tokens: We thank Y. L. Yung, J. F. Kasting, C. E. Harman, and S. Seager for a helpful discussion of atmospheric photochemistry. R.Y.H. was supported by NASA grant No. 80NM0018F0612 and grant No. 15038.004 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. The research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
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