Scientists already knew that Titan produced methane clouds in the lower layers of its atmosphere. These form the same way that low clouds do on Earth — with vapor rising, cooling and eventually becoming dense enough to fall back to the surface of the planet — but are made of methane instead of water.
But for clouds to form in the upper layers of the atmosphere, you need much colder temperatures. Otherwise, all of the moisture is trapped before it gets that high. On Earth, these kinds of clouds form more than 49,000 feet above the Earth's poles, where temperatures drop below -180 degrees Fahrenheit.
Titan is colder than Earth, but didn't seem cold enough to allow methane clouds to form in the upper atmosphere. But it turns out that the temperature varies in different locations of the moon. At its poles, researchers using data from NASA's Cassini orbiter report, there's a drop of as much as 11 degrees.
That drop seems to be enough to yield high-altitude clouds. On Titan, the clouds probably form between 98,000 and 164,000 feet above the surface.
"Titan continues to amaze with natural processes similar to those on the Earth, yet involving materials different from our familiar water," Scott Edgington, Cassini deputy project scientist for NASA, said in a statement. "As we approach southern winter solstice on Titan, we will further explore how these cloud formation processes might vary with season."