Spaceflight Insider

Atmospheric haze makes Pluto colder than previously thought

Artist’s rendition of a view from the surface of Pluto with its moon Charon in the hazy horizon

Artist’s rendition of a view from the surface of Pluto with its moon Charon in the hazy horizon. Image Credit: L. Calçada / ESO

When NASA’s New Horizons spacecraft flew by Pluto in July 2015, its instruments found the dwarf planet’s atmospheric temperature to be significantly colder than scientists had expected.

Pre-flyby estimates put the temperature of Pluto’s atmosphere at 100 kelvins, which equals minus 173 degrees Celsius or minus 280 degrees Fahrenheit.

However, measurements taken by the spacecraft’s instruments found the atmosphere to have a temperature of 70 kelvins, which equals minus 203 degrees Celsius or minus 333 degrees Fahrenheit.

Now, a new study led by Xi Zhang, an assistant professor of Earth and planetary sciences at UC Santa Cruz, attributes Pluto’s colder than expected temperatures to its hydrocarbon atmospheric haze.

The dwarf planet’s complex layered hazes surprised scientists and are one of the mission’s major discoveries.

The researchers propose haze particles in the atmosphere absorb heat from the Sun, causing them to emit infrared radiation. This then releases energy back into space and cools Pluto’s atmosphere.

“It’s been a mystery since we first got the temperature data from New Horizons,” Zhang said of the discrepancy between predicted and observed atmospheric temperatures. “Pluto is the first planetary body we know of where the atmospheric energy budget is dominated by solid-phase haze particles instead of by gases.”

Pluto's blue-sky haze

This is the highest-resolution color departure shot of Pluto’s receding crescent from NASA’s New Horizons spacecraft, taken on July 14, 2015, when the spacecraft was 120,000 miles (200,000 kilometers) away from Pluto. Shown in approximate true color, the picture was constructed from a mosaic of six black-and-white images from the Long Range Reconnaissance Imager, with color added from a lower resolution Ralph / Multispectral Visible Imaging Camera color image. The image is dominated by spectacular layers of blue haze in Pluto’s atmosphere – which the Alice instrument probed to gather compositional data. (Click for high-resolution image) Image & Caption Credit: NASA / JHU-APL / SwRI

Pluto’s many-layered haze is produced by chemical reactions in its upper atmosphere that are triggered by interaction with ultraviolet radiation from the Sun.

As a result of this interaction, nitrogen and methane become ionized and form tiny hydrocarbon particles that fall through the atmosphere. While falling, the particles begin sticking together, becoming larger before landing on the surface.

“We believe these hydrocarbon particles are related to the reddish and brownish stuff seen in images of Pluto’s surface,” Zhang reported.

He and his colleagues are interested in studying the atmospheres of similar worlds, such as Saturn’s moon Titan and Neptune’s moon Triton, to better understand the way haze particles influence their atmospheres. This same mechanism could even drive activity in the atmospheres of hazy exoplanets.

Researchers think that the James Webb Space Telescope (JWST), scheduled for launch in 2019, will be capable of detecting infrared radiation produced by Pluto’s atmospheric haze particles.

Findings of the study have been published in the November 16 issue of the journal Nature.



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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