Strange things are afoot on Saturn’s largest moon.
Alone among solar system satellites, Titan has sand dunes, lakes, rivers, rain, clouds, rainbows, and a thick nitrogen atmosphere. It even possesses organic molecules and possibly an ocean of salt water. Yet all these familiar features are twisted into unfamiliar and unearthly shapes. Methane takes the place of water in the atmosphere, lakes are made of hydrocarbons, and the ocean lies beneath a thick crust of ice. Even the composition of Titanian sand is currently unknown.
Titan’s similarity to Earth and its unique features have spurred scientific curiosity for decades.
In April 2024, NASA approved Dragonfly, a mission consisting of a flying robotic drone with four pairs of rotors to carry it from place to place on the Titanian surface, landing to perform measurements before flying to the next site. NASA plans to launch the craft in 2028, with an anticipated arrival at Titan in 2034.
The drone will carry cameras, a drill to sample surface ice, and a suite of instruments to identify molecules in the atmosphere and on the ground. In addition, it will deploy seismometers, weather monitors, and a microphone to listen to the wind.
For many, including Dragonfly principal investigator Elizabeth “Zibi” Turtle of the Johns Hopkins Applied Physics Laboratory (APL), a drone was the obvious choice for Titanian exploration. (Other options included wheeled rovers like those used on Mars and a boat to explore one of Titan’s lakes.)
“Flying from place to place rather than driving across the surface gives us lower risk and much more flexibility in terms of being able to explore different environments,” Turtle said.
The Buzz on Titan
Titan’s atmospheric and surface characteristics lend themselves to flight-based exploration. The moon is larger than the planet Mercury but has less-dense rock and more ice (composed of water and other volatile materials such as methane), so its surface gravity is less than our Moon’s. In addition, Titan’s atmospheric pressure is 1.5 times that of Earth. These properties mean it is physically easier to fly on Titan than on Earth or Mars, Turtle said.
Building a drone to explore the surface is not only practical but desirable: Even with Dragonfly spending most of its time on the ground, flying between sites will enable it to take measurements in far more locations than a rover could.
The Dragonfly team has to take every advantage because Titan is not an easy place to visit. Power generation and temperature are major issues. Not only is it sufficiently far from the Sun that solar-powered missions would need huge panels to operate, but the thick, hazy atmosphere blocks most sunlight from reaching the surface. Those factors also mean Titan is cold: a near-constant 94 K (−179°C or −290°F) at the surface. And because the Saturn system is so far from Earth, Dragonfly must be able to stay warm, navigate, and collect samples almost entirely autonomously.
Nevertheless, the extreme conditions are part of what makes the moon so scientifically intriguing.
Why Does Titan Have an Atmosphere?
With other moons of comparable size in the solar system, why is Titan the only one with a substantial atmosphere? For Sarah Hörst and other planetary scientists, the better question is, “Why doesn’t Jupiter’s moon Ganymede have a similar atmosphere?”
According to models of solar system formation, many large moons did start off with atmospheres. But Ganymede is closer to the Sun than Titan is, and that solar radiation gradually stripped molecules away from the moon’s atmosphere, leaving it the mostly bare sphere of ice we observe today. Titan was spared a similar fate by being too cold.
“There isn’t one specific thing about Titan that’s interesting,” said Sarah Hörst, a planetary scientist at Johns Hopkins University. “All of these processes that happen every day on Earth are also happening every day on Titan,” she continued, referring to Earth’s water cycle, weather systems, and surface-shaping processes.
For Hörst, Titan’s intrigue comes from the juxtaposition of the familiar with the alien.
“Being able to study all of those things in context with each other is really powerful for trying to understand some of the biggest questions that we have in planetary science: about the origin of life, about the search for life elsewhere in the universe, but also some of the smaller-scale questions too,” Hörst said.
Although Dragonfly is not specifically an origins-of-life mission, Titan’s chemistry is similar enough to that of early Earth to be intriguing. The moon’s yellow-orange atmospheric haze—and possibly the sand on its surface—contains organic molecules and hydrocarbons. Along with water, these are the chemical necessities for life as we know it.
“Anywhere there’s liquid water on Earth, there is life,” Hörst said. Titan’s liquid water is beneath the surface—as is the case on Jupiter’s moons Europa and Ganymede, Titan’s fellow Saturnian moon Enceladus, possibly Neptune’s moon Triton, and the dwarf planet Pluto. If life follows liquid water, it’s quite possible that icy worlds such as these are more common harbors of life than warm, green planets like our own.
Voyager, Boats, and Balloons
Methane was first identified in the Titanian atmosphere in 1944 by astronomer Gerard Kuiper. Although telescopes of the time weren’t able to determine the moon’s total atmospheric composition or thickness, the very presence of an atmosphere led scientists to propose Titan missions during the heady days of planetary exploration in the 1970s, many of which included balloons and other lighter-than-air craft. None of those early designs came to fruition, but they certainly inspired Dragonfly’s design concept.
Pioneer 11 was the first probe to fly past Titan, in 1979. Its findings spurred NASA researchers to divert the path of Voyager 1 to make its own flyby in 1980, providing the first real data on the density and composition of the moon’s atmosphere. (Voyager 2 also got a peek at Titan but could not survey it without sacrificing its chance to visit Uranus and Neptune.) Voyager 1’s flyby could not image the surface at all, leaving scientists’ imaginations free to operate.
In 2000, APL aerospace engineer Ralph Lorenz (now the mission architect for Dragonfly) remembers he “was trying to figure out how much power you would need to push an airship around on Titan [when] I got looking at aerospace vehicles in general and recognized that actually, a helicopter would be a great platform,” he said. “Airships and hot-air balloons are great for champagne breakfasts on your wedding anniversary if you know the weather is going to be good. But if you want to go any place and at a time of your choosing, on Earth you do that by helicopter.”
The technology for autonomous drones without something akin to GPS guidance, however, simply didn’t exist in 2000.
Meanwhile, the joint European Space Agency–NASA Cassini-Huygens mission arrived at Saturn in 2004, with the Huygens lander specifically designed to study Titan. Because some research suggested Titan’s entire surface could be covered in an ocean of liquid methane, ethane, or other hydrocarbons, mission scientists and engineers were unsure whether there was dry land for the Huygens probe to explore. That meant this “lander” was more of a “crasher.”
Huygens was instrumented primarily to study the atmosphere during its 2-hour descent. It had a parachute and was designed to float but had little cushioning and no retrothrusters.
Regardless, Huygens provided humanity’s only image from the moon’s surface in 2005: an orange-brown sandy plain strewn with rounded boulders. During its descent, the probe captured views of hydrocarbon-carved valleys and river deltas.
The Cassini orbiter provided maps of and details about Titan’s weather during multiple flybys between 2004 and 2017, as well as evidence for a possibly global water ocean beneath the surface.
Old Idea with New Technology
Cassini-Huygens was so successful that NASA included further Titan missions in its long-term list of priorities. Some of the older probe ideas got updates, whereas others were rejected. (As Lorenz noted dryly in a 2009 article, “A chemical-fueled hot air balloon was just never a good idea.”) New ideas included the Titan Mare Explorer (TiME), a floating lab dropped into the lake known as Ligeia Mare. TiME didn’t end up being accepted by NASA, to the chagrin of many researchers.
As NASA revisited ideas about missions to Titan, it also reconsidered the possibilities offered by uncrewed aerial vehicles. Autonomous drone technology had undergone revolutions in efficiency, control, and miniaturization since Lorenz and his colleagues proposed the concept more than a decade earlier. They realized the 2000 Titan helicopter idea could be modernized, leading to development of the Dragonfly probe. Meanwhile, the Ingenuity helicopter, part of NASA’s Perseverance mission to Mars, demonstrated multiple successful autonomous flights, helping convince skeptics of Dragonfly’s feasibility.
“Having had a decade plus of rovers executing science on Mars set the stage and got people thinking.”
“Having had a decade plus of rovers executing science on Mars set the stage and got people thinking,” Lorenz said, noting that Titan is substantially more difficult to navigate than the Red Planet. “You need to figure out how to operate in a cryogenic environment [and] fly in a dimly lit environment with a hazy atmosphere. It just seems overwhelming, but then the engineers’ minds kick in, and they realize that each of these challenges is actually quite soluble.”
“We’re really fortunate in that most of the technologies exist in some form for exploring other planets, including autonomous flight,” Turtle said. She also listed cameras, instruments for measuring gamma ray spectra, seismometers, meteorology equipment, and other apparatuses as being well tested on Mars or elsewhere. “We have to make sure that [the equipment] can function in a Titan environment, but it exists.”
The Curiosity and Perseverance rovers also provided off-world success stories for the radioisotope generator Dragonfly will use. This device converts the radioactive decay of plutonium-238 into about 100 watts of electricity, which charges the lithium-ion battery that will power the probe’s flight and scientific instruments. Titan takes about 16 Earth days to orbit Saturn, and because it is tidally locked with the planet, for 8 days at a time Dragonfly will be on Titan’s farside—an ideal time to recharge because no data can be sent back to Earth. A full battery charge would enable the drone to fly a few kilometers before touching down again, so Dragonfly could conceivably fly from one site to another once per Titanian day.
A Titanic Undertaking on Earth
Ensuring scientific equipment can operate under Titan’s conditions is no small undertaking.
The moon’s low gravity is impossible to simulate on Earth, and parabolic airplane flights such as those used to train astronauts are not practical for testing the car-sized probe.
Many atmospheric conditions, however, can be simulated by replicating them exactly or adjusting pressure and temperature.
To that end, APL engineers built two special chambers to test Dragonfly’s instruments under a variety of conditions. The Titan Pressure Environment Chamber is the smaller of these, and though it can’t fit the whole probe inside, the chamber provides the proper −179°C temperature and 1.5 atmospheres of pressure to test its various instruments.
“We can actually test the drill that will do sampling on Titan under Titan conditions to make sure you can break up water ice and get enough material to measure with the mass spectrometer,” Turtle said.
The larger Titan Chamber is a cube roughly 4.5 meters on each side. It’s big enough to perform tests on Dragonfly itself or, because the whole probe isn’t built yet, a full-scale mockup of the probe. The chamber goes through an impressive 750 liters of nitrogen per hour, fed from a tank outside the building that’s bigger than some small-town water towers. The interior is wired with hundreds of sensors and cameras to track everything from temperature to pressure and airflow.
Much of the chamber’s role is to check heat transfer in and out of the probe. Titan’s thick air doesn’t move much: a few meters per second at most, which is comparable to a brisk jog. “If you’re designing to stay warm [with] a couple meter-per-second breeze and you have a day with no wind…you can get too hot,” Turtle said. “Amazingly enough, there are scenarios in which we could actually get too hot on the surface of Titan, which is kind of incomprehensible!”
Because wind results from the interaction of variables such as temperature, pressure, and gravity, to simulate Titan-like breezes under Earth’s gravity, the chamber is kept at Titan temperatures but at 55% of Earth’s atmospheric pressure.
Nearly every part of the probe needs to be covered to prevent all the internal heat from escaping, but to collect scientific data, the entire thing can’t be swaddled up. Titan Chamber tests enable the Dragonfly team to place insulating foam for optimum operation.
Haze, Rocks, and Alien Sand
All of this testing and preparation, of course, is to ensure Dragonfly can do science on Titan.
No matter what it finds, the probe will provide many firsts in planetary science: first mobile geological exploration of an icy moon, first up-close surface observations of a world that has a subsurface ocean, and first detailed chemical experiments on organics on another body.
In addition, Dragonfly’s landing site is adjacent to Selk Crater, which has many researchers excited. As with Jezero Crater on Mars (Perseverance’s landing site), Selk exposes multiple layers of material on Titan’s surface.
“Titan is special because it has surface processes operating that other moons do not.”
“Titan is special because it has surface processes operating that other moons do not,” said Adeene Denton, a planetary geologist at the Southwest Research Institute in Boulder, Colo. Though Denton is not part of the mission, their interest in craters across the solar system means they will be using results from the probe.
“I’m interested in how what’s going on on the surface connects with the Titan subsurface. If there’s liquid methane on the surface, then theoretically, there’s a ground methane system,” Denton explained, but “nobody really knows!”
Dragonfly will also be flying around outside Selk, in the dune fields that dominate much of Titan’s equatorial region. Turtle explained that these dunes appear to behave similarly to those on Earth—particularly dunes in the Namib Desert in southwestern Africa—but the sand itself is chemically very different: It contains yet-to-be-identified organic compounds.
Part of identifying Titan’s organic compounds is understanding how these molecules form and evolve. This is where chemist A’Laura Hines comes in. A Ph.D. student at George Mason University in Virginia, she jumped at the chance to join the Dragonfly project as part of the NASA Guest Investigator Program.
The type of very cold chemistry occurring on Titan is not widely studied, Hines said. “We don’t have the experimental backlog to be able to talk about how these organics we’re familiar with on Earth interact in these super cold environments at high pressure.”
Hörst’s lab group at Johns Hopkins is doing their part to simulate the Titanian atmosphere and its interactions with the surface. Among other research, the group’s experiments produce reddish-brown organic grains (like those seen in Titan’s atmospheric haze) in a small simulation chamber made in part from a converted beer-brewing tank.
“Titan is fundamentally different than most of the worlds that we have ever studied in detail because the atmosphere and the surface are incredibly closely tied together,” Hörst said, pointing out that even Earth’s hydrologic cycle involves far fewer chemical interactions than the exchange between air and ground on Titan.
For instance, the methane in the moon’s atmosphere gets broken down by sunlight, converting it into hydrogen that escapes into space and ethane that falls to the surface. Somehow, new methane must also be produced because even the tiny amount of sunlight reaching Titan would have destroyed what was present at the moon’s formation 4.5 billion years ago.
In other words, understanding the surface composition of Titan requires studying the atmosphere and subsurface processes. Learning about the ocean beneath the crust means drilling into icy rock and scooping up surface materials.
A Cosmic Laboratory
What planetary scientists know so far about Titan suggests that its air, seas, surface, and subsurface are as interconnected as their counterparts on Earth. Titan’s surface-atmosphere interactions, tidal-driven quakes, and methane “groundwater” springs demonstrate that every part of the moon is connected to the others in a way that makes Dragonfly an incredibly cross-disciplinary project. It’s also a project the team is especially devoted to: Hörst has a Dragonfly-themed license plate, and Hines named her car Titan.
There is so much potential for scientific discovery on Titan that even the probe’s biggest boosters know Dragonfly can’t do it all. For one, Hörst pointed out that Dragonfly isn’t designed as a life detection mission, though some of its onboard experiments might be able to detect life if it’s close enough to the terrestrial version.
However, each new world humanity has studied has revealed something new about our cosmos. Samples returned from asteroids Ryugu and Bennu contained the building blocks of life. Explorations of icy moons and dwarf planets showed that across the solar system, cryovolcanism reshapes and revitalizes cold worlds. The extreme combinations of chemistry, pressures, and temperatures inside the gas and ice giant planets create exotic chemistry unseen on Earth. Lessons learned from each of these words will provide much-needed context for understanding what we find in the unique cosmic laboratory that is Titan.
Even if Titan—the most Earth-like world in the solar system—is lifeless, it promises to teach us something about the chemistry that made life on Earth possible more than 4 billion years ago.
—Matthew R. Francis (@BowlerHatScience.org), Science Writer