Fig. 4.

Evolution of the Sun's XUV emission normalized to the present value and scaled to the present martian orbit at 1.52 AU. The solid line indicates the evolution during the post-saturation phase (Ribas et al., 2005) with data of solar analogs (black dots) and the Sun indicated. The dotted lines indicate the approximate evolution of the saturated XUV emission estimated by ${10}^{-3.2\pm 0.3}{L}_{\mathrm{bol}}$ (Pizzolato et al., 2003), with the bolometric luminosity taken from stellar evolution tracks of a solar mass star (Baraffe et al., 1998). The shaded area indicates the expected formation time of Mars (Brasser, 2013). The dashed line shows our adopted average XUV value during the Sun's saturation phase.