A Scale-Height Estimate of the Mean Molecular Weight of K2-18 b from JWST NIRISS SOSS and NIRSpec G395H Transmission Spectra

Abstract I estimate the mean molecular weight (μ) of the sub-Neptune K2-18 b from published JWST NIRISS SOSS and NIRSpec G395H transmission spectra. I measure the excess depth of six CH₄ and CO₂ bands against neighbouring continuum, convert it to a scale height, and invert for μ. The mean amplitude is ΔD = 56.1 ± 9.1 ppm. The resulting μ ranges from 2.0 to 5.3 depending on the assumed temperature (120–255 K) and number of scale heights (N = 4–5), with a statistical error of about 16% at each choice. For 120–160 K, the range of values retrieved at 10 mbar, μ = 2.0 – 3.3. The data are consistent with both light and enriched atmospheres, and the answer is dominated by T and N rather than by measurement noise.