NASA's Dragonfly mission to Saturn's largest moon will touch down on a terrain of dunes and shattered, icy bedrock, according to a new analysis of radar imagery from the Cassini spacecraft.
Launching in 2027, Dragonfly is a rotorcraft that will arrive in 2034 and explore Titan from the air. Its range will be far greater than that of a wheeled rover, with Dragonfly capable of covering around 10 miles (16 kilometers) in each half-hour flight, according to NASA. Over the span of its two-year mission it will explore an area hundreds of miles or kilometers across. However, before taking to the sky on its own, Dragonfly must first arrive on Titan under a parachute, soft-landing on frozen terrain that is hidden from easy viewing by the dense hydrocarbon smog that fills the moon's atmosphere.
Dragonfly's landing site will be the Shangri-La dune field, close to the 50-mile-wide (80 kilometers) crater, Selk. This region was imaged by NASA's Cassini spacecraft during its mission to Saturn between 2004 and 2017, and a team of scientists led by planetary scientist Léa Bonnefoy of Cornell University has taken a new look at that data to produce the most accurate assessment of Dragonfly's proposed landing site so far.
"Dragonfly … is going to a scientifically remarkable area," Bonnefoy said in a statement. "Dragonfly will land in an equatorial, dry region of Titan. It rains liquid methane sometimes, but it is more like a desert on Earth where you have dunes, some little mountains and an impact crater."
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Selk is an interesting location. Estimated to be geologically young, perhaps a couple hundred million years old, the impact that carved it out would have melted the local ice, prompting interactions between the fresh liquid water and organic molecules present in the hydrocarbon soup on Titan's surface. Astrobiologists are particularly interested in the prebiotic chemistry — chemistry involving carbon-rich molecules but not mediated by living things — that would have resulted.
Yet Cassini's radar images of the area are limited, with the resolution at best being 1,000 feet (300 meters) per pixel. "There are probably a lot of small rivers and landscapes that we couldn't see," Bonnefoy said.
Scientists know that such rivers do exist on Titan, thanks to the European Space Agency's Huygens lander, which piggybacked on board Cassini before parachuting down to the surface of Titan in January 2005. These rivers, however, are not full of liquid water — the temperature of minus 290 degrees Fahrenheit (minus 179 degrees Celsius) is far too cold for that. Instead, liquid methane and ethane rain from the frigid sky and wash off the water-ice bedrock and into river tributaries that feed large lakes.
What Cassini's imagery did provide, however, is multiple viewing angles. Each time it flew past Titan — it enjoyed 127 close approaches of the moon during its mission — it viewed landmarks in the region of Dragonfly's landing site from different angles, ranging from inclinations of 5 degrees to 72 degrees.
By analyzing how the terrain produced different-shaped shadows based on the viewing angle, Bonnefoy's team were able to determine the topography of the region within the limits of the image resolution, finding no major show-stopping obstacles that Dragonfly would need to avoid.
The scientists also calculated the height of the rim of Selk crater, finding it to vary from less than 650 feet (200 m) tall in some parts up to 2,000 feet (600 m), which is higher than expected, indicating a less eroded crater rim.
The research was published Aug. 30 in The Planetary Science Journal.
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