JWST Detection of Hydrocarbon Ices and Methane Gas on Makemake

JWST/NIRSpec observations of Makemake reveal a chemically complex surface and evidence of gaseous CH₄. Our spectral modeling indicates a surface composition consisting of CH₄, CH₃D, and possibly CH₃OH, combined with aggregates of C₂H₂ and C₂H₆. The presence of C₂H₄ is also considered given its expected photochemical origin. Both areal and layered configurations reproduce the observed spectrum, with the latter being preferred. This composition confirms earlier hydrocarbon detections and suggests that CH₄ photolysis is either ongoing or occurred recently. The detection of CH₃D yields a D/H ratio in CH₄ ice of (3.98 ± 0.34) × 10⁻⁴, consistent within 2σ with previous estimates. We report the first detection of CH₄ fluorescence from Makemake, establishing it as only the second trans-Neptunian object—after Pluto—with confirmed volatile release. We explore two scenarios consistent with the observed CH₄ emission, though neither fully reproduces the data: an expanding coma, yielding production rates of (0.2-1.6) × 10²⁸ molecules s⁻¹ and a rovibrational temperature of ∼35 K, possibly originating from a localized plume, and a gravitationally bound atmosphere, which, if adopted, implies gas kinetic temperatures near 40 K and surface pressures of ∼10 pbar—values consistent with stellar occultation constraints and an atmosphere in equilibrium with surface CH₄ ice. Discriminating between these scenarios will require higher spectral resolution and improved signal-to-noise observations. Together, the gas-phase CH₄, intermediate D/H ratio between that in water and CH₄ in comets, and complex surface composition challenge the traditional view of Makemake as a quiescent, frozen body.