The cocktail of chemicals that make up the frozen surfaces of two of Jupiter’s largest moons is revealed in the most detailed images ever taken 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 destinations for exciting new missions in the Jovian system.
Some of the sharpest images of Jupiter’s moons ever acquired from a ground-based observatory, they reveal new insights into the processes that shape the chemical makeup of these massive moons, including geological features like the long fault-like lines that cross the surface of Europa.
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 throughout the solar system.
The Leicester team, led by Ph.D. Oliver King, used the Very Large Telescope (VLT) at the European Southern Observatory 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 to the spectra of different substances that have been measured in the laboratory.
The images and spectra of Europe, published in the Journal of Planetary Sciencereveal that Europa’s crust is mostly composed of frozen water ice with non-glacial materials contaminating the surface.
Oliver King from the University of Leicester School of Physics and Astronomy says they “have mapped the distributions of the various materials on the surface, including sulfuric acid frost which is found mainly on the side of Europe which is most heavily bombarded by gases surrounding Jupiter.”
“The modeling revealed that there could be a variety of different salts present at the surface, but suggested that infrared spectroscopy alone is generally unable to identify the specific types of salt present.”
Ganymede’s observations, published in the journal JGR: Planetsshow how the surface consists of two main types of terrain: young areas with large amounts of water ice, and old areas consisting mainly of dark gray material whose composition is unknown.
The icy areas (in blue in the images) include the polar ice caps and craters of Ganymede, where an impact event exposed the cool, 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 on Ganymede itself.
Located at high altitude in northern Chile, and with mirrors over 8 meters in diameter, the Very Large Telescope is one of the most powerful telescopes in the world.
Oliver King adds that “this has enabled us to perform detailed mapping of Europa and Ganymede, observing features on their surfaces less than 150 km in diameter, all at distances of over 600 million km from the Mapping at this fine scale was previously only possible by sending spacecraft up to Jupiter to observe the moons up close.”
Professor Leigh Fletcher, who supervised the VLT study, is a member of the science teams for ESA’s Jupiter Icy Moons Explorer (JUICE) and NASA’s Europa Clipper mission, which will explore Ganymede and Europa up close in the early 2030s. JUICE is set to launch in 2023, and scientists from the University of Leicester are playing a key role in its project to study Jupiter’s atmosphere, magnetosphere and moons.
Prof Fletcher says ‘these observations from the ground whet the appetite for our future exploration of Jupiter’s moons’.
“Planetary missions operate under difficult operational constraints and we simply cannot cover all the ground we would like, so tough decisions must be made about which areas of the moons’ surfaces deserve the most scrutiny. Observations at 150 km scale such as those provided by the VLT, and eventually its massive successor the ELT (Extremely Large Telescope), help provide a global context for spacecraft observations.”
Oliver King et al, Compositional Mapping of Europa Using MCMC Modeling of Near-IR VLT/SPHERE and Galileo/NIMS Observations, The Journal of Planetary Science (2022). DOI: 10.3847/PSJ/ac596d
Oliver King et al, Global Modeling of the Surface Composition of Ganymede: Near-Infrared Mapping of VLT/SPHERE, JGR: Planets (2022). doi.org/10.1029/2022JE007323
University of Leicester
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