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

Abstract

The atmosphere of Jupiter has east–west zonal jets that alternate as a function of latitude as tracked by cloud motions at tropospheric levels. Above and below the cold tropopause at ~100 mbar, the equatorial atmosphere is covered by hazes at levels where thermal infrared observations used to characterize the dynamics of the stratosphere lose part of their sensitivity. James Webb Space Telescope observations of Jupiter in July 2022 show these hazes in higher detail than ever before and reveal the presence of an intense (140 m s⁻¹) equatorial jet at 100–200 mbar (70 m s⁻¹ faster than the zonal winds at the cloud level) that is confined to ±3° of the equator and is located below stratospheric thermal oscillations that extend at least from 0.1 to 40 mbar and repeat in multiyear cycles. This suggests that the new jet is a deep part of Jupiter’s Equatorial Stratospheric Oscillation and may therefore vary in strength over time.

Main

Jupiter and Saturn atmospheres have zonal jets with broad prograde winds in their equatorial regions^1,2. The winds are best observed at cloud-top levels but extend 3,000 km deeper in Jupiter^3,4 and 8,000 km in Saturn^5,6. Above the cloud level, wind speeds decay with increasing altitude to nearly zero values at 20 mbar in most of Jupiter’s mid-latitudes^7,8,9. However, this pattern of vertical decay is spectacularly broken in Saturn’s equator, where an intense (400 m s⁻¹) and narrow (5° width) jet is observed tracking the motions of high-altitude hazes at the 50–60 mbar level^10,11.

In the equatorial regions of both planets, temperature retrievals through infrared spectroscopy, in combination with the thermal wind relationship^12,13, reveal the presence of intense stratospheric eastward and westward winds alternating in direction as a function of altitude in the 0.1–40.0 mbar region^8,14,15. The perturbations to the equatorial stratospheric temperatures and inferred thermal winds vary periodically on multiyear timescales^15,16,17,18, sharing characteristics with similar phenomena on Earth, such as the quasi-biennial oscillation (QBO) and semi-annual oscillation (SAO)^12,19. Like their terrestrial counterparts, Jupiter and Saturn’s equatorial stratospheric oscillations form a vertical pattern of temperature and wind perturbations that gradually descends over time^20,21,22,23. Jupiter’s equatorial stratospheric oscillation (JESO) has a variable period of 3.9 to 5.7 years with the variability linked to the occasional development of large-scale convective perturbations^20,24. Saturn’s equatorial stratospheric temperatures oscillate quasi-periodically in a 15-year cycle (that is, half a Saturn year)¹⁸, but this period can also be disrupted by large convective storms develo** at other latitudes²⁵. Variations of temperatures in Jupiter’s upper troposphere have been observed down to 300 mbar with a periodicity at the equator of 8 years and an anticorrelated relation with the JESO at near-equatorial latitudes²⁶. The complex phenomenology at the equatorial regions of Jupiter and Saturn points to connections between the stratospheric and tropospheric dynamics that are not well understood.

Elevated hazes cover Jupiter’s equatorial region around and above the 200 mbar level^27,28,29. These hazes are variable in opacity and albedo, and their brightness in the methane absorption band at 890 nm can vary up to 30% over different years^{30,8,14,15,16,17,18,20,21,22,23,24,25}. Among the latter, the different periods of JESO (3.9–5.7 years^20,24) and Saturn’s SAO (15 years¹⁸) argue for different dynamical origins. The half Saturn year cycle of Saturn’s SAO suggests a seasonal effect⁴³, while the time periods and phenomenology of the JESO cannot be related with seasons. JESO is reminiscent of the Earth’s QBO^19,44, a downward propagating easterly and westerly wind pattern that affects the 5–50 mbar portion of the Earth’s atmosphere. Although the QBO phenomenology on Earth is not fully understood, gravity waves created by convection in the lower troposphere are known to play a major role releasing energy and momentum in the Earth’s stratosphere^19,43. Similar mechanisms, such as the forcing created by gravity waves originating from convection in the troposphere, have been proposed for Jupiter’s thermal oscillations^16,45. Our JWST observation in F405N in Figs. 1 and 3, and the context views of the lower atmosphere in Supplementary Fig. 2, show bright cloud systems of possible convective nature with small-scale bright systems observable in the upper hazes.

Our JWST observations provide new insights into the troposphere-stratosphere interactions in Jupiter, and can inform us about the depth of penetration into the troposphere of the equatorial oscillation. On Earth, the QBO does not penetrate below the tropopause, but on Jupiter, the unexpected presence of this intense and elevated equatorial jet, which is not reproduced in existing simulations, suggests the stratospheric equatorial oscillations penetrate into the upper troposphere in an unanticipated way. The stratospheric oscillations are variable in both Jupiter and Saturn, and the time variability of the jet at the base of the stratosphere remains uncertain given that we have only observed Jupiter’s lower stratospheric jet during a single epoch. Thermal maps of Jupiter acquired in mid-2019 (ref. ²⁰), extrapolated in time to 2022 using the descent rate of the JESO observed during its last cycle, suggest that the ~40 mbar level had a warm anomaly during our JWST observations. This would be consistent with winds decreasing at greater depths as observed in our JWST images. This extrapolation in time also suggests a change in the sign of the thermal perturbation at 40 mbar in 2023–2024, allowing an examination of the time-variable component of the equatorial jet.

A combined view of Jupiter’s equatorial dynamics from the cloud tops to the upper stratosphere can be obtained by combining observations in the visible (500–700 mbar), near-infrared (100–200 mbar), thermal infrared (1–40 mbar), and Doppler winds from millimeter data (~4 mbar), where intense equatorial zonal winds have been observed^46,47. At 500–700 mbar, wind measurements at visible wavelengths hint at variability (<10 m s⁻¹) with possible periods of 5–7 years⁴⁸, while at stratospheric levels the periodicity in the temperature oscillations and winds is 4–6 years^20,24. In the near-infrared windows in which the upper hazes are observable at intermediate levels between the cloud tops and the stratosphere, only new observations by JWST can determine the atmospheric circulation and its expected variability. The equatorial dynamics of Jupiter and Saturn are complex, with similarities and differences that can guide us to unveil the various mechanisms that shape atmosphere dynamics at low latitudes in fast-rotating giant planets.

Methods

Image acquisition

Jupiter represents a bright target for JWST. We explored the various filters and strategies in which Jupiter could be observed with NIRCam without saturating the detectors⁴⁹. Image saturation was avoided by exposing the planet in filters centred in strong absorption bands of the Jovian atmosphere (F212N, F335M) and using the shortest integration times possible. In filters where the planet is brighter (F164N, F360M and F405N) we used subarrays (SUB640) that reduce the minimum exposure time. In all cases we used rapid readout patterns. We designed a pointing sequence in which gaps between the four NIRCam B detectors can be recovered with an appropriate dither pattern using four exposures separated in time by only a few minutes. Images obtained using a full array configuration were acquired with a dither pattern of three positions.

Image processing and composition

We used the NIRCam calibration processing pipeline⁵⁰ to produce calibrated and geometrically corrected versions of the images with different exposure levels from the original uncalibrated images. NIRCam detectors were read out non-destructively three times for each image, conserving the charge in each pixel and forming different versions of the same image (groups) with different accumulated exposures. We used a combination of one, two and three groups in images obtained in full arrays, and versions of the images from only two and three groups in images obtained using subarrays to remove saturated areas, produce sharp images without rotational smearing, and obtain high signal-to-noise ratio through our images. We used an adaptive median filter to remove bad pixels identified by the pipeline. We navigated each individual image and subframe separately comparing the position of the planet with a synthetic longitude–latitude grid using the WinJupos software due to its flexible use. These navigations were compared with navigation obtained with python code using World Coordinate System information from the telescope pointing, finding good agreement. Images obtained with small time separations of a few minutes were combined with WinJupos correcting the rotation of the planet and producing higher signal-to-noise-ratio images. These were high-pass filtered to enhance the contrast of small-scale faint features in the planet and reduce limb-darkening effects. The images were map-projected, oversampling the initial resolution of the images, that is the resolution used in the maps was greater than that of the original images, limb-darkened corrected and further enhanced with high-pass filters.

Winds analysis

Maps of the planet were compared using the PICV3 software^51,39. This software performs one- or two-dimensional correlations over two images using boxes whose size can be configured and adapted to the contrast of the region of the image where the analysis is run. Winds were measured over the final composed images except in the F164N images, where image pairs over the same B detector were compared individually producing 16 individual data sets that were later merged into a single zonal wind profile. PICV3 has a graphical user interface that allows selection of specific regions of the images. We avoided regions with large-scale features and examined the equatorial region with long boxes with typical longitudinal sizes of 20° and latitudinal sizes of 1°. The F164N images were analysed using 16 image pairs from the combination of 4 subarrays and 3 dither positions combining the results from the individual zonal wind profiles in a single zonal wind profile in that filter.

Source data

Source Data Fig. 1

Processed Jupiter images.

Source Data Fig. 2

Zip file containing csv files for all lines on Fig. 2.

Source Data Fig. 3

Jpg version of Fig. 3.

Source Data Fig. 4

Zip file containing csv files for all wind profiles on Fig. 4.

Source Data Fig. 5

Zip file containing csv files for all wind profiles on Fig. 5, plus Fig. 5 in jpg format.