THE sky is orange and dusk-dim, as ever, and a ringed gas giant looms large in the distance. From the shore, the jet-black sea doesn’t show so much as a ripple.
This is Kraken Mare, a vast hydrocarbon sea spreading some 400,000 square kilometres across the northern polar region of Titan. Saturn’s largest moon is so far from the sun and so achingly cold that there can be no liquid water on its surface – and so nothing like the chemistry that sustains life on Earth. Yet the more we learn about the weird chemistry that does exist there, the more this frozen world looks like the best place in the solar system to look for truly alien life.
“We have tonnes of places to look for life that’s based on liquid water, but we only have this one place to look for life that’s diverse,” says , a planetary scientist at Johns Hopkins University in Maryland. “And to look for that life, we have to go there.”
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It will take an audacious mission to reach Titan. But that is exactly what is in the works. The plan is to send a souped-up, sensor-packed drone to hopscotch across its surface in search of life not at all as we know it.
The Dutch astronomer Christiaan Huygens was the first to spot Titan in 1655, but most of what we know about its majestic weirdness comes from NASA’s recent Cassini mission. In 2005, it sent the Huygens probe hurtling through the clouds to touchdown on Titan’s surface. The lander survived for a few hours, just long enough to send back 350 grainy images and a trickle of data before blinking out of contact. For the next 10 years, Cassini zipped around the Saturnian system, relaying remarkable insights every time it passed Titan.
Although this moon is about 1.4 billion kilometres from Earth, it looks strangely familiar (see “Worlds together”). It has a dense atmosphere, albeit with precious little oxygen. It boasts mountain ranges with peaks rising 3300 metres into the haze, and clouds that release seasonal downpours that flow into lakes and seas. Some, like Kraken Mare, are big enough to be visible from orbit. In fact, Titan is the only place in the solar system besides Earth known to have liquids on its surface.
With Titan’s temperatures never getting warmer than -180°C, however, every last drop of its surface water is ice. Its streams flow with methane and ethane – usually gases on Earth, but dark, oily liquids in these frozen climes. And its nitrogen-heavy skies produce various carbon-containing organic compounds. In many ways, Titan is like an alternate-universe version of home, with the same features, but very different chemistry.
Liquid water is essential for life on Earth because it offers an ideal medium for chemical reactions and, as an effective solvent, an easy way to transport molecules within and between cells. Titan has none – at least not on its surface. Its suspected subsurface ocean might spew some out in occasional eruptions from ice volcanoes, and its icy ground could be melted by asteroid impacts from time to time. But even then, any liquid water wouldn’t be able to interact with organic molecules on the surface for long before freezing solid.
Second life
It might be possible, however, to base life on other liquids. “Humans are mostly water, but the most interesting parts, the hard parts, are organic chemistry,” says , also at Johns Hopkins. And although Cassini was deliberately crashed into Saturn last year to avoid the risk of contaminating its moons, the latest discoveries suggest that Titan has plenty of that, albeit in exotic forms.
In July 2017, a team led by at NASA’s Goddard Space Flight Center in Maryland reported the serendipitous results of a deep dive into data gathered by a cluster of telescopes in Chile called ALMA. The dishes would ultimately aim further afield, at stars beyond our solar system and distant galaxies, but first they were trained on Titan for calibration purposes. In the process, they picked up a clear signature of a compound called vinyl cyanide.
Here on Earth we synthesise this stuff, also known as acrylonitrile, to make acrylic fibres, synthetic rubber and plastics used in everything from cars to food packaging. In Titan’s frigid hydrocarbon lakes, however, it could form structures equivalent to the membranes that hold together the cells of most living things on Earth. Computer simulations performed in 2015 by and colleagues at Cornell University in New York suggest , vinyl cyanide could form strong and stretchy structures similarly to the way our own lipid membranes do in room-temperature water.
“Gravity is so weak on Titan that you could probably fly just by flapping your arms”
No one has yet proved that for real by making membranes from vinyl cyanide in the laboratory. But the idea that it is possible is made all the more intriguing when you consider how much of this stuff there seems to be on Titan. Palmer and her team estimated that enough of the compound could rain down into one of its largest seas, Ligeia Mare, to form up to 30 million cell membranes per cubic centimetre of liquid.
“This makes us slightly more optimistic about the chances for life on Titan,” says Palmer. “Of course, a big question is whether these molecules actually do form membranes, and we still don’t know what could be a DNA or protein equivalent in Titan conditions.”
That is important because although membranes provide shelter for life, particularly in those mysterious moments when it sparks for the first time, they aren’t enough. On Earth, living cells also require nucleic acids such as DNA to transfer genetic information from one generation to the next, and proteins to do the self-replication. Both of these components are themselves built from large, complex molecules.
The good news for astrobiologists is that two days before Palmer’s team released its findings, at University College London and his colleagues reported that Titan’s atmosphere contains ingredients that can create all manner of these macromolecules.
The discovery came from data gathered on one of Cassini’s final sweeps through the upper atmosphere, where it identified rarely spotted molecules called carbon chain anions. Based on observations of the dust clouds in interstellar space from which stars form, Desai and his colleagues know that these molecules act as catalysts for the formation of larger and more complicated organic molecules. In that sense, the presence of carbon chain anions “shows us how the complex organics on Titan might be produced”, says Desai.
How you get from reactions high in the atmosphere to life in the oily seas below isn’t clear. In the best-case scenario, carbon chain anions help to create big organic compounds that drift down to the surface where they might combine into the precursors of DNA and proteins, or whatever their analogues might be on Titan.
Precisely how that would happen is a mystery. We don’t even understand how chemical reactions gave rise to life on Earth, let alone how it would happen in hydrocarbon seas. Ultimately, the only way to find out is to return to Titan, says at Johns Hopkins. “Titan has basically been doing all these experiments into how far prebiotic chemistry can get, and the results are just sitting on the surface waiting to be analysed,” she says.
If Turtle has her way, that may happen sooner than you think. Having turned down several missions to Titan over the years, in December 2017, NASA dished out $4 million to that Turtle leads. This daring plan aims to send a sophisticated drone to buzz around the moon, landing at key sites in search of signs of prebiotic chemistry.
The team will use the money to polish the design of the Dragonfly quadcopter that gives the mission its name. It will also finesse the overall plan in the hope of convincing NASA that it should get the $850 million designated for the next mission under the , which funds solar system exploration. (The other mission in the running aims to bring back pieces of the comet 67P/Churyumov-Gerasimenko that the Rosetta probe visited a few years ago.)
By flying from place to place, a drone can cover much more ground than a rover could, and thus gather more and better data. Dragonfly could explore hundreds of kilometres of Titan’s surface over the course of a two-year mission, says Turtle.
In doing so, it would take full advantage of the unique atmospheric conditions on the moon. The air pressure at the surface is about 1.5 times greater than on Earth, and its gravity is only about 14 per cent of what we experience here – weaker even than that on our moon. What’s more, there seems to be almost no wind at the surface: Cassini’s observations suggest that if there are any ripples on Ligeia Mare, they are less than a millimetre tall. “Titan is probably the best place in the solar system to go flying,” says Hörst.
So good, in fact, that you yourself could probably fly without too much trouble. “Depending on how strong you are and how much you want to flap your arms, I’m not sure you’d even need to strap on wings,” says at the Planetary Science Institute in Colorado. Then again, with such low temperatures, you would need a very warm coat – and a reliable respirator given the lack of oxygen.
Even with a drone, touring a distant, icy moon is not without its challenges. Titan is almost 10 times further from the sun than Earth is and its dense atmosphere blocks out a lot of light, so the quadcopter can’t rely on solar power to keep it going, as the Mars rovers Spirit and Opportunity do. Instead, like any mission that ventures this far from the sun, Dragonfly will have to bring its own source of energy: a radioisotope thermoelectric generator, which uses the heat released by the decay of plutonium-238 atoms to generate electricity. This process creates more heat than the spacecraft needs to charge its battery, so the excess can be used to keep the sensitive electronics and scientific instruments from freezing up.
Turtle says that the team has already built prototypes that are slightly smaller than the final drone, which will be about the same size as Spirit and Opportunity – roughly a metre high and a couple of metres wide. All the technology required to build Dragonfly exists, she says, so it is just a matter of engineering.
But even if the Dragonfly mission is selected to launch in 2025 and arrives safely five years later, as planned, there remains a whopping great question: what exactly is it supposed to look for? Yes, it will search for different configurations of the potential building blocks of life that we know exist on Titan. But for all the recent discoveries, what kind of life that would be is still far from clear.
New forms
The problem is that seeking microbes that look just like the ones we have at home, only with different chemical compositions, probably won’t cut it. Assuming something doesn’t skitter away from the quadcopter’s landing gear as it touches down, there is a real danger that we could miss the signs of life we went to so much effort to see.
According to Turtle and Lorenz, the trick is to look for patterns. Non-biological processes tend to produce a broad range of complex molecules with relatively random abundances, but life demands efficiency. It tends to evolve to preferentially consume and produce a few different types of molecules – think of how most animals on Earth inhale oxygen and exhale carbon dioxide. “You can look for these patterns in chemical abundance that indicate non-random, organised chemistry,” says Lorenz. “That doesn’t prove life, but it’s an indicator. All we can do is try to collect as many indicators as we can.”
We don’t know exactly what those patterns will look like, but the first thing researchers will do if Dragonfly spots any will be to compare them with the indicators produced by life on Earth. Similarities might indicate that life originated in the same way in two different environments. Then again, life might instead have started in one place and hitched a ride through the solar system to end up at both Earth and Titan.
But if the patterns on Titan and Earth don’t match, it would be proof that life evolved independently on two different worlds. “If life originated twice in the solar system, that would mean it could happen all the time, all over the place,” says Lorenz.
With that in mind, it is easy to see why this far-away moon is such a tempting destination. It is the best laboratory we have to figure out how chemical reactions led to life on Earth and under what conditions life can spark elsewhere in the universe. It is also one of the only places we can plausibly reach that could answer such profound questions.
And who knows: if Dragonfly’s peregrinations ultimately reveal that water and other mundane, Earthly ingredients aren’t essential for chemistry to become biology, then we would look with fresh eyes at a vast swathe of worlds beyond our own solar system. Haze-shrouded exoplanets that once seemed barren would glow with the promise of exotic new forms of life.
This article appeared in print under the headline “Return to Titan”
