  JWST recently measured the transmission spectrum of K2-18b, a habitable-zone
sub-Neptune exoplanet, detecting CH$_4$ and CO$_2$ in its atmosphere. The
discovery paper argued the data are best explained by a habitable "Hycean"
world, consisting of a relatively thin H$_2$-dominated atmosphere overlying a
liquid water ocean. Here, we use photochemical and climate models to simulate
K2-18b as both a Hycean planet and a gas-rich mini-Neptune with no defined
surface. We find that a lifeless Hycean world is hard to reconcile with the
JWST observations because photochemistry only supports $< 1$ part-per-million
CH$_4$ in such an atmosphere while the data suggest about $\sim 1\%$ of the gas
is present. Sustaining %-level CH$_4$ on a Hycean K2-18b may require the
presence of a methane-producing biosphere, similar to microbial life on Earth
$\sim 3$ billion years ago. On the other hand, we predict that a gas-rich
mini-Neptune with $100 \times$ solar metallicity should have 4% CH$_4$ and
nearly 0.1% CO$_2$, which are compatible with the JWST data. The CH$_4$ and
CO$_2$ are produced thermochemically in the deep atmosphere and mixed upward to
the low pressures sensitive to transmission spectroscopy. The model predicts
H$_2$O, NH$_3$ and CO abundances broadly consistent with the non-detections.
Given the additional obstacles to maintaining a stable temperate climate on
Hycean worlds due to H$_2$ escape and potential supercriticality at depth, we
favor the mini-Neptune interpretation because of its relative simplicity and
because it does not need a biosphere or other unknown source of methane to
explain the data.
