Oxidants on Small Icy Bodies and Snowball Earth Yuk L. Yung (Caltech) Mao-Chang Liang (Academia Sinica)

Initiation of Snowball Hoffman, 2000  Paleoclimate Dim Sun Greenhouse by CH 4 (Kasting et al.) Loss of CH 4 : Oxidized by O 2 produced by oxygen- synthetic bacteria ice albedo (reflectance) Runaway ice-house

Atmospheric Photochemistry during Snowball?

Atmosphere of Snowball  Oxygen poor  Cold Surface temperature -40  C Low water vapor abundance Weak hydrological cycles (~1 mm/yr)  100 Myr to get rid of the Snowball, by assuming present CO 2 outgassing rate

Photochemical Calculation  One-dimensional calculation Continuity of mass  n i /  t +  i /  z = P i  L i  Close to present-day vertical mixing profile  Temperature profiles Surface temperature of 210, 240, 273 K Lapse rate of 6.5 and 10 K km -1 Deposition rate of ~10 9 molecules cm -2 s -1 for H 2 O 2

Principal Results O3O3 H2OH2O H2O2H2O2 O2O2 H2H2 CO CO 2

H2O2H2O2 ice H2O2H2O2 H2O2H2O2 H2O2H2O2 H2O2H2O2 H2OH2O H O2O2 HO 2 H2O2H2O2 OH H2O2H2O2 H2O2H2O2 H2O2H2O2 H2O2H2O2 H2O2H2O2 H2O2H2O2 1 km diffusion Schematic Diagram h (<200 nm) h (<350 nm)

Result Summary  High ratio of H 2 O 2 and H 2 O  ~10 19 mole of H 2 O 2 could be deposited ~0.1 bar of O 2  Decomposition of H 2 O 2 Release O 2 into the atmosphere  Global oxidation on the planetary surface

Evidence of the Rise of Oxygen/Oxidants  Farquhar et al. (2000) Mass-independent of sulfur isotopes  Karhu, J. A. & Holland (1996)  13 C anomaly  Kirschvink et al. (2000) Large deposition of MnO 2

Farquhar et al., (2000) SO 2 + h → SO + O

Karhu & Holland, 1996

Kirschvink et al., 2000

Impact on the Evolution of Life?

Biological Evolution Associated with Snowball  Paleoproterozic (~2.4 Ga) global oxidation event first clear fossil of eukaryotic cell (Han & Runegar, 1992)  Neoproterozic (~0.7 Ga) multi-cellular life Cambrian explosion

Ages in Millions of Years Oldest Stromatolite and Prokaryote Body Fossil Oldest Eukaryote Body Fossil Introduction of Oxygen Snowball Earth (Paleoproterozoic) Cambrian Explosion Snowball Earth (two?) (Precambrian) Archaean Proterozoic Cenozoic Mesozoic Paleozoic Courtesy of Hyman Hartman

Role of H 2 O 2 Touati, 2000