Looking for signs of life on Titan

NASA is returning to Saturn’s moon Titan, this time with a flying, drone-like rover called Dragonfly.

In the process, scientists will not only study Titan’s geology, atmosphere and weather: they will also search for signs of life, and try to better understand how life arose on our own planet.

It was an announcement that drew cheers, fist-pumps, and prolonged applause from scientists at AbSciCon 19, an astrobiology conference in Bellevue, Washington, US, which hastily arranged a special session so attendees could view a webcast of NASA’s announcement.

Astrobiologists are particularly pleased with the decision because NASA’s Cassini mission, which flew by Titan 126 times during its tour of the Saturn system from 2004 to 2017, revealed Titan to be a remarkable analogue for early Earth. 

Not that on first impression Titan looks to be all that habitable. Its surface temperature is a frosty –179 degrees Celsius – so cold that water is frozen into granite-hard ice, and the rain that falls from its skies to fill its lakes and seas is liquid methane.

But it has a thick atmosphere, in which sunlight creates chemical reactions that produce very complex organic molecules that drift to the surface, “almost like a light snow that’s always falling,” says Curt Niebur, lead program scientist for NASA’s New Frontiers program. 

“Titan’s atmosphere produces some of the most complex organics we know of in the Solar System,” adds Shannon MacKenzie, a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory (APL) in Laurel, Maryland, and a member of the Dragonfly team. 

“Cassini was able to detect molecules what were about that size of proteins in Titan’s upper atmosphere.” 

More recently, Earth-based radio astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA), in Chile, have been able to detect the signatures of additional molecules, including vinyl cyanide (C2H3CN). 

That’s important says Martin Cordiner, an astrochemist at NASA Goddard Spaceflight Centre, Greenbelt, Maryland, because laboratory experiments have shown that if you put vinyl cyanide into liquid methane, the vinyl cyanide molecules can link together like cell membranes—a process that might be a step toward the creation of primitive cells.

Adding to Titan’s intrigue, it also appears to have a subsurface ocean. If that water ever comes into contact with the complex organics on Titan’s surface, even more interesting chemistry could occur, raising the prospect that Titan might once have developed life, and might still harbour it in its subsurface waters. 

One way that could happen is if fractures in Titan’s outer surface allow liquid methane and materials dissolved in it to seep deeply enough into its crust to reach a zone where the ice that forms Titan’s bedrock is under enough pressure that it can flow – like ice at the bottom of a glacier. 

This, says Steven Vance, an astrobiologist at NASA’s Jet Propulsion Laboratory, Pasadena, California, could permit a form of solid-state convection that might feed organics all the way to the bottom of the ice and into the underlying ocean. For that matter, he says, the high-pressure ice itself might mix liquid water and organics, and be warm enough for life. 

Another way for water and surface organics to mix is if water erupts to the surface via fractures, or if an impact melts enough of it to produce a crater lake that took tens of thousands of years to refreeze. 

To explore as many of these possibilities as possible, Dragonfly will start by landing among Titan’s sprawling dune fields, which girdle much of its lower latitudes. 

These themselves are interesting, MacKenzie says, because their sand grains aren’t ordinary sand. Instead, they are tiny particles of organic matter that may have been blown from many kilometers away – meaning that the sand at any one site could come from a multitude of sources.

To find out what these grains are made of, Dragonfly will drill into the sand, then suck it up into a mass spectrometer, using a pneumatic system that the mission’s principal investigator, Elizabeth Turtle (also of APL), describes as “basically like a vacuum cleaner”. 

Dragonfly will also peer beneath the surface with a gamma ray neutron spectrometer, and look deeper beneath the crust with an onboard seismometer. “Dragonfly will let us know if the ocean is close enough to the surface to mix with all those complex molecules falling out of the atmosphere,” Niebur says. 

But eventually, Dragonfly will fly to a 50-kilometre impact crater called Selk Crater, where the heat of impact should have melted the underlying ice and allowed water and organics to have mixed. 

Laboratory experiments have found that when you take the likely chemicals on Titan’s surface “and dump them in water,” MacKenzie says, they form amino acids—the building blocks of proteins – “in a time scale of hours. So, we think that [Selk Crater] is a very interesting place to go”. 

What this all means, Niebur says, is that from a chemical perspective, Titan looks a lot like Earth might have looked like, before the dawn of life. And while we can’t go back in time to look at the early Earth, he says, “we can go to Titan and get a glimpse of what Earth was like”.

Getting there, however, will take a while. The mission is scheduled for launch in 2026, and it won’t touch down until 2034.

Once down, however, it is designed to be long-lived, powering itself not from solar energy, but from heat generated by the decay of radioactive isotopes – the same power source now being used on Mars by the Curiosity rover. 

It will also carry batteries that can store power for energy-intensive tasks like flying to the next landing site. “When we land, we can recharge,” Turtle says.

To further prolong its life, it has not four rotors, but eight, arranged in pairs. Thus, if one fails, it can still fly. In fact, MacKenzie says, Dragonfly could sustain the loss of four rotors, so long as it was lucky enough that no two came from the same pair.

Flying on Titan is easier than flying on Earth, Turtle adds, because Titan’s atmosphere is four times denser than ours, while its gravity is only one-seventh. “If you put on wings, you’d be able to fly on Titan,” she says. “It’s the best way to travel.”