The sharpest Earth-based images of Europa and Ganymede reveal their icy landscapes – Astrobiology

The chemical cocktail that makes up the frozen surfaces on two of Jupiter’s largest moons is visible in the most detailed images ever taken of them by a telescope on Earth.

Planetary scientists from the University of Leicester’s School of Physics and Astronomy have unveiled new images of Europa and Ganymede, two future targets for exciting new missions in the Jovian system.

Some of the sharpest images of Jupiter’s moons ever taken by a ground-based observatory are revealing new insights into the processes shaping the chemical composition of these massive moons — including geological features like the long, crack-like lines that criss-cross Europa’s surface.

Ganymede and Europa are two of the four largest moons orbiting Jupiter, known as the Galilean moons. While Europa is quite similar in size to our own moon, Ganymede is the largest moon in the entire solar system.

The Leicester team, led by PhD student Oliver King, used the European Southern Observatory’s Very Large Telescope (VLT) in Chile to observe and map the surfaces of these two worlds.

The new observations recorded the amount of sunlight reflected from the surfaces of Europa and Ganymede at different infrared wavelengths, producing a reflectance spectrum. These reflectance spectra are analyzed by developing a computer model that compares each observed spectrum with spectra of different substances measured in laboratories.

Images and spectra of Europa, published in the Planetary Science Journal, show that Europa’s crust is composed mostly of frozen water ice, with non-ice materials contaminating the surface.

Europe. Credit: ESO/King & Fletcher

Oliver King, from the University of Leicester’s School of Physics and Astronomy, said: “We have mapped the distribution of the various materials on the surface, including the sulphate of frost, which is mainly found on the Europa side, the most encircling of Jupiter gases is bombed.”

“The modeling indicated that a variety of different salts could be present on the surface, but suggested that infrared spectroscopy alone is generally not able to identify which specific types of salts are present.”

Ganymede’s observations, published in the journal JGR: Planets, show how the surface consists of two main types of terrain: young areas with large amounts of water ice, and old areas consisting mainly of a dark gray material, the composition of which is unknown.

The icy areas (blue in the images) encompass Ganymede’s polar caps and craters – where an impact event has exposed the fresh clean ice of Ganymede’s crust. The team mapped how the size of ice grains on Ganymede varies across the surface and the possible distributions of a variety of different salts, some of which may have originated in Ganymede itself.

Located at high altitude in northern Chile, the Very Large Telescope is one of the most powerful telescope systems in the world, with mirrors over 8 meters in diameter.

Oliver King adds: “This has allowed us to perform detailed mapping of Europa and Ganymede and observe features on their surfaces less than 150 km in diameter – all at distances of over 600 million km from Earth.” Mapping at this fine scale has previously only been possible by sending spacecraft to Jupiter to observe the moons up close.”

Professor Leigh Fletcher, who oversaw the VLT study, is a member of the science teams on ESA’s Jupiter Icy Moons Explorer (JUICE) mission and NASA’s Europa Clipper mission, which will explore Ganymede and Europa up close in the early 2030s. JUICE is scheduled for launch in 2023, and University of Leicester scientists are playing a key role in the proposed study of Jupiter’s atmosphere, magnetosphere and moons.

Professor Fletcher said: “These ground-based observations stimulate our future exploration of Jupiter’s moons.”

“Planetary missions are subject to severe operational limitations and we simply cannot cover all of the terrain we would like to have, so tough decisions must be made about which areas of the lunar surface deserve the closest scrutiny. 150km-scale observations, such as those provided by the VLT and ultimately its enormous successor, the ELT (Extremely Large Telescope), help provide a global context for the spacecraft observations.”

This work was funded by Royal Society Enhancement Award No. 180071 to Professor Leigh Fletcher of the School of Physics and Astronomy entitled “The Diversity of Jupiter’s Galilean Moons: Earth-based Pathfinder Observations in Preparation for JUICE”.

Oliver King and Leigh N. Fletcher (2022), Global modeling of Ganymede’s surface composition: near-IR mapping of VLT/SPHERE, JGR: Planets, adopted.

Oliver King, Leigh N. Fletcher, Nicolas Ligier (2022), Compositional Mapping of Europa using MCMC modeling of Near-IR VLT/SPHERE and Galileo/NIMS observations, Planetary Science Journal 3, 72.

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