Jupiter's turbulent atmosphere, Saturn's morphing rings, the tilted axis of Uranus, and Neptune's Great Dark Spot—40 years ago, humanity had a vague understanding at best of the solar system's far-out worlds. And then came the Voyager probes' tour of the solar system.

Voyager 2 actually launched first, on August 20, 1977. Voyager 1, the faster craft, left Earth 40 years ago today—September 5, 1977. With thousands of photographs, Voyagers 1 and 2 revealed the outer solar system in all its splendor for the very first time. But for as much as they told us about the gas giants planets, the legendary probes may have made their most shocking revelations by showing us that our own moon is one of the least interesting natural satellites around.

The Voyager flybys of alien moons led to profound discoveries, including active geology in the frigid outer reaches of the solar system, volcanoes that spew lava hundreds of miles high, and early indications of surface lakes and rivers of flowing hydrocarbons. General bewilderment overwhelmed the scientific community. These rocks were supposed to be dead, frozen worlds. When the opposite was proven to be true, the outer moons suddenly became as important to our understanding of the solar system as the planets themselves, if not more so.

A World of Fire and Brimstone

Linda Morabito found herself on the front lines of discovery by accident. She had landed at NASA's Jet Propulsion Laboratory (JPL) before she finished her undergraduate degree to study the orbits of Jupiter's moons. That work created the navigation strategy for the Voyager mission, and as the probes approached Jupiter, Morabito was working as a navigator for the spacecraft.

Voyager 1 computer color mosaics, shown in approximately natural color, show the Eastern (left) and Western (right) hemispheres of Io. NASA/JPL/USGS

When Galileo discovered Io, Europa, Ganymede, and Callisto, they were nothing more than little dots in the sky, but they provided evidence to challenge humanity's perception of the universe and our place in it. But when the Voyager mission got there, Morabito says, the information was "falling down on us like rain," says Morabito. She barely got any sleep during the Jupiter flybys. The first pictures she saw weren't great—in many cases, they were overexposed—but they were the closest look we ever had of these tantalizing worlds.

On the morning of March 4, 1979, as she was getting ready to go to the lab, Morabito glanced at the television just to see what the scientists were presenting. "I don't think I could have ever anticipated what they were showing. When I saw Io, that wasn't the Io I had been seeing as an overexposed satellite. They talked about this enormous heart-shaped feature on the surface of Io, and I cried! I barely had time to do that, because I was leaving to get to work."

Things got even stranger. When Morabito began examining Voyager 2's images of Io, she could see odd features around the limbs of the moon, but she couldn't quite tell what they were. Then, on March 9, she was looking over post-encounter pictures—images taken while Voyager was looking back over its shoulder as it sped on from Io. Morabito was exhausted, sleep-deprived, and weary from staring at image after image of the same rock floating out in deep space. But no matter how heavy her eyelids grew, Morabito knew immediately when she was the first person in human history to lay eyes on the lava fountains of Io.

The eruption plume of the volcano Loki rises 100 mi (160 km) over the limb of Io. NASA

There was something faint in her pictures of Jupiter's innermost moon—another moon lurking behind Io, she thought at first. But then she remembered the image she had seen on the television five days earlier, the heart-shaped feature, and suddenly it clicked. She was looking at a plume, a volcanic eruption, some 350 million miles from Earth.

It was a seismic shock to the scientific community. Almost 500 million miles away from the warmth of the sun, in the frigid vacuum of space, Io should be a dull, frozen ball of rock and ice. But it turns out that Jupiter's massive gravitational pull tugs and grinds the rock of Io, creating vast lakes and channels of liquid hot magma, and volcanoes that erupt from the surface of the moon out into space. Some lava fountains on Io reach heights of 250 miles or more. Only about a fourth the size of Earth, Io turned out to be the most volcanically active body in the solar system by a significant margin.

The Hydrocarbon World of Wonders

Carolyn Porco joined Voyager in 1983 after working on a dissertation on Saturn. The knowledge gap comparing the days before and after Voyager's Saturn flyby still astonishes her. "We didn't even know there was structure in the rings!"

Titan's thick haze layer is shown in this enhanced Voyager 1 image taken Nov. 12, 1980 at a distance of 435,000 kilometers (270,000 miles). NASA/JPL

Voyager 1 did a lot of work on Saturn and especially Titan, using a remote sensing technique called radio science occultation that measures a moon's physical characteristics. "The spacecraft went behind Titan as seen from the Earth, and its telemetry signal was occulted through the atmosphere," she says. "You could determine a lot: The density profile, the composition, the pressure and temperature of the surface."

The radio signals beamed through Titan's atmosphere by Voyager, received here on Earth, revealed more about the mysterious hydrocarbon world than anyone would have believed. Porco remembers the frenzy that swept through JPL when the first results of Voyager's Titan flyby came down.

"After the occultations I took, the first interpretation of the data by the radio science team was that the conditions of Titan's surface admitted the possibility of liquid nitrogen. Well, this bit just spread like wildfire throughout Building 264, which was where all the scientific teams were gathered for the two-week periods of these flybys. It was like, holy shit. I remember during this episode, all of Building 264 was like the Starship Enterprise and we were the crew."

This Voyager 2 photograph of Titan, taken Aug. 23, 1981 from a range of 2.3 million kilometers (1.4 million miles), shows some detail in the cloud systems on this Saturnian moon. NASA/JPL

A few days later, the science team changed their interpretation—Titan wasn't cold enough for liquid nitrogen, but it might have liquid methane. They would be proven right... eventually. You see, Voyager could not peer through Titan's thick atmosphere, and so the spacecraft was limited to studying the orange haze of Titan and guessing what lurked beneath. But two decades later, the Cassini spacecraft would deploy the Huygens probe to land on Titan's surface, and Cassini conducted extensive radar mapping of Titan as well.

This 21st century mission, long after Voyager had departed, confirmed what many in Building 264 suspected decades ago. Lakes and rivers of hydrocarbons, mostly methane and ethane, do in fact flow on Titan—a world larger than the planet Mercury. Whether life could lurk in these frigid, alien conditions, is a question that tantalizes planetary scientists to this day.

Liquid Water, 900 Million Miles From the Sun

As intriguing as the orange, hazy moon Titan may be, the moon that stuck with Porco is one of Saturn's smaller satellites, Enceladus. Before Voyager, scientists knew that Enceladus was in Saturn's second outermost ring, the E ring, and it wasn't very big. They didn't know much else about the little world.

Voyager discovered the moon's exact size, which is 310 miles across with a 157-mile mean radius, making Enceladus small enough to fit in the Gulf of Mexico. But it did so much more—Voyager's reading began a line of incredible discoveries about this moon that continues today. When Voyager visited the bright little world, it revealed that any ancient craters on Enceladus had been erased. This was quite unexpected, because when "a moon is inactive or dead, it's surface isn't moving, it is covered with craters," Porco explains. "Moons like Callisto of Jupiter are covered with craters. But there are moons that have tectonic features—the surface is cracked in some way if there's sufficient tectonic activity or you get something molten squirted up through a fracture to remove craters." Enceladus, despite everyone's expectations, was completely smoothed over.

This color Voyager 2 image mosaic shows the water-ice-covered surface of Enceladus, one of Saturn's icy moons. NASA/JPL/USGS

The craters, apparently, had been erased by cryovolcanism. "What was imagined was slushy ice, warm enough to squeeze through fragments and flow extensively," says Porco. This slushy water ice, of course, is rare in space. Voyager found the first indications that Enceladus, like Jupiter's moon Europa, could possibly have liquid water below the surface. (It was not until the Cassini mission, decades later, that scientists learned Enceladus has an entire subsurface ocean that envelops the little world.)

Nobody expected Enceladus to be the star of the show, but the discovery of an active world has kept Porco pushing on the door of Enceladus for decades. As a participant in both Voyager and Cassini, Porco has used her clout to become one of the leading advocates for further study of the small moon of Saturn. The images that Voyager beamed back of a pristine icy world, augmented by Cassini observations of blue tiger stripes crossing the smooth white crust, have led Porco to push for more complete exploration of the world.

During the Cassini mission, Porco first called Enceladus, "the Europa for the Saturn system." Europa, which is also believed to have a global subterranean ocean, is generally considered the planetary body in the solar system most likely to harbor alien life. Now, Porco wants to lead a mission back to Enceladus to study the cryovolcanism and active geology to see if it too could be habitable.

The surface of Enceladus is seen in this closeup view obtained Aug. 25, when Voyager 2 was 112,000 kilometers (69,500 miles) from this satellite of Saturn. NASA/JPL

The Voyager missions showed us that extraterrestrial life is not only likely in the vastness of the galaxy, but it could even be right here in our neighborhood, clinging to a moon in the outer solar system. The volcanoes of Io, the methane rivers of Titan, and the waters of Enceladus and Europa all point to the same underlying fact: Even worlds far away from the heat of the sun can be active, warm, covered in liquid, and possibly conducive to life.

But if we're desperate to find life, Linda Morabito says, we need only look up toward the Voyager probes. Scientists who work on the mission are often fond of bringing up each craft's long lifespan and pointing out that, hypothetically, the two spacecraft outlast all life on Earth, carrying a message from humanity on their golden records. Without a doubt, there's life in space, Morabito says. Thanks to Voyager, "we're already there."

To learn more about the Voyager mission, check out PBS's documentary about the spacecraft, The Farthest. The documentary will rebroadcast on PBS on 9/13 at 10/9c, or you stream it for free here.

This content is created and maintained by a third party, and imported onto this page to help users provide their email addresses. You may be able to find more information about this and similar content at piano.io