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18 February 2020

Return to Titan, the Remotest Land Ever Reached by Humanity

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On January 14, 2005, humans successfully achieved an incredible feat unsurpassed to date. The European Space Agency’s (ESA) Huygens probe, a metal pie-plate looking device 1.3 metres in diameter, parachuted down onto Titan, the largest of Saturn’s moons, and landed unscathed on its surface. For 90 minutes, Huygens transmitted a wealth of data from the satellite, including images from that remote spot. Huygens’ accomplishment remains the furthest ground on which a human-made probe has landed. But a return to Titan is already on the horizon. 2026 will see the launch of NASA’s Dragonfly mission, tasked with the challenge of delivering a drone capable of combing that distant world in search of the answer to an intriguing question: could there be life on Titan?

Titan is one of the Solar System’s best candidates for hosting extra-terrestrial microbial life, as it is the only moon with a dense atmosphere —composed mostly of nitrogen— and with liquid lakes on its surface, made up of hydrocarbons such as methane and ethane. However, water is also very present, at ground level only in the form of ice, but extensively in an underground liquid ocean. If life with a different chemistry than on Earth is possible, Titan could be the place to find microorganisms based on methane instead of water, whose existence is still only hypothetical.

Photograph of the Saturn moon Titan in False Color, taken by the Cassini space probe with ultraviolet and infrared camera.Credit: NASA / JPL / Space Science Institute

Interest in exploring the Saturn system more deeply led NASA, ESA and the Italian Space Agency to collaborate on the design of the Cassini-Huygens mission, launched in 1997 and consisting of the Cassini orbiter and the Huygens landing probe. Cassini entered Saturn’s orbit in 2004 and studied the ringed planet and its moons for 13 years. On December 25, 2004, Cassini released Huygens in the first attempt by humans to land an instrument on the surface of an outer Solar System world.

Huygens was designed to land on solid ground as well as to float, since it was not known whether it would find anything solid on the surface. After a two-and-a-half-hour descent through the atmosphere, and with the help of three parachutes, the probe landed on a ground having the consistency of soft, wet sand, populated by boulders of frozen water. The scientific instruments revealed the conditions on Titan’s surface: a temperature of -170°C, an atmospheric pressure somewhat higher than that of the Earth, light winds and the presence of carbon dioxide and organic compounds.

The mission was a success, although a partial failure in the communication system with its orbiting link, Cassini, caused the loss of half of the photos taken by the probe. Despite this glitch, the 350 images that were able to be recovered were a treasure trove that showed the researchers an orange world enveloped in a thick fog, with obvious traces of liquid erosion.

Valuable clues to return

The data from Cassini and Huygens have not only helped scientists to understand in greater detail what is, for the team on the new Dragonfly mission, “in many ways the most Earth-like body in the solar system,” but have also provided valuable clues for the design of this return to Titan. “Huygens gave us direct measurements of the atmospheric conditions on Titan including the winds, and Cassini data also tells us what Titan’s landscape is like,” planetary scientist Ralph Lorenz of the Johns Hopkins University (JHUAPL) Applied Physics Laboratory, who was part of the Huygens science team and is now the architect of the Dragonfly mission, tells OpenMind.

What is more, as Elizabeth Zibi Turtle, also a planetary scientist at JHUAPL and the mission’s principal investigator, explained to OpenMind, “the timing of the launch in 2026 means that Dragonfly will arrive at Titan in 2034, basically one Titan year after Huygens’ atmospheric descent and landing.” Thus, “the Huygens data give us direct information about the atmosphere during the season when Dragonfly will arrive at Titan,” she added.

Without doubt, the most novel and striking aspect of the mission is the use of a drone with four double rotors, each approximately one metre in diameter, powered by a thermoelectric radioisotope generator —in essence, a nuclear battery. The device will be able to fly up to a height of 4 kilometres and at a speed of 36 km/h, which will allow it to cover in a single flight a distance greater than that covered by the Martian rovers in an entire decade. The concept offers so many possibilities that NASA will also apply it to its next Martian mission: Mars 2020. However, the Dragonfly drone will be enormously more powerful and capable, given that Titan’s dense atmosphere, unlike that of Mars, is optimal for supporting a large rotorcraft.

To understand prebiotic chemistry

According to Turtle, the mission concept has been made possible by major technological advances developed for land-based drones over the past decade. “We haven’t had to invent new technologies,” she says. But since this device will be flying on another world, its design must be adapted to the specific conditions on Titan. “Dragonfly’s design, like the shape of the rotors, is optimized for the cold dense atmosphere there,” says Lorenz. With those cold temperatures, “we have to keep the vehicle electronics and battery warm with our main power source,” adds the researcher. Finally, since radio signals take more than an hour to reach Titan from Earth, it is not possible to operate the device with a joystick; “the autopilot must do that,” Lorenz says. “And there isn’t GPS or a magnetic field, so Dragonfly must use cameras to navigate.”

And all this, with a central purpose: “Dragonfly’s primary objective is to understand prebiotic chemistry,” summarizes Turtle. After a long stretch when astrobiology has not been a priority on NASA missions, the agency has recently begun to intensify its efforts to make this science an integral part of its future missions. Dragonfly is not designed to search for possible microbes on Titan today, but with its four instruments it will be able to analyse whether conditions are suitable for the development of life, as occurred in the so-called primordial soup of early Earth.

“While we wouldn’t expect life as we know it under Titan’s conditions, Dragonfly would be able to detect chemical biosignatures were chemistry to have taken the leap to biology on Titan at some point in its past, either as a water-based system or potentially something completely different like a hydrocarbon-based system,” Turtle explains. We still don’t know if that leap has occurred on another world in the Solar System, and it may never have happened. But we now have the tools to unravel this mystery.

Javier Yanes


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