FIG. 1.
(a) The stellar energy distribution at a planet receiving the same amount of total energy flux that the Earth received ∼2.5 billion years ago for three different stars: the Sun, AD Leo, and T3100 (a model M dwarf that has no chromosphere). (b) The bottom panel is an expansion of the UV region of the top panel, with a logarithmic y axis. The bottom panel also shows the absorption cross section of CH3SH, units for which are on the right y axis (also logarithmic). Color images available online at www.liebertonline.com/ast