The Ultimate ‘Moon Shadow’ Captured On Jupiter As NASA Reveals New Insights Into Its Planet-Sized Source

What in our Solar System is bigger than Mercury and Pluto yet isn’t a planet (or the Sun)? It’s Ganymede, Jupiter’s monster-sized moon, which NASA’s Juno spacecraft has been seeing a lot of lately.

During its most recent close flyby—perijove number 40, in fact—it glimpsed the shadow of Ganymede passing across the cloud-tops of Jupiter. This image, above, was captured by Juno’s JunoCam imager and sent back to Earth via NASA’s Deep Space Network, then downloaded and carefully processed by citizen scientists Kevin M. Gill.

The fabulous new image appear online this week just as scientists as the 53rd Lunar and Planetary Science Conference (LPSC) in Houston, Texas were discussing new findings about Ganymede from Juno’s close flyby on June 7, 2021.

Ganymede has a diameter of 3,273 miles/5,268 kilometers. It’s the largest moon and the ninth-largest object in the Solar System.

Our best views of Ganymede for decades, Juno had a close-up from 1,000 kilometeres of its fractured, cratered northern hemisphere, allowing scientists to make some impressive discoveries:

Tros Crater is the coldest place observed on Ganymede

On June 7, 2021 Juno’s Microwave Radiometer (MWR) was able to partially map Ganymede’s ice shell in six channels to “see” the temperatures across its surface. The microwave spectra reveal spectral differences according to different types of terrain visible on Ganymede. “The darker regions are the warmest while the lighter regions are the coldest,” said Shannon Brown at NASA’s Jet Propulsion Laboratory presenting research during LPSC. “The coldest microwave feature we observed was the Tros Crater.” It’s the largest bright ray crater on Ganymede.

Juno’s 3D map of Ganymede

Juno’s images are on the scale of about 1-2.5 kilometres per pixel and of the northern hemisphere of Ganymede, allowing planetary scientists to see new detail in geological features on the moon’s surface. Since its images overlap scientists—also presenting their research at LPSC—were able to developed stereoscopic images, meaning about nine million square kilometres of its surface could be recreated in 3D. That allowed the depths and scale of some of the features to be more accurately determined.

Tros appears in the images to be less than a kilometer deep while a three kilometer-high dome was also spotted, though the topography of the moon more generally is very low. The images also revealed eight new “paterae” on Ganymede, volcano caldera-like depressions with raised rims, which could be the result of collapse or explosions—scientists don’t know.

Ganymede has aurora

The June 7, 2021 flyby also gave scientists their first good ultraviolet map of Juno, which the Hubble Space Telescope hasn’t been able to provide purely because of the Ganymede’s distance from Earth.

As well as detecting reflected sunlight, Juno observed auroral ovals around the north and south poles produced by the impact of electrons accelerated by Ganymede’s magnetosphere into its tenuous atmosphere. That’s something Hubble has seen before, but Juno was able to measure their width and shape, according to more research presented at the LPSC.

“The aurorae are caused by electrons being accelerated within Ganymede’s magnetic fields towards the poles where they excite the atmosphere,” said Philippa Molyneux, Southwest Research Institute, Texas. “We had seen these emissions using Hubble, but now we’re able to see the exact locations of the emissions, and much more precisely, which is important because the location of the emissions changes as Jupiter’s magnetic field rotates.” By observing how the aurora changes—which scientists will be able to do once the European Space Agency’s JUpiter ICy moons Explorer (JUICE) probe reaches Ganymede (and Europa and Callisto) in 2026—it may be possible to constrain the presence and saltiness of Ganymede’s suspected subsurface ocean.

All of that from one single flyby of Jupiter—and from four images.

It all bodes well for the upcoming missions to the Jovian system, JUICE and Europa Clipper (due to reach Jupiter in 2030). “From this single flyby with Juno we’ve already vastly improved our knowledge of Ganymede in the ultraviolet (UV)—and JUICE and the Europa Clipper will both carry UV instruments that are very similar to those on Juno,” said Molyneux. “So we can expect these missions to give us the same improved spatial resolution in coverage at all of the three icy moons—so watch this space!”

Wishing you clear skies and wide eyes.