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Centaurs are minor solar system bodies with orbits transitioning betweenthose of Trans-Neptunian Scattered Disk objects and Jupiter Family comets.39P/Oterma is a frequently active Centaur that has recently held both Centaurand JFC classifications and was observed with the JWST NIRSpec instrument on2022 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 lowestdetection of CO$_2$ of any Centaur or comet. CO and H$_2$O were not detecteddown 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 notwater, and show a significant difference between the coma abundances of29P/Schwassmann-Wachmann 1, another Centaur at a similar heliocentric distance,which may be explained by thermal processing of 39P's surface during itsprevious Jupiter-family comet orbit. To help contextualize the JWST data wealso acquired visible CCD imaging data on two dates in July (Gemini North) andSeptember (Lowell Discovery Telescope) 2022. Image analysis and photometrybased on these data are consistent with a point source detection and anestimated effective nucleus radius of 39P in the range of $R_{nuc}= $2.21 to2.49~km.

First detection of CO$_2$ emission in a Centaur: JWST NIRSpec observations of 39P/Oterma