As a celestial body, hanging on to your atmosphere isn't always easy – just ask Mars – and a new study suggests that the gases surrounding Pluto are now vanishing, turning back into ice as the dwarf planet drifts further away from the Sun.
Pluto's atmosphere, already on the thin side, is made up largely of nitrogen with a few dabs of methane and carbon monoxide. As temperatures drop on the surface, it seems that this is causing nitrogen to freeze up again, causing the atmosphere to fade.
The assessment was made using what's known as occultation: using a distant star as a backlight for telescopes on Earth to take a look at what's happening on Pluto. It's a tried and tested observation technique used widely in astronomy.
"Scientists have used occultations to monitor changes in Pluto's atmosphere since 1988," says planetary scientist Eliot Young from the Southwest Research Institute (SwRI) in Texas.
"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."
Pluto's atmosphere is created from the vaporized ice on the surface, with small changes in temperature leading to significant changes in the bulk density of the atmosphere. The largest known nitrogen glacier is Sputnik Planitia, the western part of the heart-shaped Tombaugh Regio area visible on Pluto's surface.
The dwarf planet currently takes 248 Earth years to make one orbit around the Sun, at one point getting as close as 30 astronomical units (AUs) from the Sun – that's 30 times the distance between Earth and the Sun.
That distance is growing though, leaving Pluto with less sunlight and lower temperatures. The boost in atmosphere density noticed in 2015 is most likely due to thermal inertia – residual heat trapped in the nitrogen glaciers that has a delayed reaction to the increasing distance between Pluto and the Sun.
"An analogy to this is the way the Sun heats up sand on a beach," says SwRI planetary scientist Leslie Young. "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."
Above: Telescopes near the middle of the shadow's path observed a 'central flash' caused by Pluto's atmosphere refracting light into a region at the center of the shadow.
Pluto might not count as a planet any more – still a source of some controversy amongst experts – but it very much remains a planetary body of interest for astronomers. We continue to learn new information about this distant rock all the time.
In recent years, astronomers have been able to ascertain that there are snowcapped mountains on Pluto, and liquid oceans under its surface – two discoveries that can tell us more about how the dwarf planet's atmosphere operates (and both coming as a result of the 2015 New Horizons flyby).
The 2018 observations benefited from a 'central flash', indicating that the telescopes used were looking straight at Pluto while atmospheric measurements were calculated, and further adding to their credibility.
"The central flash seen in 2018 was by far the strongest that anyone has ever seen in a Pluto occultation," says Young. "The central flash gives us very accurate knowledge of Pluto's shadow path on the Earth."
The findings have been shared at the American Astronomical Society Division for Planetary Sciences Annual Meeting.