More than 20 years ago, a spaceship from Earth reached Saturn, the ringed giant. Called Cassini, its voyage took seven long years. For the next 13 years, it circled the planet, taking pictures and collecting data. Those images showed details of the planet’s many moons. Cassini even discovered seven new ones.
On Christmas Day 2004, Cassini sent a smaller ship — called a probe — to explore Titan, Saturn’s largest moon. This craft parachuted through Titan’s hazy atmosphere. During that hours-long descent, the probe collected data to send back to Earth. It recorded the temperature of the air and the speed of the winds. It took pictures of the moon’s surface. Later, it analyzed the hard stuff that it landed on.
From above, Titan looks much like Earth. It has a thick atmosphere with clouds and smog. “It’s the only icy moon with an atmosphere,” notes Elizabeth Turtle. A planetary geologist who studies such distant worlds, she works at the Johns Hopkins Applied Physics Laboratory in Laurel, Md.
Wind whips across Titan’s solid surface. Dunes — some of the tallest in the solar system — rise from big deserts near its equator. Rain falls from the sky. Riverbeds streak across the surface and empty into lakes and giant oceans. It’s the only place in the solar system besides Earth where scientists have found flowing liquids.
Yet it’s also very alien.
Less than half Earth’s size, Titan has a surface temperature around -179° Celsius (-290° Fahrenheit). Like Earth, its atmosphere contains mostly nitrogen. Unlike Earth, there’s no detectable oxygen. Someone walking on Titan would freeze to death if they didn’t suffocate first.
Titan — Saturn’s biggest moon — has oceans, coastlines and mountains like Earth. Still, its chemistry is very different from Earth’s. Astrobiologists now want to know: Can anything live there? University of Arizona/JPL/NASA
What appears to be a rocky surface is solid ice. The stuff that flows through its rivers is not water but methane and ethane. On Earth, these chemical hydrocarbons exist as gases. But because Titan’s so cold, they’re liquid here.
And those large dunes? They’re not sand. They’re grains of hydrocarbons frozen in ice. They look more like coffee grounds or soot. Also unlike Earth, Titan probably has a liquid ocean beneath its icy crust.
Titan is similar enough to Earth that scientists already have plans to return and explore more. In 2028, NASA plans to send a robocopter, called Dragonfly. This giant drone will hover and skitter over Titan’s surface as scientists seek a better understanding of the super-frigid world and look for signs of alien life.
Our solar system is full of such strange and surprising worlds. Humans have explored few of them. Planets get plenty of attention in schools and news articles. Earth’s moon does, too. But not these distant celestial satellites.
Scientists will be taking a closer look at many of our solar system’s moons in coming years. And there are plenty to choose from.
So many moons
According to NASA, 298 moons have officially been recognized throughout our solar system. But that only includes those around the major planets and Pluto. Astronomers have identified more than 400 moons orbiting asteroids and “dwarf” planets. And the International Astronomical Union, which tracks many celestial objects, no longer counts or names new ones unless they’re deemed especially important.
Mars has two moons. Astronomers have spotted and named 146 moons around Saturn. Jupiter has 95 — but those are only the ones with names. Thousands of smaller space rocks also orbit it. Neptune has 16 known moons. They all take their names from sea gods and creatures in Greek mythology.
Uranus has 28 moons, named for characters in famous plays and poems. One of them, Miranda, is only one-seventh as large as Earth’s moon but has canyons 12 times as deep as the Grand Canyon. Because the biggest is so deep and Miranda’s gravity is so weak, a rock thrown from the canyon’s top would take 10 minutes to reach the bottom.
This 10-minute video focuses on Jupiter’s moons, including many less well-known oddities. It’s a bit out of date — only 80 of this planet’s now 95 known moons were recognized when this video was made.
Pluto, which was demoted to a minor planet in 2006, hosts five moons. Charon, its largest, is about half Pluto’s size. Charon has long been a mystery. Scientists think most moons began as big space rocks that either collided with a planet or were captured by its gravity. Some moons, like Jupiter’s four biggest, may have formed from the gas and dust left over from the planet’s formation. But Charon is too big to fit those origin stories.
In January, astronomers presented evidence for a new idea. They think Charon and Pluto came together when the two worlds “kissed” — that is, collided and didn’t break apart. Over time, the bodies drifted apart. Then they settled into a linked pair as host and moon.
Venus has no moons, but it does have Zoozve. This asteroid serves as a quasi-moon. It appears to orbit Venus, but its orbit is unstable. One day, the gravity of Venus and the sun will fling Zoozve out into deep space. Earth has quasi-moons, too. There are seven known asteroids that orbit the sun but from one vantage point seem to orbit Earth.
Once asteroid 2002VE68 was trapped by the gravity of Venus, this body began orbiting the planet in the irregular pattern shown here. Zoozve is not a true moon but a temporary orbiter that eventually will be kicked off into interstellar space.Phoenix7777/HORIZONS system/JPL/NASA
Why reach for these moons?
Dragonfly isn’t the only upcoming mission that will visit a far-flung moon. The European Space Agency and NASA both have craft on their way to study Jupiter’s moons. The Japanese Space Agency plans to land a ship on the Martian moon Phobos and have it bring back samples.
Scientists hope these missions will help us get to know our solar system better. Moons may contain rock or ice, or a mixture of both. Some may have liquid water or liquid methane. On some moons, plumes blast water vapor hundreds or thousands of miles into space.
Others are explosive — like Io, a volcanic moon of Jupiter. Or they can be quiet and dark. Two of Pluto’s moons look like giant footballs spinning in space. Some look round; others lumpy. In studying this variety of worlds, scientists hope to answer big questions, such as how these worlds and their parent planets formed in the first place.
But there’s another question behind all this exploration: Are we alone in the cosmos? Indeed, if life does exist elsewhere in the universe, these moons may be the best, nearest places to find it.
This short video shows the design of Japan’s Martian Moons eXploration (MMX) spacecraft, now on its way to photograph and collect samples from Phobos.
How distant moons came into focus
Ancient civilizations believed the moon that orbits Earth was a deity. Chang’e was the Chinese goddess of the moon. According to Chinese mythology, she stole a drink that would enable her to live forever, then fled to the moon. In Greek mythology, Selene is the goddess of this moon. Her name is Luna in Roman mythology. Khonsu was the ancient Egyptian god of the moon.
Until hundreds of years ago, our moon was the only one people could recognize. The rest, if they were visible at all, were but faint lights in the sky. One night in January 1610 that all changed. The Italian astronomer Galileo Galilei turned a homemade telescope on the sky and spied bright dots near Jupiter. He thought they were distant stars. But he was wrong. After studying them for many nights, he realized they were four smaller worlds circling the giant gas planet.
Galileo’s observation showed that smaller worlds orbited bigger ones. This supported the heliocentric theory of the solar system. It holds that Earth orbits the sun, and not the other way around.
Here’s a short animated tour of Io, showing volcanic plumes and a cutaway of its fiery interior that likely drives those eruptions.
These four moons of Jupiter are still known as the Galilean moons. Hundreds of years later, astronomers settled on names for each: Io, Europa, Ganymede and Callisto. Of those, Ganymede is the largest — and reigns as the solar system’s biggest known moon. In fact, it’s bigger than Mercury and Pluto. It’s also the only moon known to have a magnetic field.
In the 400 years since Galileo, telescopes and technology have improved. So have scientists’ ideas about the architecture of the solar system.
The inner planets — Mercury and Venus — lack moons. Their gravity isn’t strong enough to compete with that of the sun. The heft of our sun would hurl any potential moons there deep into space.
How Earth got its moon
Planetary scientists think our moon formed when a Mars-sized chunk of rock smashed into Earth. The rocky mess left over from that collision likely formed the moon. Scientists suspect that the moons of Mars were once asteroids. The Red Planet’s gravity likely kidnapped them as were passing by.
Jupiter and beyond
More distant planets are larger and farther from the pull of the sun. Moon formation gets more complicated in the hinterlands. Many of Jupiter’s larger moons, such as Europa, probably formed from the big disk of dust and debris that was left after the gas giant formed. Jupiter has so many moons that it looks like a mini solar system.
(Scientists also believe that in some cases, moons might have moons. The scientific name for such a body is moonmoon.)
The last century or so has brought in a flood of new data on moons. Much of that has come from missions over the last 50 years. That’s especially true for the moons of Jupiter and Saturn.
“They’ve gone from lights in the sky to real, working, geologic worlds,” says Diana Blaney. She’s an astronomer at NASA’s Jet Propulsion Laboratory (JPL) near Pasadena, Calif. (Astronomers are reluctant to identify any one moon as their fave. But Blaney admits that “Europa and the Galilean system have always been close to my heart.”)
In the early 1970s, NASA sent two missions to the outer planets. Called Pioneer 10 and 11, they sent back photos of Jupiter and blurry pics of its big moons. In the late 1970s, the Voyager 1 and 2 spacecraft sent back even better images. One standout was Jupiter’s moon Europa. These pictures revealed a white world crisscrossed with reddish streaks. Astronomers compared it to a broken eggshell.
Pioneer 10 (seen in this artist’s drawing) was one of the first spacecraft to explore the outer planets (those beyond Earth) and their moons. NASA
Scientists now suspect those patterns have an exciting origin. Europa may have a liquid ocean beneath its icy crust. Cracks in the crust let water rise and freeze. Like a Zamboni machine at an ice rink, the moon may refresh its surface with liquid water. Those processes might make it possible to host life on this moon.
More missions followed. In 1989, the Galileo mission returned to orbit Jupiter and its big moons. Cassini launched in 1997. Both missions had a dramatic finale when they ran out of fuel. Galileo crashed into the thick atmosphere of Jupiter. Cassini plunged into Saturn. This was intentional. If they’d accidentally crashed onto a moon, the craft might have contaminated it for future studies.
In April 2023, the European Space Agency launched a spaceship toward the moons of Jupiter. It’s due to arrive in 2031. And in October 2024, NASA launched a ship called the Europa Clipper. Starting in 2030, it will make nearly 50 flybys of Europa.
Clipper isn’t designed to look for aliens. It will, however, study whether conditions on Europa could support some form of life.
NASA’s Europa Clipper will orbit Jupiter and conduct 49 flybys of its moon Europa to study its habitability and geology. Scientists think a liquid ocean may hide beneath its icy surface.
Studying moons “changes our perspective on what makes a habitable world,” explains Lynnae Quick. She’s a planetary geophysicist working for NASA at the Goddard Space Flight Center in Greenbelt, Md. While Europa is her favorite moon, she’s interested in others. She’s an expert on cryovolcanoes. They blast out gases and chemicals, not just melted rock. She’s got a hand in planning the Dragonfly mission to Titan, too.
Scientists also study moons with telescopes. Those include the Hubble Space Telescope and the James Webb Space Telescope. Both have captured stunning images of these alien worlds.
Every time humans explore some place new with better instruments, “we have surprised ourselves,” says Blaney, who helped design some of Clipper’s devices. “I think we’re going to get there and be really surprised at what we see. We’re going to have to rethink a lot of what we think is going on.”
Creating alien moons in the lab
The study of distant moons combines many scientific areas. One of those is planetary science. Researchers in this field want to understand how these moons form and change. The extreme gravity and radiation around Jupiter have shaped its moons.
On Io, which orbits closest to Jupiter, those forces smash the moon’s rocky layers together. These melt and form magma. It bursts through the surface in more than 400 known volcanoes. The hot liquid rock reaches 1,000 °C (1,832 °F).
Astrobiologists are also interested in these moons. These scientists study how life might arise outside Earth. Among them is Osama Alian (whose last name is pronounced “alien”). He studies habitability and the origins of life at JPL. He won’t admit to having a favorite moon. He doesn’t want to be biased, he says.
Let’s learn about Jupiter’s moons
“But I am partial to Enceladus because its plumes are cool,” he says.
Enceladus orbits Saturn. It’s tiny — only about 1/25 the size of Earth. But it shoots up big plumes. In 2005, the Cassini mission flew through one of these plumes. (Quick, who works on Clipper and Dragonfly, says the Cassini mission got her hooked on moons. “We saw these big, beautiful plumes erupting from the South Pole.”)
Cassini detected water, ice and organic particles. An organic material contains carbon. Those are exciting because on Earth, every living thing contains carbon. “Complex carbon molecules don’t happen very easily on their own,” Alian says. “So any organics that we detect would be super exciting to analyze.”
In 2023, images from the James Webb Space Telescope showed that these plumes can reach 9,700 kilometers (6,000 miles). That’s nearly 20 times the width of the moon itself.
After studying biology in college, Alian became interested in planets. Astrobiology brought these two interests together. Now, he works in a lab studying hydrothermal vents. These are cracks in Earth’s ocean floor that release plumes of hot water. Unusual forms of life thrive there. Some biologists think life began in hydrothermal vents.
Alian is currently designing an experiment that will act like a hydrothermal vent, but in the lab. Using it, he and other scientists can adjust conditions — like minerals, temperature and composition of the water — to see how these can affect living things.
Earth’s vents support life. Hydrothermal vents may exist on other worlds, too. The Cassini mission found evidence that similar vents may churn on the floor of the underground ocean on Enceladus. Missions to Mars have turned up chemicals seen at vents, too. And maybe Europa, orbiting Jupiter, has plumes where minerals, water and heat mix.
Scientists can’t wait to see what the missions and telescopes of today and tomorrow reveal about the exotic moons in our solar system. They can help answer questions about the many ways that worlds form. They may also point to how life begins.
“If we see this thing on Earth, you can’t help but wonder,” Alian says. “It has to be happening elsewhere.” Titan, for instance, may host conditions similar to Earth just before life emerged.
