Sharpest Earth-based images of Europa and Ganymede reveal their icy landscapes

Europa. Credit:ESO/King & Fletcher

The chemical concoction that makes up the frozen surfaces of two of Jupiter's biggest moons is revealed in the most comprehensive photographs of them yet captured by an Earth-based observatory. Planetary experts from the University of Leicester's School of Physics and Astronomy have released fresh photos of Europa and Ganymede, two potential sites for exciting new Jovian expeditions. Some of the finest views of Jupiter's moons yet obtained by a ground-based telescope, they give fresh insights into the processes determining the chemical composition of these huge moons, including geological features like Europa's lengthy rift-like linae.

Ganymede and Europa are two of Jupiter's four biggest moons, known as the Galilean moons. While Europa is roughly the size of our Moon, Ganymede is the biggest moon in the Solar System. The Leicester team, coordinated by Ph.D. student Oliver King, saw and mapped the surfaces of these two planets using the European Southern Observatory's Very Large Telescope (VLT) in Chile. The new measurements measured the quantity of sunlight reflected from the surfaces of Europa and Ganymede at various infrared wavelengths, yielding a reflectance spectrum. These reflectance spectra are examined by creating a computer model that compares each observed spectrum to spectra obtained in laboratories of various substances.

Ganymede. Credit: ESO/King & Fletcher

The photos and spectra of Europa, which were published in the Planetary Science Journal, show that the planet's crust is mostly made of frozen water ice, with non-ice elements polluting the surface. According to Oliver King of the University of Leicester School of Physics and Astronomy, they studied the distributions of several minerals on the surface, including sulfuric acid frost, which is mostly present on the side of Europa that is most heavily battered by the atmosphere around Jupiter.

The modeling discovered that a range of different salts might be present on the surface, but that infrared spectroscopy alone is often incapable of identifying which precise forms of salt are there. Ganymede's findings, published in the journal JGR: Planets, demonstrate that the surface is divided into two types of terrain: youthful areas with a lot of water ice, and old areas with a lot of dark gray material whose composition is unknown.

The frozen spots (blue in the photos) include Ganymede's polar caps and craters where an impact event has revealed Ganymede's crust's new pristine ice. The researchers studied how the size of ice grains changes over Ganymede's surface, as well as the likely distributions of various salts, some of which may have originated on Ganymede itself. The Very Large Telescope, located at a high altitude in northern Chile, is one of the world's most powerful telescope facilities, with mirrors over 8 meters broad.

Oliver King adds, "This has enabled us to conduct detailed mapping of Europa and Ganymede, observing features on their surfaces smaller than 150 km across all at distances of over 600 million kilometers from Earth." Mapping at this precise scale was previously only achievable by sending spacecraft all the way to Jupiter to closely monitor the moons.

Professor Leigh Fletcher, who oversaw the VLT research, is a member of the science teams for ESA's Jupiter Icy Moons Explorer (JUICE) and NASA's Europa Clipper missions, both of which will conduct close-up explorations of Ganymede and Europa in the early 2030s. JUICE is set to launch in 2023, and scientists from the University of Leicester will play major roles in its intended research of Jupiter's atmosphere, magnetosphere, and moons.

According to Professor Fletcher, these ground-based observations whet our thirst for future investigation of Jupiter's moons.

Planetary missions must work within strict operational limits, and we simply cannot cover all of the land that we would like to, therefore difficult judgments must be made regarding which regions of the moons' surfaces merit the most attention. Observations at a size of 150 kilometers, such as those given by the VLT and, eventually, its huge replacement, the ELT (Extremely Large Telescope), aid in providing a global context for satellite observations.

Journal Information: Oliver King et al, Compositional Mapping of Europa Using MCMC Modeling of Near-IR VLT/SPHERE and Galileo/NIMS Observations, The Planetary Science Journal (2022). DOI: 10.3847/PSJ/ac596d

Oliver King et al, Global Modeling of Ganymede's Surface Composition: Near-IR Mapping from VLT/SPHERE, JGR: Planets (2022).

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