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Various icy compounds make up Pluto’s atmospheric haze

Pluto's atmospheric haze. Credit: NASA/JHUAPL/SwRI

Pluto’s atmospheric haze. Credit: NASA/JHUAPL/SwRI

Composed of various icy organic compounds, Pluto’s famous blue atmospheric haze likely formed differently from the hazes surrounding Saturn’s moon Titan and Neptune’s moon Triton.

All three worlds have an abundance of nitrogen, methane, and carbon monoxide in their atmospheres, which initially led scientists to think their hazes formed similarly and have similar compositions.

Cassini image of haze surrounding Saturn’s moon Titan. Image Credit: NASA/JPL-Caltech/Space Science Institute

Now, a new study finds that Pluto’s and Triton’s hazes formed very differently from that of Titan.

Using computer models of haze formation, a team of scientists found that because Pluto’s atmosphere is approximately 175 degrees Fahrenheit (80 degrees Celsius) lower than that of Titan, the chemical reactions that produce Titan’s haze would create only half of the haze particles NASA’s New Horizons spacecraft detected around Pluto.

One reason for this is chemical reactions occur much more slowly in colder environments.

On Titan, these chemical reactions in the upper atmosphere are triggered by sunlight. The result is simple organic compounds, such as hydrogen cyanide, that through additional chemical reactions go on to form more complex organic molecules.

Ultraviolet solar radiation triggers similar chemical reactions in Pluto’s upper atmosphere, but because of the colder temperatures, simple compounds such as hydrogen cyanide, freeze, producing ice particles.

As the small planet’s gravity pulls these ice particles downward toward the surface, other gases condense around the particles, causing the haze to grow thicker.

Voyager 2 image of Neptune’s moon Triton. Image Credit: NASA/JPL

The additional chemical reactions that occur in Titan’s atmosphere, producing more complex organic molecules, do not occur in Pluto’s atmosphere, as there is less interaction between solar radiation and the icy particles that comprise Pluto’s haze.

This results in Pluto’s haze being composed mostly of various ices. Because Triton’s atmosphere is even colder than Pluto’s, its chemical reactions occur even more slowly and likely make its haze even more icy than Pluto’s.

According to the research team, the computer model accurately predicted the amounts, sizes, and atmospheric distributions of the various ices New Horizons detected in Pluto’s haze.

“The comparison of Titan, Pluto, and Triton illuminates the important differences in the mechanisms of haze formation in planetary atmospheres,” noted Panayotis Lavvas of the University of Reims Champagne-Ardenne in Reims, France.

Lavvas is lead author of a paper on the findings published in the journal Nature Astronomy.

Pluto's blue atmospheric haze. Image Credit: NASA/JHUAPL/SwRI

Pluto’s blue atmospheric haze. Image Credit: NASA/JHUAPL/SwRI


Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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