Our solar system is home to more than 200 moons. All the major planets, apart from Mercury and Venus, have moons, and dozens orbit around giants like Saturn and Jupiter. And while Earth's moon is undeniably a wonderful thing, other moons in our solar system are downright wacky in comparison.
Io, one of the four Galilean moons of Jupiter, is our solar system’s most volcanically active object. Gravitational pulls from its fellow moons and Jupiter itself can make its solid surface swell by as much as 330 feet. The resulting frictional heating creates a massive amount of magma—and more than 400 active volcanoes. When one of these giants erupts, the lava fountains can reach dozens of miles into the thin atmosphere, with plumes sometimes shooting 300 miles high.
Of Neptune’s 13 moons, Triton is the largest—and it’s a real oddity. Triton orbits in the opposite direction of Neptune’s rotation, making it the only large moon within our solar system with a retrograde orbit. Because of this, scientists believe Triton originated in the Kuiper Belt and was snagged by the Neptune's gravity millions of years ago.
Triton is also extraordinarily cold. It’s so frigid, much of its nitrogen-rich atmosphere is actually frost. When Voyager 2 passed by Triton in 1989, things only got stranger. The spacecraft captured images that revealed plumes of icy material spewing from the moon’s surface. The cause of this activity remains a mystery. NASA’s Trident mission aims to find out more, with a proposed launch date in October 2025.
Jupiter’s Ganymede is the solar system’s biggest moon—it’s more massive than Mercury or Pluto. The giant satellite is also the only known moon with its own magnetic field, which in turn causes auroras at the north and south poles. Strong evidence suggests an underground ocean could be hiding below Ganymede’s icy surface. Some scientists believe Ganymede might have distinct ocean layers split by different types of ice. In theory, the layers could support life that has adapted to each ocean’s specific conditions.
The smallest and innermost of Saturn's major moons has an ominous look about it. Visually, the pocked moon is dominated by the 81-mile-wide Herschel crater. The impact that caused the crater almost shattered Mimas entirely. It also left it looking worryingly like the Death Star from Star Wars, prompting many “That’s no moon” jokes.
Europa is the smallest of Jupiter’s four Galilean moons. Besides Earth, Europa is generally considered the most promising place within our solar system to support life, rendering it the subject of various proposed missions. Scientists believe there could be a 40– to 100-mile-deep ocean lying beneath the moon’s water-ice crust.
Saturn’s Enceladus largely remained a mystery until Voyager 1 and Voyager 2 passed nearby in the early ‘80s. The bright, icy satellite was then surveyed in greater detail by the Cassini spacecraft in 2005. It revealed a moon far more interesting than initially thought, with jets of water vapor and ice particles bursting from the surface. The fact that Enceladus hides an ocean of salty water below its surface means it may have the potential to support life.
The Ringed Planet's third largest moon is a strange-looking satellite. It’s sometimes called Saturn’s yin and yang moon due to its two distinct hemispheres: one dark and one bright. Iapetus also features a massive equatorial mountain range. This ridge hugs more than 75 percent of the moon, with peaks as high as 12.4 miles. Some scientists theorize the ridge was formed by debris from a sub-satellite that once orbited Iapetus until the moon’s gravitational pull tore it to pieces.
Saturn's largest moon is the most Earth-like body in the solar system. Like Earth, Titan’s atmosphere is dense and made primarily of nitrogen. This moon is the only object apart from our home planet to feature rivers, lakes, and seas on its surface. Titan also has an Earth-like water cycle, complete with evaporation, rain, and clouds. That doesn’t mean it’s a nice place for a vacation: The surface temperature is a chilly -179° C, and the rivers and lakes are made of liquid methane and ethane. The source of the methane remains a mystery, and has prompted discussions about the possibility of methane-based life on Titan.
Charon is Pluto’s largest moon—the gray satellite is almost half the size of the dwarf planet. The two bodies are only 12,200 miles apart, and they spin around a center of gravity that actually lies outside of Pluto. Because of this deep connection, Pluto and Charon are considered to be the solar system’s only known example of a double planetary system.
Spare a thought for Phobos, the larger of the two Martian moons. The irregularly shaped moon orbits the Red Planet three times a day. Phobos's quickly orbiting days are numbered, however, as it inches 1.8 centimeters closer to Mars each year. In around 50 million years, the moon will either collide with the planet or be torn to pieces.
Despite being Jupiter’s second-largest moon, Callisto was long thought to be “a dead and boring moon, just a hunk of rock and ice.” But in 1998, new data from NASA's Galileo spacecraft suggested there could be something hidden beneath Callisto’s surface, possibly a salty ocean and the ingredients for life. Any future missions to explore this theorized ocean won’t be easy.