Supporting Information

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1 Supporting Information Ueno et al /pnas SI Text Details for UV Shielding Molecules. To evaluate isotopic fractionation for SO 2 photolysis given UV shielding, we calculate Eq. 10 in the main article when the transparency term e ( ) is from 0.01 to When e ( ) is 0.01, the atmosphere is so optically thick that SO 2 is not photolyzed, whereas, when e ( ) is larger than 0.99, the shielding effect is negligibly small. We first consider potential major species (CO 2,O 2,O 3,H 2 O, NH 3,CS 2, OCS, and SO 2 ), which attenuate UV flux in the nm region. The CO 2 molecule presents significant cross-sections in the nm range (1, 2). These cross-sections are a combination of real extinction coefficients ( nm) and Rayleigh scattering factors ( nm). See refs. 1 and 2 for a more detailed discussion on the UV absorption-scattering properties of CO 2. Because the cross-sections of CO 2 are smaller than those of SO 2, the overhead column of this gas was calculated from to molecules/cm 2. The O 2 molecule has cross-sections on the order of cm 2 on the 190-nm shoulder (3, 4); consequently it is of significance for the opacity term. The large cross-sections between 190 and 203 nm belong to the highly-structured Schuman-Runge band (3). At energies 203 nm, the Hezberg continuum presents cross-sections of the order of cm 2 (4). The overhead column was calculated from to molecules/cm 2. The O 3 has large cross-sections at energies 180 nm reaching a maximum of cm 2 at 260 nm (5). Due to these large absorption cross-sections, the overhead column for O 3 was calculated from to molecules/cm 2. The H 2 O molecule presents a comparatively smaller absorption cross-section ( 220 nm) (6), yet it is still important to analyze its possible interference with the SO 2 spectra; the overhead column was calculated for to molecules/cm 2. The NH 3 molecule presents large absorption cross-sections between 160 and 220 nm (7). The overhead column for NH 3 was calculated from to molecules/cm 2. The CS 2 molecule also presents large absorption crosssections between 190 and 210 nm (8). The overhead column for CS 2 was calculated from to molecules/cm 2. The OCS molecule presents a large absorption cross-section between 190 and 260 nm (9). The overhead column for OCS was calculated from to molecules/cm 2. The effects of SO 2 self-shielding were also analyzed. The SO 2 cross-sections have been published by our group (10). For this overhead column, the natural isotopic distribution was used ( 32 SO %, 33 SO %, and 34 SO %). Columns ranging from to molecules/cm 2 were calculated. We have not included the possible temperature dependence of the wavelength-resolved fractionation constant in the model. These measurements have been made for N 2 O (11), where a significant effect is seen. However, these measurements have not been made for SO 2. It is not expected that the intensity of the SO 2 hot bands will be unusually large, as is the case for N 2 O. The N 2 O transition becomes allowed as the molecule bends, meaning that hot bands arising from the excited bending states have significantly larger intensities than the ground state (12). A similar variation in the transition intensity with vibrational coordinates is not expected for SO 2. In these molecules, OCS and O 3 preferentially attenuate 202 nm of UV flux and are thus potential candidates for causing the negative 33 S observed in Archean sulfate deposits (see Results). For examining alternative molecules, UV spectra of 490 atmospherically relevant molecules were scanned (MPI- Mainz-UV-VIS Spectral Atlas of Gaseous Molecules; For first screening, the following criteria was used: r ( )d 202 ( )d 3 To shift the 33 E to positive value by the preferential UVshielding, the molecule should absorb nm region of UV by at least 3 times nm region (i.e., r 3). Among the 490 species, only 6 molecules met this screening criteria (BrNO, ICl, OCS, CH 3 C(O)CH 2 Cl, F 2, and O 3 ). Of these six, the halogenated species would not be common in the atmosphere of a planet with an ocean, and electronegative fluorine would certainly not be found free in the atmosphere in the oxidized elemental form. Many other halogen compounds, like HCl, absorb preferentially in the 202-nm region. By the process of elimination, the only relevant shielding candidates are OCS and O 3. UV Emission of the Early Sun. The Archean solar UV spectrum may have been different from that of the present-day Sun. This difference may affect the estimated isotope effect of SO 2 photolysis. The surface temperature of the Sun 4.5 billion years ago would have been 200 K lower than at present (5779 K) (13). To address how this variation in the solar spectrum affects the calculated isotope effects, the blackbody emission curves were calculated at 5,779 K and 5,586 K (Fig. S3). The difference between the calculated blackbody spectra at the two temperatures produces a change in 34 and 33 of 1. The calculation shows that solar evolution would have had a negligible effect on isotopic fractionation via SO 2 photolysis. The difference in isotopic fractionation between the measured solar spectrum and the blackbody curve is significant (20 for 34 and 28 for 33 ). However, both measured and blackbody spectra give a negative 33 E value because the lower energy UV flux at wavelengths longer than 202 nm is 4 10 more intense than the high-energy region (Fig. 1 and Fig. S3). Hence, the exact details of the Archean solar spectrum are not expected to have a large effect on our results. In addition, the early sun may have emitted more VUV 150 nm (14), although the enhanced flux seems to be an order of magnitude smaller than that of the lower energy nm region. The early enhanced VUV flux derived from an intense magnetic dynamo is unlikely to be significant relative to 5,500 K thermal emission at wavelengths longer than 190 nm (14), where the SO 2 absorption cross-section maxima occur. Thus, the VUV effect is unlikely to affect our result. Instead, enhanced VUV may have allowed photoionization deeper in the atmosphere than occurs today (possibly down to the mesosphere), affecting the overall chemistry and composition of the upper atmosphere. We do not consider this effect, but it should be considered in the future. Comparison to Previous Experiments. Farquhar et al. (15) reported experiments of SO 2 photolysis in the laboratory showing wavelength-dependent MIF effects. Such experimental results may be useful to test our model. One of their experiments, using a 1of11

2 solar-like continuum (Xe lamp), yielded sulfate with positive 33 S and 34 S (15). This trend is approximately similar to our prediction that both 33 and 34 are negative for SO 2 photolysis by solar UV spectrum without an UV-shielding molecule (Fig. 2), which should result in sulfate aerosol with a positive 33 S and 34 S. However, this similarity could be accidental, because SO 2 photolysis may not be the only source of sulfur MIF observed in product sulfate in such an experiment. We performed additional runs of our reaction model assuming Torr pure SO 2, the experimental condition used by Farquhar et al. (15). The results suggest that the MIF effect of SO 2 photoexcitation (SO 2 h 3 *SO 2 ) and subsequent photooxidation (SO 2 *SO 2 3 SO 3 SO), if assumed, are more significant sources of the isotope fractionation transferred into aerosol species than SO 2 photolysis. This high SO 2 partial pressure allows transformation of isotopically fractionated *SO 2 into sulfate or sulfur aerosol species through the photooxidation reaction. In natural conditions, however, the excited *SO 2 is rapidly quenched to SO 2. Hence, the isotopic signature of excited *SO 2 is unlikely to be transferred into aerosol species in natural systems unless unusually high SO 2 concentration is maintained locally (for example within a volcanic plume). Therefore, our model based on the estimated isotope effect in SO 2 photolysis is only relevant for reducing atmospheric conditions, and not to previous experiments in pure SO 2. Furthermore, the experiments using a single line UV source (193 nm and 248 nm; see ref. 15) are problematic for testing our hypothesis because the isotope effect is very sensitive to wavelength (10). For example, a change of only 0.3 nm from nm to nm shifts 33 E from 600 to 350. Thus, for calculating the precise isotope effect, one would require much more information about the experimental conditions; for example, the central wavelength and linewidth of the excimer laser, the sample temperature, and the material and thickness of the window. The photolysis rates of the individual isotopologues are sensitive to these factors, which will greatly alter the resulting isotope distribution. Further photolysis experiments should be designed to test our hypothesis. 1. Parkinson WH, Rufus J, Yoshino K (2003) Absolute absorption cross section measurements of CO 2 in the waelength region nm and the temperature dependence. Chem Phys 290: Karaiskou A, Vallance C, Papadakis V, Vardavas IM, Rakitzis TP (2004) Absolute absorption cross-section measurements of CO 2 in the ultraviolet from 200 to 206 nm at 295 and 373 K. Chem Phys Lett 400: Minschwaner K, Anderson GP, Hall LA, Yoshino K (1992) Polynomial coefficients for calculating O 2 Schumann-Runge cross sections at 0.5 cm 1 resolution. J Geophys Res 97: Cheung ASC, Yoshino K, Parkinson WH, Freeman DE (1986) Molecular spectroscopic constants of O 2 (B3 -u): The upper state of the Schumann-Runge bands. J Mol Spec 119: Yoshino K, Esmond JR, Freeman DE, Parkinson WH (1993) Measurements of absolute absorption cross sections of ozone in the 185- to 254-nm wavelength region and the temperature dependence. J Geophys Res 98: Parkinson WH, Yoshino K (2003) Absorption cross-section measurements of water vapor in the wavelength region nm. Chem Phys 294: Cheng BM et al. (2006) Absorption cross sections of NH 3,NH 2D, NHD 2, and ND 3 in the spectral range nm and implications for planetary isotopic fractionation. Astrophys J 647: Chen FZ, Wu CYR (1995) High, room and low temperature photoabsorption cross sections of CS 2 in the Å region. Geophys Res Lett 22: Molina LT, Lamb JJ, Molina MJ (1981) Temperature dependent UV absorption cross sections for carbonyl sulfide. Geophys Res Lett 8: Danielache SO, Eskebjerg C, Johnson MS, Ueno Y, Yoshida N (2008) High-precision spectroscopy of 32 S, 33 S and 34 S sulfur dioxide: Ultraviolet absorption cross sections and isotope effects. J Geophys Res 113:D von Hessberg P, et al. (2004) Ultra-violet absorption cross sections of isotopically substituted nitrous oxide species: 14 N 14 NO, 15 N 14 NO, 14 N 15 NO, and 15 N 15 NO. Atmos Chem Phys 4: Johnson MS, Billing GD, Gruodis A, Janssen MHM (2001) Photolysis of nitrous oxide isotopomers studied by time-dependent Hermite propagation. J Phys Chem 105: Sackmann IJ, Boothroyd AI, Kraemer KE (1993) Our Sun. III. Present and future. Astrophys J 418: Ribas I, Guinan EF, Güdel M, Audard M (2005) Evolution of the solar activity over time and effects on planetary atmospheres. I. High-energy irradiances (1 1700Å). Astrophys J 622: Farquhar J, Savarino J, Airieau S, Thiemens MH (2001) Observation of wavelengthsensitive mass-independent sulfur isotope effects during SO 2 photolysis: Applications to the early atmosphere. J Geophys Res 12: of11

3 1.8x x SO2 33 SO2 34 SO2 1.4x10-17 Cross section [cm 2 ] 1.2x x x x x x Wavelength [nm] Fig. S1. UV absorption spectra of 32 SO 2, 33 SO 2, and 34 SO 2 between 200 and 210 nm (1). The isotopologues maximum intensity, peak width, and structure change in an unexpected and complex manner. 1. Danielache SO, Eskebjerg C, Johnson MS, Ueno Y and Yoshida N (2008) High-precision spectroscopy of 32 S, 33 S and 34 S sulfur dioxide: UV absorption cross sections and isotope effects. J Geophys Res 113:D of11

4 Reduced sulfur pool O, O2 Oxidized sulfur pool 1 SO2 3 SO2 O2 H2S HCO HO2 H O OH SH H S HCO OH H2 OH CO, CO2 S S2 S3 CO CO CO O2 CO2 OCS SO O2 HO2 CO2 (SO)2 SO2 OH O HSO3 O2 O SO3 H2O Aerosol S4 S8 Aerosol Fig. S2. Schematic diagram of simplified sulfur photochemistry used in the model. Bold molecules (SO 2, OCS, and H 2 S) represent relatively long-lived species. 4of11

5 Actinic Flux [photon/s/cm 2 /nm] Measured (Rottman et al., 2006): 34 ε = -33.7, 33 Ε = Blackbody at 5779 K (present day): 34 ε = 13.0, 33 Ε = Blackbody at 5586 K (ca. 4.5 Ga): 34 ε = 12.1, 33 Ε = Wavelength [nm] Fig. S3. Comparison of measured solar spectrum (1) and blackbody radiation curves at 5,779 K and 5,586 K, which represent present day and 4.5 Ga Sun (2). Isotope effects of SO 2 photolysis ( 33 and 33 ) for the three spectra are also shown. 1. Rottman GJ, Woods TN, McClintock W (2006) SORCE solar UV irradiance results. Adv Space Res 37: Sackmann IJ, Boothroyd AI, Kraemer KE (1993) Our Sun. III. Present and future. Astrophys J 418: of11

6 Actinic flux [photon/s/cm 2 /nm] Atmosphere 1 CO2 : 1% CO : 0.1% Atmosphere 2 CO2 : 0.1% CO : 1% A1 A2 A Wavelength [nm] Wavelength [nm] 190 Atmosphere 3 CO2 : 0.1% CO : 1% OCS: 5 ppm 0 km 5 km 10 km 20 km Wavelength [nm] Number density [molecules/cm 3 ] B1 B2 B3 OCS SO S8 +60 C1 C2 C3 Δ 33 S [ ] S Fig. S4. Results of the numerical simulation for the model Atmospheres 1, 2, and 3. (A1 A3) Calculated actinic flux spectra at an altitude of 0, 5, 10, and 20 km in the three models. The black line shows solar UV spectrum at the top of the atmosphere. Vertical axis is the common in the three graphs. (B1 B3) Concentrations of OCS and SO 2 and total amounts of product H 2 SO 4 and S 8 as a function of time (hour) after injecting 10 ppm SO 2 at an altitude of 10 km. (C1 C3) Isotopic compositions ( 33 S) of the total H 2 SO 4 and S 8 as a function of time (hour) after injecting 10-ppm SO 2 at an altitude of 10 km. 6of11

7 A Δ 33 S [ ] Altitude [km] Δ 33 S [ ] B C 60 Atmosphere amount 40 S8 10 ~ 100% 1 ~ 10% 0.1 ~ 1% δ 34 S [ ] Atmosphere S Δ 33 S [ ] Atmosphere S δ 34 S [ ] Fig. S5. Results from the atmospheric reaction model. (A) The plots show the isotopic composition of the resulting sulfate and elemental sulfur at an altitude of 5 km. Three data points for each model show 10, 100, and 1,000 h after injection of SO 2. More than 10% of the injected SO 2 was converted into S 8 in our model (shown by the size of the symbols). (B) Vertical profile of the 33 S values of aerosols at 1,000 h after injection of 10 ppm SO 2, when compositions of sulfur species are almost stabilized. (C) The 34 S and 33 S relationships of the product aerosols at 1,000 h after injection of 10-ppm SO 2. Note that S 8 production is negligibly small in Atmosphere 1 and thus is not shown. 7of11

8 Table S1. Compositions of atmosphere used for model calculations Model Atmosphere 1 Atmosphere 2 Atmosphere 3 Total N 2 pressure at surface, bar O 2, PAL CO 2, % CO, % OCS, ppm 5 H 2, ppm of11

9 Table S2. Rate constants for photolysis reactions Rate constant at 5 km altitude, s 1 No. Reaction* Atmosphere 1 Atmosphere 2 Atmosphere 3 Ref. R1a 32 SO 2 h 3 32 SO O 1.420E E E-06 1 R1b 33 SO 2 h 3 33 SO O 1.277E E E-06 1 R1c 34 SO 2 h 3 34 SO O 1.324E E E-06 1 R2 SO h 3 S O 2.86E E E-05 2 R3 SO 3 h 3 SO 2 O 2.12E E E-06 3 R4 H 2 SO 4 h 3 SO 2 2OH 5.30E E E-08 4 R5 SO 2 h 3 sso E E E-05 5 R6 SO 2 h 3 tso E E E-08 5 R7 H 2 S h 3 SH H 2.33E E E-05 6 R8 S 2 h 3 S S 1.03E E E-05 7 R9 S 3 h 3 S 2 S 1.03E E E-05 5 R10 S 4 h 3 S 2 S E E E-05 5 R11 OCS h 3 CO S 3.44E E E-07 8 R12 O 2 h 3 O O( 1 D) 2.66E E E-07 5 R13 O 2 h 3 O O 5.10E E E-09 5 R14 O 3 h 3 O( 1 D) O E E E-04 5 R15 O 3 h 3 O O E E E-05 5 R16 H 2 O h 3 H OH 8.30E E E-07 5 R17 HO 2 h 3 OH O 5.79E E E-05 5 R18 CO 2 h 3 CO O 1.43E E E-11 5 R19 CO 2 h 3 CO O( 1 D) 1.57E E E-09 5 *Singlet and triplet state of sulfur have been denoted by s and t respectively. 1. Danielache SO, Eskebjerg C, Johnson MS, Ueno Y, Yoshida N (2008) High-precision spectroscopy of 32 S, 33 S and 34 S sulfur dioxide: ultraviolet absorption cross sections and isotope effects. J Geophys Res 113:D Phillips LF (1981) Absolute absorption cross sections for SO between 190 and 235 nm. J Phys Chem 85: Burkholder JB, McKeen S (1997) UV absorption cross sections for SO 3. Geophys Res Lett 24: Kasting JF (1990) Bolide impacts and the oxidation state of carbon in the Earth s early atmosphere. Origins Life Evol Biosph 20: Pavlov AA, Brown LL, Kasting JF (2001) UV shielding of NH 3 and O 2 by organic hazes in the Archean atmosphere. J Geophys Res 106: Wu CYR, Chen FZ (1998) Temperature-dependent photoabsorption cross sections of H 2S in the Å region. J Quant Spectrosc Radiat Transfer 60: DeAlmeida AA, Singh PD (1986) Photodissociation lifetimes of 32 S 2 molecule in comets. Earth Moon Planets 36: Molina LT, Lamb JJ, Molina MJ (1981) Temperature dependent UV absorption cross sections for carbonyl sulfide. Geophys Res Lett 8: of11

10 Table S3. Rate constants for two- and three-body reactions No. Reaction Rate constant Ref. Sulfur chemistry R20 S O 2 3 SO O 2.30E-12 1 R21 S OH 3 SO H 6.60E-11 2 R22 S HO 2 3 SO OH 1.50E-11 2 R23 S O 3 3 SO O E-11 3 R24 S CO 2 3 SO CO 1.00E-20 3 R25 S S M 3 S 2 M 1.00E-30* 4 R26 S 2 M 3 S S M 9.12E-80 4 R27 S S 2 M 3 S 3 M 1.00E-30* 4 R28 S 3 M 3 S S 2 M 9.42E-50 4 R29 S S 3 3 S 2 S E-14 4 R30 S 2 S 2 3 S S E-44 4 R31 S 2 S 2 M 3 S 4 M 2.80E-31* 3 R32 S S 3 M 3 S 4 M 2.80E-31* 3 R33 S 4 S 4 M 3 S 8 M 2.80E-31* 3 R34 SO O 2 3 SO 2 O 8.72E-17 1 R35 SO HO 2 3 SO 2 H 2.30E-11 2 R36 SO O M 3 SO 2 M 6.00E-31* 2 R37 SO OH 3 SO 2 H 8.60E-11 2 R38 SO O 3 3 SO 2 O E-14 3 R39 SO CO 2 3 SO 2 CO 2.13E-43 4 R40 SO SO 3 SO 2 S 8.30E-15 3 R41 SO SO 3 3 SO 2 SO E-15 3 R42 SO 2 O M 3 SO 3 M 1.80E-33* 1 R43 SO 2 OH M 3 HSO 3 M 3.30E-13* 1 R44 SO 2 O 3 3 SO 3 O E-22 1 R45 SO 3 H 2 O 3 H 2 SO E-15 3 R46 HSO 3 O 2 3 SO 3 HO E-13 3 R47 HSO 3 O 3 SO 3 OH 1.00E-11 3 R48 HSO 3 OH 3 SO 3 H 2 O 1.00E-11 3 R49 SO 2 H 3 SO OH 6.23E-30 4 R50 SO H 3 S OH 2.45E-26 4 R51 SO 3 CO 3 SO 2 CO E-30 4 R52 SO 2 CO 3 SO CO E-47 4 R53 HSO 3 H 3 SO 3 H E-11 3 R54 sso 2 3 tso E 03 3 R55 sso 2 3 SO E 04 3 R56 sso 2 M 3 tso 2 M 1.00E-12 3 R57 sso 2 M 3 SO 2 M 1.00E-11 3 R58 sso 2 O 2 3 SO 3 O 1.00E-16 3 R59 sso 2 SO 2 3 SO 3 SO 4.00E-12 3 R60 tso 2 3 SO E 03 3 R61 tso 2 M 3 SO 2 M 1.50E-13 3 R62 tso 2 SO 2 3 SO 3 SO 7.00E-14 3 R63 S H 2 S 3 SH SH 9.77E-16 4 R64 S H 2 3 SH H 2.56E-25 4 R65 H 2 S M 3 SH H M 3.14E-72 4 R66 S H 2 O 3 OH SH 1.12E-36 4 R67 S HCO 3 SH CO 5.00E-11 3 R68 S HO 2 3 SH O E-11 3 R69 SH SH 3 H 2 S S 1.50E-11 4 R70 S SH 3 S 2 H 4.50E-11 4 R71 H 2 S OH 3 H 2 O SH 4.75E-12 4 R72 H 2 S H 3 SH H E-13 3 R73 H 2 S O 3 OH SH 2.28E-14 1 R74 SH OH 3 H 2 O S 2.50E-12 4 R75 SH O 3 H SO 1.60E-10 1 R76 SH O 2 3 OH SO 4.00E-19 1 R77 SH HO 2 3 H 2 S O E-11 3 R78 SH HCO 3 H 2 S CO 5.00E-11 3 R79 SH H 2 O 3 H 2 S OH 3.17E-32 4 R80 SH H 2 3 H 2 S H 9.94E-23 4 R81 SH H 3 H 2 S 2.50E-11 4 R82 SH H M 3 H 2 S M 1.00E of 11

11 No. Reaction Rate constant Ref. R83 S 2 H 3 S SH 8.74E-23 4 R84 S 3 H 3 S 2 SH 1.80E-13 4 R85 S CO M 3 OCS M 1.07E-34* 4 R86 S 2 CO 3 OCS S 5.30E-38 4 R87 S 3 CO 3 OCS S E-40 4 R88 SH CO 3 OCS H 3.44E-25 4 R89 SH CO 2 3 OCS OH 2.45E-41 4 R90 SO 3 OCS 3 CO 2 (SO) E-26 4 R91 (SO) 2 OCS 3 CO SO 2 S E-20 4 R92 OCS M 3 CO S M 2.21E-61 4 R93 S OCS 3 CO S E-15 4 R94 S 2 OCS 3 CO S E-48 4 R95 H OCS 3 CO SH 1.80E-14 4 R96 OH OCS 3 CO 2 SH 2.01E-15 4 O-C-H chemistry R97 O O 2 M 3 O 3 M 6.00E-34* 3 R98 O O M 3 O 2 M 2.94E-33* 3 R99 O( 1 D) H 2 O 3 OH OH 2.20E-10 3 R100 O O 3 3 O 2 O E-15 3 R101 O( 1 D) O 2 3 O O E-11 3 R102 O( 1 D) M 3 O M 2.60E-11 3 R103 OH OH 3 H 2 O O 1.89E-12 3 R104 OH O3 3 HO 2 O E-14 2 R105 OH O 3 H O E-11 2 R106 H OH M 3 H 2 O M 6.78E-31 2 R107 H O 2 M 3 HO 2 M 5.50E-32 2 R108 H O 3 3 OH O E-11 2 R109 H H 2 O 3 OH H E-25 4 R110 H HO 2 3 H 2 O E-12 3 R111 H HO 2 3 H 2 O O 1.62E-12 3 R112 H HO 2 3 OH OH 7.29E-11 3 R113 H 2 O( 1 D) 3 OH H 1.00E-10 3 R114 H 2 O 3 OH H 2.94E-18 3 R115 H 2 OH 3 H 2 O H 7.00E-15 3 R116 CO OH 3 CO 2 H 1.50E-13 3 R117 CO O M 3 CO 2 M 4.54E-36* 3 R118 H CO M 3 HCO M 1.18E-34* 3 R119 HCO O 2 3 HO 2 CO 5.62E-12 3 R120 HCO H 3 H 2 CO 1.20E-10 3 R121 HCO OH 3 H 2 O CO 5.00E-11 3 R122 HCO O 3 H CO E-10 3 R123 HCO O 3 OH CO 1.00E-10 3 The rate constant units are cm 3 /s for two-body reactions unless indicated. For three-body reactions (*), rate constant units are cm 6 /s. M represents molecules acting as a third-body collision partner. 1. Sander SP, et al. (2006) Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling, Evaluation No. 15, JPL Publication 06-2, Jet Propulsion Laboratory, Pasadena, CA. 2. Kasting JF (1990) Bolide impacts and the oxidation state of carbon in the Earth s early atmosphere. Origins Life Evol Biosph 20: Pavlov AA, Brown LL, Kasting JF (2001) UV shielding of NH 3 and O 2 by organic hazes in the Archean atmosphere. J Geophys Res 106: Krasnopolsky VA (2007) Chemical kinetic model for the lower atmosphere of Venus. Icarus 191: of 11