The dwarf planet’s atmosphere, much like the Earth, is primarily made up of nitrogen. On Pluto, this is supported by the vapour pressures of ice on its surface. This means that if Pluto increases in temperature, the density of its atmosphere can be significantly altered.
Over the past 25 years, Pluto has been moving continually further from the Sun and therefore its surface temperature is going down. This has resulted in the atmosphere of the small celestial body becoming colder – until it becomes closer to the Sun during other parts of its irregular orbit.
Pluto takes 248 Earth years to complete one full orbit around the Sun, and its distance varies from its closest point by about 20 astronomical units (the distance from the Earth to the Sun).
Until 2018, despite Pluto moving away from the Sun, its surface pressure and atmospheric density was increasing. This is due to a process called thermal inertia.
“An analogy to this is the way the Sun heats up sand on a beach,” said Dr Leslie Young, a scientist from Southwest Research Institute who specialises in modelling the interaction between the surfaces and atmospheres of icy bodies in the outer solar system.
“Sunlight is most intense at high noon, but the sand then continues soaking up the heat over the course of the afternoon, so it is hottest in late afternoon. The continued persistence of Pluto’s atmosphere suggests that nitrogen ice reservoirs on Pluto’s surface were kept warm by stored heat under the surface. The new data suggests they are starting to cool.”
The astronomers discovered this strange phenomenon by using telescopes in the United States and Mexico to observe Pluto’s atmosphere when it passed in front of a star. This occultation event – which only took two minutes – allowed them to measure the atmosphere by tracking the rate at which the star disappeared and reappeared.
“Scientists have used occultations to monitor changes in Pluto’s atmosphere since 1988,” said Dr Eliot Young, a senior program manager in SwRI’s Space Science and Engineering Division.
“The New Horizons mission obtained an excellent density profile from its 2015 flyby, consistent with Pluto’s bulk atmosphere doubling every decade, but our 2018 observations do not show that trend continuing from 2015.”
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