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NASA scientists call it the ‘impossible’ cloud. It’s made up of a compound of carbon and nitrogen called dicyanoacetylene, one of the many compounds responsible for the moon’s brownish-orange atmosphere. Scientists have struggled to explain it ever since Voyager 1 first saw it more than 30 years ago.
“The appearance of this ice cloud goes against everything we know about the way clouds form on Titan,” says Carrie Anderson, a CIRS co-investigator. CIRS stands for Cassini’s composite infrared spectrometer.
Why is this cloud so weird? Remember learning about how clouds form on Earth back in school? The cycle of evaporation and condensation creates clouds. That same cycle happens on Titan with methane instead of water.
This process alters slightly in Titan’s stratosphere (where the ice cloud was spotted). Circulation patterns push warm gas downward at the poles. As this gas sinks through the cooler layers of the polar stratosphere, it condenses. Instead of rising and condensing, gas sinks and condenses here. But the vapor and ice have to reach a perfect equilibrium, which is determined by air temperature and pressure. Without the equilibrium, you don’t get a cloud.
“This equilibrium is mandatory, like the law of gravity,” says Robert Samuelson, a co-author of the paper.
Because this equilibrium is constant, scientists can figure out the amount of vapor where ice is present. And this is where the cloud gets its ‘impossible’ name. There should be at least 100 times more dicyanoacetylene vapor to form the ice cloud spotted by Cassini.
But the team have come up with a solution. The scientists call it “solid-state chemistry.” They got the idea from Earth’s stratosphere. Up there, polar stratospheric clouds form as chlorine-bearing chemicals stick to water ice crystals. The chemical reactions that follow release chlorine molecules.
Titan’s ice cloud didn’t form via condensation. It formed from reactions with other ice particles.
Here’s what the scientists propose.
Ice particles made from cyanoacetylene (HC3N) is coated by hydrogen cyanide (HCN) as it dives through Titan’s stratosphere. Now you have an ice particle with a core of cyanoacetylene and a shell of hydrogen cyanide. Every so often, a photon of UV light gets inside the frozen shell and sets off chemical reactions in the ice. This reaction can start in the core or the shell. It doesn’t matter. What’s left is dicyanoacetylene ice and hydrogen.
This solves the vapor conundrum. Because dicyanoacetylene ice doesn’t make direct contact with the atmosphere, the ice/vapor numbers don’t have to match up.
Here’s Michael Flasar talking about the similarities and differences between Earth and Titan.
“The compositions of the polar stratosphere of Titan and Earth could not differ more,” said Flasar. “It is amazing to see how well the underlying physics of both atmospheres has led to analogous cloud chemistry.”
Research like this will continue long after Cassini plunges into Saturn’s atmosphere next year. The hardy spacecraft’s mission is entering its final year. On September 15, 2017, Cassini’s mission will come to a close by diving directly the planet’s atmosphere. Friction with Saturn’s atmosphere will result in a spectacular, fiery end for NASA’s Saturn explorer.