Europa Clipper is one of two missions on their way to see if Jupiter’s moons could support life
Mike Sori :Assistant Professor of Planetary Science, Purdue University
Jupiter’s moons hide giant subsurface oceans.
On Oct. 14, 2024, NASA launched a robotic spacecraft named Europa Clipper to Jupiter’s moons. Clipper will reach the ice-covered Jovian moon Europa in 2030 and spend several years collecting and sending valuable data on the moon’s potential habitability back to Earth.
On Oct. 14, 2024, NASA launched a robotic spacecraft named Europa Clipper to Jupiter’s moons. Clipper will reach the ice-covered Jovian moon Europa in 2030 and spend several years collecting and sending valuable data on the moon’s potential habitability back to Earth.
Clipper isn’t the only mission highlighting researchers’ interest in Jupiter and its moons.
On April 13, 2023, the European space Agency launched a rocket carrying a spacecraft destined for Jupiter. The Jupiter Icy Moons Explorer – or JUICE – will spend at least three years on Jupiter’s moons after it arrives in 2031.
I’m a planetary scientist who studies the structure and evolution of solid planets and moons in the solar system.
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There are many reasons my colleagues and I are looking forward to getting the data that Europa Clipper and JUICE will hopefully be sending back to Earth in the 2030s. But perhaps the most exciting information will have to do with water. Three of Jupiter’s moons – Europa, Ganymede and Callisto – are home to large, underground oceans of liquid water that could support life.
Four moons next to a large red spot on the surface of Jupiter.
This composite image shows, from tOP to bottom, Io, Europa, Ganymede and Callisto next to Jupiter. NASA, CC BY-ND
Meet Io, Europa, Ganymede and Callisto
Jupiter has dozens of moons. Four of them in particular are of interest to planetary scientists.
Io, Europa, Ganymede and Callisto are, like Earth’s Moon, relatively large, spherical complex worlds. Two previous NASA missions have sent spacecraft to orbit the Jupiter system and collected data on these moons. The Galileo mission orbited Jupiter from 1995 to 2003 and led to geological discoveries on aLL four large moons. The Juno mission is still orbiting Jupiter today and has provided scientists with an unprecedented view into Jupiter’s composition, structure and space environment.
These missions and other observations revealed that Io, the closest of the four to its host planet, is abuzz with geological activity, including lava lakes, volcanic eruptions and tectonically formed mountains. But it is not home to large amounts of water.
Europa, Ganymede and Callisto, in contrast, have icy landscapes. Europa’s surface is a frozen wonderland with a young but complex history, possibly including icy analogs of plate tectonics and volcanoes. Ganymede, the largest moon in the entire solar system, is bigger than Mercury and has its own magnetic field generated internally from a liquid metal core. Callisto appears somewhat inert compared to the others, but serves as a valuable time capsule of an ancient past that is no longer accessible on the youthful surfaces of Europa and Io.
Most exciting of all: Europa, Ganymede and Callisto all almost certainly possess underground oceans of liquid water.
A diagram showing a cutaway of Europa.
Warmth from Europa’s interior and tidal energy from Jupiter likely maintain a massive liquid ocean beneath the moon’s icy surface. NASA/JPL-Caltech/Michael Carroll
Ocean worlds
Europa, Ganymede and Callisto have chilly surfaces that are hundreds of degrees below zero. At these temperatures, ice behaves like solid rock.
But just like Earth, the deeper underground you go on these moons, the hotter it gets. Go down far enough and you eventually reach the temperature where ice melts into water. Exactly how far down this transition occurs on each of the moons is a subject of debate that scientists hope to resolve with JUICE and Europa Clipper. While the exact depths are still uncertain, scientists are confident that these oceans exist.
The best evidence of these oceans comes from Jupiter’s magnetic field. Saltwater is electrically conductive. So as these moons travel through Jupiter’s magnetic field, they generate a secondary, smaller magnetic field that signals to researchers the presence of an underground ocean. Using this technique, planetary scientists have been able to show that the three moons contain underground oceans. And these oceans are not small – Europa’s ocean alone might have more than double the water of all of Earth’s oceans combined.
An obvious and tantalizing next question is whether these oceans can support extraterrestrial life. Liquid water is an important piece of what makes for a habitable world, but far from the only requirement for life. Life also needs energy and certain chemical compounds in addition to water to flourish. Because these oceans are hidden beneath miles of solid ice, sunlight and photosynthesis are out. But it’s possible other sources could provide the needed ingredients.
On Europa, for example, the liquid water ocean overlays a Rocky interior. That rocky seafloor could provide energy and chemicals through underwater volcanoes that could make Europa’s ocean habitable. But it is also possible that Europa’s ocean is a sterile, inhospitable place – scientists need more data to answer these questions.
Artist's impression of the JUICE spacecraft approaching Jupiter and the jovian moons.
The Jupiter Icy Moons Explorer spacecraft will travel for eight years before reaching Jupiter. ESA/ATG medialab/NASA/JPL/University of Arizona/J. Nichols
Upcoming missions from ESA and NASA
Europa Clipper and JUICE are set up to give scientists game-changing information about the potential habitability of Jupiter’s moons. While both missions will gather data on multiple moons, JUICE will spend time orbiting and focusing on Ganymede, and Europa Clipper will make dozens of close flybys of Europa.
Both of the spacecraft will carry a suite of scientific instruments built specifically to investigate the oceans. Onboard radar will allow Europa Clipper and JUICE to probe into the moons’ outer layers of solid ice. Radar could reveal any small pockets of liquid water in the ice, or, in the case of Europa, which has a thinner outer ice layer than Ganymede and Callisto, hopefully detect the larger ocean.
Magnetometers will also be on both missions. These tools will give scientists the opportunity to study the secondary magnetic fields produced by the interaction of conductive oceans with Jupiter’s field in great detail and will hopefully give researchers clues to salinity and volumes of the oceans.
Scientists will also observe small variations in the moons’ gravitational pulls by tracking subtle movements in both spacecrafts’ orbits, which could help determine if Europa’s seafloor has volcanoes that provide the needed energy and chemistry for the ocean to support life.
Finally, both craft will carry a host of cameras and light sensors that will provide unprecedented images of the geology and composition of the moons’ icy surfaces.
Maybe one day, a spacecraft will be able to drill through the miles of solid ice on Europa, Ganymede or Callisto and explore oceans directly. Until then, observations from spacecraft like Europa Clipper and JUICE are scientists’ best bet for learning about these ocean worlds.
When Galileo discovered these moons in 1609, they were the first objects known to directly orbit another planet. Their discovery was the final nail in the coffin of the theory that Earth – and humanity – resides at the center of the universe. Maybe these worlds have another humbling surprise in store.
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Article
What moons in other solar systems reveal about planets like Neptune and Jupiter
Bradley Hansen: Professor of Physics and Astronomy, University of California, Los Angeles
What is the difference between a planet-satellite system as we have with the Earth and moon, versus a binary planet – two planets orbiting each other in a cosmic do-si-do?
I am an astronomer interested in planets orbiting nearby stars, and gas giants – Jupiter, Saturn, Uranus and Neptune in our solar system – are the largest and easiest planets to detect. The crushing pressure within their gassy atmosphere means they are unlikely to be hospitable to life. But the rocky moons orbiting such planets could have conditions that are more welcoming. Last year, astronomers discovered a planet-sized exomoon orbiting another gas giant planet outside our solar system.
In a new paper, I argue that this exomoon is really what is called a captured planet.
Is the first detected ‘exomoon’ really a moon?
True Earth analogues, that orbit Sun-like stars, are very hard to detect, even with the large Keck telescopes. The task is easier if the host star is less massive. But then the planet has to be closer to the star to be warm enough, and the star’s gravitational tides may trap the planet in a state with a permanent hot side and a permanent cold side. This makes such planets less attractive as a potential location that could harbor life. When gas giants orbiting Sun-like stars have rocky moons, these may be more likely places to find life.
In 2018, two astronomers from Columbia University reported the first tentative observation of an exomoon – a satellite orbiting a planet that itself orbits another star. One curious feature was that this exomoon Kepler-1625b-i was much more massive than any moon found in our solar system. It has a mass similar to Neptune and orbits a planet similar in size to Jupiter.
Astronomers expect moons of planets like Jupiter and Saturn to have masses only a few percent of Earth. But this new exomoon was almost a thousand times larger than the corresponding bodies of our solar system – moons like Ganymede and Titan which orbit Jupiter and Saturn, respectively. It is very difficult to explain the formation of such a large satellite using current models of moon formation.
In a new model I developed, I discuss how such a massive exomoon forms through a different process, wherein it is really a captured planet.
ALL planets, large and small, start by gathering together asteroid-sized bodies to make a rocky core. At this early stage in the evolution of a planetary system, the rocky cores are still surrounded by a gaseous disk left over from the formation of the parent star. If a core can grow fast enough to reach a mass equivalent to 10 Earths, then it will have the gravitational strength to pull gas in from the surrounding space and grow to the massive size of Jupiter and Saturn. However, this gaseous accumulation is short-lived, as the star is draining away most of the gas in the disk, the dust and gas surrounding a newly formed star.
If there are two cores growing in close proximity, then they compete to capture rock and gas. If one core gets slightly larger, it gains an advantage and can capture the bulk of the gas in the neighborhood for itself. This leaves the second body without any further gas to capture. The increased gravitational pull of its neighbor drags the smaller body into the role of a satellite, albeit a very large one. The former planet is left as a super-sized moon, orbiting the planet that beat it out in the race to capture gas.
A remnant core as a look back into history
Viewed in this context, the captured planet is unlikely to be habitable. Growing planetary cores have gaseous envelopes, which make them more like Uranus and Neptune – a mix of rocks, ice and gas that would have become a Jupiter if it had not been so rudely cut off by its larger neighbor.
However, there are other implications that are almost as interesting. Studying the cores of giant planets is very difficult, because they are buried under several hundred Earth masses of hydrogen and helium. Currently, the JUNO mission is attempting to do this for Jupiter. However, studying the properties of this exomoon may enable astronomers to see the naked core of a giant gaseous planet when it is stripped of its gaseous envelope. This can provide a snapshot of what Jupiter may have looked like before it grew to its current enormous size.
This exomoon system Kepler-1625b-i is right at the edge of what is detectable with current technology. There may be many more objects like this that could be uncovered with future improvements in telescope capabilities. As astronomers’ census of exoplanets continues to grow, systems like the exomoon and its host highlight an issue that will become more important as we go forward. This exomoon reveals that the properties of a planet are not solely a consequence of its mass and position, but can depend on its history and the environment in which it formed.