Saturn’s largest moon, Titan, is a fascinating and mysterious world, a world literally shrouded in mystery due to thick clouds that cameras imaging in the visible spectrum cannot penetrate. This was made apparent when NASA’s Pioneer 11 became the first spacecraft to fly past Titan in 1979, and then NASA’s Voyager 1 and 2 in 1980 and 1981, respectively. All three spacecraft were equipped with cameras that were unable to penetrate Titan’s atmosphere of thick clouds, although atmospheric data from Voyager 1 suggested Titan might be the first body, aside from Earth, where liquid might exist on its surface.
It wasn’t until the NASA’s Cassini spacecraft had its first encounter with Titan in October 2004 when Saturn’s largest moon was no longer able to hide its secrets beneath the hazy atmosphere. Cassini revealed a world of liquid methane and ethane lakes, sand dunes encircling the equator, and evidence for a possible internal ocean likely comprised of water or ammonia. In December 2004, Cassini released the European Space Agency’s Huygens probe, which had been mounted to the orbiting spacecraft prior to launch. Huygens entered Titan’s atmosphere and landed on the surface in January 2005 after taking 2 hours and 27 minutes to descend through the thick atmosphere, and ultimately operating for an additional one hour and 10 minutes after touchdown. Data during the descent and post-touchdown images revealed rounded rocks and a suite of onboard instruments taught us much about Titan’s atmosphere. Even though Cassini’s mission ended in September 2017 when the spacecraft intentionally plunged into Saturn, scientists continue to pore over scores of data and images that Cassini revealed about Saturn’s largest moon.
Titan is like Earth with its rivers, lakes, and seas filled by rain, but as stated, this is liquid methane and ethane as opposed to liquid water on Earth. Like Earth, Titan also has sand dunes, but they are made of hydrocarbons instead of silicate-based substances. Also, much like Earth, Titan is known for having a seasonal liquid transport cycle, also known as the water cycle on Earth, linking atmosphere, land, and oceans.
A recent study published in Geophysical Research Letters discusses a new model for this transport cycle on Titan, showing how this drives the movements of grains over Titan’s surface.
“Our new model adds a unifying framework that allows us to understand how all of these sedimentary environments work together,” said Dr. Mathieu Lapôtre, an assistant professor of geological sciences at Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth). “If we understand how the different pieces of the puzzle fit together and their mechanisms, then we can start using the landforms left behind by those sedimentary processes to say something about the climate or the geological history of Titan – and how they could impact the prospect for life on Titan.”
While researchers think similar processes on Earth helped form the dunes, plains, and labyrinth terrains on Titan, the sediments on Saturn’s largest moon are composed of solid organic compounds, as opposed to silicate-derived rocks found on Earth, Mars, and Venus. The ability for these organic compounds to grow into sediment grains that can be transported around Titan has left scientists puzzled until now.
Lapôtre and his research team found an answer by studying sediments on Earth called ooids, which are small, spherical grains most often found in shallow tropical seas, such as the Bahamas. These ooids form when calcium carbonate is pulled from the water column and attached in layers around a grain, such as quartz. These ooids form through chemical precipitation, while the simultaneous process of erosion slows the growth as the grains are smashed together by waves and storms. Balancing each other out over time, these two competing mechanisms form a constant grain size – which the research team suggest could also be happening on Titan. While the compounds might differ, this study nonetheless demonstrates that Titan is not that dissimilar from Earth.
“Titan has been viewed as a potential analog to Earth for a variety of reasons,” said Lapôtre. “Notably, its atmosphere is thought to be conducive to the type of prebiotic chemistry that may have given rise to life on Earth. Titan’s modern landscapes are also to a great extent analogous to Earth’s landscapes, with lakes, rivers, and fields of sand dunes. In our study, we proposed a unifying hypothesis to explain how a global sedimentary cycle, driven by Titan’s climate, may generate the observed distribution of Titan’s landscapes. Such models, in turn, will allow us to decipher any sedimentary record on Titan once we get to explore Saturn’s moon in situ. Sedimentary “rocks” (which on Titan could be made of complex organics and ices) offer a prime target to better understand past environmental conditions, and thus, the history of Titan’s surface and atmosphere.”
While Cassini and Huygens were instrumental in teaching more about this mysterious world, NASA’s upcoming Dragonfly mission will deliver an 8-bladed rotorcraft to Titan hopes to further unlock the mysteries of Saturn’s largest moon, and is a mission that Dr. Lapôtre is very excited about.
“Any observations made by Dragonfly will be groundbreaking,” said Lapôtre. “Currently, we only have one picture of Titan’s surface that was acquired from the ground by the Huygens lander in 2005. Everything else we’ve seen of Titan’s surface was from orbit at low resolution. High-resolution ground observations of organic sand grains blown by winds as well as constraints on their chemical composition, for example, will help test our hypothesis.“
Dragonfly is slated to launch in 2027 and arrive at Titan in 2034. During its 2.7-year (32-month) baseline mission, this rotorcraft will sample and examine dozens of promising sites around Saturn’s icy moon and advance our search for the building blocks of life.
What further secrets will Dragonfly unlock about this alien, but similar, world? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!