NASA describes Saturn's moon Titan as the most Earth-like world they have discovered so far.
In a new study published in the Geophysical Research Letters, researchers presented what could be considered as another Earthlike quality of the moon: a seemingly impossible cloud that may have been created by weather processes similar to those seen on Earth.
This impossible cloud was first discovered by the infrared instrument onboard NASA's Voyager 1 spacecraft decades ago. The cloud was composed of a carbon and nitrogen based compound known as dicyanoacetylene (C4N2), which is partially responsible for the brownish-orange haze of the moon
NASA's Cassini-Huygens spacecraft,which has been orbiting Saturn found a second example of the cloud recently. Just like with the earlier discovery, the cloud contains dicyanoacetylene and the surrounding stratosphere was found to contain less than 1 percent of this compound required for the cloud to exist.
Condensation is needed for clouds to form and this takes place when the air temperature and pressure can condense vapor into ice.
Clouds in the moon's stratosphere form when the ice and methane vapor are in a state of equilibrium, but the amount of dicyanoacetylene vapor found here is supposedly not enough to keep the ice trapped in the cloud in equilibrium. Scientists said that at least 100 times more vapor is needed in Titan's stratosphere to form an ice cloud.
"For clouds that condense, this equilibrium is mandatory, like the law of gravity," said study co-author Robert Samuelson, from NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Scientists, however, think they have found an explanation to this phenomenon in certain clouds on earth that forego condensation. These clouds form through a kind of "solid-state" chemistry.
Earth's stratosphere has scant moisture, but polar stratospheric clouds can form when chlorine-bearing chemicals enter the atmosphere as pollution and stick to crystals of water ice causing a chemical reaction that produces ozone-destroying chlorine molecules.
A similar process could create the ice clouds on Titan. Cyanoacetylene, which contains hydrogen, carbon and nitrogen, could become coated with hydrogen cyanide as it moved down an icy crystals into the stratosphere. Ultraviolet rays from the sun that hit one of these dual-layer ice crystals could result in a chemical reaction that would release dicyanoacetylene and hydrogen to form a cloud.
"As our analysis of the Cassini Composite Infrared Spectrometer 478 cm-1 ice emission feature demonstrates, this solid-state photochemistry mechanism eliminates the need for the relatively high C4N2 saturation vapor pressures required (even though they are not observed) when the ice is produced through the usual procedure of direct condensation from the vapor," the researchers wrote in their study.