Pluto’s atmosphere appears to be diminishing as the distant dwarf planet drifts into colder regions of its orbit, according to new research. Scientists observed a 16% reduction in atmospheric pressure between 2021 and 2023, possibly indicating that Pluto’s thin atmosphere is starting to freeze back onto its icy surface.
The research team, led by Amanda Sickafoose from the Planetary Science Institute, used starlight to monitor changes in Pluto’s atmosphere. Their study, published in The Planetary Science Journal, analyzed data collected between 2017 and 2023, several years after NASA’s New Horizons mission visited the dwarf planet.
Following its closest approach to the sun in 1989, Pluto is now venturing into the colder regions of its elongated orbit. Scientists are closely observing how its atmosphere responds to these cooling conditions.
This marks the first occasion since Pluto’s 1930 discovery that researchers have tracked its atmosphere during this phase of its orbit. These insights may shed light on the long-term interactions between Pluto’s surface ice and atmospheric gases.
Unveiling Pluto’s Atmosphere Using Stellar Occultations
Currently, no spacecraft is close enough to directly examine Pluto’s atmosphere. From Earth, the dwarf planet is too small and dim for telescopes to resolve fine atmospheric details.
To overcome this, scientists observe Pluto as it crosses in front of distant stars during events called stellar occultations. These allow them to analyze variations in starlight caused by the atmosphere’s presence.
When starlight filters through Pluto’s atmosphere, it bends and scatters, altering the star’s brightness before it disappears behind the dwarf planet. Observing these light changes gives clues about atmospheric density and pressure.
According to the team headed by Amanda Sickafoose, Pluto’s atmospheric pressure remained largely steady from the 2015 New Horizons flyby until mid-2021. Afterwards, measurements from mid-2021 through July 2023 revealed a 16% decline in overall pressure.
The study included analysis of 10 occultation events recorded between 2017 and 2023. Four of these were observed simultaneously from multiple Earth locations, aiding validation and providing diverse perspectives of Pluto’s atmosphere.
Dimming Atmospheric Haze on Pluto
Pluto’s atmosphere mainly consists of nitrogen, with traces of methane and carbon monoxide. Interaction of these gases produces haze layers, similar to those detected by New Horizons in 2015.
The researchers incorporated these haze layers into atmospheric models since they affect the behavior of light passing through. If the pressure drop continues, these hazes are expected to thin out and settle closer to the surface.
The study explains that Pluto’s atmosphere is closely connected to its surface ices. The frozen terrain and overlying gases continually interact, making the dwarf planet’s climate systems complex to unravel.
“I’m constantly amazed at how the simple technique of watching starlight dim and reappear allows us to study a thin atmosphere—a few millionths of Earth’s—on a world two-thirds the size of our moon and 30 times farther from the sun,” said Sickafoose.
Further Observations Needed to Understand Atmospheric Trends
While the recent decrease in atmospheric pressure is significant, additional data is essential to determine whether this marks a sustained decline or a short-term fluctuation.
Sickafoose emphasized that ongoing observations in the years and decades ahead will be crucial to track changes in Pluto’s atmosphere as it continues its path around the sun.
“We’re at a particularly interesting point for Pluto,” Sickafoose explained. “Our work suggests that the atmosphere has recently started decreasing in pressure. I am hopeful that we’ll be able to get more data in the coming years to decades to specifically confirm or refute this trend.”
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