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Mildly irradiated mini-Neptunes have densities potentially consistent withthem hosting substantial liquid water oceans (`Hycean' planets). The presenceof CO2 and simultaneous absence of ammonia (NH3) in their atmospheres has beenproposed as a fingerprint of such worlds. JWST observations of K2-18b, thearchetypal Hycean, have found the presence of CO2 and the depletion of NH3 to<100 ppm; hence, it has been inferred that this planet may host liquid wateroceans. In contrast, climate modelling suggests that many of thesemini-Neptunes, including K2-18b, may likely be too hot to host liquid water. Wepropose a solution to this discrepancy between observation and climatemodelling by investigating the effect of a magma ocean on the atmosphericchemistry of mini-Neptunes. We demonstrate that atmospheric NH3 depletion is anatural consequence of the high solubility of nitrogen species in magma atreducing conditions; precisely the conditions prevailing where a thick hydrogenenvelope is in communication with a molten planetary surface. The magma oceanmodel reproduces the present JWST spectrum of K2-18b to < 3 sigma, suggestingthis is as credible an explanation for current observations as the planethosting a liquid water ocean. Spectral areas that could be used to rule out themagma ocean model include the >4um region, where CO2 and CO features dominate:Magma ocean models suggest a systematically lower CO2/CO ratio than estimatedfrom free chemistry retrieval, indicating that deeper observations of thisspectral region may be able to distinguish between oceans of liquid water andmagma on mini-Neptunes.

Distinguishing oceans of water from magma on mini-Neptune K2-18b