What's out of sight?
Pluto photographed by the New Horizons probe. NASA/Johns Hopkins University Applied Physics Laboratory/SouthwestErres/Alex Parker
In 2016, as Pluto skimmed past a distant star (the so-called "occultation" phenomenon occurred), astronomers found that its atmosphere was thickening, confirming a conjecture since 1988.
Now, Pluto's atmosphere is accelerating its disappearance. In July 2019, Pluto passed in front of a star again, and astronomer Ko Arimatsu and others at the University of Tokyo were surprised to find that Pluto's atmospheric pressure was more than 20% lower than in 2016.
As Pluto approaches the Sun, its frozen surface sublimates as it is baked by the Sun's heat, producing nitrogen, methane, and carbon dioxide, thickening its thin atmosphere. And as it moves away from the sun, its atmosphere gradually freezes on the surface.
Pluto began to move away from the Sun after reaching perihelion at the end of 1989. But astronomers have found that its atmosphere is still thickening at this stage.
After the New Horizons probe flew by Pluto in 2015, astronomers believed they knew why. Pluto's surface is complex. In one of its hemispheres, there is a huge basin – the white "Sponnick" ice sheet.
The Sponnick ice sheet may play a key role in changes in Pluto's atmosphere. It releases gas when it is bathed in sunlight. And that may be the reason why Pluto's atmosphere is still thickening as it moves away from the sun.
Theories predict that the Sponnick ice sheet will begin to cool down in 2015 and cause Pluto's atmosphere to gradually solidify. But the problem is that the theory predicts that Pluto's atmospheric pressure will only drop by up to 1% after 2016, while Japanese scientists have found that the actual decline in Pluto's atmospheric pressure has reached 20%.
This suggests that some unknown cause is accelerating the disappearance of Pluto's atmosphere. What is the reason for this unknown? Could it be related to Another Hemisphere of Pluto that we haven't seen before? It is still unknown.
This may also be an error, because it is not easy to observe Pluto's atmosphere through "occultation", and there are many uncertainties. As Pluto moves farther and farther away from the Sun, it is also moving away from the plane of the Milky Way, and the probability of its occultation with suitable stars is getting lower and lower. In the future, if people want to observe Pluto's atmosphere through occultation, it will obviously be more and more difficult to solve the mystery of Pluto's atmosphere disappearance.
reference:
pluto’s strange atmosphere just collapsed
https://astronomy.com/news/2020/05/plutos-strange-atmosphere-just-collapsed