The evolution of comet-impact products in Jupiter’s atmosphere using JWST-MIRI observations

The 1994 impact of comet Shoemaker Levy 9 on Jupiter triggered the injection of new chemical species into the planet’s stratosphere – including H₂O, CO, HCN and CS – that had never been detected before [1,2,3]. Recent studies have also proposed that similar cometary impacts may have delivered similar species to other outer planets [4,5,6].

The temporal and spatial evolution of these SL9-derived species has since been monitored in Jupiter. These molecules were originally injected at 44S, confined to a narrow layer at 0.1 mbar. Over the following years, these molecules have diffused vertically, reaching pressure levels of 3 mbar by 2017, and have also dispersed horizontally across the planet. Notably, recent ALMA observations revealed that CO is now evenly distributed across latitudes [7]. However, other molecules, such as HCN and CO₂, show unexpected spatial patterns that challenge current transport and chemical models. First, HCN is well mixed at mid-latitudes but it exhibits polar depletion [7]. Second, CO₂, a daughter molecule of the SL9-derived CO and H₂O, presented an enhanced abundance in the South Polar Region in 2000 as revealed by Cassini/CIRS observations [8].

We analyzed James Webb Space Telescope (JWST) Mid InfraRed Instrument (MIRI) medium-resolution spectroscopy observations from latitudes of 17S to 26S, and from 45S towards the south pole to retrieve the molecular abundances of CO₂, H₂O, and HCN (see Figure 1) by coupling a radiative transfer code with an inversion algorithm. This model was used to retrieve, in the first place, the temperature from the CH4 ν4 band in first place and, in the second place, the abundance of the species for the different latitudes.

Our results show a complex latitudinal structure for each molecule. Regarding HCN, comparisons with ALMA (2017) and Cassini/CIRS (2000) allow us to assess its temporal evolution. Using a simple transport model, we could suggest a possible origin for its polar depletion, that could be linked to the presence of stratospheric aerosols. These aerosols could trigger heterogeneous chemical reaction that can lead to the adsorption of HCN onto these aerosols. For the oxygenated molecules, this is the first time H₂O’s meridional distribution has been mapped with such resolution , and the first time CO₂ has been observed in Jupiter since 2000 with Cassini/CIRS. We observed the column density to be affected at the regions where the auroral oval is present. Interestingly, both H₂O and CO₂ show variations linked to Jupiter’s southern auroral oval, with CO₂ exhibiting strong depletion and H₂O showing enhancement in the same region. This potential anti-correlation is not well understood and may indicate the involvement of an unknown mechanism — possibly initiated by particle precipitation or complex meridional transport — that affects the oxygen exchange between these molecules.

While these findings mark a significant step forward in understanding post-impact chemistry and atmospheric dynamics, several questions remain unclear, particularly the chemical relationship between the oxygenated molecules. Upcoming JWST Cycle 4 observations (PI: Rodriguez-Ovalle) will provide a full meridional coverage of these molecules (see figure 2), and are expected to shed some light on the chemistry and transport behind H₂O and CO₂.

Figure 1. Top panel: Example of Jupiter spectral region (zonally averaged) encompassing CH4 lines and the H2O ν2 lines at 75S (red), 65S (blue) and 25S (black). The vertical lines display the spectral features of CH4 (black) and NH3 (orange). Bottom left panel: Same but for HCN ν2 band (zonally averaged) next to C2H2 lines. Bottom right panel: Same but for CO2 ν2 band. The grey regions mark the positions of the emission lines of each molecule. The spectra at 65S is shifted to match the continuum emission of the 75S spectrum. The offset values in nW.cm−2.sr−1 / cm−1 are indicated in the legend of each panel.

 

Figure 2: MIRI/MRS footprints on Jupiter for a single dither for channels 1 (blue) and 3 (red). The order of the observation is labeled from 1 to 10. The northern auroral oval is marked in fuchsia.

 

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