Pluto’s atmosphere is slowly disappearing, scientists find

As a celestial body, it’s not always easy to cling to your atmosphere – just ask Mars – and a new study suggests that the gases surrounding Pluto are now disappearing, turning to ice as the planet dwarf moves away from the Sun.

Pluto’s already thin atmosphere is made up largely of nitrogen with a few touches of methane and carbon monoxide. As temperatures drop at the surface, it appears to cause the nitrogen to freeze again, causing the atmosphere to discolour.

The assessment was done using what’s called occultation: using a distant star as a backlight for telescopes on Earth to take a look at what’s going on on Pluto. It is a proven observation technique widely used in astronomy.

Pluto and its atmosphere, observed by New Horizons. (NASA / JHU-APL / SwRI)

“Scientists have used occultations to monitor changes in Pluto’s atmosphere since 1988,” says planetologist Eliot Young of the Southwest Research Institute (SwRI) in Texas.

“The New Horizons mission achieved an excellent density profile from its 2015 flyby, consistent with Pluto’s global atmosphere doubling every decade, but our 2018 observations do not show this trend continuing from 2015. “

Pluto’s atmosphere is created from ice vaporized on the surface, with small changes in temperature causing large changes in the bulk density of the atmosphere. The largest known nitrogen glacier is Sputnik Planitia, the western part of the Tombaugh Regio heart-shaped region visible on the surface of Pluto.

The dwarf planet currently takes 248 Earth years to orbit the Sun, at any given time approaching up to 30 Astronomical Units (AU) from the Sun, or 30 times the distance between Earth and the Sun.

This distance increases, however, leaving Pluto with less sun and lower temperatures. The increase in the density of the atmosphere observed in 2015 is most likely due to thermal inertia – the waste heat trapped in nitrogen glaciers that has a delayed reaction to the increasing distance between Pluto and the Sun.

“An analogy to this is how the Sun heats sand on a beach,” says SwRI planetologist Leslie Young. “The sunlight is most intense at midday, but the sand then continues to absorb heat in the afternoon, so it gets warmer in the late afternoon.”

(NASA / SwRI)

Above: Telescopes near the middle of the shadow’s path observed a “central flash” caused by Pluto’s atmosphere refracting light in a region in the center of the shadow.

Pluto may no longer count as a planet – still a source of controversy among experts – but it remains a planetary body of interest to astronomers. We keep learning new information about this distant rock all the time.

In recent years, astronomers have been able to establish that there were snow-capped mountains on Pluto and liquid oceans below its surface – two findings that can tell us more about how the dwarf planet’s atmosphere (and both following the new horizon flyover).

The 2018 observations benefited from a “central flash,” indicating that the telescopes used were looking directly at Pluto while atmospheric measurements were being calculated, further adding to their credibility.

“The center flash seen in 2018 was by far the most powerful anyone has ever seen in a Pluto occultation,” Young explains. “The central flash gives us very precise knowledge of the path of Pluto’s shadow on Earth.”

The results were shared at the annual meeting of the American Astronomical Society Division for Planetary Sciences.