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tokens: This work was aided greatly by discussions with L. Coogan, N. Cowan, C. Goldblatt, A. Lenardic, L. Schaefer, N. Sleep, and K. Zahnle. We thank the anonymous referee for helpful comments that greatly improved the manuscript. We thank the Kavli Summer Program in Astrophysics for the setting to perform this research and the hospitality of the program members and community at the University of California, Santa Cruz. T.D.K. acknowledges support from NASA headquarters under the NASA Earth and Space Science Fellowship Program Grant PLANET14F-0038. D.S.A. acknowledges support from the NASA Astrobiology Institute Virtual Planetary Laboratory, which is supported by NASA under cooperative agreement NNH05ZDA001C.