2025NatAs...9..370F

The James Webb Space Telescope can potentially explore the geological diversity of the surfaces of rocky exoplanets, especially due to its access to mid-infrared wavelengths. Here we investigate the level of geological detail that it could be possible to observe with the low-resolution spectroscopy and photometric modes of the mid-infrared instrument onboard the James Webb Space Telescope. We used new emissivity measurements of 15 basaltic samples between 2 μm and 25 μm to produce synthetic spectra and photometric fluxes. We found that the mid-infrared instrument can, in principle, distinguish several specific mineralogical and bulk chemical signals among relatively similar rocks. In particular, hydrous minerals, such as amphibole and serpentine, which would signal the existence of past or present water, can have observable characteristics in both low-resolution spectroscopy observations (with the precision of 5 eclipses) and the integrated fluxes over mid-infrared instrument filter bandwidths (20-100 eclipses). Photometric fluxes are also sensitive to bulk compositions (for example, wt% Al ₂ O ₃ ), which reflect magmatic processes. Our work demonstrates the potential for the James Webb Space Telescope and future observatories to access a fuller picture of exoplanet surface geology.