Abstract: Early JWST observations of the giant planets in our solar system are summarized, including both imaging and spectroscopy.
The giant planets in our solar system, Jupiter, Saturn, Uranus, and Neptune, share similar characteristics. They are dominated by hydrogen and helium, and are much larger than the Earth, without a solid surface, and they all have strong magnetic fields – see Figure 1 for an overview. However, these planets have significant differences that render each planet unique. Jupiter, the largest planet, is one of the brightest objects in the night sky, and its familiar stripes, known as belt and zones, define its visual appearance, along with the Great Red Spot, the largest storm in the solar system.
Saturn, with its striking ring-system, is the second largest planet in our solar system. It has been intensively studied over the past decades, driven largely by the presence of the Cassini-Huygens mission between 2004-2017. The mission revealed several intriguing features of the system, including active cryo-volcanicity at the moon Enceladus, the sixth largest moon (Dougherty et al., 2006). The mission characterised the seasonal change in the atmosphere, produced by the large axial tilt of the planet (Fletcher et al., 2016). Later in the mission observations of the aurora became a significant focus, trying to elucidate their origins (Badman et al., 2012; Lamy et al., 2013; Melin et al., 2016).
Uranus and Neptune are classed as ice giants, with significant amounts of methane, water, and ammonia, both being about four times larger than the Earth. Far away from the Sun, they receive little sunlight – at the orbit of Neptune, the solar flux is only ∼0.1% compared to the flux received at Earth. Uranus stands out as the oddball in the solar system. It has an axial tilt of 98°, which means that its rotational axis is closely aligned with the ecliptic plane: the planet rotates on its side, perhaps because of a large collision during the formation of the solar system (Ida et al., 2020). This collision could also potentially explain the complete absence of any internal heat at Uranus. This is in stark contrast to Neptune, which has the largest internal heat flux of all. The internal heat has consequences for the atmospheres of these planets, since it drives their turbulent convection; the fastest tropospheric winds are seen at Neptune, largely driven by this internal heat.
Whilst emissions from the troposphere and stratosphere of these planets are relatively bright, the tenuous upper atmosphere is harder to observe from the ground. This region is an important interface region between the atmosphere, the magnetic field, and the surrounding space environment. The upper atmospheres of all the giant planets are observed to be several hundreds of degrees hotter than solar input predicts. This is one of the largest outstanding questions in planetary science and is colloquially referred to as the giant planet energy crisis. There are two main potential solutions to this problem. Firstly, the auroral process drives very strong currents at the magnetic poles, and heats it in the process, analogous to a current running through a resistor, which generates frictional heat. This renders auroral regions hotter than the rest of the planet, and this energy could potentially be re-distributed globally. However, the challenge is that these planets are very fast rotators, which generate very strong Coriolis forces, effectively prohibiting the redistribution of energy towards lower latitudes. Secondly, gravity waves generated in the turbulent lower atmosphere can propagate up in altitude where these waves break and release their energy. However, the efficacy of this process is still being debated (Yelle and Miller, 2004).
Whilst all four giant planets have been visited by numerous spacecraft, significant questions remain. In particular, very little is known about either Uranus or Neptune, having only been visited by Voyager 2, in 1986 and 1989, respectively. Given that these planets are the best analogues available for a vast number of exoplanets outside of our solar system, understanding these worlds is a matter of priority. Both the 2022 US Planetary Science and Astrobiology Decadal Survey (National Academies of Sciences and Medicine, 2023) and the ESA Voyage 2050 (Fletcher et al., 2021) have given high priority to a flagship style mission to either Uranus or Neptune. This may take the form of an international collaboration between ESA and NASA, as well as other stakeholders, perhaps building on the highly successful Cassini-Huygens mission to Saturn and Titan.
Observations of the giant planets from the James Webb Space Telescope offer unique opportunities to understand these complex and dynamic systems (Norwood et al., 2016; Villanueva and Milam, 2023). The telescope offers capabilities that were not previously available, either via ground-based telescopes, or by in-situ spacecraft. Firstly, the unrivalled sensitivity of JWST offers access to very faint emission and absorption features currently invisible using existing facilities, and secondly, offers much higher spectral resolution than near and mid-infrared instruments flying on orbital missions (e.g., Cassini VIMS, Cassini CIRS, and Juno JIRAM). Consequently, JWST provides the only means to address some of the most pressing questions at these planets.
The first JWST images of Jupiter were released in July 2022 (ERS programme #1373), which are shown in Figure 2. This image reveals striking cloud structures within the belts and zones cutting across the disk of Jupiter, as well as stunning near-infrared views of the Great Red Spot. By comparing images taken on successive rotations, tracking the movement of individual clouds, pole-to-pole stratospheric wind profiles can be derived. Hueso et al. (2023) discovered a new narrow equatorial jet, moving at a speed of 140 m s-1, likely forming part of a stratospheric circulation system that varies over time. Figure 2 also shows the northern and southern aurora in red. These are emissions from the molecular ion H3+ in the upper atmosphere, which traces the ionisation of molecular hydrogen by precipitating auroral electrons, delivered by vast magnetospheric currents.
The first observations of Saturn were obtained by JWST MIRI in November 2022 (Figure 3) providing an opportunity to characterise the change in the planet’s troposphere and stratosphere since the demise of Cassini-Huygens in 2017. Fletcher et al. (2023) found that ammonia was significantly elevated around the equator, suggesting that Saturn exhibit similar stratospheric circulation patterns as Jupiter does. They also observed atmospheric changes consistent with the changing seasons, with equinox occurring in June 2025.
This brief summary highlights that JWST observations of the giant planets provide insights previously impossible from any other facility, be it orbiting spacecraft or telescopes on the ground. These observations can sample altitudes from the troposphere all the way out to the upper atmosphere, highlighting the versatility of JWST in exploring these planets. In particular, Uranus and Neptune are ideally suited targets, given their small angular sizes, and faint emission features, which coupled with the fact that they are relatively unexplored, provides the exciting opportunity for uncovering fundamental features of these systems.
Acknowledgments:
HM was supported by an STFC James Webb Fellowship (ST/W001527/1).
References:
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