1. 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.

2. Alice Pevyhouse (MS), SJSU Hamadi McIntosh (BS), SJSU Xander Tielens, Leiden Univ. Michael Frenklach, UC Berkeley Vikki Meadows, U. Washington Sean Raymond, U. Colorado Collaborators:

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

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

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

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

11. PAHs are the most abundant form of condensible carbon in terrestrial planet- forming region of disks: H 2 + CO Condensible carbon + OH, H

12. 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

13. Cyclopropene Benzene Naphthalene Acenaphthene Phenanthrene Pyrene PAH and related compounds A3-C 2 H

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

15. Model results: 1200 K, starting with HCN: PAHs destroyed ~10 3 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 NH 3 is initially present, & vice versa

17. Carr & Najita (Science 2008) H 2 O 1.3 OH 0.18 HCN 0.13 C 2 H 2 0.016 CO 2 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 H 2 O)

18. Model results for T = 1100 K, P = 10 -6 atm. Input: Pyrene, water, CO and H 2 only. Abundances, relative to CO: observed model (peak value) (Carr & Najita 2008) H 2 O 1.31 OH 0.18 3 x 10 -6 (shocks, UV, x-rays?) C 2 H 2 0.016 0.1 CO 2 0.004-0.26 0.002 HCN 0.13 ~0.1 (highly dep. on t and NH 3 )

19. 10 10 5 log(e-folding time for PAH destruction, sec) 1000150020002500 Temperature (K) 300 years ~1 day ~10 6 years

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

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 CO 2, C 2 H 2, CH 4 and HCN can persist for > 10 5 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 mountain dust sand rock boulder smoke increasing particle size 0.1 mm shooting stars fireballs Anders 1989

24. Exogenous influx at 4 Ga would have been >> than today: Most stars have debris disks for 300 Myr timescale ~ Late heavy bombardment Flux ~ 10 6 x today Beuzit et al, ESO/Obs. Grenoble  -Pictoris

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

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

28. 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

29. Products released during Murchison flash-heating experiments Major products: CO, CO 2, H 2 O (as expected) CH 4, SO 2 and H 2 S (interesting!) Other products (very interesting!): Hydrocarbons Numerous functionalized polycyclics (PAHs) Various heterocycles

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

31. CH 4 - 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 --> CH 4 : modern CH 4 formation rate from micrometeorites ~10 8 g yr -1 compare to modern abiotic CH 4 formation rate ~10 13 g yr -1 At 4 Ga, CH 4 form. rate ~ 10 14 g yr -1 (~ total modern rate)

32. Hydrocarbons (e.g. CH 4, C 2 H 6 ) play key role in smog/haze formation PAHs provide pre-O 3 UV protection? Disequilibrium chemistry : false positive biosignature in exoplanet atmosphere?...More implications....... more than just prebiotic organics!

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

34. Altitude of Release Affects Fate Consider methane (CH 4 ) at 100 km: CH 4 destroyed by photochemistry before it can be mixed by atmospheric motions at 70 km: CH 4 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 kmif released at 70 km photochemical lifetime~3-4 days~ 8 months vertical mixing~1 month zonal mixing~ 3-4 days< 1 day

35. 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!

36. 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 studies

37. 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 science

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

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