Juno’s June 7, 2021 flyby of Ganymede is the closest a spacecraft has come to Jupiter’s largest moon since NASA’s Galileo spacecraft made its close approach back on May 20, 2000.
This image of Ganymede was obtained by the JunoCam imager aboard NASA’s Juno spacecraft during its June 7, 2021, flyby of the icy moon. This image is a preliminary product — Ganymede as seen through JunoCam’s green filter. Juno is a spin-stabilized spacecraft (with a rotation rate of 2 rpm), and the JunoCam imager has a fixed field of view. To obtain Ganymede images as Juno rotated, the camera acquired a strip at a time as the target passed through its field of view. These image strips were captured separately through the red, green, and blue filters. To generate the final image product, the strips must be stitched together and colors aligned. At the time this preliminary image was generated, the spice kernels (navigation and other ancillary information providing precision observation geometry) necessary to properly map-project the imagery were not available. The red, and blue filtered image strips were also not available. When the final spice kernel data and images from the two filters are incorporated, the images seams (most prevalent on lower right of sphere) will disappear and a complete color image will be generated. Image credit: NASA / JPL-Caltech / SwRI / MSSS.
Ganymede has a diameter of 5,268 km (3,273 miles), around 8% larger than that of the planet Mercury and much larger than Pluto.
Discovered in 1610, it is the only moon in the Solar System to have its own magnetosphere, which causes aurorae in regions circling its north and south poles.
As Ganymede has no atmosphere, the surface at its poles is constantly being bombarded by plasma from the magnetosphere of Jupiter. This bombardment has a dramatic effect on the moon’s ice.
It has three main layers: a sphere of metallic iron at the center, a spherical shell of rock surrounding the core, and a spherical shell of mostly ice surrounding the rock shell and the core. The ice shell on the outside is very thick, maybe 800 km (497 miles) thick.
“Ganymede’s ice shell has some light and dark regions, suggesting that some areas may be pure ice while other areas contain dirty ice,” said Juno principal investigator Dr. Scott Bolton, a researcher at the Southwest Research Institute.
This image of the dark side of Ganymede was obtained by the Stellar Reference Unit star camera aboard NASA’s Juno spacecraft during its June 7, 2021, flyby of the icy moon. Usually used to the spacecraft on course, the navigation camera was able to obtain an image of the moon’s dark side because it was bathed in the dim light scattered off Jupiter; the camera operates exceptionally well in low-light conditions. Image credit: NASA / JPL-Caltech / SwRI.
The Juno probe came within 1,038 km (645 miles) of Ganymede’s surface on June 7, 2021 at 1:35 p.m. EDT (10:35 a.m. PDT).
“This is the closest any spacecraft has come to this mammoth moon in a generation,” Dr. Bolton said.
“We are going to take our time before we draw any scientific conclusions, but until then we can simply marvel at this celestial wonder.”
Using its green filter, the spacecraft’s JunoCam visible-light imager captured almost an entire side of the water-ice-encrusted moon.
Later, when versions of the same image come down incorporating the camera’s red and blue filters, imaging experts will be able to provide a color portrait of Ganymede.
In addition, Juno’s Stellar Reference Unit, a navigation camera that keeps the spacecraft on course, provided a black-and-white picture of Ganymede’s dark side bathed in dim light scattered off Jupiter.
“The conditions in which we collected the dark side image of Ganymede were ideal for a low-light camera like our Stellar Reference Unit,” said Dr. Heidi Becker, Juno’s radiation monitoring lead at NASA’s Jet Propulsion Laboratory.
“So this is a different part of the surface than seen by JunoCam in direct sunlight. It will be fun to see what the two teams can piece together.”
