Monitoring the temporal evolution of SL9-derived species in the stratosphere of Jupiter with ALMA

Giant planet atmospheres are continuously affected by external sources, including cometary impacts, interplanetary dust particles (IDPs), and material originating from rings and satellites, that modify their composition over long timescales. At Jupiter, the last major comet impact occurred in July 1994, when the 21 fragments of comet Shoemaker–Levy 9 (SL9) collided with the planet in its southern hemisphere over the course of one week (Noll et al. 1996). This rare event provided a unique opportunity to investigate the shock-induced chemistry recombining jovian atmosphere and cometary material, leading to the detection of several new species in Jupiter’s stratosphere (e.g., CO, CS, and HCN; Lellouch et al. 1996).

On longer timescales, monitoring these newly formed species months and years after the impacts provides valuable constraints on the chemical and dynamical evolution of exogenic material in Jupiter’s atmosphere. First, the temporal evolution of their abundances traces the chemical loss processes. For instance, Cavalié et al. (2023) reported an HCN loss of a factor of 2–8 in 2017 ALMA observations compared to the 1995–1998 period. Second, their horizontal and vertical transport constrains atmospheric dynamics and mixing as a function of longitude, latitude, and altitude. At first located at the impact sites, the longest-lived species progressively spread in longitude over a few months, in latitude over a few years (e.g., Lellouch et al. 2006; Moreno et al. 2003), and vertically from pressures near 0.1 mbar initially to several mbar (Cavalié et al. 2023; Rodriguez-Ovalle et al. 2025). Owing to horizontal mixing, these molecules were expected to become uniformly distributed across Jupiter’s disk on a timescale of about 10 years (Moreno et al. 2003). This prediction was confirmed in 2017 for CO, which exhibited a constant latitudinal distribution (Cavalié et al. 2023).

However, HCN exhibits an unusual distribution from the mid-latitudes to the polar regions (Cavalié et al. 2023; Rodriguez-Ovalle et al. 2025). This behaviour is unexpected because HCN and CO share the same cometary origin and should therefore display similarly uniform distributions. Two remarkable features were identified by Cavalié et al. (2023). First, they observed an HCN depletion towards the poles, coincident with the position of the auroral regions. This depletion is seen vertically at pressures greater than 0.1 mbar and has been tentatively attributed to adsorption onto aerosols forming and growing at these pressure levels in the polar atmosphere (Friedson et al. 2002). Second, and more surprisingly, HCN exhibits a high-altitude abundance enhancement reaching up to 20 times the low-to-mid latitude abundance. This enhancement is spatially confined to the southern auroral hot spot (Sinclair et al. 2023) at 0.01 mbar, i.e., above the pressure levels where depletion is observed in the polar regions. Such an HCN enhancement could result from local production through ion–neutral chemistry under auroral conditions.

In this study, we aim to monitor the temporal evolution of SL9-produced species in the stratosphere of Jupiter, in order to (i) follow their horizontal and vertical transport to constrain the mixing properties at the probed levels; (ii) constrain the chemical losses over time ; and (iii) assess their distribution in the polar regions and understand the auroral related chemistry.

We use mapping observations of Jupiter performed with Atacama Large Millimeter/submillimeter Array (ALMA) recorded on three occasions in September 2022, i.e., almost 30 years after the SL9 impacts. We detect the HCN (J=4-3), CO (J=3-2), and CS(J=7-6) emission lines emanating from the stratosphere of Jupiter all along the planetary limb. While HCN and CO were already observed with ALMA in 2017, we present the first high resolution map of CS. Here, we will present radiative transfer calculations and vertical profile retrievals performed at various latitudes to illustrate the horizontal and vertical distributions of the targeted species. We will compare our results with those obtained from observations performed over the last 30 years.

References:

Cavalié et al. 2021, A&A 647, 8

Cavalié et al. 2023, Nat. Astron. 7, 1048

Friedson et al. 2002, Icarus 158, 389

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Lellouch et al. 2006, Icarus 184, 478

Moreno et al. 2003, P&SS 51, 591

Noll et al. 1996, Cambridge University Press

Rodriguez-Ovalle et al. 2025, A&A 696, 173

Sinclair et al. 2023, Planet. Sci. J. 4, 76