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A comprehensive analysis of 38 previously published Wide Field Camera 3(WFC3) transmission spectra is performed using a hierarchy of nested-samplingretrievals: with versus without clouds, grey versus non-grey clouds, isothermalversus non-isothermal transit chords and with water, hydrogen cyanide and/orammonia. We revisit the "normalisation degeneracy": the relative abundances ofmolecules are degenerate at the order-of-magnitude level with the absolutenormalisation of the transmission spectrum. Using a suite of mock retrievals,we demonstrate that the normalisation degeneracy may be partially broken usingWFC3 data alone, even in the absence of optical/visible data and withoutappealing to the presence of patchy clouds, although lower limits to the mixingratios may be prior-dominated depending on the measurement uncertainties. WithJames Webb Space Telescope-like spectral resolutions, the normalisationdegeneracy may be completely broken from infrared spectra alone. We find notrend in the retrieved water abundances across nearly two orders of magnitudein exoplanet mass and a factor of 5 in retrieved temperature (about 500 to 2500K). We further show that there is a general lack of strong Bayesian evidence tosupport interpretations of non-grey over grey clouds (only for WASP-69b andWASP-76b) and non-isothermal over isothermal atmospheres (no objects). 35 outof 38 WFC3 transmission spectra are well-fitted by an isothermal transit chordwith grey clouds and water only, while 8 are adequately explained by flatlines. Generally, the cloud composition is unconstrained.

Retrieval analysis of 38 WFC3 transmission spectra and resolution of the normalisation degeneracy