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What Do We Know About the Atmosphere of Pluto?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

A Tail?

Scientists have known about the atmosphere of Pluto thanks to many occultations it made with stars in the past, but the actual size of it was unknown until the flyby of New Horizons. Measuring in at 1,650 miles above the surface, the atmosphere was not only larger than expected but also colder and more dense than anticipated (see the section on haze for more on that).

But what was even more surprising was what the SWAP instrument detected behind Pluto: a comet-like tail spanning 48,000–68,000 miles! The low gravity of Pluto means it cannot hang onto any gases for long, so they escape the surface, being deflected by the solar wind into a tail. It is made of ionized nitrogen and forms a plasma, which allows the solar wind to carry away up to 500 tons of it each hour.

Despite all of this, Pluto maintains a 7–10 microbar surface pressure (about 0.001% that of Earth's), and has only lost half a foot of total atmosphere depth to space since the formation of the solar system (as opposed to the old-school though of a half mile) (Thompson "First", Calderone, Betz "Pluto's icy," Stern "The Pluto" 26, Talcott).

The haze layers revealed.

The haze layers revealed.

Compounds of the Atmosphere

What else is in the atmosphere? On July 24, 2015, just 10 days post flyby, NASA released a picture of Pluto backlit by the Sun. The outlined dwarf planet shows two haze layers with one 30 miles above the surface and the other 50 miles above, going up to 80 miles high. These layers are not that dense - just 100,000 times less than Earth's atmosphere. The production of the haze is a little complicated, so buckle up.

Methane, carbon monixide, and nitrogen gases in the atmosphere of Pluto get broken down by UV light and converts it to ethylene, acetylene, hydrogen cyanide, and other complex hydrocarbons called tholins which are red in color, produce a blue appearance similar to what nitrogen and oxygen in our atmosphere scatters, and are 0.1 to 0.5 microns in size. This also releases heat via infrared rays and cool the haze to about 70 Kelvin. The tholins and hydrogen cyanides settle to the lower levels of the atmosphere below 125 miles in altitude where they freeze into icy particles and become haze. They act as seedlings upon which other chemicals can latch onto, building up much like a rolling snowball does. It is possible that these can be broken down further by UV light and other particles into even more tholins which help give a red tint to the dwarf planet. Others that break down simply begin the freezing process all over again. All of this means Pluto gives off less heat than expected and keeps it trapped inside the atmosphere. Some clouds were even spotted, somehow correlated in their movement to the start and ends of a Pluto day (Thompson "New," Powell, Betz "Pluto Surprises," Stern "The Pluto" 26, Stern "Hot" 30, Wenz, Stephens, Johnson).

ALICE readings on Pluto.

ALICE readings on Pluto.

Penitentes

In an interesting twist, Pluto became only the second known location to have penitentes. Located in the Tartarus Dorsa region, these formations happen on Earth at high latitudes and result from interactions with light and methane ice according to work by John Moores (York University in Canada). But on Pluto, they rise up to 500 meters tall, way higher than their Earth counterparts! They formed because of the extreme cold temperatures combined with the low atmosphere allowed for nitrogen and methane ices to directly sublimate into water vapor and combining that with reflections on the surface, and viola! Or course, other explanations for the features are out there, including glaciation or wind sculpting, but without far side data it will be tough to tell (Dockrill, Stern "Puzzled" 24)

What else will we uncover about Pluto's atmosphere in the years ahead? Who knows, it is all up in the air at the moment...

Works Cited

Betz, Eric. "Pluto’s icy plains, pits, and mountains take shape in Tombaugh Regio." Astronomy.com. Kalmbach Publishing Co., 17 Jul. 2015. Web. 19 Aug. 2015.

---. "Pluto Surprises With Blue Skies, Red Water." Astronomy Jan. 2016: 16. Print.

Calderone, Julia. "NASA just found something big hiding out behind Pluto." BusinessInsider.com. Business Insider Inc., 17 Jul. 2015. Web. 19 Aug. 2015.

Dockrill, Peter. "NASA has discovered gigantic ice towers on Pluto, standing 500 metres tall." Sciencealert.com. Science Alert, 10 Jan. 2017. Web. 06 Feb. 2017.

Johnson, Scott K. "Like Titan, Pluto's atmosphere is hazy, but for a different reason." Astronomy.com. Kalmbach Publishing Co., 28 Dec. 2020. Web. 20 Jun. 2021.

Powell, Corey S. "The Passion of the Pluto-Philes." Discover Dec. 2015: 71. Print.

Stephens, Tim. "Pluto's hydrocarbon haze keeps dwarf planet colder than expected." innovations-report.com. innovations report, 16 Nov. 2017. Web. 12 Mar. 2019.

Stern, Alan. "Hot Results from a Cool Planet." Astronomy May 2016: 30. Print.

---. "Puzzled by Pluto." Astronomy Sept. 2017. Print. 24.

---. "The Pluto System Explored." Astronomy Nov. 2015: 26. Print.

Talcott, Richard. "New Horizons Unleashes Torrent of Pluto Science." Astronomy Mar. 2016: 15. Print.

Thompson, Amy. "First Pluto data reveals lots of terrain that is “not easy to explain” ars technica. Conte Nast., 17 Jul. 2015. Web. 19 Aug. 2015.

---. "New Horizons data shows Pluto’s atmosphere, surface features." ars technica. Conte Nast, 27 Jul. 2015. Web. 19 Aug. 2015.

Wenz, John. "Pluto's Hazy Atmosphere Could Have Large Clouds." Astronomy Feb. 2017: 13. Print.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Leonard Kelley

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