JWST Detection of Hydrocarbon Ices and Methane Gas on Makemake

Abstract

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.