Atmospheric jets are large-scale bands of fast flowing winds that are common in many planets. On Earth, jet streams form at different latitudes and drive westerly winds that meander across latitudes and can reach velocities of 400 km/hr. In Jupiter and Saturn, atmospheric jets are one of the main features of the atmosphere of both planets and they are so perfectly aligned with the parallels that they are called zonal jets. The zonal jets are observed in the motion of the ammonia clouds that cover both planets at pressure levels of 600 mbar in Jupiter and around 1000 mbar in Saturn. The zonal jets alternate their direction in latitude and the most intense jets reach velocities of 500 and 1500 km/h in Jupiter and Saturn respectively. The zonal alignment is a consequence of the fast rotation of the planets (both Jupiter and Saturn have a rotation period of about 10 hr), which results in a balance between Coriolis forces and latitudinal gradients of pressure. In Jupiter and Saturn the jets are mostly stable in time with only minor changes at cloud level observed over years and decades, or with intense changes linked only to rare planetary-scale disturbances on localized latitudes.

On July 27, 2022 the James Webb Space Telescope (JWST) observed Jupiter’s atmosphere as part of an “Early Release Science” program. The goals of that program were to demonstrate the wide range of capabilities of the telescope for Solar System research, with observations of the atmosphere in imaging and spectroscopic modes, spectroscopy of the large satellites Ganymede and Io, and image observations of the rings and the minor satellites around Jupiter. In spite of decades of previous observations of the Jupiter system from other telescopes and spacecraft, the unique capabilities of JWST resulted in a treasure of new data for the different targets of the program. In Hueso et al. (2023) we report some of the results obtained from the analysis of the gorgeous images of the planet taken by JWST.

The JWST images of Jupiter opened new fresh views of the planet that were unlike any previous observation of the gas giant. Since Jupiter is a very bright target for JWST (which has a light collecting area that is 6.3 times larger than the Hubble Space Telescope), the images were acquired in wavelengths in which most of the light is absorbed in its path through the Jovian atmosphere by atmospheric gases such as methane and hydrogen. Thus, the selection of filters used in the observations included some of the wavelengths where the planet is the darkest; meaning that in many of these wavelengths Jupiter had never been observed at enough spatial resolution to resolve the weather systems in the atmosphere. Combining different filters that are sensitive to different amounts of atmospheric absorptions, the set of images obtained offers a three-dimensional view of Jupiter’s cloud systems that is remarkably sensitive to aerosols in the upper atmosphere near the tropopause, but deepening also in the troposphere and covering pressure levels from 1 bar to 100 mbar. Elevated clouds appear bright in these images, and the regular cloud systems observed in most ground-based and spacecraft images appear dark. In addition, the observations were designed to produce an instantaneous view of the atmospheric dynamics and motions of the Jovian atmosphere. This was done by acquiring two sets of images separated by a full Jovian rotation. The two sets of images can then be used to examine the relative motions of small-scale cloud systems tracking the winds with respect to Jupiter’s rotation.

The result was surprising. At the equator, zonal winds obtained in a set of filters sensitive to the upper hazes showed a narrow and intense jet located slightly off the equator. The jet observed at the hazes reaches peak velocities of 500 km/h, or twice as much as the winds at the cloud level. Our analysis of the altitudes sensed at the equator in these images results in a pressure level of 200-100 mbar, or 22-36 km above the cloud level where previous studies of the winds have focused. Thus, the new zonal jet has a very intense vertical shear with winds that increase in altitude by 7-10 km/hr per kilometer of altitude variation. Outside of the central jet, wind speeds decrease with height from the cloud tops to the hazes, resulting also in strong meridional and vertical shears at the haze level all through the equatorial region.

While these findings were surprising, the three-dimensional structure of Jupiter’s equatorial jet is similar to the complex equatorial winds in Saturn. Cloud-tracking winds obtained during the Cassini mission found also a narrow and elevated jet with strong meridional and vertical shears. The vertical shear in Jupiter’s equatorial jet is, however, much more intense and the presence of the jet defies expectations. In both planets, the fast equatorial jet is traced by the motions of equatorial hazes that are located near the tropopause in the transition region between the troposphere, where vertical motions can develop, and the stratosphere, where vertical motions are inhibited by the thermal inversion of temperatures. The new jet near the tropopause is probably a deep counterpart of a complex phenomenon that has been observed for decades in the stratospheres of Jupiter, Saturn and Earth: regular oscillations of temperatures and winds that occur at stratospheric levels. In Jupiter these equatorial thermal oscillations have been observed in the range of pressure levels from 0.1 to 50 mbar (i.e. 30-150 km above the equatorial hazes where the jet discovered on JWST images has been observed), and have a periodicity of 4–6 years. If the new jet in Jupiter is linked to these stratospheric oscillations, then it should be a variable phenomenon in both Jupiter and Saturn at altitude levels near the tropopause. The physical origin of stratospheric oscillations on Earth, Jupiter and Saturn is not well understood, and neither are the near tropopause equatorial jets of Saturn and Jupiter. These intriguing phenomena occur near the equatorial tropopauses of gas giants, right where atmospheric dynamics change by the vanishing effect of Coriolis forces, and where the thermal properties of the atmosphere change drastically. Future JWST observations of the tropopause level of both Jupiter and Saturn can shed new light in these puzzling phenomena.

This research is published in Nature Astronomy:

Hueso et al. (2023). An intense narrow equatorial jet in Jupiter's lower stratosphere observed by JWST. https://www.nature.com/articles/s41550-023-02099-2