The mixture of chemicals that make up the frozen surfaces on two of Jupiter’s largest moons have been revealed in the most detailed images of them ever taken by a telescope on Earth.
Planetary scientists from the UK's University of Leicester’s School of Physics and Astronomy unveiled new images of Europa and Ganymede, two future destinations for new missions to the giant planet’s system.
The images provide new insights into the chemical composition of the moons, including geological features such as long rift-like lines cutting across Europa’s surface.
Ganymede and Europa are two of the four largest moons orbiting Jupiter, known as the Galilean moons.
Europa is similar in size to Earth’s Moon, while Ganymede is the largest moon in the whole Solar System.
PhD student Oliver King, from the University of Leicester’s School of Physics and Astronomy, said: “We mapped the distributions of the different materials on the surface, including sulphuric acid frost which is mainly found on the side of Europa that is most heavily bombarded by the gases surrounding Jupiter.
“The modelling found that there could be a variety of different salts present on the surface, but suggested that infrared spectroscopy alone is generally unable to identify which specific types of salt are present.”
The researchers used the European Southern Observatory’s Very Large Telescope (VLT) in Chile to observe and map the surfaces of the moons.
They recorded the sunlight reflected from Europa and Ganymede’s surfaces at different infrared wavelengths, producing a reflectance spectrum.
This was analysed by developing a computer model that compares the observations to different substances that have been measured in laboratories.
The images and spectra of Europa, published in the Planetary Science Journal, reveal that Europa’s crust is mainly composed of frozen water ice with non-ice materials contaminating the surface.
The observations of Ganymede, published in the journal JGR:Planets, show how the surface is made up of two main types of terrain.
These are young areas with large amounts of water ice, and ancient areas made up of an unknown dark grey material.
The icy areas – which appear blue in the images – include Ganymede’s polar caps and craters – where an impact event has exposed the fresh clean ice of Ganymede’s crust.
Researchers also mapped how the size of the grains of ice on Ganymede varies across the surface and the possible distributions of a variety of different salts, some of which may originate from within the moon itself.
Located at high altitude in northern Chile, and with mirrors over eight metres across, the Very Large Telescope is one of the most powerful telescope facilities in the world.
Mr King said: “This has allowed us to carry out detailed mapping of Europa and Ganymede, observing features on their surfaces smaller than 150km across – all at distances over 600 million kilometres from the Earth.
“Mapping at this fine scale was previously only possible by sending spacecraft all the way to Jupiter to observe the moons up close.”
Professor Leigh Fletcher, who supervised the VLT study, is a member of the science teams for the European Space Agency’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 scheduled to launch in 2023, with University of Leicester scientists playing a key role in its proposed study of Jupiter’s atmosphere, magnetosphere and moons.
Prof Fletcher said: “These ground-based observations whet the appetite for our future exploration of Jupiter’s moons.
“Planetary missions operate under tough operating constraints and we simply can’t cover all the terrain that we’d like to, so difficult decisions must be taken about which areas of the moons’ surfaces deserve the closest scrutiny.
“Observations at 150km scale such as those provided by the VLT, and ultimately its enormous successor the ELT (Extremely Large Telescope), help to provide a global context for the spacecraft observations.”
