Chemical evolution of presolar compounds: from disks to earth-like planets.

1. ASU: Nov 9, 2009. 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

2. 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

3. Origins & Astrobiology: Interstellar medium --> planets --> life http://www.spaceflight.esa.int/users/images/commonpic/ISM.jpe

4. Outline • PAHs in space and in meteorites • Destruction of PAHs in planet-forming disks • Delivery of organics to Earth via micrometeorites pyrene

5. 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

6. PAHs in astrophysical environments Ames Astrochemistry Lab

7. “protoplanetary disks”

8. 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

9. Carbonaceous chondrites contain abundant aromatic carbon (G. Cody, Carnegie) http://www.gl.ciw.edu/~cody/meteorite_files/IMAGE006.JPG

10. Carbon in primitive meteorites is mostly aromatic Cody & Alexander 2005

11. PAHs are the most abundant form of condensible carbon in terrestrial planet-forming region of disks: Condensible carbon + OH, H H2 + CO

12. 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’

13. PAH and related compounds Phenanthrene Cyclopropene Naphthalene Benzene Acenaphthene Pyrene A3-C2H

14. Pathways to destroying PAH T = 1000 K (started with A2 initially)

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

16. 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

17. High abundances of simple organics exist in the inner regions of planet-forming disks Abundances relative to CO H2O 1.3 OH 0.18 HCN 0.13C2H2 0.016 CO2 0.004-0.26 T = ~500-1000 K (Unlabeled features are H2O) Carr & Najita (Science 2008)

18. 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 1 OH 0.18 3 x 10-6(shocks, UV, x-rays?) C2H2 0.016 0.1 CO2 0.004-0.26 0.002 HCN 0.13 ~0.1 (highly dep. on t and NH3)

19. 1010 log(e-folding time for PAH destruction, sec) 105 1000 1500 2000 2500 Temperature (K) ~106 years 300 years ~1 day

20. Terrestrial planets form from solids not gas Solids agglomerate for ~1 Myr time 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.

21. 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

22. (c) Tezel 2001 Micrometeorites are very strongly heated as they enter the atmosphere

23. 30,000,000 kg of meteorites fall to Earth every year increasing particle size 0.1 mm shooting stars fireballs mountain dust sandrockboulder smoke Anders 1989

24. -Pictoris Beuzit et al, ESO/Obs. Grenoble 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

25. What happened to the carbon in these strongly-heated micrometeorites? ~100 m in diameter; olivine, magnetite, glass... metal sulfide

26. unmelted ~10m 50%wt C Don Brownlee

28. Experiment: Simulate atmospheric entry Grind up bulk Murchison matrix into ~300 m particles Flash-heat in pyroprobe: 500 K/sec to ~900-1000 K Volatile products analyzed with GC

29. 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

30. Flash heating of Murchison Meteorite Powder Organics Detected Alkylbenzenes Phenol Alkylthiophenes Benzonitrile Benzothiophene Hydrocarbons Naphthalene Styrene Contaminant ... 710 °C @ 500 °C/sec GC retention time 610 °C @ 500 °C/sec G. Cody, Carnegie

31. 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)

32. ...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!

33. At what altitude are organics released in Earth’s atmosphere? (Alice Pevyhouse, MS Thesis) Entry angle = 80o from vertical

34. 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

35. 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!

36. 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?

37. 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?

38. Disks are complex regions Data/ Constraints/ Tests of models: Numerical experiments observations of disks laboratory experiments

39. 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