Titan's Deepest Winter
Winter in the depths of space, under oily rains on icy lakeshores: a glimpse into the coldest season on an alien landscape.
By Elise Wall
Images courtesy of NASA/JPL-Caltech
Titan is a world of cognitive dissonance. Saturn’s largest moon receives one percent of the sunlight Earth enjoys; a typical day would find you shivering, cursing the negative 290 degree Fahrenheit temperature as a foul drizzle settled lazily on your spacesuit visor in gravity that is a mere one-seventh of Earth’s. But despite this miserable alien imagery, Titan still seems almost Earth-like.
“Countless lakes like our own!” the vacation pamphlet might seductively promise. “Water! Vast amounts of water! Precipitation pouring down from clouds floating in a thick, luscious atmosphere!” Suddenly Titan's weather seems downright familiar. Sort of. If you ignore that the rain is closer to natural gas than water, and that any water not frozen rock-hard must be heavily polluted by something nasty like ammonia.
Now that you've decided on Titan as a VACATION destination, you’re about to find out how terrible a trip planner you are.
The 23 degree tilt of Earth's axis alternates between exposing the northern and southern hemispheres to the sun's warmth, causing the quarterly temperature changes we call seasons. Titan and Saturn are tilted as a unit, 26 degrees away from vertical. Saturn's orbit is much larger than Earth's, and it takes 29 Earth-years to complete a full orbit. If you’ve signed on for a tour of Titan lasting a full season, you’ll be sticking around for seven Earth-years.
Even accounting for axial tilt, Saturn's seasonal counterparts are not equal. Its orbit is not perfectly centered around the sun, dragging Titan with it as it swings 6 percent further from the sun than average as the south turns to winter, and 6 percent closer in southern summers. This means that southern winter is the darkest, and probably coldest, season Titan sees in its long year. As of this writing, Titan's southern hemisphere is moving into the deepest day of winter as the Summer Solstice approaches in May 2017. (Solstices are named relative to the northern hemisphere.)
There are some important differences between seasons on Earth and Titan, though. Titan's surface temperature does not vary as drastically as Earth's. Even comparing the poles to the equator, the difference is only 7 degrees Fahrenheit. For comparison, the difference between sunning yourself on Earth's equator and exploring a polar glacier is about 100 degrees Fahrenheit. However, Titan's climate is very close to the liquid-gas phase boundaries of its main liquids, which means even small changes can evaporate a puddle or cause rain to condense.
It might be a little misleading to get excited about this winter as Titan's coldest, simply due to the small scale of the temperature change. As the north transitioned from late winter to spring, the change was less than 1 degree Fahrenheit—a chill so minor even a Southern Californian might not complain. But even these small changes seem to have visible effects on Titan's global landscape.
So what might happen during Titan's southern winter?
Titan and Saturn may go through seasons together, but Saturn's seasonal variation as a gas giant is comparably abstract: The winter pole takes on a blue tinge and the temperatures of neighboring gas bands change pattern. Titan's weather spreads over a rocky landscape—an easier image to imagine.
Those travel brochures mentioned rain, but it’s not the kind of rain you might be thinking of—Titan’s atmosphere is practically free of water vapor. Instead, Titan’s precipitation ranges from a fine waxy snow to a thick drizzle, made up respectively of complex and simple hydrocarbons. Titan is cold enough that what we call natural gases on Earth (which are mostly methane) condense, fall and collect as liquids on the moon's surface.
Methane—the simplest hydrocarbon there is—goes through a cycle on Titan, much like water goes through its own cycle back on Earth. Methane reacts in the upper atmosphere to form more complex structures; this high-altitude melting pot is a hot mess of methane, nitrogen, solar photons, and particles charged by Saturn's magnetosphere. The end result is a flurry of bigger organics that drift down in a fine flakey snow. Lower in the atmosphere, methane can form a thick fog, a drizzle, or a downpour. Remote images have shown changes in surface features which could have been caused by the more dramatic of these storms.
The polar vortex, a high-altitude feature seen at the north pole during its winter, has shifted to the now-wintry south pole. Titan's thick, orange atmosphere circulates in a global rising and sinking cycle, which reverses with the seasonal change. Warm air rises in the sunny summer hemisphere and circulates to the winter pole, where it sinks. There, the cold gases condense and crystallize, forming masses visible from space. This swirling cloud is Titan’s harbinger of winter, and is the most pronounced seasonal change known thus far. Now that the south pole is under enough shadow to protect the vapors that condense to form ice, the cloud is there to stay for the season. Particles of frozen hydrogen cyanide mere micrometers in size, along with the more obvious methane ice, are the components of this cloud.
If you set up your vacation bungalow at the equator, you might choose a spot in one of the large fields of equatorial dunes: Titan's "Equatorial Band." The changing wind patterns could have drastic effects on the shifting dunes, which can rise over 100 meters in height. The windy season is just getting started, according to some models. Unless you don't mind risking being buried for the sake of SCIENCE (to find out if these dunes do indeed shift with the seasonal winds), you might head towards the poles instead.
Titan’s poles are home to its most unique feature, the one the travel brochures would brag about most—its lakes. Methane drizzle collects to form lakes, rivers and seas. These glassy liquid bodies are far more common in the northern hemisphere, which hosts seas as large as Kraken Mare. This is possibly because of the unequal seasons. The north pole of Titan is tilted away from the sun when Saturn's orbit brings the whole moon closer, sparing it from the hotter summer of the south that might make it harder for lakes to persist.
Northern lakes were observed growing throughout their winter, so you might see them shrink now as the sparse southern lakes swell. With the temperature drop in the south and the reversal of global weather patterns, maybe the lonely lake in the south, Lacus Ontario, will expand. This may mean that the winter is a “wetter” season, but it is still a guess—it could be a number of other causes. In other words, your beach-side cabin may be submerged before the season is over.
At this point, we can only speculate on the finer details. Even though many of Titan's secrets have been revealed in the past few years, like the methane-hydrocarbon cycle and the existence of equatorial dunes, more observations are needed to really pin down how weather has affected Titan's landscape. Cassini, the spacecraft observing Titan, is perfectly positioned to observe the current seasonal shift.
If you took that astro-travel agent up on a trip to Titan, you’d be cold. Really, really cold. You would need an umbrella that can stand up to methane rainstorms and sludgey hydrocarbon snow, and one hell of a spacesuit to keep warm. And that's not even touching on how you'd manage to breathe or eat.
But you would also be treated to a dynamic seasonal landscape, and an incredible eclipse of Saturn at each equinox. With a view like this, maybe life on Titan wouldn’t be so bad after all.
If you want to fight about Star Trek, you've found a worthy opponent: Elise is a Minneapolis-based Environmental Health and Safety chemist who spends her down time glassblowing, sailing, and learning about neat plants. Find her (to debate Star Trek or otherwise) on Instagram @sparrowsoup.