spaceSpace and Physics

Scientist Have Discovered An "Impossible" Ice Cloud On Titan


Jonathan O'Callaghan

Senior Staff Writer


A true-color image of Titan (note, there is no image of this new cloud feature available). NASA/JPL-Caltech/Space Science Institute

Scientists at NASA have discovered a strange new feature on Titan that doesn’t fit in with our current understanding of this moon of Saturn. It does, though, point to this world being even more Earth-like than we thought.

What the scientists discovered, using the composite infrared spectrometer (CIRS) on the Cassini spacecraft, was a cloud made of a compound of carbon and nitrogen called dicyanoacetylene (C4N2). This compound is responsible for giving the moon its brownish-orange atmospheric hue.


The cloud was found in Titan’s stratosphere (a similar feature was found by Voyager 1 in 1980), the upper level of the atmosphere. But the confusing thing is that there is not enough C4N2 in vapor form to explain how this cloud formed. As NASA puts it, Titan’s stratosphere is as “dry as a desert”, but this cloud has still somehow been able to form from this frozen chemical.

"The appearance of this ice cloud goes against everything we know about the way clouds form on Titan," said Carrie Anderson, a CIRS co-investigator at NASA's Goddard Space Flight Center in Greenbelt, Maryland, in a statement. Anderson is the lead author of a study on this discovery published in Geophysical Research Letters.

Clouds on Titan generally form like they do on Earth. On our world, water evaporates and condenses in a cycle, but on Titan the process takes place using methane instead. At a high enough altitude, the evaporated methane turns into ice, forming a cloud.

But for this cloud to have formed, NASA said there would have to be 100 times more vapor than has been detected. To explain it, the researchers suggest it may have formed through a different process called solid-state chemistry. How that works, well, we’ll let NASA explain.


How scientists think solid state chemistry may work on Titan. NASA/JPL-Caltech/GSFC

“The first step in the proposed process is the formation of ice particles made from the related chemical cyanoacetylene (HC3N),” NASA said. “As these tiny bits of ice move downward through Titan's stratosphere, they get coated by hydrogen cyanide (HCN).

“At this stage, the ice particle has a core and a shell comprised of two different chemicals. Occasionally, a photon of ultraviolet light tunnels into the frozen shell and triggers a series of chemical reactions in the ice. These reactions could begin either in the core or within the shell. Both pathways can yield dicyanoacteylene ice and hydrogen as products.”

A similar process can occur on Earth, when chlorine-bearing chemicals stick to crystals of water ice. The stratospheres of Titan and Earth are vastly different in composition, but this research suggests some of the underlying physics may be the same.


"It's very exciting to think that we may have found examples of similar solid-state chemical processes on both Titan and Earth," said Anderson.


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