  James Webb Space Telescope's NIRSpec infrared imaging spectrometer observed
the outer solar system dwarf planets Eris and Makemake in reflected sunlight at
wavelengths spanning 1 through 5 microns. Both objects have high albedo
surfaces that are rich in methane ice, with a texture that permits long optical
path lengths through the ice for solar photons. There is evidence for N2 ice
absorption around 4.2 um on Eris, though not on Makemake. No CO ice absorption
is seen at 4.67 um on either body. For the first time, absorption bands of two
heavy isotopologues of methane are observed at 2.615 um (13CH4), 4.33 um
(12CH3D), and 4.57 um (12CH3D). These bands enable us to measure D/H ratios of
(2.5 +/- 0.5) x 10-4 and (2.9 +/- 0.6) x 10-4, along with 13C/12C ratios of
0.012 +/- 0.002 and 0.010 +/- 0.003 in the surface methane ices of Eris and
Makemake, respectively. The measured D/H ratios are much lower than that of
presumably primordial methane in comet 67P/Churyumov-Gerasimenko, but they are
similar to D/H ratios in water in many comets and larger outer solar system
objects. This similarity suggests that the hydrogen atoms in methane on Eris
and Makemake originated from water, indicative of geochemical processes in past
or even ongoing hot environments in their deep interiors. The 13C/12C ratios
are consistent with commonly observed solar system values, suggesting no
substantial enrichment in 13C as could happen if the methane currently on their
surfaces was the residue of a much larger inventory that had mostly been lost
to space. Possible explanations include geologically recent outgassing from the
interiors as well as processes that cycle the surface methane inventory to keep
the uppermost surfaces refreshed.
