For almost 200 years humans have been watching the Great Red Spot (GRS) on Jupiter and wondering what’s behind it. Thanks to NASA’s Juno mission, we’ve been getting better and better looks at it. New images from JunoCam reveal some of the deeper detail in our Solar System’s longest-lived storm.
JunoCam is the visible light instrument onboard NASA’s Juno mission to Jupiter. It’s not part of the Juno spacecraft’s primary scientific payload. It was included in the mission just to engage and thrill us, and it hasn’t disappointed. But as it turns out, JunoCam’s high-resolution images are serving a scientific purpose.
A new study led by Agustín Sánchez-Lavega (University of the Basque Country, Spain) has used the detailed images from JunoCam to look more closely at the morphology of the clouds that make up the GRS. Up until now most of what we know about the GRS has come from previous missions to Jupiter. First were the Voyager missions, then the Galileo mission, and of course the Hubble Space Telescope. The image resolution of each succeeding mission has improved, but nothing close to JunoCam’s resolution.
As image quality improved from as poor as 150 km/pixel to as fine as 7 km/pixel, our understanding of the GRS has improved along with it. The paper from Sanchez-Lavega focuses on five particular morphological features of the storm: compact cloud clusters, mesoscale waves, spiraling vortices, the central turbulent nucleus, and filament structures.
- Compact cloud clusters resemble altocumulus clouds in Earth’s atmosphere and may suggest the condensation of ammonia.
- Mesoscale waves are wave packets that could indicate regions of stability.
- Spiralling vortices are eddies with a radius of about 500 km that indicated intense horizontal wind shear.
- The central turbulent nucleus of the GRS is about 5200 km long, or about 40% of Earth’s diameter.
- Large dark, thin, undulating filaments from 2,000 to 7,000 km in length move at very high speed around the outside of the vortex. They may have a different composition than other features or they could be a different altitude.
The study determines that although the size of the GRS has changed dramatically over the last 140 years, the winds have changed only modestly since 1979, when the Voyager missions visited Jupiter. The authors suggest that a “deeply rooted dynamical circulation” maintains these wind speeds. Further, they suggest that the rich morphologies in the top of the GRS reflect the dynamics at the cloud tops.
From the study:
A comparison with high-resolution images from previous missions suggests a high temporal variability in the dynamics of this layer, strongly enforced by the interaction of the GRS with phenomena close in latitude (Sánchez-Lavega et al. 1998, 2013). However, while the size of the GRS has changed strongly in the last 140 years (Rogers 1995; Simon et al. 2018), the wind field in the GRS shows modest changes during the period 1979–2017 (Figure 6) implying a deeply rooted dynamical circulation. The rich GRS cloud-top morphologies embedded in these winds reflect the dynamics at the top of the system.
Scientist’s are still working on a deeper understanding of Jupiter’s atmosphere and how the GRS is formed and maintained. Instruments on the Juno spacecraft will help with this, as will the Hubble. Juno’s Microwave Radiometer (MWR) is designed to study the hidden structure beneath Jupiter’s morphologically stunning cloud tops. The MWR should be able to probe the Jovian atmosphere to a depth of 550 km. It has already revealed that some atmospheric features visible on the surface actually extend to a depth of at least 300 km.
The authors of the study sum it up best: “Our knowledge about the GRS dynamics will increase further, thanks to the ongoing studies on the vertical gravity soundings and the observations with the MWR instrument onboard Juno, together with a supporting campaign from the HST, Earth-based telescopes, and the planned future James Webb Space Telescope (Norwood et al. 2016) of this unique and fascinating phenomenon.”