Iconic missions to the outer solar system like Voyager 1 and 2, Galileo, and Cassini gave us a close look at the giant planets’ largest moons. Once thought of as inactive, cold conglomerates of ice and rock, we now know these distant moons are planet-like worlds with rich histories.

While our search for life in the universe was once restricted to Earth-like planets, with terrestrial atmospheres and surface oceans, such icy moons with potentially habitable underground oceans offer new horizons.

Jupiter’s three largest icy moons—Europa, Ganymede and Callisto—all show hints of hosting liquid water oceans beneath their crusts. On Earth, life thrives in the deepest, darkest parts of our oceans near hydrothermal vents. Could life similarly evolve or survive in the oceans floors of these moons? The European Space Agency’s (ESA) boldest mission to date aims to find out.

The JUpiter ICy moons Explorer, or JUICE, will give us insights on how icy worlds around Jupiter and Jupiter-like exoplanets evolve. By finding out if Jupiter’s icy moons are habitable, JUICE will greatly expand possible places to look for life in the universe.

How will JUICE study Jupiter’s moons?

JUICE is planned to launch in mid-2022 and arrive at Jupiter in 2029. It will spend 2.5 years orbiting Jupiter, often flying within 200 to 1,000 kilometers of the icy moons. In this first phase of the mission, the solar-powered spacecraft will fly by Europa twice, and 12 times past Ganymede and Callisto each, enabling repeated close studies of these moons in unprecedented detail. In the next and final mission phase, JUICE will orbit Ganymede, studying it closely for at least nine months. This would be the first time a spacecraft orbits any moon other than our own.

At 4,800 kilograms, JUICE is a heavy spacecraft. The complex trajectories JUICE will need to execute to closely study Jupiter’s icy moons requires it to carry almost 3,000 kilograms of fuel. JUICE will carry 10 state-of-the-art instruments, which together only weigh 104 kilograms thanks to experience gained by ESA in past missions.

How will JUICE map Jupiter’s icy moons and study the planet’s effects on them?

JUICE will use a visible-and-infrared spectrometer to detect what the surfaces of the icy moons are made of. It will help us identify and map minerals across Ganymede’s surface, understand the formation of some of Europa’s and Callisto’s complex landforms, and find organic molecules—building blocks of life—on each of the moon’s surfaces as signs of their potential habitability. A laser-altimeter onboard will complement this data by mapping 3D shapes of features on the moons.

NASA’s Hubble space telescope has observed hints of water vapor in Ganymede’s exosphere (thin atmosphere) and water plumes erupting on Europa. JUICE’s ultraviolet spectrometer and charged particle detector can confirm such activity in the moons’ exospheres by identifying their molecules. JUICE will also sport a novel sub-millimeter wave spectrometer, which detects light frequencies between infrared and microwave, to complement surface mapping and exospheric data taken by the other two spectrometers.

Part of the JUICE mission involves studying Jupiter because it affects the icy moons and conditions on them. Jupiter has a massive magnetic field, one that is 20 times stronger than Earth’s. Europa and Ganymede in particular, being fairly close to Jupiter, are showered in intense radiation particles traveling along Jupiter’s magnetic field lines. JUICE’s magnetometer, particle detector, and plasma instrument will together map and measure Jupiter’s magnetic field, detect particles speeding within it, and how they alter material of the moons’ surfaces.

How will JUICE confirm if Ganymede has a liquid water ocean below its surface?

Ganymede is the only moon in our solar system with its own magnetic field. JUICE’s magnetometer will measure it, explore how Jupiter’s field interacts with it, and study how charged particles raining down along Jupiter’s field lines cause glowing aurorae on Ganymede.

Hubble’s aurorae observations of Ganymede and the Galileo spacecraft’s magnetic field measurements strongly suggest the presence of a 100 kilometers thick ocean—10 times deeper than Earth’s—buried under 150 kilometers of ice. Such an ocean would affect magnetic field lines passing through it. JUICE’s magnetometer can sense such changes in the field and confirm the presence of an ocean. Similarly, JUICE will help solidify or nullify previous hints of a subsurface ocean at Callisto.

How will JUICE see below the surface of Jupiter’s icy moons?

The Italian Space Agency and NASA are providing JUICE’s radar that can penetrate the surfaces of the icy moons as deep as 9 kilometers, showing us their internal structures for the first time. While the oceans of all three worlds are expected to lie further below, the radar will tell us about the nature of the icy terrain leading to it, if there are any pockets of liquid water sandwiched in between (especially on Europa), how the crusts of the moons evolved over time, and how they contributed to making the surfaces look the way they do today.

JUICE will also measure the gravity fields of the icy moons. As JUICE passes over denser and rarer regions of the moons, its position and velocity will be impacted by the changes in gravity. Two radio instruments onboard will note these finer movements  of the spacecraft, allowing scientists to infer the gravity fields of the moons and thus provide insights into their interiors.

Jovian space odyssey

JUICE is part of ESA’s Cosmic Vision 2015-2025 program, which undertakes ambitious space science missions that take over a decade to realize. In May 2012, ESA selected JUICE as the program’s first of the three large-class missions, the other two being Athena and LISA. ESA completed a detailed study of JUICE’s design and goals in 2014, and then set out to build their Jovian explorer. The spacecraft, its parts, and instruments have been undergoing extensive testing over the last four years at various facilities across Europe.

NASA is planning to launch Europa Clipper in 2024, which will arrive at Jupiter around the same time as JUICE. It will perform multiple flybys of Europa to help us better understand the moon’s ability to support life. China is planning a mission to Jupiter's moons too, one of its leading proposals being a Callisto orbiter and lander. Together, these missions will inform us not only about the habitability of the icy moons of Jupiter but that of icy worlds around giant planets across the universe.

Originally published at The Planetary Society.

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