FIG. 4.

Transmission spectra of an Earth-like atmosphere with different total atmospheric pressures. (a) Difference in flux from the stellar flux. (b) Percent of stellar flux absorbed by the atmosphere. The 1.06 μm dimer feature is strong only in the spectra corresponding to atmospheric pressures greater than ∼0.5 bar. The spectra for atmospheres with pressure ≥1 bar are nearly identical except for an offset because there is a fundamental limit on which heights in an atmosphere can be probed by transmission spectroscopy. For an Earth-like atmosphere around the M5V used here, only the top 0.9 bar can be probed for all atmospheres at all wavelengths. This pressure corresponds to an altitude of ∼1 km above the surface for a 1 bar atmosphere and 19 km above the surface for a 10 bar atmosphere. The offset is due to the flux blocked by layers in the atmosphere with pressure greater than 0.9 bar.