An intense narrow equatorial jet in Jupiter's lower stratosphere observed by JWST

Jupiter and Saturn's atmospheres exhibit east-west zonal jets varying with latitude, primarily observed at tropospheric cloud levels. Above and below the tropopause (~100 mbar), equatorial hazes hinder infrared observations of stratospheric dynamics. Previous studies have revealed intense stratospheric winds in both planets, alternating in direction with altitude (0.1–40 mbar), exhibiting multiyear cyclical variations similar to Earth's quasi-biennial oscillation (QBO) and semi-annual oscillation (SAO). These oscillations exhibit a vertical pattern of temperature and wind perturbations gradually descending over time. Jupiter's JESO has a variable period (3.9–5.7 years), linked to large-scale convective perturbations. Saturn's equatorial stratospheric temperatures oscillate quasi-periodically in a 15-year cycle. Understanding the connections between stratospheric and tropospheric dynamics in these equatorial regions remains a key challenge. Equatorial hazes obscure the view of lower stratospheric dynamics in Jupiter, preventing high-quality wind measurements at these altitudes. The high-resolution capabilities of the JWST offer a unique opportunity to address these limitations.

Studies of Jupiter's and Saturn's atmospheres have primarily focused on cloud-top level wind observations, with some insights gained from infrared spectroscopy and thermal wind relationships at stratospheric levels. These studies have revealed the presence of intense stratospheric eastward and westward winds, alternating with altitude in the 0.1–40 mbar region. The perturbations to equatorial stratospheric temperatures and inferred thermal winds vary periodically on multiyear timescales, resembling terrestrial QBO and SAO. The downward propagation of these oscillations over time has also been observed. However, the complexity of equatorial dynamics, including the interaction between stratospheric and tropospheric processes, remains poorly understood. Previous attempts to observe winds within the equatorial hazes have been limited by image quality, leading to uncertainties in wind measurements.

Observations were conducted on July 27, 2022, using the JWST's Near Infrared Camera (NIRCam). To avoid detector saturation, filters centered in strong atmospheric methane absorption bands (F164N, F335M, F360M, F405N) and the H₂-H₂ and H₂-He collision-induced absorption band at 2.12 μm (F212N) were selected. Full-disk images were obtained, and observations were repeated after one planetary rotation to track atmospheric features. Image processing involved correcting for planetary rotation, combining individual frames, and applying high-pass spatial filters to enhance fine-scale details. The two-way penetration depth of solar radiation was calculated for a simplified model atmosphere without aerosols, considering optical depths from τ=1 to τ=5 to estimate the altitude sensitivity of each filter. Wind measurements involved tracking atmospheric features in the F164N, F212N, and F335M images using image correlation software. Regions covered by large atmospheric systems (e.g., the Great Red Spot) were excluded. The software's capabilities for adapting box sizes to image contrast ensured accurate analysis in various regions. For the F164N images, individual data sets from subarrays were merged into a single zonal wind profile.

The JWST observations revealed an intense equatorial jet (140 m s⁻¹) at 100–200 mbar, exceeding the zonal wind speeds at the cloud level by 70 m s⁻¹. This jet is remarkably narrow, confined to within ±3° of the equator. The zonal wind profiles obtained outside the equatorial zone closely resembled previous measurements of Jupiter's zonal winds at cloud tops, confirming the absence of elevated aerosols in those regions. However, within the equatorial zone (±10°), a clear difference was observed, with the derived zonal winds at higher altitudes showing significant differences compared to the cloud-top level. The intense central jet showed a wind speed increase with altitude up to the tropopause, contrasting with a wind speed decrease with altitude outside the central jet. The wind measurements from the F164N, F212N, and F335M filters revealed strong vertical wind shear within the haze levels, with the most intense winds observed in the F335M images likely corresponding to the highest levels. Significant vertical wind shears were estimated (40–70 m s⁻¹ per scale height from cloud tops to hazes, with more uncertain values within the hazes). The observed latitudinal asymmetry in winds might be related to varying haze opacities north and south of the equator. This intense jet's existence was confirmed by comparison with previous wind measurements during the Cassini flyby of 2000.

The discovery of this intense, narrow equatorial jet near Jupiter's tropopause reveals striking similarities between Jupiter's and Saturn's equatorial circulations. Both planets exhibit a doubly peaked equatorial jet at the main cloud level, and Jupiter now shows a sharply peaked jet at the equator in the hazes above the cloud deck. This jet's location coincides with a brighter region in the equatorial zone at 889 nm and F335M. Despite these similarities, differences exist: Jupiter's equatorial zone is narrower than Saturn's, and the cloud-level meteorology is distinct. The significantly stronger vertical shear in Jupiter's equatorial jet compared to Saturn's requires further investigation. The relationship between the jet and the observed stratospheric thermal oscillations, and its temporal variability, are also important considerations given our single-epoch observations. The unexpected penetration of the stratospheric equatorial oscillations into the upper troposphere challenges existing simulations, highlighting the need for improved models that incorporate the interactions between tropospheric and stratospheric processes.

This study, utilizing the JWST's high-resolution capabilities, provides unprecedented insights into Jupiter's lower stratospheric dynamics, revealing an intense and narrow equatorial jet. The findings highlight the close relationship between Jupiter and Saturn's equatorial circulations and underscore the importance of troposphere-stratosphere interactions. Further investigation, including observations across multiple epochs, is crucial for understanding the jet's temporal variability and its connection to stratospheric oscillations. The unexpected deep penetration of the equatorial oscillation warrants the development of improved atmospheric models.

This study is based on observations from a single epoch. This limits the ability to determine the temporal variability of the newly discovered equatorial jet. Also, the simplified model atmosphere used to assess the filter penetration depths may not fully capture the complexity of Jupiter's actual atmospheric structure, potentially influencing the precision of altitude estimations.