The clearest images yet of Europa and Ganymede from Earth

Astronomers have revealed the most stunning and detailed images of two of Jupiter’s largest moons ever obtained from Earth’s surface.

The images show the icy surfaces and details of the processes that shape the chemical composition of Europe and Ganymedetwo of Jupiterthe four Galilean moons, named after the astronomer who first observed them. And scientists hope that the new images, captured by the very large telescope (VLT) in Chile, will lay the groundwork for future space missions centered on these worlds.

The highly detailed observations reveal geological features on the two frozen Jovian moons, including a long rift-like marking that cuts across Europa’s surface, a type of deformation planetary scientists call a ‘linae’.

Related: Juno Photos Reveal More Stunning Glimpses of Jupiter’s Ice-Covered Moon Europa

Sightings of Europa, which is about the size of earth moonand Ganymede, the largest moon in the solar system and larger than Mercury, note the amount of sunlight reflected from the icy surfaces of the moons. This observation produces what astronomers call a reflectance spectrum, which they can then analyze through a process called spectroscopy to reveal the chemical “fingerprints” of specific elements. These reflectance spectra allowed scientists to determine the chemical composition of the two Jovian moons.

Analysis of Europa showed that its crust is mostly composed of water ice frozen with materials other than ice, including a variety of different salts that reflectance spectroscopy could not identify, through its surface.

“We mapped the distributions of the different materials on the surface, including sulfuric acid frost, which is mainly on the side of Europa that is most heavily bombarded by the gases surrounding Jupiter,” said the head of team and the University of Leicester School of Physics and Astronomy. Ph.D. student Oliver King, said in a statement. “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.”

Observations by the Ganymede team, meanwhile, revealed two types of terrain on the moon’s surface. The younger areas were marked by large amounts of water ice than the much older regions, which are made of dark gray material that the team has not yet been able to identify.

Jupiter’s moons Europa and Ganymede orbiting Jupiter. (Image credit: ESO/King & Fletcher. Jupiter background image: NASA, ESA, A. Simon (Goddard Space Flight Center) and MH Wong (University of California, Berkeley) and the OPAL team)

Seen as blue-colored areas in the VLT image, Ganymede’s icy regions include its polar caps and craters where asteroid impacts have freshly exposed the ice that makes up the Galilean moon’s crust. King and his colleagues also used the images to map the size of ice grains on Ganymede’s surface and determine how different salts may be distributed on it.

“[The VLT] enabled us to carry out detailed mapping of Europa and Ganymede, observing features on their surfaces less than 150 kilometers [90 miles] across – all at distances of over 600 million miles [370 million miles] from Earth,” King said. “Mapping at this fine scale was previously only possible by sending a spacecraft up to Jupiter to observe the moons up close.”

This research does not mean that future missions to these moons are not possible. On the contrary, this cartography of Europa and Ganymede makes the prospect of these space missions even more enticing.

Preparing the ground for future exploration

Leigh Fletcher oversaw the VLT survey of Europa and Ganymede and is also a member of ESA’s Jupiter Icy Moons Explorer science teams (JUICE) and NASA European Clipper mission, each intended to explore these Jovian moons in the early 2030s.

Missions will begin when JUICE launches in 2023 to study Europa and Ganymede, as well as a third Jovian moon, Callistoplus the gas giant’s atmosphere and magnetic field.

“These ground-based observations whet the appetite for our future exploration of Jupiter’s moons,” Fletcher said. “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 merit the most scrutiny.” Now, these decisions can incorporate additional context.

NASA’s Europa Clipper mission will follow JUICE to the launch pad in 2024 and will focus on determining if the ocean beneath the moon’s icy surface has conditions favorable for life. Unlike JUICE, Europa Clipper will focus firmly on Europa, analyzing both the inner ocean and the dynamics of the ice shell.

The team’s work is published in two separate articles; Europa research is detailed in The Journal of Planetary Science and the team’s Ganymede findings are discussed in a paper accepted for publication in the journal JGR:Planets and currently available on the preprint server ArXiv.

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