High-Altitude Methane Cloud Detected In The Stratosphere Of Saturn’s Moon Titan

Chuck Bednar for redOrbit.com – Your Universe Online
Scientists have unexpectedly identified a high-altitude methane ice cloud floating above the north pole of Saturn’s moon Titan – a cloud that is similar to the exotic clouds found far above Earth’s poles, NASA announced on Friday.
The cloud was first spotted by the Cassini spacecraft in 2006, and while methane clouds were already known to exist in the lowest layer of Titan’s atmosphere, the troposphere, the discovery of such clouds forming this high up is “completely new,” Cassini mission scientist Carrie Anderson said in a statement.
Anderson, who works at the Goddard Space Flight Center and is lead author of a study detailing the findings currently available online in the journal Icarus, explained that scientists at the space agency had never considered the possibility that the moon’s stratosphere could contain methane clouds.
The methane cloud was part of the winter cap of condensation over Titan’s north pole, and like rain and snow clouds on Earth, those clouds form as part of a cycle of condensation and evaporation. Like on Earth, vapor rises from the surface of the moon, encounters increasingly cooler temperatures and then returns to the ground in the form of precipitation – except on Titan, the vapor involved in the process is methane, not water.
Instead of developing in the troposphere, this newly identified cloud formed in the layer above it, the stratosphere. Earth also has polar stratospheric clouds, which typically form above the North Pole and South Pole between 49,000 and 82,000 feet. These clouds are rare and only form at temperatures of minus 108 degrees Fahrenheit.
“Other stratospheric clouds had been identified on Titan already, including a very thin, diffuse cloud of ethane, a chemical formed after methane breaks down,” NASA said. “Delicate clouds made from cyanoacetylene and hydrogen cyanide, which form from reactions of methane byproducts with nitrogen molecules, also have been found there.”

Image Above: Earth’s polar stratospheric clouds. Credit: L. NASA/JPL/U. of Ariz./LPGNantes; R. NASA/GSFC/M. Schoeberl
“But methane clouds were thought unlikely in Titan’s stratosphere,” the agency added. “Because the troposphere traps most of the moisture, stratospheric clouds require extreme cold. Even the stratosphere temperature of minus 333 degrees Fahrenheit (minus 203 degrees Celsius), observed by Cassini just south of the equator, was not frigid enough to allow the scant methane in this region of the atmosphere to condense into ice.”
Anderson and co-author Robert Samuelson, also from Goddard, reported in their new study that the temperatures in Titan’s lower stratosphere differ based on their latitudes. Using data from Cassini’s Composite Infrared Spectrometer and its radio science instrument, they were able to demonstrate that the high-altitude temperature near the north pole was actually far colder than the conditions just south of the moon’s equator.
“It turns out that this temperature difference – as much as 11 degrees Fahrenheit (minus 12 degrees Celsius) – is more than enough to yield methane ice,” NASA explained. While early observations of the cloud system were said to be “consistent with small particles composed of ethane ice,” further analysis revealed that come regions tended to be denser and clumpier, suggesting that there could be multiple forms of ice present in the clouds.
“The team confirmed that the larger particles are the right size for methane ice and that the expected amount of methane – one-and-a-half percent, which is enough to form ice particles – is present in the lower polar stratosphere,” the agency added, noting that the mechanism for forming stratospheric clouds appeared to differ from those formed at lower altitudes due to global circulation patterns that carry warm air from the summer hemisphere up from the surface, into the stratosphere and towards the winter pole, where it sinks, cools and forms methane clouds.
Like high-altitude clouds on Earth, Titan’s methane cloud was located near the winter pole, above 65 degrees north latitude, according to Anderson and Samuelson. The study authors estimate that this type of cloud system, which is known as subsidence-induced methane clouds (SIMCs), could develop between 98,000 to 164,000 feet above the moon’s surface.
“Cassini has been steadily gathering evidence of this global circulation pattern, and the identification of this new methane cloud is another strong indicator that the process works the way we think it does,” said Michael Flasar, Goddard scientist and principal investigator for Cassini’s Composite Infrared Spectrometer (CIRS).
“Titan continues to amaze with natural processes similar to those on the Earth, yet involving materials different from our familiar water,” added Scott Edgington, Cassini deputy project scientist at NASA’s Jet Propulsion Laboratory (JPL) facility in Pasadena, California. “As we approach southern winter solstice on Titan, we will further explore how these cloud formation processes might vary with season.”
—–
Follow redOrbit on Twitter, Facebook and Pinterest.
—–