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SOFIA data sheds new light on endurance of Pluto’s atmosphere

High-resolution image of Pluto’s layered hazes, as captured by New Horizons. Image Credit: NASA/JHUAPL/SwRI

Data captured by the Stratospheric Observatory for Infrared Astronomy (SOFIA), an airborne observatory that studied Pluto’s atmospheric hazes just two weeks before New Horizons‘ 2015 flyby, indicates the hazes in Pluto’s atmosphere are regularly replenished.

Analysis of that data reveals the hazes are made up of tiny particles that remain in Pluto’s atmosphere for some time rather than quickly falling to the surface.

The atmosphere is created when sunlight causes particles on Pluto’s surface to vaporize.

SOFIA, a modified Boeing 747 airplane carrying a nine-foot telescope, observed Pluto as the dwarf planet occualted or passed in front of a star, casting a faint shadow on Earth’s surface. Scientists use such occultations to observe distant objects during the intervals between spacecraft flybys.

During an occultation, the object observed is backlit by the background star it eclipses. If, like Pluto, the foreground object has an atmosphere, that atmosphere can be studied and analyzed.

In anticipation of the New Horizons flyby, SOFIA focused on the middle layers of Pluto’s atmosphere, imaging them in both optical and infrared light. New Horizons complemented SOFIA’s observations by using radio waves and ultraviolet light to study the atmosphere’s upper and lower layers.

Image from artist’s animation of Pluto occulting a star. Image Credit: NASA

Together, the SOFIA and New Horizons observations, along with those taken with NASA’s Kuiper Airborne Observatory (KAO), which initially confirmed the existence of Pluto’s atmosphere, provide a detailed, complete understanding of that atmosphere.

Pluto reached its perihelion, or closest distance to the Sun, in 1989. One concern of the scientists who spent 25 years advocating a Pluto mission was that its atmosphere would collapse before a spacecraft would reach the dwarf planet, making studies of its atmosphere impossible.

That did not happen. Instead, New Horizons found that Pluto’s atmosphere rises higher above its surface, in relative terms, than Earth’s atmosphere rises above our planet. The spacecraft’s Alice imaging spectrograph observed Pluto’s atmosphere as high as 1,000 miles (1,600 km) above its surface.

Pluto’s atmosphere, like Earth’s, is composed primarily of nitrogen gas. It also contains low levels of carbon monoxide and methane. The atmospheric hazes, composed of particles between 0.06 and 0.10 microns thick, form more than 20 miles above Pluto’s surface. Interaction between atmospheric methane, other gases, and sunlight eventually causes the particles to slowly fall to the surface below.

New Horizons detected these tiny atmospheric particles, which scatter blue light, and are therefore responsible for the hazes’ blue color. SOFIA‘s data revealed just how tiny the particles are.

“Pluto is a mysterious object that is constantly surprising us. There had been hints in earlier remote observations that there might be haze, but there wasn’t strong evidence to confirm it really existed until the data came from SOFIA. Now, we’re questioning if Pluto’s atmosphere is going to collapse in the coming years–it may be more resilient than we thought,” said Michael Person of the Massachusetts Institute of Technology (MIT) Wallace Astrophysical Observatory and lead author of a paper on the findings published in the journal Icarus.

Analysis of SOFIA data shows that Pluto’s atmosphere alternately thickens and thins in a cycle that lasts only a few years, rather than along longer timescales based on its 248-year orbit around the Sun. The fact that the haze particles form quickly also indicates that the atmosphere may be regulated by Pluto’s elliptical orbit rather than by its changing distance from the Sun.

Additionally, because Pluto rotates on its side, like Uranus, various regions on its surface receive different amounts of sunlight at varying times during its orbit. Increasing sunlight may cause icy regions to vaporize and thicken the atmosphere, producing more haze particles. Decreasing sunlight may cause the atmosphere to thin out.

Scientists have observed these changes even as Pluto recedes from the Sun into the colder regions of the Kuiper Belt.

“There’s still a lot we don’t understand, but we’re forced now to consider earlier predictions. Pluto’s atmosphere may collapse more slowly than previously predicted, or perhaps not at all. We have to keep monitoring it to find out,” Person said.




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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