Going back in time to see what Earth was like billions of years ago could answer some questions about why ours is the only planet of its kind in the solar system and how life began.
Ancient rocks and fossils of early, simple life forms have provided some information. But if we could explore, today, another world that was similar, at least in some ways, to the young Earth, what might we learn about the way life formed and evolved?
Titan, the largest of Saturn’s 62 known moons, offers an opportunity to do just that.
The atmosphere is closer to that which Earth experienced billions of years ago. It’s about 95% nitrogen and 5% methane, with small amounts of other carbon-rich compounds. It is much colder, though, with a temperature around -290°F. The terrain is made up of vast expanses of organic-covered surfaces consisting of dunes, rocks, and rivers and lakes of liquid methane with subsurface ice and water. How a frigid environment evolved into one that supports life is a mystery that data from Titan could help solve (See Distant Encounters).
Humans can’t survive there unprotected for many of the same reasons we didn’t exist on early Earth. But a robotic explorer could get the science started, just as robots have done on Mars. As the first icy moon planned to host a lander, Titan is shaping the development of a new kind of robot – a rotorcraft similar to a remotely piloted drone.
Images recorded by the European Space Agency’s Huygens probe descent imager/spectral radiometer between 11 and 5 miles were assembled to produce this panoramic mosaic. The probe ground track is indicated as points in white. North is up. Narrow dark linear markings, interpreted as channels, cut through the brighter terrain. The complex channel network implies precipitation (likely as methane “rain”) and possibly springs. Image credit: ESA/NASA/JPL/University of Arizona)
Titan’s atmosphere is four times denser than Earth’s, and it has one-seventh the gravity, making excellent conditions for flying.
“It’s safer to fly than to drive,” says Ralph Lorenz. “The Perseverance rover has a helicopter scout, a little subvehicle, that’s supposed to explore, scouting ahead 300 feet or so with high-resolution vision. But we do that with our own vehicle.”
A scientist with the Johns Hopkins Applied Physics Laboratory who specializes in research on Titan, Lorenz helped develop NASA’s Dragonfly mission. He says the lander will be on the ground most of the time, but flying to various locations will make it possible to avoid many of the obstacles wheeled vehicles contend with—ice crevasses, boulders and steep slopes. Flight will also make it possible to explore the varied surface of the moon more quickly than a wheeled vehicle.
Ligeia Mare, shown here in a false-color image from NASA’s Cassini mission, is the second-largest known body of liquid on Titan. It is filled with liquid hydrocarbons, such as ethane and methane, and is one of the many seas and lakes in Titan’s north polar region. Image credit: NASA/JPL-Caltech/ASI/Cornell
The drone-like lander will fly from place to place, leveraging existing technology to get around autonomously.
“The autonomy is not cutting-edge artificial intelligence or something like that,” explains Lorenz. “For example, the vehicle has onboard sensors that can measure wind. We will tell it, ‘We would like you to fly over there and scout out this place and then land at this other place.’ It will then check conditions autonomously before it takes off.”
This multirotor craft will also be programmed to act as its own weather balloon, rising up to various points in the atmosphere to profile how the wind and methane moisture change with altitude. While most planetary probes do that once during a parachute descent, Dragonfly will measure changes at various times of day and locations.
Dragonfly will visit many locations on Titan, analyzing the organic materials on its surface. The dense atmosphere and low gravity make flying easy, and the probe won’t have to negotiate the sort of obstacles most planetary landers encounter. Image credit: Johns Hopkins Applied Physics Laboratory
Dragonfly will benefit from data from NASA’s Cassini mission. That spacecraft used radar and infrared instruments to peer through Titan’s dense atmosphere and discern details on the surface. It deployed the Huygens probe to the surface, which gathered valuable atmospheric measurements throughout the descent. The probe then sent back the first pictures of Titan’s surface and transmitted in situ data that’s informing the Dragonfly design decisions.
“The environment is a factor,” says Lorenz. “We need a heat source, and since we can’t use solar power due to low sunlight, we’ll power the craft with a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG). The MMRTG generates a lot of heat.” A fluid loop will take “waste heat” into the vehicle to keep electronics and scientific instruments warm, and foam layers will limit heat leakage.
The Cassini spacecraft used a narrow-angle camera with a spectral filter sensitive to wavelengths of near-infrared light to see through Titan’s hazy atmosphere to take the first pictures of the distant moon’s surface. Image credit: NASA/JPL/University of Arizona
One of the scientific instruments, a gamma ray spectrometer used to measure the composition of surface material, usually requires refrigeration down to cryogenic temperatures to keep it operational. Dragonfly will have the spectrometer mounted on the outside to capitalize on the frigid environment, eliminating the weight and the energy demands of refrigeration equipment.
Navigating across a remote icy surface presents another unique challenge.
“We don’t have GPS, so the vehicle navigates itself, in part by comparing the images taken throughout the mission,” explains Lorenz. “That sort of vision-based, terrain relative navigation has been done by cruise missiles since the ’80s, so we’ve implemented that in simulations and field tests.”
Computer simulations incorporating the diffuse light on Titan, where it’s always overcast, are necessary to “teach” algorithms to “see” without the strong shadows on which current technology depends. Feature-mapping algorithms must perform better than they do now in low light if they’re to function successfully.
Lidar, a surveying method that uses lasers to measure distance with the reflected laser light and a sensor, assesses the hazard level of potential landing sites based on a roughness measurement. The vehicle will be programmed to “retain the best three places it’s flown over. So, if it detects a fault in its systems, it will immediately land at one of those locations,” Lorenz explains. All of that autonomy is being carefully designed and tested, “but it’s not advancing the state of the art by a great deal.”
Dragonfly is a no-frills flight without perks such as an orbiting satellite to assist with communications. Everything it needs has to be onboard. This mission is all about trying a new approach and getting results quickly. If it’s successful on this first icy moon, aspects of it might work well on the others (See Exploring Icy Moons).
Lorenz notes that the goal for this robotic mission is similar to what is taking place on Mars.
“The big difference, perhaps, is that on Mars we don’t expect to find much organic material at all,” he says. “Whereas on Titan, there’s lots and lots of organic material, so sifting through that will be interesting” (See Insight).
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