  Centaurs are minor solar system bodies with orbits transitioning between
those of Trans-Neptunian Scattered Disk objects and Jupiter Family comets.
39P/Oterma is a frequently active Centaur that has recently held both Centaur
and JFC classifications and was observed with the JWST NIRSpec instrument on
2022 July 27 UTC while it was 5.82 au from the Sun. For the first time, CO$_2$
gas emission was detected in a Centaur, with a production rate of Q$_{CO_2}$ =
(5.96 $\pm$ 0.80) $\times$ 10$^{23}$ molecules s$^{-1}$. This is the lowest
detection of CO$_2$ of any Centaur or comet. CO and H$_2$O were not detected
down to constraining upper limits. Derived mixing ratios of Q$_{CO}$/Q$_{CO_2}$
$\leq$2.03 and Q$_{CO_2}$/Q$_{H_2O}$ $\geq$0.60 are consistent with CO$_2$
and/or CO outgassing playing large roles in driving the activity, but not
water, and show a significant difference between the coma abundances of
29P/Schwassmann-Wachmann 1, another Centaur at a similar heliocentric distance,
which may be explained by thermal processing of 39P's surface during its
previous Jupiter-family comet orbit. To help contextualize the JWST data we
also acquired visible CCD imaging data on two dates in July (Gemini North) and
September (Lowell Discovery Telescope) 2022. Image analysis and photometry
based on these data are consistent with a point source detection and an
estimated effective nucleus radius of 39P in the range of $R_{nuc}= $2.21 to
2.49~km.
