MYSTERIOUS MAGNETIC FIELD

One of Ganymede's most remarkable features is its intrinsic magnetic field, discovered by the Galileo mission. No other moon in the Solar System is known to have one, and only two other solid bodies (Mercury and the Earth) generate magnetic dipole fields. Ganymede's miniature magnetosphere lies within Jupiter's much larger magnetosphere, with complex interactions happening between the two. JUICE aims to study these interactions in detail. This includes looking into how particles in the near-Ganymede space environment affect the composition of the moon's surface, how auroras develop on the satellite, and how Ganymede's magnetic field influences auroras on Jupiter. To do this, some of the mission's instruments will measure the moon's magnetic field, both its intrinsic component and that induced by Jupiter. Other instruments will characterise the plasma environment around the moon, studying the particles in it, and will investigate the outer layers of the moon's thin atmosphere.

Artist's impression of aurorae on Ganymede - Credit: NASA, ESA, and G. Bacon (STScI)

HIDDEN OCEAN

The Galileo spacecraft's measurements of Ganymede's magnetic field suggested there could be a subsurface layer of salt water, which unlike ice is a good conductor of electricity. However, this evidence was not conclusive because of the complex interactions between the magnetic fields of Jupiter and Ganymede. More recent observations of Ganymede's auroras made with the Hubble Space Telescope further hinted at the existence of the under-ice ocean. As such, scientists now have firm evidence that Ganymede, like Europa, conceals an ocean under its icy shell, one that may contain more water than all surface water on Earth combined.

However, we still don't know at what depth this ocean starts, how far down it extends, and how it interacts with both the deep interior of Ganymede and the icy crust above it. Finding out more about Ganymede's liquid layer, including its composition and conductivity, is a main objective of JUICE since the ocean might be habitable.

The mission's instruments will make detailed measurements of Ganymede's magnetic field and determine the electric currents at the moon to constrain the extent and conductivity of its subsurface ocean. JUICE will also use other techniques to find out the size of the liquid layer and the icy crust. These include detailed measurements of the moon's rotation rate, and well as a precise determination of Ganymede's gravity field, and of shape and surface changes at the moon due to Jupiter's tidal influence.

A COMPLEX CORE

Scientists hope these measurements will help them find out more about the composition and interior structure of Ganymede. With a mass about half of Mercury's and a diameter greater than that planet's, the moon has a relatively low density, indicating that it's made up of both rocks and ices. In addition, the magnetic-field data from Galileo points to the existence of a liquid, iron-rich core. JUICE will reveal more about the thickness of the various layers in the moon's interior and their composition, which will improve our understanding of how the moon evolved and how it was able to acquire its differentiated structure. JUICE's measurements will also help us find out more about how Ganymede's core is able to generate and maintain a magnetic field.

INVESTIGATING THE ICE

Another of JUICE's objectives is to explore and characterise Ganymede's icy crust, which will be a completely new science endeavour. The mission will use an ice-penetrating radar to probe the moon's subsurface down to a depth of about 9 km. The aim is to determine the crust's minimum thickness, possibly detect liquid water in the shallow subsurface, and explore the moon's tectonic features and geological evolution.

In addition to the subsurface studies, the mission will explore the very top of the icy shell. Ganymede's surface has very old, dark, and densely-cratered regions (like most of Callisto), as well as lighter, younger, grooved regions (more similar to Europa's surface). The intricate pattern of grooves and ridges on Ganymede is thought to be due to tectonic activity, though icy volcanism could also have played a role in the formation of these regions. JUICE will combine stereo imaging and laser altimetry to investigate the moon's surface and determine the geological processes that helped shape it. The spacecraft will also carry out high-resolution investigations of selected regions on Ganymede's surface to better understand the local geology.

SEARCHING FOR BIOSIGNATURES

This high-resolution mapping of the surface can help constrain the moon's composition and mineralogy, and assess how habitable Ganymede could be by searching for biosignatures. Observations at various wavelengths will allow astronomers to study non-water-ice material to determine the distribution of biologically essential elements—such as carbon or oxygen—and other important elements—such as magnesium and iron—on the planetary body. The mission will also shed light on the origin and evolution of the materials on the surface by exploring which substances form at Ganymede and which are brought in from the plasma environment around the moon.

To study Ganymede in detail, JUICE will enter orbit around it, becoming the first spacecraft to orbit a moon in the outer Solar System. The dedicated orbital tour is expected to last about eight months and will be the final stage of the mission.

At about the same size as Earth's moon, Europa is the smallest of the Galilean moons. Like the other Galilean satellites, it is tidally locked, always showing the same face towards Jupiter during its 3.5-day journey around the planet. Because of the moon's elliptical orbit, the gravitational pull from Jupiter varies as the moon orbits, creating tides that flex and relax Europa's surface. The resulting tidal heating provides energy to the moon's shell and, possibly, its interior layers.

CURIOUS CRACKS

Europa's surface, one of the smoothest of any solid body in the Solar System, is another puzzle that JUICE scientists hope to solve. The moon has cracks and streaks etched onto it but very few craters, which points to the surface being relatively young and the moon tectonically active. Indeed, recently, researchers found evidence for a global plate-tectonic system at Europa using images from the Galileo mission. The observations show that some of the features on the moon's surface could be like the mid-ocean ridges on Earth, that is, regions where new surface material is formed. Other cracks could be similar to subduction faults, locations where terrain moves under a second (overriding) plate at the surface. Researchers also saw evidence for cryovolcanoes on this overriding plate, but little is known about them.

These active zones, which could provide a way to exchange material between the moon's surface and its ocean, will be a particular focus of JUICE. There is still mystery surrounding the formation of the various features on Europa's surface, both its bright, striated plains and the darker, smeared regions. JUICE will identify and investigate the geological processes responsible for the distinctive formations on the moon, as well as determine to which degree different processes can exchange material between the surface and the liquid reservoirs underneath it.

Blood-red scars and veins on Europa - Credit: NASA/JPL/University of Arizona

CHEMISTRY AND COMPOSITION

JUICE's instruments will probe the chemistry of Europa's surface at these active regions, searching for elements essential to life and substances that could provide evidence of past or present life on the moon. JUICE's detailed study of the chemistry and composition of Europa also aims to provide clues into how specific elements arrive at the surface. Which substances originate underground and are brought up by tectonics or cryovolcanism, and which particles arrive at Europa from the other moons via Jupiter's magnetosphere?

The mission will also investigate Europa's icy crust and how it interacts with the ocean underneath it. JUICE will use an ice-penetrating radar to determine if pockets of liquid water exist within the first few kilometres of the subsurface. Depending on how thick the icy crust is, the radar will also be able to probe the interface between the shell and the subsurface ocean.

JUICE will flyby this moon twice, at middle latitudes over both of Europa's hemispheres. At closest approach, the spacecraft will be at an altitude of about 400 km.

Other Icy moon: CALLISTO

To better contrast and compare Jupiter's icy worlds, in addition to investigating Ganymede and Europa, JUICE will also study Callisto, the outermost and second-largest Galilean satellite. Like Ganymede, the moon is similar in size to Mercury but, with less than a third of that planet's mass, it is the least dense of all the Galilean moons. Like both Ganymede and Europa, Callisto is thought to have a large ocean beneath its surface, although the evidence for this is less compelling than for the other two moons.

Despite these similarities, in many ways this Galilean moon is very different from its icy companions. Because of its remote location, orbiting beyond Jupiter's main radiation belts, Callisto sees lower radiation levels and less influence from the Jovian magnetosphere than Ganymede and Europa. The 1 880 000 kilometre distance from Jupiter, and the fact that Callisto is relatively isolated from the other Galilean satellites, also means that tidal heating at the moon is not significant, making it far more geologically stable than the inner Galilean moons.

A FOSSIL FROM THE FORMATION ERA

Callisto has an old, heavily marked landscape, with impact features covering the moon's entire surface and with landforms showing signs of intense erosion. In fact, Callisto's surface is the oldest and most densely cratered of any body in the Solar System. The lack of resurfacing or other signs of geologic activity on the moon's surface point to Callisto being a long-dead world, a remnant of the early Jovian system. As such, it can provide scientists with a unique window to explore the early stages of formation of the Galilean moons.

JUICE will investigate Callisto's cratered surface in detail to find out more about the moon's past activity. The mission's observations will allow us to constrain the moon's surface ages, both at the global and local scales, and to study erosion and asteroid-dust deposition processes on Callisto's landforms. Scientists hope to find out how much of a role tectonics and other geologic processes played in moulding the moon's surface in the past, and how much influence impact activity has had in Callisto's evolution.

At the surface, JUICE will study the composition and chemistry of Callisto's rocky and icy materials. Researchers aim to identify the various organic and non-organic compounds that make up the moon's surface, relate them to the moon's geology, and understand how high-energy particles from the Jovian space environment can alter Callisto's surface composition, a process called space weathering.

DISENTANGLING A DIFFERENTIATED INTERIOR

The mission will also study the moon's subsurface and interior. Callisto is thought to have some differentiated layers in its interior: a small rocky inner core surrounded by a mantle mostly made up of ice. In addition, the Galileo spacecraft found evidence for a possible subsurface ocean. That mission detected an induced magnetic field at Callisto with characteristics that suggest the moon could have an underground layer of liquid salty water.

By investigating the moon's gravity field and shape, JUICE's instruments will be able to probe Callisto's interior structure, including its degree of differentiation. If a subsurface ocean does exist, JUICE will study its properties, and constrain the thickness of the icy crust above it. In addition, JUICE's ice-penetrating radar will map the moon's subsurface down to a few kilometres to investigate the icy shell.

BETWEEN SURFACE AND SPACE

Galileo's measurements of an induced magnetic field at Callisto could also be explained, in part, by an ionosphere capable of conducting electric current. A key goal for JUICE is to find out the role the ionosphere plays in producing the measured field, to separate this contribution from that of the interior liquid layer and to better constrain the ocean's properties.

Finally, JUICE will investigate Callisto's thin atmosphere, which is mostly composed of carbon dioxide. Because it is so rarefied, the atmosphere would escape to space in a few days; as such, there has to be a process at Callisto that continually replenishes carbon dioxide. A possibility is that carbon-dioxide ice at the moon's surface slowly sublimates away into the atmosphere. JUICE will help to uncover this mystery.

JUICE will fly past Callisto 12 times, at altitudes between 400 km and 200 km at closest approach.

Jupiter's cratered moon: Callisto - Credit: NASA/JPL/DLR	Landslides on Callisto - Credit: NASA/JPL/ASU

 

JUICE'S Tertiary TARGET: THE JUPITER SYSTEM

Although much of the focus of the JUICE mission is on the icy Galilean moons, its work of exploring how habitable worlds emerge around gas giants would not be complete without a comprehensive overview of the Jovian system. With a multitude of moons representing unique environments, and a central object with a composition similar to that of the Sun, the Jovian system is akin to a small-scale star system. By studying it we can learn more about how the Solar System and exoplanetary systems work, how planets form, and how life can emerge under different conditions.

  

Jupiter and three of its largest moons - Credit: NASA, ESA, Hubble Heritage Team	Surface changes on Io - Credit: NASA/JPL/University of Arizona

VOLCANIC IO

While JUICE's focus is on the icy moons, the mission will also gather information about the innermost Galilean satellite, Io, the most volcanically active object in our Solar System. At Io, the inward gravitational pull from Jupiter and the outward pull from the other Galilean moons results in intense tidal heating, which drives the moon's geological activity. Eruptions from its 400 active volcanoes release lava onto Io's surface, while gases and other substances escape to the moon's thin atmosphere and Jupiter's magnetosphere. JUICE will monitor the volcanic activity of Io and determine the composition of different surface materials.

INNER MOONS AND DUSTY RINGS

JUICE will also remotely explore the smaller inner moons of Jupiter and its faint, dusty rings. Metis, Adrastea, Amalthea, and Thebe orbit the gas giant from about 127 000 kilometres to 222 000 kilometres, respectively, passing through Jupiter's ring system. Dust ejected by these small satellites is believed to provide the material in the rings, though the exact composition and age of the system is unknown. Another uncertainty is how the dust in the rings is replenished: due to the violent processes at Jupiter, which remove small particles, the tenuous ring system would not be a long-lived feature without new material regularly being added to it. JUICE will study Jupiter's small inner satellites by investigating their shape and composition and shedding light on how these objects relate to the rings. To find out more about the origin and dynamics of the rings, the mission will also explore their physical properties and chemical composition.

Much of JUICE's focus will also be on Jupiter itself, extending the discoveries made with the Galileo spacecraft and complementing the work of NASA's Juno mission.

AN EVER-CHANGING ATMOSPHERE

JUICE will study Jupiter's complex and ever-changing atmosphere—from the churning clouds of the troposphere up to the high thermosphere—with emphasis on its vertical structure, composition, and the processes that shape it. JUICE's investigations of Jupiter—the archetypical giant planet—will provide new insight into the myriad processes shaping giant planet weather, climate, and atmospheric chemistry, both in our Solar System and beyond.

  

Jupiter at a glance - Credit: NASA, ESA, A. Simon (GSFC), M. Wong (UC Berkeley), and G. Orton (JPL-Caltech)

 

The lowest part of the atmosphere, the weather layer, is characterised by a complicated system of hazes and clouds. Depending on their opacity, the clouds appear as light zones or dark belts, giving Jupiter its famous banded appearance. JUICE will repeatedly map Jupiter's atmosphere from the equator to the poles to determine the mechanisms responsible for maintaining and regenerating these colourful bands.

JUICE will considerably expand our knowledge of Jupiter's poorly explored middle and upper atmosphere, high above the visible clouds. The suite of remote sensing instruments will reveal the processes that connect the different atmospheric layers together, such as wave propagation and energy transport. JUICE's instruments will measure, for the first time, the winds in the planet's middle atmosphere. The mission will also investigate the composition of the various atmospheric layers and determine how external impacts—from asteroids, or comets such as Shoemaker-Levy 9—influence the atmosphere.

JUICE will reveal how energy is transported within Jupiter's atmosphere, both vertically from the weather layer into the middle and upper atmosphere, and horizontally from the smallest thunderstorms and turbulent eddies to the largest wind jets bordering the belts and zones.

Through close monitoring and tracking of atmospheric processes over time, JUICE hopes to answer questions such as: What mechanisms drive Jupiter's atmospheric circulation and dynamics? How do the various atmospheric processes relate to each other? And how are jets and long-lived storms, such as the Great Red Spot, maintained?

JUICE's comprehensive exploration of Jupiter will give astronomers a global picture of how its atmosphere connects the planet's deep interior to the near-Jupiter space environment, including the Jovian magnetosphere.

MYSTERIOUS MAGNETOSPHERE

Jupiter's magnetosphere—created by the planet's large magnetic field—occupies an extremely vast area around the gas giant and protects it from the solar wind. The region has its own plasma environment, with Io's volcanoes being the main source of plasma. Jupiter's fast rotating magnetic field accelerates particles, creating one of the most intense radiation environments in the Solar System. The complicated interactions between Jupiter's rotation, the planet's magnetic field, the plasma loading from Io, and the solar wind create different regions within the vast magnetosphere. The exact configuration and global dynamics of Jupiter's magnetosphere, and how the diverse plasma environments relate to each other, is one of the outstanding mysteries of our Solar System.

JUICE will explore Jupiter's magnetosphere in unprecedented detail to shed light on the processes occurring within it. It will also probe how Jupiter's magnetic field interacts with the Galilean satellites. While Io is a source of plasma, the icy moons—especially Ganymede and Europa—are plasma sinks with material transported by Jupiter's magnetic field affecting the satellites' composition. The details of these processes, and how much the energetic particles in the magnetosphere affect the composition of the icy moons, are unknown. JUICE aims to unveil these mysteries.