tokens: We thank the anonymous referee for comments that significantly improved the manuscript. We additionally thank X. Bai, R. Fu, and J. Bryson for useful discussions. We gratefully acknowledge support from the NASA Emerging Worlds program (grant No. NNX15AH72G) , the U.S. Rosetta program (CREI 1576768), and Thomas F. Peterson Jr. Software: Astropy (Astropy Collaboration et al. 2013 ), matplotlib (Hunter 2007 ), NumPy (Van Der Walt et al. 2011 ), SciPy (Jones et al. 2001 ).
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