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We present a ground-based transmission spectrum and comprehensive retrievalanalysis of the highly inflated Saturn-mass planet WASP-39b. We obtainedlow-resolution spectra ($R \approx 400$) of a transit of WASP-39b using theACAM instrument on the 4.2m William Herschel Telescope as part of theLRG-BEASTS survey. Our transmission spectrum is in good agreement with previousground- and space-based observations of WASP-39b, and covers a wavelength rangeof 4000-9000A. Previous analyses of this exoplanet have retrieved waterabundances that span more than four orders of magnitude, which in turn lead toconclusions of a subsolar or highly supersolar atmospheric metallicity. Inorder to determine the cause of the large discrepancies in the literatureregarding WASP-39b's atmospheric metallicity, we performed retrieval analysesof all literature data sets. Our retrievals, which assume equilibriumchemistry, recovered highly supersolar metallicities for all data sets. Whenrunning our retrievals on a combined spectrum, spanning 0.3-5$\mu$m, werecovered an atmospheric metallicity of $282^{+65}_{-58} \times$ solar. We findthat stellar activity has a negligible effect on the derived abundances andinstead conclude that different assumptions made during retrieval analyses leadto the reported water abundances that differ by orders of magnitude. This inturn has significant consequences for the conclusions we draw. This is thefourth planet to be observed as part of the LRG-BEASTS survey, which isdemonstrating that 4m class telescopes can obtain low-resolution transmissionspectra with precisions of around one atmospheric scale height.

LRG-BEASTS: transmission spectroscopy and retrieval analysis of the highly-inflated Saturn-mass planet WASP-39b