Fractionation factor associated with oxygen escape. From top to bottom, the panels show ¹⁸f_s-e, ¹⁸f_ph, ¹⁷f_s-e, and ¹⁷f_ph. The definitions of these fractionation factors are described in Section 4.3. The horizontal axis is the fractional contribution of atomic oxygen to O-bearing escaping species, corresponding to α in Equation (4). The solid, dashed, and dotted blue lines represent the results obtained using the standard eddy diffusion coefficient, a coefficient one order of magnitude smaller, and a coefficient one order of magnitude larger, respectively. The black dashed–dotted horizontal lines in the second and fourth panels represent ¹⁸f_ph,esc and ¹⁷f_ph,esc, respectively.