NASA scientists have identified an unexpected high-altitude methane ice cloud on Saturn’s moon Titan that is similar to exotic clouds found far above Earth’s poles.
This lofty cloud, imaged by NASA’s Cassini spacecraft, was part of the winter cap of condensation over Titan’s north pole. Now, eight years after spotting this mysterious bit of atmospheric fluff, researchers have determined that it contains methane ice, which produces a much denser cloud than the ethane ice previously identified there.
“The idea that methane clouds could form this high on Titan is completely new,” said Carrie Anderson, a Cassini participating scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the study. “Nobody considered that possible before.”
Methane clouds were already known to exist in Titan’s troposphere, the lowest layer of the atmosphere. Like rain and snow clouds on Earth, those clouds form through a cycle of evaporation and condensation, with vapor rising from the surface, encountering cooler and cooler temperatures and falling back down as precipitation. On Titan, however, the vapor at work is methane instead of water.
The newly identified cloud instead developed in the stratosphere, the layer above the troposphere. Earth has its own polar stratospheric clouds, which typically form above the North Pole and South Pole between 49,000 and 82,000 feet (15 to 25 kilometers) — well above cruising altitude for airplanes. These rare clouds don’t form until the temperature drops to minus 108 degrees Fahrenheit (minus 78 degrees Celsius).
Other stratospheric clouds had been identified on Titan already, including a very thin, diffuse cloud of ethane, a chemical formed after methane breaks down. Delicate clouds made from cyanoacetylene and hydrogen cyanide, which form from reactions of methane byproducts with nitrogen molecules, also have been found there.
This cloud in the stratosphere over the north pole of Titan is similar to Earth’s polar stratospheric clouds. NASA scientists have determined that Titan’s cloud contains methane ice, which was not previously thought to form in that part of the atmosphere. Image Credit: NASA/JPL/University of Arizona/LPGNantes.