Detection of propadiene (CH$_2$CCH$_2$), propene (C$_3$H$_6$) and non-detection of propane (C$_3$H$_8$) in Jupiter's northern polar stratosphere

Abstract

We report the first detection of stratospheric propadiene (CH$_2$CCH$_2$) and propene (C$_3$H$_6$) at Jupiter's mid-to-high northern latitudes using IRTF-TEXES measurements recorded on March 5-6, 2025. Using radiative transfer software to quantitatively test for the presence of propadiene and propene, we report a $>$12-$σ$ detection of propadiene and a $>$17-$σ$ detection of propene inside Jupiter's northern auroral region (henceforth 'NAR'), where the species are most concentrated. For example, at 62$^\circ$N inside Jupiter's NAR, we derive a 1-mbar propadiene abundance of 2.0 $\pm$ 0.2 ppbv, which is 40 $\pm$ 3 higher than abundances predicted by the Moses & Poppe (2017) photochemical model (henceforth 'MP17'), and significantly higher than the 1.2-ppbv upper limit abundance derived at 42$^\circ$N (the lowest latitude sampled by the observations). Similarly, we derive a 1-mbar propene abundance of 8.1 $\pm$ 0.5 ppbv at 62$^\circ$N inside Jupiter's NAR, which is 28 $\pm$ 2 higher than the MP17 predicted abundance and higher than the 6-ppbv 1-mbar upper limit abundance derived at 42$^\circ$N. The fact that propadiene and propene are most enriched inside Jupiter's NAR strongly suggests that perturbations to the chemistry by auroral-related heating and exogenous ions/electrons are responsible for their significant enrichment. Spectral features of propane (C$_3$H$_8$) were not detected at any of the locations sampled by the data: 3-$σ$ upper limits of 10 ppbv were derived at the 10-mbar level at 62$^\circ$N inside Jupiter's NAR. The non-detection of propane could, in part, be explained by the vertical sensitivity of its spectral features to deeper pressures, where there is negligible auroral-related heating. The results of this work advocate for development of ion-neutral chemistry models of Jupiter's polar stratosphere.