Comets as a Source of Prebiotic Organic Molecules for the Early Earth

  • C. F. Chyba
  • C. Sagan

Abstract

Life on Earth originated during the final throes of the heavy bombardment, in which the Earth—Moon system, as well as the rest of the inner solar system, was subjected to an intense bombardment of comets and asteroids. This bombardment may have rendered the Earth’s surface inhospitable for life for hundreds of millions of years subsequent to terrestrial formation. It may also have delivered to the Earth’s surface the bulk of the current terrestrial volatile inventory, in the form of a late-accreting impact veneer. Delivering intact prebiotic organic molecules of interest for the origins of life is much more difficult. However, several mechanisms seem likely to have been delivering exogenous organics to the surface of the Earth, or shock-synthesizing them in impacts. In an early carbon dioxide-rich terrestrial atmosphere, these mechanisms would have quantitatively rivaled or exceeded terrestrial organic synthesis in situ. In an early reducing (methane-rich) atmosphere, the exogenous sources would have been quantitatively unimportant compared to atmospheric production.

Preview

Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.

References

  1. Alvarez, L.W., Alvarez, W.A., Asaro, F., and Michel, H.V. (1980), Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208, 1095–1108.ADSCrossRefGoogle Scholar
  2. Anders, E. (1989), Pre-biotic organic matter from comets and asteroids. Nature 342, 255–57.ADSCrossRefGoogle Scholar
  3. Baldwin, R.B. (1987a), On the relative and absolute ages of seven lunar front face basins. I. From viscosity arguments. Icarus 71, 1–18.MathSciNetADSCrossRefGoogle Scholar
  4. Baldwin, R.B. (1987b), On the relative and absolute ages of seven lunar front face basins. II. From crater counts. Icarus 71, 19–29.ADSCrossRefGoogle Scholar
  5. Barak, I. and Bar-Nun, A. (1975), The mechanisms of amino acid synthesis by high temperature shock-waves. Origins of Life 6: 483–506.ADSCrossRefGoogle Scholar
  6. Bar-Nun, A. and Shaviv, A. (1975), Dynamics of the chemical evolution of Earth’s primitive atmosphere. Icarus 24, 197–210.ADSCrossRefGoogle Scholar
  7. Bar-Nun, A., Bar-Nun, N., Bauer, S.H., and Sagan, C. (1970), Shock synthesis of amino acids in simulated primitive environments. Science 168, 470–473.ADSCrossRefGoogle Scholar
  8. Basaltic Volcanism Study Project (BVSP) (1981), Basaltic Volcanism on the Terrestrial Planets. Pergamon, New York.Google Scholar
  9. Becker, L., Bada, J.L., Winans, R.E., Hunt, J.E., Bunch, T.E., and French, B.M. (1994), Fullerenes in the 1.85-billion-year-old Sudbury impact structure. Science 265, 642–645.ADSCrossRefGoogle Scholar
  10. Becker, L., Poreda, R.J., and Bada, J.L. (1996), Extraterrestrial helium trapped in fullerenes in the Sudbury impact structure. Science 272, 249–252.ADSCrossRefGoogle Scholar
  11. Cameron, A.G.W. (1983), Origin of the atmospheres of the terrestrial planets. Icarus 56, 195–201.ADSCrossRefGoogle Scholar
  12. Cameron, A.G.W. (1986), The impact theory for origin of the Moon. In Origin of the Moon ( W.K. Hartmann, R.J. Phillips, and G.J. Taylor, eds.), Lunar and Planetary Inst., Houston, pp. 609–616.Google Scholar
  13. Ceplecha, Z. (1992), Earth influx of interplanetary bodies. Astronomy and Astrophysics 263, 361–366.ADSGoogle Scholar
  14. Chou, C.-L. (1978), Fractionation of siderophile elements in the earth’s upper mantle. Proc. Lunar Planet. Sci. Conf. 9, 219–230.ADSGoogle Scholar
  15. Chyba, C.F. (1990a), Impact delivery and erosion of planetary oceans in the early inner solar system. Nature 343, 129–133.ADSCrossRefGoogle Scholar
  16. Chyba, C.F. (1990b), Extraterrestrial amino acids and terrestrial life. Nature 348, 113–114.ADSCrossRefGoogle Scholar
  17. Chyba, C.F. (1991), Terrestrial mantle siderophiles and the lunar impact record. Icarus 92, 217–233.ADSCrossRefGoogle Scholar
  18. Chyba, C.F. (1993a), The violent environment of the origins of life: Progress and uncertainties. Geochim. Cosmochim. Acta 57, 3351–3358.ADSCrossRefGoogle Scholar
  19. Chyba, C.F. (1993b), Explosions of small Spacewatch objects in the Earth’s atmosphere. Nature 363, 701–703.ADSCrossRefGoogle Scholar
  20. Chyba, C.F. and McDonald, G. (1995). The origin of life in the Solar System: Current issues. Annu. Rev. Earth Planet Sci. 24, 215–249.ADSCrossRefGoogle Scholar
  21. Chyba, C. and Sagan, C. (1991), Electrical energy sources for organic synthesis on the early Earth. Origins Life 21, 3–17.CrossRefGoogle Scholar
  22. Chyba, C. and Sagan, C. (1992), Endogenous production, exogenous delivery and impact-shock synthesis of organic molecules: An inventory for the origins of life. Nature 355, 125–132.ADSCrossRefGoogle Scholar
  23. Chyba, C.F., Thomas, P.J., Brookshaw, L., and Sagan, C. (1990), Cometary delivery of organic molecules to the early Earth. Science 249, 366–373.ADSCrossRefGoogle Scholar
  24. Chyba, C.F., Thomas, P.J., and Zahnle, K.J. (1993), The 1908 Tunguska explosion: Atmospheric disruption of a stony asteroid, Nature 361, 40–44.ADSCrossRefGoogle Scholar
  25. Chyba, C.F., Owen, T.C., and Ip, W.-H. (1995), Impact delivery of volatiles and organic molecules to Earth. In Hazards Due to Comets and Asteroids (ed. T. Gehrels ) pp. 9–58, University of Arizona Press, Tucson.Google Scholar
  26. Clark, B.C. (1988), Primeval procreative comet pond. Origins of Life 18, 209–238.ADSCrossRefGoogle Scholar
  27. Dalrymple, G.B. and Ryder, G. (1993), ArAr age spectra of Apollo 15 impact melt rocks by laser step-heating and their bearing on the history of lunar basin formation. J. Geophys. Res. 98, 13085–13095.ADSCrossRefGoogle Scholar
  28. Delsemme, A.H. (1992), Cometary origin of carbon, nitrogen and water on the Earth. Origins of Life 21, 279–298.Google Scholar
  29. DesMarais, D.J. (1985), In The Carbon Cycle and Atmospheric CO 2 : Natural Variations Archean to Present. (eds. E.T. Sundquist and W.S. Broecker), American Geophysical Union, Washington DC, pp. 602–611.Google Scholar
  30. Dreibus, G. and Wänke, H. (1987), Volatiles on Earth and Mars: A comparison. Icarus 71, 225–240.ADSCrossRefGoogle Scholar
  31. Dreibus, G. and Wanke, H. (1989), Supply and loss of volatile constituents during the accretion of terrestrial planets. In Origin and Evolution of Planetary and Satellite Atmospheres ( S.K. Atreya, J.B. Pollack, and M.S. Matthews, eds.), University of Arizona Press, Tucson, pp. 268–288.Google Scholar
  32. Eberhardt, P., Dolder, U., Schulte, W., Krankowsky, d., Lämmerzahl, P., Berthelier, J.J., Woweries, J., Stubbemann, U., Hodges, R.R., Hoffman, J.H., and Illiano, J.M. (1987), The D/H ratio in water from comet P/Halley. Astron. Astrophys. 187, 435–437.Google Scholar
  33. Fernandez, J.A. (1985), The formation and dynamical survival of the comet cloud. In Dynamics of Comets: Their Origin and Evolution ( A. Carusi and G.B. Valsecchi, eds.), Reidel, Dordrecht, pp. 45–70.CrossRefGoogle Scholar
  34. Fernandez, J.A. and Ip, W.-H. (1983), On the time evolution of the cometary influx in the region of the terrestrial planets. Icarus 54, 377–387.ADSCrossRefGoogle Scholar
  35. Fishman, G.J., Bhat, P.N., Mallozzi, R., Horack, J.M., Koshut, T., Kouveliotou, C., Pendleton, G.N., Meegan, C.A., Wilson, R.B., Paciesas, W.S., Goodman, S.J., and Christian, H.J. (1994), Discovery of intense gamma-ray flashes of atmospheric origin. Science 264, 1313–1316.Google Scholar
  36. Fiske, P.E., Nellis, W.J., Lipp, M., Lorenzana, H., Kikuchi, M., and Syono, Y. (1995), Pseudotachylites generated in shock experiments: Implications for impact cratering products and processes. Science 270, 281–283.Google Scholar
  37. Folinsbee, R.E., Douglas, J.A.V. and Maxwell, J.A. (1967), Revelstoke, a new Type I carbonaceous chondrite. Geochim. Cosmochim. Acta 31, 1625–1635.ADSCrossRefGoogle Scholar
  38. Gilvarry, J.J. and Hochstim, A.R. (1963), Possible role of meteorites in the origin of life. Nature 197, 624–626.ADSCrossRefGoogle Scholar
  39. Greenberg, M.J. (1981), Chemical evolution of interstellar dust-a source of prebiotic material? In Comets and the Origin of Life ( C. Ponnamperuma, ed.), Reidel, Dordrecht, pp. 111–127.CrossRefGoogle Scholar
  40. Grinspoon, D.H. (1988), Large impact events and atmospheric evolution on the terrestrial planets. Ph.D. thesis, University of Arizona.Google Scholar
  41. Hartmann, W.K. (1980), Dropping stones in magma oceans: Effects of early lunar cratering. In Proceedings of the Conference on the Lunar Highland Crust, pp. 155–171 ( Lunar and Planetary Institute, Houston ).Google Scholar
  42. Hartmann, W.K. (1987), A satellite-asteroid mystery and a possible early flux of scattered C-class asteroids. Icarus 71, 57–68.ADSCrossRefGoogle Scholar
  43. Hartmann, W.K. (1990), Additional evidence about an early intense flux of C asteroids and the origin of Phobos. Icarus 87, 236–240.ADSCrossRefGoogle Scholar
  44. Hartmann, W.K. (1995), Planetary cratering I. The question of multiple impactor populations: Lunar evidence. Meteoritics 30, 451–467.ADSCrossRefGoogle Scholar
  45. Hayes, J.M., I.R. Kaplan, and K.W. Wedeking (1983), Precambrian organic geochemistry, preservation of the record. In Earth’s Earliest Biosphere (J.W. Schopf, ed.), Princeton University Press, Princeton, NJ, pp. 93–134.Google Scholar
  46. Hochstim, A.R. (1963), Hypersonic chemosynthesis and possible formation of organic compounds from impact of meteorites on water. Proc. Nat. Acad. Sci. 50, 200–208.ADSCrossRefGoogle Scholar
  47. Horowitz, N.H. (1986), To Utopia and Back: The Search for Life in the Solar System. Freeman, New York.Google Scholar
  48. Ip, W.-H. (1977), On the early scattering processes of the outer planets. In Comets-AsteroidsMeteorites: Interrelations, Evolution and Origin ( A.H. Delsemme, ed.), University of Toledo Press, Toledo, OH, pp. 485–490.Google Scholar
  49. Kasting, J.F. (1990), Bolide impacts and the oxidation state of carbon in the Earth’s early atmosphere. Origins of Life 20, 199–231.CrossRefGoogle Scholar
  50. Kasting, J.F. (1993), Earth’s early atmosphere. Science 259, 920–925.ADSCrossRefGoogle Scholar
  51. Kasting, J.F. and Ackerman, T.P. (1986), Climatic consequences of very high carbon dioxide levels in the Earth’s early atmosphere. Science 234, 1383–1385.ADSCrossRefGoogle Scholar
  52. Kasting, J.F., Whitmire, D., and Reynolds, R. (1993), Habitable zones around main sequence stars. Icarus 101, 108–128.ADSCrossRefGoogle Scholar
  53. Kerr, R.A. (1994), Atmospheric scientists puzzle over high-altitude flashes. Science 264, 1250–1251.ADSCrossRefGoogle Scholar
  54. Krinov, E.L. (1966), Giant Meteorites. Pergamon, Oxford.Google Scholar
  55. Kyte, F.T. and Wasson, J.T. (1986), Accretion rate of extraterrestrial matter: Iridium deposited 33 to 67 million years ago. Science 232, 1225–1229.ADSCrossRefGoogle Scholar
  56. Lasaga, A.C., Holland, H.D., Dwyer, M.J. (1971), Primordial oil slick. Science 174, 53–55.ADSCrossRefGoogle Scholar
  57. Lewis, J. (1974), The temperature gradient in the solar nebula. Science 186, 440–443.ADSCrossRefGoogle Scholar
  58. Lewis, J., Barshay, S.S. and Noyes, B. (1979), Primordial retention of carbon by the terrestrial planets. Icarus 37, 190–206.ADSCrossRefGoogle Scholar
  59. Love, S.G. and Brownlee, D.E. (1993), A direct measurement of the terestrial mass accretion rate of cosmic dust. Science 262, 550–553.ADSCrossRefGoogle Scholar
  60. Mason, B. (1971), Handbook of Elemental Abundances in Meteorites. Gordon and Breach, New York.Google Scholar
  61. McKay, C.P. (1986), Exobiology and future mars missions: The search for Mars’ earliest biosphere. Adv. Space Res. 6, 269–285.ADSCrossRefGoogle Scholar
  62. McKay, C.P. (1991), Urey Prize lecture: planetary evolution and the origin of life. Icarus 91, 93–100.ADSCrossRefGoogle Scholar
  63. McKay, C.P., Scattergood, T.W., Pollack, J.B., Borucki, W.J., and Van Ghyseghem, H.T. (1988), High-temperature shock formation of N2 and organics on primordial Titan. Nature 332, 520–522.ADSCrossRefGoogle Scholar
  64. McKay, C.P., Borucki, W.R., and Kojiro, K.R., and Church, F. (1989), Shock production of organics during cometary impact. Lunar Planet. Sci. Conf. 20, 671–672.ADSGoogle Scholar
  65. McKinnon, W.B., C.R. Chapman, and K.R. Housen (1990), Cratering of the uranian satellites. In Uranus ( J.T. Bergstrahl, E.D. Miner, and MS. Matthews, eds.), University of Arizona Press, Tucson, pp. 629–692.Google Scholar
  66. Melosh, H.J. (1989), Impact Cratering: A Geologic Process. Oxford University Press, New York.Google Scholar
  67. Melosh, H.J. and A.M. Vickery (1989), Impact erosion of the primordial atmosphere ofGoogle Scholar
  68. Mars. Nature 338 487–489.Google Scholar
  69. Miller, S.L. and Urey, H.C. (1959), Organic compound synthesis on the primitive Earth. Science 130, 245–251.ADSCrossRefGoogle Scholar
  70. Mukhin, L.M., Gerasimov, M.V., and Safonova, E.N. (1989), Origin of precursors of organic molecules during evaporation of meteorites and mafic terrestrial rocks. Nature 340, 46–48.ADSCrossRefGoogle Scholar
  71. Noyes, W.A. and Leighton, P.A. (1941), The Photochemistry of Gases. Reinhold, New York. Oberbeck, V.R. and Aggarwal, H. (1992), Comet impacts and chemical evolution on the bombarded Earth. Origins of Life 21, 317–338.Google Scholar
  72. Oberbeck, V.R. and G. Fogleman (1989), Estimates of the maximum time required to originate life. Origins Life 19, 549–560.CrossRefGoogle Scholar
  73. Oberbeck, V.R., McKay, C.P., Scattergood, T.W., Carle, G.C. and Valentin J.R. (1989), TheGoogle Scholar
  74. role of cometary particle coalescence in chemical evolution. Origins of Life 19 39–55.Google Scholar
  75. Orb, J. (1961), Comets and the formation of biochemical compounds on the primitive Earth. Nature 190, 389–390.ADSCrossRefGoogle Scholar
  76. Pollack, J.B., Podolak, M., Bodenheimer, R, and Christofferson, B. (1986) Icarus 67 409443.Google Scholar
  77. Prinn, R.G. and B. Fegley (1989), Solar nebula chemistry: Origin of planetary, satellite and cometary volatiles. In Origin and Evolution of Planetary and Satellite Atmospheres ( S.K. Atreya, J.B. Pollack, and M.S. Matthews, eds.). University of Arizona Press, Tucson, pp. 78–136.Google Scholar
  78. Rabinowitz, D.L. (1993), The size distribution of the Earth-approaching asteroids. Astrophys. J. 407, 412–427.ADSCrossRefGoogle Scholar
  79. Rabinowitz, D.L., Gehrels, T., Scotti, J.V., McMillan, R.S., and Perry M.L. (1993), The terrestrial asteroid belt: A new population of near-Earth asteroids. Nature 363, 704706.Google Scholar
  80. Robbins, E.I. and Iberall, A.S. (1991), Mineral remains of early life on Earth? On Mars? Geomicrobiology J. 9, 51–66.CrossRefGoogle Scholar
  81. Ryder, G. (1990), Lunar samples, lunar accretion and the early bombardment of the Moon. Eos 71, 313, 322–323.ADSGoogle Scholar
  82. Ryder, G. and Wood, J.A. (1977), Serenitatis and Imbrium impact melts: Implications for large-scale layering in the lunar crust. Proc. Lunar Sci. Conf. 8, 655–668.ADSGoogle Scholar
  83. Sagan, C. and Chyba, C. (1996), The early faint sun “paradox”: organic shielding of UV-labile greenhouse gases. In preparation.Google Scholar
  84. Sagan, C. and Thompson, W.R. (1984), Production and condensation of organic gases in the atmosphere of Titan. Icarus 59, 133–161.ADSCrossRefGoogle Scholar
  85. Schidlowski, M.A. (1988), 3,800-million-year isotope record of life from carbon in sedimentary rocks. Nature 333, 313–318.Google Scholar
  86. Schlesinger, G. and Miller, S.L. (1983a), Prebiotic synthesis in atmospheres containing CH4, CO, and CO2. I. Amino acids. J. Molec. Evol. 19, 376–382.CrossRefGoogle Scholar
  87. Schlesinger, G. and Miller, S.L. (1983b), Prebiotic synthesis in atmospheres containing CH4, CO, and CO2. II. Hydrogen cyanide, formaldehyde, and ammonia. J. Molec. Evol. 19, 383–390.CrossRefGoogle Scholar
  88. Schmidt, R.M. and K.R. Housen (1987), Some recent advances in the scaling of impact and explosion cratering. Int. J. Impact Engng. 5, 543–560.ADSCrossRefGoogle Scholar
  89. Schonland, B.F.J. (1928), The interchange of electricity between thunderclouds and the Earth. Proc. Roy. Soc. A 118, 252–262.ADSCrossRefGoogle Scholar
  90. Schonland, B.F.J. (1953), Atmospheric Electricity Methuen, London.Google Scholar
  91. Schopf, J.W. (1993), Microfossils of the early Archean apex chert: New evidence of the antiquity of life. Science 260, 640–646.ADSCrossRefGoogle Scholar
  92. Schopf, J.W. and Walter, M.R. (1983), Archean microfossils: New evidence of ancient microbes. In Earth’s Earliest Biosphere (J.W. Schopf, ed.). Princeton University Press, Princeton, NJ, pp. 214–239.Google Scholar
  93. Shoemaker, E.M. and R.F. Wolfe (1984), Evolution of the Uranus—Neptune planetesimal swarm. Proc. Lunar Planet. Sci. Conf. 15, 780–781.ADSGoogle Scholar
  94. Sleep, N.H., K.J. Zahnle, J.F. Kasting, and H.J. Morowitz ( 1989. Annihilation of ecosystems by large asteroid impacts on the early Earth. Nature 342, 139–142.ADSCrossRefGoogle Scholar
  95. Stacey, F.D. (1977), Physics of the Earth. Wiley, New York.Google Scholar
  96. Stevenson, D.J. (1983), The nature of the Earth prior to the oldest known rock record: The Hadean Earth. In Earth’s Earliest Biosphere: Its Origin and Evolution (J.W. Schopf, ed.). Princeton University Press, Princeton, NJ, pp. 32–40.Google Scholar
  97. Stevenson, D.J. (1990), Fluid dynamics of core formation. In Origin of the Earth (H.E. Newsom and J.H. Jones, eds.). Oxford University Press, New York, pp. 231–249.Google Scholar
  98. Stribling, R. and Miller, S.L. (1987), Energy yields for hydrogen cyanide and formaldehyde synthesis: The HCN and amino acid concentrations in the primitive ocean. Origins of Life 17, 261–273.ADSCrossRefGoogle Scholar
  99. Strom, R.G. (1987), The Solar System cratering record: Voyager 2 results at Uranus and implications for the origin of impacting objects. Icarus 70, 517–535.ADSCrossRefGoogle Scholar
  100. Sun, S.-S. (1984), Geochemical characteristics of archaean ultramafic and mafic volcanic rocks: Implications for mantle composition and evolution. In Archean Geochemistry ( A. Kröner, G.N. Hanson, and A.M. Goodwin, eds.). Springer-Verlag, Berlin, pp. 2546.Google Scholar
  101. Swindle, T.D., M.W. Caffee, C.M. Hohenberg, and S.R. Taylor (1986), I—Pu—Xe dating and the relative ages of the Earth and Moon. In Origin of the Moon ( W.K. Hartmann, R.J. Phillips, G.J. Taylor, eds.). Lunar and Planetary Institute, Houston, pp. 331–357.Google Scholar
  102. Tera, F., Papanastassiou, D., and Wasserburg, G. (1974), The lunar timescale and a summary of isotopic evidence for a terminal lunar cataclysm. Lunar Planet. Sci 5, 792.ADSGoogle Scholar
  103. Tilton, G.R. (1988), Age of the Solar System. In Meteorites and the Early Solar System ( J.F. Kerridge and M.S. Matthews, eds.). University of Arizona Press, Tucson, pp. 259–275.Google Scholar
  104. Tingle, T.N, Tyburczy, J.A., Ahrens, T.J. and Becker, C.H. (1992), The fate of organic matter during planetary accretion: Preliminary studies of the organic chemistry of experimentally shocked Murchison meteorite. Origins of Life 21, 385–397.Google Scholar
  105. Veizer, J. (1983), Geologic evolution of the Archean-Early Proterozoic Earth. In Earth’s Earliest Biosphere (J.W. Schopf, ed.). Princeton University Press, Princeton, NJ, pp. 240–259.Google Scholar
  106. Walker, J.C.G. (1977), Evolution of the Atmosphere. Macmillan, New York.Google Scholar
  107. Walker, J.C.G. (1986), Carbon dioxide on the early Earth. Origins Life 16, 117–127.ADSGoogle Scholar
  108. Walter, M.R. (1983), Archean stromatolites: evidence of the Earth’s earliest benthos. In Earth’s Earliest Biosphere (J.W. Schopf, ed.). Princeton University Press, Princeton, NJ, pp. 187–213.Google Scholar
  109. Wetherill, G.W. (1977), Evolution of the earth’s planetesimal swarm subsequent to the formation of the earth and moon. Proc. Lunar. Sci. Conf. 8, 1–16.ADSGoogle Scholar
  110. Wetherill, G.W. (1990), Formation of the Earth. Annu. Rev. Earth Planet. Sci. 18, 205–256.ADSCrossRefGoogle Scholar
  111. Wilhelms, D.E. (1984), Moon. In The Geology of the Terrestrial Planets (M.H. Carr, ed.), pp. 107–205. NASA SP-469.Google Scholar
  112. Wilhelms, D.E. (1987), The Geologic History of the Moon. U.S. Geological Survey professional paper 1348. U.S. Government Printing Office, Washington, DC.Google Scholar
  113. Wilkening, L.L. (1978), Carbonaceous chondritic material in the solar system. Naturwiss. 66, 73–79.ADSCrossRefGoogle Scholar
  114. Zahnle, K. (1986), Photochemistry of methane and the formation of hydrocyanic acid (HCN) in the Earth’s early atmosphere. J. Geophys. Res. 91, 2819–2834.ADSCrossRefGoogle Scholar
  115. Zahnle, K. (1990), Atmospheric chemistry by large impacts. In Global Catastrophes in Earth History (V.I. Sharpton and P.D. Ward, eds.). Geological Society of America SP-247, Boulder, pp. 271–288.Google Scholar
  116. Zahnle, K. and Grinspoon, D. (1990), Comet dust as a source of amino acids at the Cretaceous/Tertiary boundary. Nature 348, 157–159.ADSCrossRefGoogle Scholar
  117. Zahnle, K.J. and Walker, J.C.G. (1982), Evolution of solar ultraviolet luminosity. Rev. Geophys. Space Phys. 20, 280–292.ADSCrossRefGoogle Scholar
  118. Zhao, M. and Bada, J.L. (1989), Extraterrestrial amino acids in Cretaceous/Tertiary boundary sediments at Stevns Klint, Denmark. Nature 339, 463–465.ADSCrossRefGoogle Scholar

Copyright information

© Springer Science+Business Media New York 1997

Authors and Affiliations

  • C. F. Chyba
  • C. Sagan

There are no affiliations available

Personalised recommendations