Simulated transmission spectra of LHS 1140 b with weak extinction from hazes in solid lines as shown in Fig. 5. Upper panel: H₂-dominated atmosphereswith low, medium, and high CH₄ concentrations corresponding to scenarios 1a (blue), 1b (green), and 1c (orange), respectively. Lower panel: CO₂ -dominated atmosphereswith low, medium, and high CH₄ concentrations corresponding to scenarios 10a (blue), 10b (green), and 10c (orange), respectively. The shaded region shows the contribution of the minor species to the full spectrum. Expected error bars for a single transit observation assuming scenario 1c (upper panel) and 30 co-added transits assuming scenario 10c (lower panel) using JWST NIRSpec PRISM (0.7–5 μm) and JWST MIRI LRS (5–12 μm), binned to R = 30. Strongest contribution of minor atmospheric molecular absorption bands are indicated by the color of the scenario or in gray when all scenarios have a significant contribution.