Spectral range of ACS MIR containing PH₃ absorption features. Panel a: modelled transmission spectrum contributions from 10 ppbv of PH₃ from a 20 km tangent height. Panel b: single spectra extracted from orders 142–145 for 12 consecutive tangent heights (2–26 km) in the lower atmosphere of Mars using ACS MIR secondary grating position 9. All visible absorption lines result from CO₂, which also impacts the broad continuum that decreases transmission towards lower wavenumbers, where the CO₂ν₃ vibration-rotation band is centred. Boxes in panel a indicate the spectral windows covering PH₃ absorption features in order 143 used to determine upper limits.