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(Universal Life) Consequences of Heavy Bombardment Periods on Chemistry of the Early Earth

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As the Sun formed from its molecular cloud, it was accompanied by a disk of material that consisted of gas and small dust particles. Over a few tens of millions of years,  these dust particles accumulated and formed the planets that we see today. This process occurred in several stages in the terrestrial planet zone, eventually culminating in massive, potentially moon-forming impacts on the proto-Earth.(1) Then, following the solidification of the Moon around 4.5 Ga, the initially heavy impactor flux declined(2) and increased again during the Late Heavy Bombardment (LHB) some 4-3.85 Ga.(2) Best models for the origin of the LHB link it to a dynamic instability in the outer solar system (the so-called Nice model(3),(4)), when Jupiter’s orbit changed as a result of close encounters with ice giants and small cometary bodies. These changes resulted in the release of impactors from their previously stable asteroidal and cometary reservoirs. The synthesis of observation and theoretical constraints indicates that the impactor flux on the Earth was around 10 times higher at the LHB  than in the period immediately preceding the LHB and that this flux slowly decayed afterwards.(5),(6),(7) At the peak, the LHB most likely involved an impact frequency of 109 tons of material per year.(5) The typical impact speeds are estimated to have increased from around 9 to 21 km/s once the LHB began. The ratio of the gravitational cross-sections of Earth and the Moon is found to be approximately 17:1. Thus, for every lunar basin, such as Orientale or Imbrium, approximately 17 basins should have formed on the Earth.(8)

Such a huge impact activity also had extensive implications for the evolution of early Earth:(9) the atmosphere was partly eroded and transformed,(10),(11) and the hydrosphere was enriched by water.(12),(13) Crucially, these impact-related processes most likely also contributed to the transformation of biomolecules and their precursors on Earth’s surface, which would have relevant consequences on the origin of life.(14),(15)  Our recent findings demonstrate that extraterrestrial impacts, which were an order of magnitude more abundant during the late heavy bombardment period than before and after, could not only destroy the existing ancient life forms, but they could also contribute to the creation of biogenic molecules. In our pioneering works,(16) we simulated the high-energy synthesis of aminoacids in simple mixtures of molecular gases with compositions (CO2–N2–H2O and CO–N2–H2O) and total pressure close to that of the Earth´s early atmospheres. Glycine, alanine, serine and asparagine were found using HPLC analytical technique. In subsequent experiments, we demonstrated synthesis of all the RNA canonical nucleic bases from formamide during such an impact of an extraterrestrial body.(17) Again, high-power laser has been used to induce the dielectric breakdown of the plasma produced by the impact. The experimental results together with plasma chemistry models and quantum chemistry calculations demonstrate that initial dissociation of the formamide molecule produces a large amount of highly reactive CN and NH radicals, which could further react with formamide to produce adenine, guanine, cytosine, and uracil. Also, according to the optical spectra, LIDB plasma surely contains excited carbon and nitrogen ions, atoms and molecules, and CN radical (also C2 and C3).(18) We also show that sugars can be synthetized from formaldehyde in high energy environment of the shock wave plasma together with basic molecule necessary for catalytic conversion of formaldehyde to sugars in such environments – glycoladehyde (condensation reactions over borates etc.).(19) We can conclude that high energy chemistry serves as a powerful trigger of biomolecules synthesis and it possibly contributed to origin of life on early Earth.

References

1. Canup RM, Asphaug E (2001) Origin of the Moon in a giant impact near the end of the Earth’s formation. Nature 412:708–712.

2. Koeberl C, Reimold WU, McDonald I, Rosing M (2000) in IMPACTS AND THE EARLY EARTH , LECTURE NOTES IN EARTH SCIENCES ., ed Gilmour, I and Koeberl C (SPRINGER-VERLAG BERLIN , HEIDELBERGER PLATZ 3 , D-14197 BERLIN , GERMANY), pp 73–97.

3. Tsiganis K, Gomes R, Morbidelli A, Levison HF (2005) Origin of the orbital architecture of the giant planets of the Solar System. Nature 435:459–461.

4. Nesvorny D, Morbidelli A (2012) Statistical Study of the Early Solar System’s Instability with Four, Five, and Six Giant Planets. Astron J 144 . 5. Koeberl C (2006) Impact processes on the early Earth. Elements 2:211–216.

6. Geiss J, Rossi AP (2013) On the chronology of lunar origin and evolution Implications for Earth, Mars and the Solar System as a whole. Astron Astrophys Rev 21.

7. Morbidelli A, Marchi S, Bottke WF, Kring DA (2012) A sawtooth-like timeline for the first billion years of lunar bombardment. EARTH Planet Sci Lett 355:144–151.

8. Bottke WF et al. (2012) An Archaean heavy bombardment from a destabilized extension of the asteroid belt. Nature 485:78–81. 9. Lunine JI (2006) Physical conditions on the early Earth. Philos Trans R Soc B-BIOLOGICAL Sci 361:1721–1731.

10. De Niem D, Kuehrt E, Morbidelli A, Motschmann U (2012) Atmospheric erosion and replenishment induced by impacts upon the Earth and Mars during a heavy bombardment. Icarus 221:495–507.

11. Ferus M, Matulkova I, Juha L, Civis S (2009) Investigation of laser-plasma chemistry in CO-N-2-H2O mixtures using O-18 labeled water. Chem Phys Lett 472:14–18.

12. Morbidelli A et al. (2000) Source regions and timescales for the delivery of water to the Earth. Meteorit Planet Sci 35:1309–1320.

13. Cavosie AJ, Valley JW, Wilde SA (2005) Magmatic delta O-18 in 4400-3900 Ma detrital zircons: A record of the alteration and recycling of crust in the Early Archean. EARTH Planet Sci Lett 235:663–681.

14. Chyba C, Sagan C (1992) Endogenous Production, Exogenous Delivery and Impact-Shock Synthesis of Organic Molecules – an Inventory for the Origin of Life. Nature 355:125–132.

15. Chyba CF, Thomas PJ, Brookshaw L, Sagan C (1990) Cometary Delivery of Organic Molecules to the Early Earth. Science (80- ) 249:366–373.

16. Civis S, et al. (2004) Amino acid formation induced by high-power laser in CO2 /CO-N-2-H2O gas mixtures. Chem Phys Lett 386(1–3):169–173.

17. Ferus M, et al. (2015) High-energy chemistry of formamide: A unified mechanism of nucleobase formation. Proc Natl Acad Sci 112(3):657–662.

18. Ferus M, et al. (2016) Small Radicals and they Role in Prebiotic Plasma Chemistry – Network in Reduction Atmospheres. Phys Chem Chem Phys:In Review Process.

19. Civis S, et al. (2016) TiO2-catalyzed synthesis of sugars from formaldehyde in extraterrestrial impacts on the early Earth. Sci Rep 6. doi:10.1038/srep23199.

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