Planets get most of the attention when it comes to astronomical discussions. Their moons, on the other hand, are a bit more obscure. Neptune, Jupiter and Saturn are well-known names, but what about Enceladus, Io, and Titan?
A whopping 293 moons, most of which are small enough to resemble asteroids, dance around the planets of our solar system. The universe is not concerned about equality here, either: For example, Venus has no moons, Earth has one, and Saturn has a staggering 146.
These moons, or natural satellites as they’re called, are also not built equally. From plasma-spewing hellscapes to a frigid land with air so thick a human could don a winged suit and take flight, these extraterrestrial moons are fascinating worlds in their own right, and may even be unexpected candidates for harboring life.
1. Io
(Credit: Gwengoat/Shutterstock)
We’ll start this list with a bang. Io — one of Jupiter’s 92 moons — is the most volcanically active body known to science, with hundreds of volcanoes dotting its surface, spewing lava dozens of miles into its thin atmosphere. But just what makes Io, named after a mortal woman in love with Zeus in Greek mythology, so fussy?
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The answer: tidal heating. Much like how the moon’s gravity pushes and pulls on the oceans to generate tides, Jupiter’s gravitational might combines with that of its other moons, Europa and Ganymede, to push and pull on Io. Only instead of sloshing water, as on Earth, the planets are bending Earth itself. The most extreme tides on Earth shift the water level by around 60 feet; Io’s influencers displace the ground itself by over 300 feet every orbit.
These shifts are capable of generating gargantuan amounts of hot friction. In fact, Io’s entire core is heated by this cosmic tug-of-war. A surface of molten sulfur is constantly remodeling itself as a subsurface ocean of magma ebbs and flows. But that’s not all; Io is also a gigantic generator. Solar radiation caught in Jupiter’s magnetic field charges up volcanic gasses in Io’s tenuous atmosphere, ejecting them into orbit.
As Jupiter’s magnetic field shifts, 400,000 volts of electric potential develop across Io’s conductive surface. The current then travels through the ionized plasma to Jupiter, creating massive bursts of lightning.
These electric arcs are so strange and enchanting that they inspired science fiction author Michael Swansick’s “The Very Pulse of the Machine,” a short story about Io’s true identity as a vast sentient computer. (The narrative was later adapted by the animated sci-fi anthology series Love, Death & Robots.)
2. Europa and 3. Enceladus
Jupiter’s 92 moons include Io, Europa, and Ganymede, the largest moon in our solar system. (Credit: alexaldo/Getty Images)
Not one to be upstaged by its hotheaded neighbor, one of Jupiter’s other moons, Europa, is a lot cooler, but no less mysterious. At first glance, Europa looks like a giant ball of ice reminiscent of Pluto: surely, a dead world. But just like Io, Europa is also affected by tidal flexing from Jupiter and is likely volcanically active, according to a 2020 study published in Geophysical Research Letters. What’s more, this action helps renew the crust and pumps out key ingredients necessary for life.
Read More: Astronomers Catch Water Erupting from Plumes on Jupiter’s Icy Moon Europa
Europa isn’t as much of a showoff as Io, but scientists hypothesize it may also have volcanoes. It’s still unclear whether that potential volcanic activity is simply subsurface reactions generating key minerals and nutrients, or if hydrothermal vents like those seen on Earth are spewing out this soup of life. In any case, cracks and grooves on the otherwise smooth icy surface point to activity below. Europa’s surface may even be host to ice-powered cryovolcanoes — icy volcanoes that belch volatile compounds, like methane, instead of lava, created as freezing water expands and generates immense pressures.
Much of Europa’s data comes from the Galileo spacecraft that orbited Jupiter from 1995 to 2003. Fortunately, NASA’s Europa Clipper mission, slated to launch in October 2024, aims to shed further light on this frozen enigma, possibly definitively determining if the world has the potential for life.
Beyond Europa, Saturn’s Enceladus, also an iceball, is another promising option. Observations from the Cassini probe have confirmed tectonic activity and a watery ocean beneath its surface, as evidenced by the moon’s watery geysers. Even Mimas, a tiny moon of Saturn, was revealed to have an ocean in a 2024 study published in Nature.
4. Titan
(Credit: Ianm35/Getty Images)
Speaking of Saturnian satellites, few are as baffling as Titan, which is also the ringed planet’s largest moon. With a gravitational pull only 14 percent as powerful as the Earth’s, it’s a wonder that it’s been able to hold on to an atmosphere over 50 percent denser than ours, composed primarily of nitrogen. Titan’s air is so thick in comparison to its gravity that a human with wings could take off at running speed.
Read More: Titan-in-a-Test-Tube: Earthbound Chemists Search for Alien Life
Titan’s atmosphere also allows the surface to host rivers, lakes and oceans — all containing liquid methane and ethane. These hydrocarbon stockpiles surpass our oil and gas reserves several hundredfold. Beyond that, Titan is unique as the only other place in our solar system with its own weather cycle; liquid methane evaporates, forms clouds and rains back down into rivers.
Still, Titan’s average surface temperature hovers around -290 degrees Fahrenheit, making liquid water on the surface impossible. Of course, that hasn’t stopped Titan from developing its own water-based oceans; they’re just buried deep beneath its icy crust.
From orbit, Titan looks like a hazy ball of bronze, thanks to hydrocarbon particles suspended high in the sky. These solids, called tholins, also accumulate on the surface and form a sort of sand-like plastic. Unlike virtually every other moon, Titan also has seasons thanks to the angle of its orbit.
Read More: Sponge-Like Snow Could Explain Titan’s Magic Islands
Due to its low gravity, radiation-shielding atmosphere, and abundance of hydrocarbon fuels, some astronomers have proposed Titan as a long-term goal for colonization. But until we find a way of hauling life support systems across a roughly 750 million mile distance, the ambition to furnish Titan as a home away from home will remain a moonshot.
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