorites just yet!
Biggest challenge to delivering organics to Habitable planets:
Getting below as much of the
atmosphere as possible!
Further studies
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?
VPL science
• 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?
Disks are complex regions
Data/ Constraints/ Tests of models:
Numerical experiments
observations of disks
laboratory experiments
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