Prof. Patrick Irwin

Abstract:

The origin of the red colouration in Jupiter's Great Red Spot (GRS) is a long-standing question in planetary science. While several candidate chromophores have been proposed, no clear conclusions have been reached regarding its nature, evolution, or relationship to atmospheric dynamics. In this work, we perform microphysical simulations of the reddish haze over the GRS and quantify the production rates and timescales required to sustain it. Matching the previously reported chromophore column mass and effective radius in the GRS requires column-integrated injection fluxes in the range 1×10⁻¹²–7×10⁻¹² kg m⁻² s⁻¹, under low upwelling velocities in the upper troposphere (v<inf>trop</inf>≲1.5×10⁻⁴ m s⁻¹) and particle charges of at least 20 electrons/μm. Such rates exceed the mass flux that standard photochemical models of Jupiter currently supply via NH<inf>3</inf>–C<inf>2</inf>H<inf>2</inf> photochemistry at 0.1–0.2 bar, the most popular chromophore pathway in recent literature. We find a lower limit of 7 years on the haze formation time. We also assess commonly used size and vertical distribution parameterisations for the chromophore haze, finding that eddy diffusion prevents the long-term confinement of a thin layer and that the extinction is dominated by particles that can be represented by a single log-normal size distribution.

Abstract:

We report the first detection of stratospheric propadiene (CH₂CCH₂) and propene (C₃H₆) 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 at high latitudes inside Jupiter’s auroral region, where the species are most concentrated. For example, at 62 °N (planetocentric) inside Jupiter’s northern auroral region (henceforth ‘NAR’), we derive a 1-mbar propadiene abundance of 2.0 ± 0.2 ppbv, which is 40 ± 3 higher than abundances predicted by the Moses and Poppe (2017) photochemical model (henceforth ‘MP17’), and significantly higher than the 1.2-pbbv upper limit abundance derived at 42 °N (the lowest latitude sampled by the observations). Similarly, we derive a 1-mbar propene abundance 8.1 ± 0.5 ppbv at 62 °N inside Jupiter’s NAR, which is 28 ± 2 higher than the MP17 predicted abundance and significantly higher than the 6-ppbv 1-mbar upper limit abundance derived at 42 °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, as has been observed for other unsaturated/aromatic hydrocarbon species. Spectral features of propane (C₃H₈) were not detected at any of the locations sampled by the data (poleward of 42 °N): 3-σ upper limits of ~10 ppbv at 10 mbar were derived at 62 °N inside Jupiter’s NAR, which is ~2.5 times the MP17 predicted abundance. The non-detection of propane could, in part, be explained by the vertical sensitivity of its mid-infrared emission lines to deeper pressures, where there is negligible auroral-related heating to warm the line forming region. The results of this work strongly advocate for development of ion-neutral chemistry models of Jupiter’s polar stratosphere to quantify how strong auroral-related heating and magnetospheric particles modify the reaction pathways that produce higher-order hydrocarbons.

Abstract:

Previous analysis of Neptune’s atmosphere using near-infrared Gemini/NIFS observations found the strongest spectral signature of hydrogen sulphide (HS) to be at the planet’s south pole. Conversely, analysis of microwave observations with the Atacama Large Millimeter/submillimeter Array in 2019 suggested a distribution of HS that peaks in the mid-latitudes and is strongly depleted towards the pole. We analyse near-infrared observations from VLT-SINFONI in 2018 and fit a parametrized cloud model to the data using nested sampling. By prescribing a latitudinally varying methane (CH) profile previously derived from visible light observations, we find general agreement with the microwave analysis, with an enhancement of HS by a factor of 4 at the southern mid-latitudes compared to polar latitudes. The stronger spectral signature at the pole is explained with a deeper cloud top, resulting in a higher cloud-top HS column abundance in this region. Our results are indicative of deep upwelling at the mid-latitudes, with downwelling at the pole and possibly near the equator.

Abstract:

The extreme conditions in Venus’s lower atmosphere make robust calibration of in situ observations challenging. Consequently, measurements from past entry probes provided mixed evidence regarding the existence of a near-surface particulate layer (NSPL). Although the Venera 11 (1978) and Venera 13 and 14 (1982) landers performed in situ spectrophotometric observations during descent, the original datasets were later lost. However, a subset has been reconstructed by digitising graphical outputs produced during the missions’ initial data-processing phase [1]. Following careful analysis to identify and mitigate errors and other artefacts, the reconstructed dataset retains the reliable downward-looking spectra acquired by the three landers from ~62 km altitude to the surface.Previous retrievals from the reconstructed Venera 13 indicated an NSPL centred at ~3.5–5 km, with particulate optical properties consistent with a basaltic composition [2]. Following the methodology of [2], we use NEMESIS, a radiative transfer and retrieval code [3], to perform near-surface retrievals from the reconstructed Venera 11 and Venera 14 datasets. The results from Venera 11, 13, and 14 retrievals are compared with reported detections and non-detections from other instruments on earlier in situ missions, to explore potential formation pathways for the NSPL in light of the combined observational record.References:[1] Ignatiev, N. I., Moroz, V. I., Moshkin, B. E., Ekonomov, A. P., Gnedykh, V. I., Grigor’ev, A. V., and Khatuntsev, I. V. Cosmic Research 35(1), 1–14 (1997).[2] Kulkarni, S. V., Irwin, P. G. J., Wilson, C. F., & Ignatiev, N. I. Journal of Geophysical Research: Planets, 130, e2024JE008728, (2025).[3] Irwin, P. G., Teanby, N. A., de Kok, R., Fletcher, L. N., Howett, C. J., Tsang, C. C., Wilson, C. F., Calcutt, S. B., Nixon, C. A., and Parrish, P. D. Journal of Quantitative Spectroscopy and Radiative Transfer 109(6), 1136–1150 (2008). 

Abstract:

To extract, or 'retrieve' atmospheric properties from the observed radiance spectra from a planetary atmosphere requires software that can generate the expected radiances from a guessed atmospheric model, compare the radiances with those measured, determine how the model should be updated to reduce any discrepancy between the modelled and observed radiances, and then iterate these steps until these differences are minimised. One such retrieval model is NEMESIS (Nonlinear optimal Estimator for MultivariatE Spectral analySIS), which was initially developed by myself and my colleagues in the 1990s, and which has since been continually updated and enhanced. NEMESIS has now been used in more than 300 papers retrieving atmospheric properties from observed thermal and solar-reflected radiance spectra from all the planetary atmospheres in our solar system and also some beyond. NEMESIS uses the Optimal Estimation framework for atmospheric retrievals and is written in FORTRAN. Recently, more Bayesian frameworks have become computationally possible and favoured, especially for exoplanetary retrievals where prior constraints are almost entirely absent. Hence, NEMESIS has recently been updated to Python (ArchNEMESIS), and combined with PyMultiNest to allow nested sampling retrievals that can better explore the degeneracy between different atmospheric properties. I will review how NEMESIS retrievals have improved our understanding of planetary atmospheres over the last 30 years and how the development of ArchNEMESIS has breathed new life into the NEMESIS/ArchNEMESIS project.