Distribution of Jovian South Polar Aerosols in the Near-Infrared with JWST/NIRSpec IFU

We present observations of Jupiter’s south polar region using the JWST/NIRSpec IFU spectrometer, providing high resolution spectral imaging across the near-infrared that probes the vertical and latitudinal distribution of aerosols and gaseous species.  Near-infrared spectroscopic mapping is a powerful diagnostic for studying giant planet atmospheres, enabling constraints on cloud structure in the weather-forming lower troposphere, and haze in the radiatively-controlled lower stratosphere.  The 1.7-5.3 µm range sampled here provides access to reflected sunlight both inside and outside of strong methane absorption bands; ionospheric emission from CH₄ and H₃⁺; and deep thermal emission from the 5 µm window where gaseous absorption is relatively low; sensing the 1 – 10 bar region. Locating and constraining the aerosol layers of Jupiter’s atmosphere is of great importance for photochemical models, as these hazes are produced in high-altitude photochemistry [1], and affect the efficiency of radiative heating and cooling of the upper troposphere and stratosphere [2]. The highest latitudinal zonal jets entrain a cold polar vortex, with clear transitions in aerosol properties (and potentially gaseous abundances) across the polar vortex boundary. Reflective aerosols may also be created by auroral particle precipitation [3] which allows magnetospheric phenomena to be traced onto the jovian clouds [4]. We will invert the JWST NIRSpec observations to explore how aerosol and gaseous properties differ between the south polar vortex and the mid-latitudes.

JWST/NIRSpec observed the south pole of Jupiter on 24 December 2022 as part of ERS-1373 (Co-PIs: de Pater & Fouchet), using the high-resolution, long-wavelength filter/grating combination F290LP/G395H (2.8 – 5.3µm). This provides exceptional spectral resolution (R~2700) over the latitude range 42 – 85^oS at spatial resolutions as high as 74km/spaxel. Six tiles were taken using this grating, giving 100^o of longitudinal coverage, and three tiles were taken with F170LP/G235H (1.7 – 3.2 µm). This covers the full near-IR range, from short-wavelength reflected sunlight to long-wavelength thermal emission. Hubble and Juno also observed the region at a similar time (2022/11/12 and 2022/12/15) allowing comparison of features in visible light and tracking over time in the same wavelength range.

The vertical structure of aerosols and gases will be determined using spectral modelling and inversion package NEMESIS [5]. We vary the vertical profile of phosphine, ammonia, water, germane, and arsine to obtain a good fit, and build an aerosol model of several layers of Mie-scattering spherical particles. We find that a three-layer cloud structure fits best, with a deep cloud at ~2 bar, a main cloud at ~0.7 bar and a haze layer in the lower stratosphere. Once these gas profiles and aerosols are fitting well at low latitudes, the model will be applied to zonally-averaged spectra across the full latitude range of the observations in order to study the changes in chemistry and aerosol properties that occur over the  polar vortex boundary. This neutral atmosphere model can then be used to compare against the north pole of Jupiter, or to extract clean spectra of the ionospheric emissions for later study.