The Disappearing Act on Saturn’s Largest Moon

Ligeia Mare, shown here in a false color image from NASA's Cassini mission, is the second-largest known body of liquid on Titan, a moon of Saturn.
Ligeia Mare, shown here in a false color image from NASA's Cassini mission, is the second-largest known body of liquid on Titan, a moon of Saturn.NASA / JPL-Caltech / ASI / Cornell

In the summer of 2013, Jason Hofgartner and his professors at Cornell University were looking at radar images of Titan, Saturn’s largest moon, when they noticed something strange. There were bright spots inside Ligeia Mare, a large lake in the moon’s northern region, that had never shown up in data before. By the start of 2015, the features had vanished again.

The astronomers dubbed them “magic islands” because of their shape and disappearing act. Nearly three years later, scientists still don’t know what they are or what’s causing them, but they’re days away from getting one last shot to find out.

On Saturday, as Cassini begins its slow descent into Saturn’s atmosphere for the end of its mission, the spacecraft will make one final flyby of Titan. Cassini’s imaging instruments will target Ligeia Mare at a certain angle that will magnify the brightness of these mystery features, if they still exist, and make them more visible. The data that reaches Earth a few days later could help crack the case.

“If anyone’s going to solve it, it’s likely to be this upcoming Cassini observation,” said Hofgartner, a member of Cassini’s radar analysis team.

Hofgartner and his fellow Cornell researchers say there are three potential explanations for “magic islands,” which measure about 10 kilometers (six miles) across. The spots could be some kind of solid suspended in Ligeia Mare, which is a body of liquid mostly made of methane and larger than Lake Superior. They could be bubbles, brought to the surfaces by interactions between methane, ethane, and nitrogen. Or, as Hofgartner suspects, the islands could actually be waves, pushed into existence by wind just like on Earth.

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“Imagine a smooth lake surface on a calm day and then a breeze picks up that roughens part of the lake surface with some fairly small waves,” Hofgartner said.

Other scientists imagine it differently. In a study published this week, an international team of astronomers, led by a researcher from the the University of Reims Champagne-Ardenne in Paris, describe a bubble-forming process known as exsolution that they say could be the source of the “mystery islands.” Titan’s lakes are made of a swirling mixture of methane and ethane liquids. Methane-rich liquid at the surface flows downward into the depths, where there is more ethane. Changes in temperature and pressure at the bottom then cause nitrogen gas to separate out and rise to the surface, where bubbles can pool and fizz to create a layer large enough to seen by radar-imaging instruments.

Lakes on Titan, in other words, are kind of like giant cans of soda. Carbonated drinks contain carbon dioxide, packed inside under pressure so that it dissolves inside the liquid, much like the nitrogen in the moon’s lakes. When a can is opened, the gas escapes as bubbles.

The “magic islands” are another sign that Titan is very much alive in the geological sense. Aside from Earth, the moon is the only place that has rivers, lakes, seas, and even rainfall. Thanks to the frigid temperatures, methane flows in liquid form and evaporates, forming methane clouds that release methane rain drops. If humans ever want to explore the seas of another world, Titan is their best bet.

Scientists might never understand the mysterious bright spots, but their existence alone reveals a transience that is rare in our solar system. On other planets and moons, “you don’t find as much geologic activity,” Hofgartner said. “It’s more about reading their history and knowing what happened in the past. Titan is one of those places where you can actually go and look and see things happening today.”

And so Cassini will go and look, one last time.