Spending $1.7 billion and traveling hundreds of millions of kilometers just to listen to the sound of the ocean there.

Jupiter is the largest planet in the solar system. Theoretical studies and observations have shown that Europa, Ganymede and Callisto may have underground saltwater oceans. Saltwater oceans are conducive to the survival and evolution of halophilic microorganisms. Therefore, these satellites may have life outside the Earth. In order to obtain important properties of these satellites in various aspects and confirm whether they are suitable for life, the European Space Agency (ESA) has spent many years and finally successfully launched the Jupiter Icy Moons Explorer (Juice). Juice will obtain detailed information on the underground seas, topography, geology, surface chemicals, magnetic fields, internal structures and gravity fields of these icy satellites, helping humans to fully and deeply understand these icy planets and even find evidence of the existence of extraterrestrial life.

Written by | Wang Shanqin

At 12:14 UTC on April 14, 2023 (20:14 Beijing time), the European Space Agency (ESA)'s Jupiter Icy Moons Explorer (Juice or JUICE) was launched from the French Guiana Space Center on an Ariane 5 heavy-lift rocket. [Please go to "Fanpu" to watch the video]

Ariane 5 rocket carrying Juice into space. Image source:
ESA/CNES/Arianespace/Optique Vidéo du CSG/JM Guillon

Juice cost 1.6 billion euros and its mission is to explore the Jupiter system, especially Jupiter's three largest icy satellites: Europa, Ganymede and Callisto. They were independently discovered by Galileo Galilei (1564-1642) and Simon Marius (1573-1625) more than 400 years ago, along with Io, and were later named the "Galilean satellites". [Note 1]

A puzzle of Jupiter and its four "Galilean satellites". From left to right, Io, Europa, Ganymede and Callisto are shown at the top; part of Jupiter is shown at the bottom. Jupiter, Io, Europa and Ganymede were photographed by the Galileo probe, and Callisto was photographed by the Voyager probe. Image credit: NASA/JPL/DLR

Juice's primary mission is to explore Ganymede, followed by Europa, Callisto, and Jupiter, and Io. Through the various instruments it carries, Juice will deeply explore the various properties of these celestial bodies. In this short article, we will introduce Juice's development history, missions, instruments, and other topics.

Artist's impression of Juice (upper left) exploring the Jovian system. Ganymede is at the lower right, with Jupiter in the center; the remaining moons from left to right are Io, Europa, and Callisto. Image credit: ESA / NASA / DLR

Development History

The origin of Juice can be traced back to February 2008, when NASA and ESA decided to cooperate in developing an Outer Planet Flagship Mission. In February 2009, the Europa Jupiter System Mission-Laplace was given priority and will be used to replace the Galileo mission that ended its mission and continue to explore Jupiter's icy satellites.

According to the plan, EJSM-Laplace consists of the Jupiter Europa Orbiter (JEO) managed by NASA and the Jupiter Ganymede Orbiter (JGO) managed by ESA. The two will be launched independently around 2020, and after several years of cruising, they will orbit and conduct in-depth research on Europa and Ganymede respectively.

However, JGO, which ESA is responsible for, must compete with other important projects to obtain priority launch rights [Note 2], so NASA prepared a separate emergency plan for JEO, which it is responsible for, and thus seemed half-hearted. More importantly, the two probes were not very interdependent, and the cooperation between the two parties broke down in April 2011, and they began to develop their own probes independently. ESA changed JGO to "Jupiter Icy Moons Explorer", but still took the exploration of Ganymede as its primary goal. NASA's JEO was replaced by the "Europa Clipper" project in June 2015.

In April 2012, Juice won the competition with two other projects and was subsequently identified by ESA as the first large-scale (L-level) project of the Cosmic Vision 2015–2025 Programme. [Note 3]

In 2014, ESA completed a detailed study of Juice's design and objectives. In July 2015, Airbus Defence and Space was selected as the lead manufacturer to design and build Juice, and has since manufactured Juice's various hardware and instruments across Europe. Over the past few years, Juice's components and instruments have passed various tests, and then were assembled and transferred to the launch site.

The logo of the Ariane 5 rocket. Two years ago, ESA asked children around the world to submit Juice's logo. 10-year-old Yaryna's work was successfully selected and was painted on the top of the rocket. Image source: Manuel Pédoussaut/ESA

Juice's Mission

According to the plan, Juice's primary mission is to study the various characteristics of Ganymede, followed by studying Europa, Callisto and Jupiter, and then studying Io and other satellites of Jupiter. We can categorize its mission as follows. [1]

1. Determine whether Ganymede has an underground ocean; if so, indirectly study its properties. Ganymede has an average radius of 2,634 kilometers, making it the largest satellite in the Solar System. Its radius is larger than the average radius of Mercury (2,440 kilometers). Astronomers speculated in the 1970s that Ganymede has an underground ocean. Observations of Ganymede by Galileo and the Hubble Space Telescope (Hubble) further supported the speculation that Ganymede has an underground ocean. It is estimated that the total amount of Ganymede's ocean water may be 6-8 times the total amount of Earth's ocean water, with a depth of up to 100 kilometers and buried under a 150-kilometer ice shell. [2, 3]

The ultraviolet false-color image of Ganymede's aurora observed by Hubble is superimposed with the image of Ganymede taken by Galileo. Based on the swinging angle of the aurora, astronomers infer that Ganymede has an underground ocean, which produces an induced magnetic field that partially prevents the swinging effect of Ganymede's aurora caused by Jupiter's magnetic field. Image credit: NASA/ESA

2. Study the surface topography, geology, chemical composition of surface materials, physical properties of the ice layer, and properties of the thin atmosphere of Ganymede.

3. Ganymede's magnetic field and its interaction with Jupiter's magnetic field. The intrinsic magnetic field generated by Ganymede's iron-nickel core (Ganymede is the only satellite in the solar system with an intrinsic magnetic field) and the induced magnetic field generated by its possible underground salt water sea will be important objects of Juice's research.

4. Detect Europa's underground sea and determine its characteristics. The detection of Galileo's magnetometer led astronomers to infer that Europa has an underground saltwater sea, which generates an induced magnetic field. It is estimated that the amount of water in Europa may be three times the total amount of water on Earth, and the average depth may be 100 kilometers. The water plumes erupting from some areas of Europa observed by Hubble further confirmed that Europa may indeed have an underground sea.

A composite image of water plumes erupting from Europa, taken by Hubble on January 26, 2014. The image of Europa was synthesized from data from the Galileo and Voyager probes. The plume erupted at an altitude of 160 kilometers. The plume indicates that there may be an ocean of water beneath Europa. Image credits NASA, ESA, W. Sparks (STScI), and the USGS Astrogeology Science Center

5. Study the topographical features, geological characteristics, surface chemical substances, and chemical composition of non-water ice substances of Europa, and determine the minimum thickness of the ice layer in the most geologically active part of it.

Conamara Chaos (left) on Europa and its location on Europa (right). Some process destroyed the previously existing cross-cutting ridges. Blue represents water ice, and reddish-brown areas represent non-water ice. These images are a composite of data acquired by Galileo in 1996-1997. Image credit: NASA/JPL

6. Determine whether Callisto really has an underground ocean, map the topography, geology and chemical composition of surface materials in different regions of Callisto, and study the physical properties of Callisto's ice layer. Callisto has not experienced any geological activity since its formation, and all the craters left by meteorite impacts on its surface are preserved. Studying the specific topography of Callisto is of great value in understanding the formation history of celestial bodies in the solar system.

Callisto, as photographed by Galileo in 2001. Image credit: NASA/JPL/DLR

7. Study the characteristics of the internal mass distribution, dynamics and evolution of icy satellites, and measure the gravity field at various locations on icy satellites.

8. Measure Jupiter’s magnetosphere, study the interaction between Jupiter’s magnetic field and the magnetic fields of its icy satellites, and study the effects of charged particles accelerated by Jupiter’s magnetic field on the surfaces of its icy satellites.

9. Observe Io and some irregularly shaped satellites of Jupiter at a relatively long distance.

One of the core tasks of Juice is to confirm the existence of saltwater oceans on icy satellites and study their properties (if any). Saltwater oceans are conducive to the survival and evolution of halophilic microorganisms. Therefore, Juice may achieve a breakthrough in human exploration of life outside the Earth.

If Juice fails to find evidence of life, but confirms the existence of underground oceans and obtains more specific information about their properties, it will also be an important step forward, because liquid water provides one of the important clues to the evolution of celestial bodies.

An artistic conception of water from beneath Europa's ice layer erupting through cracks in the ice layer, forming plumes. The plumes carry out molecules from the underground ocean, and charged particle radiation from space bombards and decomposes some of the molecules. Volcanoes erupting on the seabed (see left) provide heat for the seawater. Image credit: NASA/JPL-Caltech

Juice Instruments

There are a total of 10 scientific instruments on Juice, including 3 spectrometers, 2 mapping and remote sensing instruments, 2 particle detectors, and the remaining 3 instruments are cameras, magnetometers and radars.

The three spectrometers are the UV Imaging Spectrograph (UVS), the Moons and Jupiter Imaging Spectrometer (MAJIS), and the Sub-millimeter Wave Instrument (SWI). The observation/working wavelengths are 55-210 nanometers (ultraviolet), 0.4-5.7 microns (optical and infrared), and about 0.3 mm/about 0.6 mm respectively.

UVS studies the atmospheric exosphere of icy satellites, the upper atmosphere of Jupiter, the auroras of Jupiter and its icy satellites, and searches for water plumes erupting from the surface of Europa. MAJIS observes cloud features and trace gases in the troposphere of Jupiter's atmosphere, identifies and studies the complex ice and mineral composition of the surface of Jupiter's icy satellites, and searches for organic molecules on their surfaces to determine whether they are suitable for life. [4] SWI studies Jupiter's stratosphere and troposphere, as well as the atmospheric exosphere and surface of icy satellites. When observing Jupiter, the resolutions of MAJIS and UVS are 100 km and 250 km, respectively; when observing Ganymede, the resolutions of MAJIS and UVS can reach up to 75 meters and 500 meters, respectively. [4]

Artist's impression of water plumes erupting from the surface of Europa. Image credit: Ron Miller

The two surveying and remote sensing instruments are the Ganymede Laser Altimeter (GALA) and the Gravity and Geophysics of Jupiter and Galilean Moons (3GM). The main function of the former is to map the three-dimensional terrain through laser ranging, with a vertical resolution of 10 cm; the main function of the latter is to measure the gravity field at various locations, determine the structure of the underground ocean of the icy satellite, and determine the structure of the atmosphere and ionosphere of the icy satellite. [4]

The two particle detectors are the Particle Environment Package (PEP) and the Radio and Plasma Wave Investigation (RPWI), which are used to directly detect neutral particles, charged particles, and the radio waves they emit.

The camera is "Jovis, the Amorum ac Natorum Undique Scrutator, JANUS"[Note 4]. Its observation wavelength range is 0.36-1.1 microns, and it has three-dimensional imaging capabilities. It mainly photographs the surfaces of Ganymede and Callisto, with a resolution of up to 2.4 meters. [4]

The magnetometer is the Juice Magnetometer (J-MAG). It can detect the magnetic field of the icy satellites, thus confirming or denying the inference that Ganymede and Callisto have underground oceans. In addition, it can also detect the interaction between the magnetic fields of Europa and Ganymede and the magnetic field of Jupiter.

The radar is the Radar for Icy Moons Exploration (RIME). Its 16-meter-long antenna emits radar waves with a wavelength of about 33 meters, which can penetrate 9 kilometers below the ice surface, then reflect back and be received by the antenna. Through penetration and reflection, the ice layer structure within 9 kilometers below the surface of the icy moon can be obtained, and its vertical resolution can reach up to 30 meters. [4]

RIME was developed by NASA's Jet Propulsion Laboratory (JPL) and the Italian Space Agency (ASI), with JPL providing the radar's transmitter, receiver, and electronics. Image credit: NASA/JPL-Caltech

In addition to the above 10 instruments, Juice's antenna can also transmit specific signals. The Very Long Baseline Interferometry (VLBI) on Earth receives these signals and accurately measures the gravitational field of Jupiter and its icy satellites. This is the Planetary Radio Interferometer and Doppler Experiment (PRIDE).

The location distribution map of Juice's 10 instruments, with the black squares representing the solar panels. Image source: ESA

Thanks to the advanced technology used, the 10 instruments are extremely lightweight, with a total mass of only 104 kg.[3]

Juice uses solar panels to generate electricity to power its instruments, and its operating power reaches 820 watts. Near Jupiter's orbit, the intensity of the sun's radiation is only about 4% of that near Earth. In order to obtain sufficient power, not only does the battery performance need to be very good, but the area of ​​the solar panel also needs to be very large - its area is about 85 square meters. [5]

In addition to instruments and batteries, another important component is the navigation equipment, which accurately guides the probe along the predetermined route.

Run Path

In order to save rocket fuel and increase speed, Juice will use the previously widely used gravity slingshot (gravity assist) technology to increase speed with the help of the huge gravity of the planet. According to the plan, it will fly past the Earth-Moon system in August 2024, fly past Venus on August 31, 2025, and fly past the Earth for the first and second time on September 29, 2026 and January 18, 2029. [4]

After these flybys, Juice will gain enough speed to head to Jupiter. On the way, it may fly by the asteroid 223 Rosa on October 15, 2029,[4] and conduct a close-up exploration of it.

Schematic diagram of Juice's orbit. The time nodes on the straight line below the figure are from left to right: launch, flyby of the Earth-Moon system, flyby of Venus, first flyby of the Earth, second flyby of the Earth, arrival at Jupiter, exploration of the Jupiter system and completion of 35 flybys, orbiting Ganymede. Image source: ESA

According to the plan, after 8 years of cruising, Juice will begin to enter the Jupiter system in July 2031, and change its orbit through maneuvers and the slingshot effect of gravity's Jupiter satellites, entering an orbit around Jupiter. After about 3.5 years of orbiting exploration, it will leave Jupiter's orbit in December 2034 and head for Ganymede.

After completing these flybys, Juice will enter orbit around Ganymede, becoming the Ganymede Orbiter, which will also make it the first probe to orbit a natural satellite other than the Moon.

This image of Ganymede taken by the Juno spacecraft on June 7, 2021. Image credit:
NASA/JPL-Caltech/SwRI/MSSS

When Juice first orbited Ganymede, the average radius of its orbit was very large (5,000 km). After several orbit changes, its orbit became circular and its altitude from Ganymede's surface was only 500 km.

From entering the Jupiter system to orbiting Ganymede at low altitude, Juice flew past Europa, Ganymede, and Callisto 35 times in total,[5] conducting close-range exploration of these three icy satellites. In addition to several flybys before entering Jupiter to achieve gravity slingshot, Juice had to perform about 40 maneuvers to complete these flybys. These maneuvers would consume a large amount of propellant, so the mass of the propellant carried by Juice was about 3,000 kilograms.[6]

Juice will orbit Ganymede for about one year (at least nine months) at an orbit of 500 km. [7] If there is still propellant left, Juice will continue to maneuver, orbiting at an altitude of 200 km. After the propellant is exhausted, Juice will impact Ganymede in late 2035. These close orbits and the final approach before the impact will allow Juice to observe Ganymede with unprecedented resolution.

The structural model of Ganymede proposed by astronomers. According to theoretical inference, Ganymede consists of a solid core rich in iron, a liquid core rich in iron and sulfide, a rocky mantle, deep ice (which becomes type VI tetragonal ice due to excessive pressure), a liquid ocean, and an outer layer of ice (ordinary type Ih hexagonal ice). The surface of Ganymede is covered with craters, grooves, light terrain, and dark terrain. Image source: Kelvinsong

A combination of techniques to detect Jupiter's icy satellites

If all goes well, Juice will greatly improve our understanding of the various properties of Ganymede, Europa, Callisto and some of Jupiter's properties. In the most optimistic case, it may even provide evidence of life on Jupiter's icy moons.

The Europa Clipper, which will be launched in 2024, will arrive at the Jupiter system in 2030 (one year earlier than Juice). It will observe Europa in detail through multiple close flybys (there will be no orbiting mode because Jupiter's magnetic field is too strong at Europa's orbit), and will cooperate with Juice, which will arrive later, to provide each other with experience and compare data.

In addition, my country's Tianwen-4 project, which is being planned, also takes Jupiter and its icy satellites as its main exploration targets. If it is approved, it will likely be launched in 2029, and its main probe will reach Jupiter in 2035 and become an orbiter around Callisto to obtain various detailed information about Callisto.

If Juice, Europa Clipper and Tianwen-4 can all successfully achieve their goals, humanity's understanding of the various properties of Jupiter's icy satellites will be comprehensively enhanced.

We wish them all great success.

Notes

[Note 1] The Galilean satellites are the four largest satellites of Jupiter. They are called "Galilean satellites" because Galileo published his observations of the four satellites of Jupiter earlier than Marius. Academician Xi Zezong pointed out in his 1981 paper that Gan De, a famous astronomer in the Warring States Period of China, may have observed Ganymede more than 2,000 years ago ("Suixing is in Zi, ... if there is a small red star attached to its side, it is called an alliance"). However, the literature describes the star discovered by Gan De as a red star ("small red star"), which is puzzling because such a dark star cannot be distinguished by color. Therefore, this suspected discovery has not yet been recognized.

[Note 2] This is extremely important for planetary probes, because such probes have a certain time window, and if they miss it, they will have to wait for many years.

[Note 3] The "Cosmic Vision 2015-2025" program is a large space project group project of ESA, in which the probes are divided into three levels: small (S), medium (M) and large (L). There are only three projects in the L level, and Juice is the first one (L1).

[Note 4] Jupiter’s English name is Jupiter, which is the god in Roman mythology and Zeus in Greek mythology. Marius named Jupiter’s satellites after Zeus’ lovers in Greek mythology: Io, Europa, Ganymede, and Callisto.

References

[1]https://sci.esa.int/web/juice/-/50068-science-objectives

[2]https://www.nasa.gov/press/2015/march/nasa-s-hubble-observations-suggest-underground-ocean-on-jupiters-largest-moon

[3]https://www.planetary.org/space-missions/juice

[4]https://en.wikipedia.org/wiki/Jupiter_Icy_Moons_Explorer

[5]https://www.esa.int/Science_Exploration/Space_Science/Juice

[6]https://sci.esa.int/web/juice/-/61498-juice-inner-structure

[7]https://www.nature.com/articles/d41586-023-01256-x

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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