Astronomers using the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument on ESO’s Very Large Telescope have imaged Jupiter’s icy moons Europa and Ganymede in infrared light.
This image from the SPHERE instrument on ESO’s Very Large Telescope shows Europa; the different colored regions provide clues to the composition and history of the moon. Image credit: ESO / King & Fletcher.
The Galilean moons — Io, Europa, Ganymede, and Callisto — are Jupiter’s four largest moons.
They collectively form a solar system in miniature around Jupiter, with environments ranging from the volcanic and rocky Io to the icy Europa, Ganymede, and Callisto.
Europa, the second Galilean moon from Jupiter, is the smallest of the four moons. It has a core of silicate rock with an outer crust of liquid water and water ice that may only be 20 km thick.
The moon is tidally locked, meaning that it always presents the same face to Jupiter, and tidal heating is sufficient to maintain a liquid subsurface ocean between the surface and silicate interior.
The presence of the ocean is supported by induced magnetic field measurements by NASA’s Galileo mission. The subsurface ocean and its direct contact with Europa’s silicate interior make Europa one of the most likely candidates in the Solar System to be able to support habitable conditions.
Europa is geologically active, with transient cryovolcanic plumes of water vapor tentatively detected in Hubble observations. It has a very smooth surface, with few impact craters, implying that the surface is geologically very young, with an average age of 50 million years.
Europa’s surface is mainly composed of water ice and is covered with a series of intersecting linear features or lineae, the largest of which are over 1,000 km long. These lineae are thought to be caused by tidal stresses on Europa’s surface that open fissures, exposing the warmer ice layers beneath.
This image from the SPHERE instrument on ESO’s Very Large Telescope shows Ganymede; the bright regions, that consist mainly of water in the form of ice with hints of various salts, are believed to have formed more recently than the older darker patches, whose composition still remains a mystery. Image credit: ESO / King & Fletcher.
Ganymede, the largest of Jupiter’s Galilean moons, is the largest moon in the Solar System, with a radius of 2,631 km.
It has a strongly differentiated interior and intrinsic magnetic field, likely caused by convection in its iron core.
Ganymede is believed to have a subsurface ocean, likely sandwiched between multiple layers of ice.
The moon’s crust is mainly composed of water ice with significant contamination from non-ice materials on the surface.
Much of the current understanding of Ganymede’s surface composition comes from the Galileo mission that orbited Jupiter from 1995 to 2003 with repeated flybys of the Galilean satellites.
Observations with Galileo’s Near-Infrared Mapping Spectrometer and panchromatic cameras identified a surface made up of contrasting dark and light terrain, covering about 1/3 and 2/3 of the surface, respectively.
The dark terrain is heavily cratered, suggesting it is over 4 billion years old and appears to be covered with a thin layer of dark material.
The light terrain is much younger, with fewer craters and cleaner exposed ice, suggesting the light regions were caused by tectonic events, potentially through rift like formation or cryovolcanic resurfacing.
“Whilst Europa is quite similar in size to our own Moon, Ganymede is the largest moon in the whole Solar System — it’s even bigger than the planet Mercury,” said University of Leicester researcher Oliver King and colleagues.
“Their orbits around Jupiter are slightly elliptical, so they get closer and further away from the planet as they orbit it.”
“This results in the moons being stretched and squeezed by the gravitational pull from Jupiter at periodical intervals.”
“This creates frictional heat, warming the insides of the moons, which has made them geologically active.”
“Europa in particular is likely to have active plumes and geysers erupting from the oceans of liquid water beneath the thick ice cover that makes up the surface.”
These images shows Jupiter’s icy moons Ganymede and Europa with Jupiter in the background. Image credit: ESO / King & Fletcher / NASA / ESA / A. Simon, NASA’s Goddard Space Flight Center / M.H. Wong, University of California, Berkeley / OPAL Team.
To estimate the abundances of chemical species on the surfaces of Europa and Ganymede, the authors analyzed new images and spectra obtained by the SPHERE instrument on ESO’s Very Large Telescope.
They found evidence for high acid abundance and larger ice grains on the trailing hemisphere of Europa and a variety of potential hydrated salts.
“Future studies will help to further constrain Europa’s surface composition by expanding the spatial and spectral coverage and resolution of near-infrared observations,” they said.
“Higher spectral resolution observations (e.g., Webb and ELT/HARMONI) and laboratory reference spectra measurements will help to constrain salt abundances by identifying narrow characteristic features in the spectra, while high spatial resolution spacecraft observations will enable accurate geolocation of compositional features.”
The researchers also found that the bright regions of Ganymede consist mainly of water in the form of ice with hints of various salts, and that they have formed more recently than the older darker patches, whose composition still remains a mystery.
“Our results show how Ganymede’s surface is made up to two main types of terrain: young areas have large amounts of water ice, whereas ancient areas mainly consist of a dark grey material which we were unable to identify,” they said.
“We detected sulfuric acid near Ganymede’s poles, which is likely to originate from the gasses which surround Jupiter.”
“A range of different salts were detected, some of which may originate from within Ganymede itself.”
“These surface composition maps will be useful for understanding the processes happening on, and under, Ganymede’s surface.”
“They will also help to plan for the robotic space missions which are due to explore Ganymede up-close in the coming decades.”
The results appear in two paper in the Planetary Science Journal and the Journal of Geophysical Research: Planets.
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Oliver King et al. 2022. Compositional Mapping of Europa Using MCMC Modeling of Near-IR VLT/SPHERE and Galileo/NIMS Observations. Planet. Sci. J 3, 72; doi: 10.3847/PSJ/ac596d
Oliver King & Leigh N. Fletcher. 2022. Global Modelling of Ganymede’s Surface Composition: Near-IR Mapping from VLT/SPHERE. JGR: Planets, in press; doi: 10.1029/2022JE007323
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