When the New Horizons spacecraft swept past Pluto and Charon in 2015, it revealed two amazingly complex worlds and an active atmosphere on Pluto. Those snapshots redefined our understanding of the system. Now, new observations using the James Webb Space Telescope (JWST) taken in 2022 and 2023, show that Pluto’s atmosphere is completely different from any other one in the Solar System. For one thing, it contains haze particles that rise and fall as they are heated and cooled.
Pluto’s atmosphere is a complicated haze of nitrogen, methane, and carbon monoxide. Based on the JWST data, the haze particles control the energy balance of the atmosphere as they heat up and cool off. That’s very unusual and hasn’t been seen at other solar system worlds. The observations were inspired by an idea astronomer Xi Zhang (University of Californa – Santa Cruz) proposed in 2017. “It was a crazy idea,” said Zhang. However, he and the co-authors on the paper felt confident enough to predict that if a haze is cooling Pluto, it should be emitting strong mid-infrared radiation. If so, then an infrared-sensitive telescope should be able to “see” the phenomenon.
Inspired by that prediction, a team of astronomers led by Tanguy Bertrand of the Observatoire de Paris, used JWST to study the haze control of Pluto’s atmospheric heat balance. “We were really proud, because it confirmed our prediction,” Zhang said. “In planetary science, it’s not common to have a hypothesis confirmed so quickly, within just a few years. So we feel pretty lucky and very excited.”
Pluto, Charon, and their Atmospheres
The atmosphere at Pluto is a chemically rich melange of nitrogen, methane, and carbon monoxide. In contrast, Charon has no appreciable atmosphere, although it may experience seasonal outgassing.
A map of Charon’s surface showing deposits of materials from Pluto. Courtesy NASA/JHUAPL/SwRI
The haze we see at Pluto in the New Horizons flyby images and data is an active experiment in nitrogen and methane photochemistry. In that regard, it’s similar to the hazes we see at Titan. Understanding how that experiment works required longer-term observations than the New Horizons spacecraft could accomplish. The JWST observations of Pluto and Charon done in 2022 focused the MIRI instrument on the hazes and atmosphere of Pluto. It also made measurements 18, 21, and 25 microns at both worlds. However, to truly understand the atmospheric activity at Pluto, scientists wanted to get data only about Pluto’s atmosphere. In 2023, MIRI turned its attention to Pluto and provided atmospheric and haze data in the mid-infrared (4.9 – 27 microns) range. That allowed scientists to get a more complete picture of the atmospheric changes and activity at Pluto.
The results revealed variations in surface thermal radiation – that is, temperature changes – at both Pluto and Charon during their rotation. By comparing these data with thermal models of the two worlds, the researchers were able to place strong constraints on the thermal inertia, emissivity, and temperature of different regions of Pluto and Charon. These properties are what drive the global ice distributions on Pluto and pushes material from Pluto to Charon.
The seasonal cycles of volatile ice distribution across the surface drive a migration of ice deposits across the Pluto surface. It’s almost as if various ice deposits are “picked up” and redistributed elsewhere. Some of that material also gets pulled completely away from Pluto and deposited on Charon. As far as scientists know, this doesn’t happen anywhere else in the Solar System.
Controlling Temperatures
The new data show that Pluto’s atmosphere is unique among Solar System planetary atmospheres. Its radiative energy equilibrium – that is, the balance between incoming sunlight and its heat loss to space – is controlled primarily by haze particles instead of gas molecules, as happens at other worlds. According to Zhang, that makes Pluto even more interesting to study. It also gives some insight into Earth’s early atmosphere, which was almost entirely nitrogen and a mixture of hydrocarbons. “By studying Pluto’s haze and chemistry, we might get new insights into the conditions that made early Earth habitable.”
The JWST studies are just a first step toward understanding the complexity of interaction in Pluto’s atmosphere, as well as its contribution to materials found on Charon.
“Pluto sits in a really unique spot in the range of how planetary atmospheres behave. So this gives us a chance to expand our understanding of how haze behaves in extreme environments,” Zhang explained. “And it’s not just Pluto—we know that Neptune’s moon Triton and Saturn’s moon Titan also have similar nitrogen and hydrocarbon atmospheres full of haze particles. So we need to rethink their roles, too.”
For More Information
‘Crazy Idea’ about Cooling Effects of Pluto’s Haze Confirmed by New James Webb Telescope Data
Evidence of Haze Control of Pluto’s Atmospheric Heat Balance from JWST/MIRI Thermal Light Curves
Haze Heats Pluto’s Atmosphere, Yet Explains Its Cold Temperature