paper

JWST Detection of Hydrocarbon Ices and Methane Gas on Makemake

arXiv:2509.06772 · doi:10.3847/1538-4357/adf4e7

Abstract

JWST/NIRSpec observations of Makemake reveal a chemically complex surface and evidence of gaseous CH. Our spectral modeling indicates a surface composition consisting of CH, CHD, and possibly CHOH, combined with aggregates of CH and CH. The presence of CH 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 CH photolysis is either ongoing or occurred recently. The detection of CHD yields a D/H ratio in CH ice of , consistent within with previous estimates. We report the first detection of CH fluorescence from Makemake, establishing it as only the second trans-Neptunian object$\unicode{x2014}$after Pluto$\unicode{x2014}$with confirmed volatile release. We explore two scenarios consistent with the observed CH emission, though neither fully reproduces the data: an expanding coma, yielding production rates of $(0.2\unicode{x2013}1.6) \times 10^{28}$ molecules s 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$\unicode{x2014}$values consistent with stellar occultation constraints and an atmosphere in equilibrium with surface CH ice. Discriminating between these scenarios will require higher spectral resolution and improved signal-to-noise observations. Together, the gas-phase CH, intermediate D/H ratio between that in water and CH in comets, and complex surface composition challenge the traditional view of Makemake as a quiescent, frozen body.

arXiv comment23 pages, 11 figures, 2 tables. Accepted for publication in The Astrophysical Journal Letters

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