Saturn’s moon, Enceladus, is a gleaming beacon that captivates our intellectual curiosity. Its clean, icy surface makes it one of the most reflective objects in the entire Solar System. But it’s what’s below that ice that really gets scientists excited.
Under its icy shell is an ocean of warm, salty water, and the ESA says investigating the moon should be a top priority.
Enceladus is Saturn’s sixth-largest moon. It’s only about 500 km (300 miles) in diameter. But despite its small size, it may harbour a buried ocean containing 15 million cubic km of water. (Earth has about 1.4 billion cubic kilometres of water.)
The Cassini spacecraft spotted plumes of water coming from under the ice, and ever since then, scientists have hungered for a closer look at the moon. The European Space Agency (ESA) aims to give them one.
“The mission concepts that we have recommended would provide tremendous scientific return, driving forward our knowledge, and would be fundamental for the successful detection of biosignatures on icy moons.”
The ESA’s long-term plan for exploring the Solar System is called Voyage 2050. In 2021, the ESA settled on an overarching theme for their Voyage 2050 activities called “Moons of the Giant Solar System Planets.” The ESA struck a committee of top planetary scientists to flesh out their ideas, and that committee laid out the priorities. According to them, the ESA should focus on one of the ocean moons and explore its habitability by investigating links between its environment and its interior. The ESA should also search for signs of life, either extant or ancient, and try to identify any surface chemistry that could enable life.
Dr. Zita Martins, an astrobiologist at Instituto Superior Técnico, chaired the team of planetary scientists. “The mission concepts that we have recommended would provide tremendous scientific return, driving forward our knowledge, and would be fundamental for the successful detection of biosignatures on icy moons,” said Dr. Martins.
“I am very happy to have been part of this process, seeing first-hand the early steps that will potentially lead to the investigation of the moons of the giant planets by ESA,” said Dr. Martins. “The search for habitable conditions and for signatures of life in the Solar System is challenging from a science and technology point of view but very exciting!”
But which moon should the ESA focus on? Candidates include Jupiter’s moon Europa and Saturn’s moons, Enceladus and Titan. Strong scientific cases can be made for each of these, as each one hosts liquid water.
Europa, Enceladus, and Titan all have subsurface oceans, and all three are targets for potential exploration. Image Credits: NASABut each moon is unique, and any mission to either of these moons would be uniquely complex. And expensive. Working alongside the science committee was a team of engineers from the ESA’s Concurrent Design Facility (CDF). Their job was to think ahead to the types of technologies that would be needed, and if they would be possible within a couple of decades.
“We commissioned three CDF studies focused on the most promising moons: Jupiter’s Europa and Saturn’s Enceladus and Titan,” elaborates Dr Frederic Safa, head of ESA’s Future Missions Department. “The team of scientists worked closely with the CDF engineers on the objectives of each study. The outcomes helped pin down what can be done with the resources that we will have in the 2040s.”
One had to be chosen, and the ESA chose Enceladus. Titan is second on the list, and Europa is third. (NASA is launching a mission to Europa in October 2024, and the ESA launched its JUICE mission to Jupiter last year.)
Enceladus has many qualities that attract planetary scientists interested in habitability: it has liquid water, an energy source, and some specific chemicals.
Data from the Cassini spacecraft is behind this global infrared mosaic of Saturn’s moon Enceladus. The intriguing ‘tiger stripes’ feature is prominent. Image Credit: NASA/JPL-Caltech/University of Arizona/LPG/CNRS/University of Nantes/Space Science InstituteEnceladus’ plumes are salty and chemically rich. Along with sodium, chlorine, and carbon trioxide, there are nitrogen, carbon dioxide, and hydrocarbons like methane and formaldehyde. There are also some simple organic compounds and larger organic molecules like benzene.
The water is kept liquid by the warmth from tidal heating. As Enceladus orbits Saturn, the gigantic planet tugs on the moon and deforms it. Each time it does, friction heats the moon. The moon also has a rocky core, and some of that rock is probably melted, creating magma chambers. It all adds up to an icy moon with a liquid ocean where the water interacts with the rock core, a critical part of it all. And it’s all kept warm despite a lack of radionuclides.
Unlike Earth’s core, Enceladus has no radionuclides to generate warmth. Instead, tidal heating keeps the moon warm and drives the movement of water. Image Credit: Surface: NASA/JPL-Caltech/Space Science Institute; interior: LPG-CNRS/U. Nantes/U. Angers. Graphic composition: ESAAnybody who follows planetary science news knows some of this, and they know that Enceladus is begging to be explored. A mission to Enceladus would be great for everybody interested in planetary science but would be especially rewarding for the ESA itself.
“An investigation into signs of past or present life around Saturn has never been achieved before. It would guarantee ESA leadership in planetary science for decades to come,” said ESA Director of Science, Prof. Carole Mundell.
The ESA launched its JUICE (Jupiter Icy Moons Explorer) mission one year ago. It’ll reach the Jovian system in 2031 and explore Jupiter’s moons Europa, Ganymede, and Callisto. Together with an eventual mission to Enceladus and NASA’s Europa Clipper mission, we’re on the cusp of learning an awful lot more about icy ocean moons.
The mission won’t be launched until the early 2040s and would take about a decade to reach its target. It could explore the Saturn system with far more technologically advanced science instruments than its predecessor, Cassini-Huygens. It could mimic that mission by exploring the system before a grand finale took it up close to Enceladus for our best-ever look at the icy ocean moon.
The science team developing the mission concept says that collecting a sample from Enceladus’ plumes is a must. A lander could do it, though that introduces an order of magnitude more complexity and expense. But an orbiter could do it too, by flying through the plumes, collecting a sample, and examining it in an onboard lab.
The discovery of ocean moons with icy shells has changed our understanding of planetary science, our Solar System, habitability, and the search for life. If there are this many ocean moons in our Solar System, how many are there out there in the Milky Way?
Learning more about Enceladus, Europa, and the rest could teach us a lot about life in the Universe and potential exomoon habitability.
