Titan, Saturn’s largest moon, is bigger than the planet Mercury and has the thickest atmosphere of any known planetary satellite. Its blanket of gases is incredibly similar in some ways to our own: it is made predominantly of nitrogen, for example, and the surface pressure—roughly 100 times greater than that of Mars and 100 times less than that Venus—resembles Earth’s as well. It even has clouds and precipitation.  

In other ways, however, Titan’s atmosphere is utterly alien. At an average temperature of -290 degrees Fahrenheit, the clouds and raindrops are made not of water but of liquid methane and ethane that evaporate from hydrocarbon lakes and seas near the poles. The water ice that makes up the moon’s surface is so cold that it acts like bedrock. Combined with Titan’s low gravity—just 1/7th of Earth’s—the thickish atmosphere has led some to speculate that humans could fly under their own power with synthetic wings strapped to their arms.

A particularly fascinating aspect of Titan is the thick haze of organic chemicals that dominates the atmosphere and obscures the surface. It is reminiscent of the smog in Los Angeles, but hundreds of miles thick. The haze is composed of microscopic, oil-like snowflakes and is formed high in the atmosphere through the interaction of atmospheric nitrogen and methane with ultraviolet light and high energy particles. Although this photochemistry happens in other oxygen-poor atmospheres including those of Pluto, Jupiter, Saturn and, billions of years ago, even the early Earth, the sheer abundance of Titan’s haze and the extended length of the Cassini mission, which orbited Saturn from 2004 until its plunge into the planet in 2017, make it the most striking and well-studied example of this process.

Oddly, images of Titan show a very high layer of haze that is detached from the main hazy atmosphere. The formation of the detached layer is still an active area of debate, with some arguing that it’s caused by a change in the shape of the particles that form the layer, while others argue that winds drive the phenomenon. Not only does the detached haze layer encircle the entire moon, but its altitude changes over time. It was very high in the atmosphere—about 350 miles above the surface—during Titan’s northern winter and just after the equinox in 2009, which aligns with the start of Titan’s northern spring. The layer then fell more than 50 miles in a few months before slowly merging into the main hazy atmosphere below in 2012.

A recent study in Nature Astronomy describes the movement of this haze layer including its disappearance in 2012 and subsequent reappearance high in the atmosphere in 2016. It was unexpected, but the movement of the haze helps researchers understand and explain the formation of the detached haze layer. This vanishing act is thought to be driven by the shifting balance between upward winds and falling particles as the seasons change. These particles also act like tracers that allow for observation of the atmospheric circulation.

Previous measurements from the Voyager spacecraft in 1980 observed the detached haze layer at exactly the same altitude as Cassini did, one Titan “year” (approximately 29.5 Earth years) earlier. These observations indicate that the seasonal cycle of the detached haze layer is surprisingly consistent over the course of its year. The reappearance of the detached haze layer, however, was more complicated than we previously expected based on computer models. Computer simulations of Titan’s atmosphere predicted the detached layer would re-form high in the atmosphere a few years after disappearing and slowly decrease in altitude for a few years before stabilizing until the next equinox. However, the recent reappearance started with transient detached layers high in the atmosphere that dissipated within months of forming. The haze had not formed a stable detached layer by the end of the Cassini mission in 2017.

Without Cassini, which left a treasure trove of amazing observations, it might be a long time before we can determine when the detached haze layer returns to stability, or whether it will during this northern summer season. This enduring spacecraft taught us a lot about Titan and the chemical and physical processes occurring in the atmosphere. Future studies will have to make do with ground-based and Earth-orbiting telescopes, at least until the next mission. Recently, the Dragonfly probe to Titan was selected as a finalist by NASA. If given the go-ahead, this mission would send a quadcopter to fly to a series of landing sites on Titan, taking a number of unique measurements about the surface-to-atmosphere exchange of materials; meteorology on Titan; and the composition of the surface, including biologically relevant molecules. This mission could launch as early as 2025.

 [KS1]Q: Should this be “particles that form the layer” or “particles from the layer”?