NASA's Cassini spacecraft has found evidence of a "monstrous" ice cloud that sweeps over the southern polar region of Saturn's frigid moon Titan.

Cassini first spotted the cloud in 2012, but those observations only saw the tip of the iceberg. These new findings reveal a massive ice cloud system in Titan's lower stratosphere. The cloud has a density similar to fog on Earth, but is flatter on top. This is the first time Cassini has been able to watch Titan's transition from autumn to winter because the moon's seasons each last about seven-and-a-half years.

"When we looked at the infrared data, this ice cloud stood out like nothing we've ever seen before," said Carrie Anderson of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "It practically smacked us in the face."

Ice clouds on Titan form much differently than rain clouds here on Earth. For Earthly rain clouds, water evaporates from the surface and rises until it reaches cooler temperatures, the combination of cold and low air pressure then transform it into condensation. Titan's polar clouds form when circulation in the atmosphere moves gas from the pole in the warm hemisphere to the pole in the cold hemisphere. At the cold pole, warm air sinks in a process called subsidence. Once the sinking gas encounters colder temperatures closer to the surface, it condenses into cloud-like formations.

"Titan's seasonal changes continue to excite and surprise," said Scott Edgington, Cassini deputy project scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "Cassini, with its very capable suite of instruments, will continue to periodically study how changes occur on Titan until its Solstice mission ends in 2017."

The new findings confirm that Titan's winter is much more severe than scientists initially believed.

"The opportunity to see the early stages of winter on Titan is very exciting," said Robert Samuelson, a Goddard researcher working with Anderson. "Everything we are finding at the south pole tells us that the onset of southern winter is much more severe than the late stages of Titan's northern winter."

The findings were presented at the annual Meeting of the Division of Planetary Sciences of the American Astronomical Society.