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Pluto’s atmosphere is disappearing

Pluto's atmosphere: View of a silhouetted planet with a thin, white glowing layer around the edge.
Pluto’s atmosphere is visible when the planet is backlit, as in this 2015 image from New Horizons, taken as it sped away from the dwarf planet into deeper space. Image via NASA/ Johns Hopkins/ SwRI.

Pluto’s atmosphere is disappearing

Distant Pluto has a highly elliptical orbit around the sun. The shape of its orbit brings Pluto closer to the sun than Neptune for about 20 years in every Plutonian “year” (248 Earth-years). Pluto was last closer than Neptune from 1979 to 1999. And, while it was still relatively close to Earth, scientists rushed to send a spacecraft to Pluto. In 2015, when the New Horizons spacecraft made its dramatic sweep past Pluto, data it returned to Earth suggested Pluto’s atmosphere was doubling in density every decade. But more recently, during a 2018 occultation of a star by Pluto – an event that backlit Pluto’s atmosphere – data suggested Pluto’s atmosphere was starting to decrease and would eventually vanish.

The scientists presented these results on October 4, 2021, at the 53rd annual DPS meeting. They said their new work confirms the idea that, as Pluto gets farther from the sun again in its highly elliptical orbit, its atmosphere is freezing and falling back onto its surface. Eliot Young of the Southwest Research Institute (SwRI) commented:

The New Horizons mission obtained [excellent data on density of Pluto’s atmosphere] 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.

August 15, 2018, occultation

Astronomers on Earth first discovered Pluto’s atmosphere in 1988, during an occultation of a star by Pluto. At that time, all of humanity considered Pluto the ninth planet in our solar system. Since 2006, it’s been categorized as a dwarf planet. During the 1988 occultation, the star’s light gradually dimmed just before it disappeared behind Pluto. The dimming demonstrated the presence of Pluto’s thin, greatly distended atmosphere. New Horizons was then able to analyze that atmosphere from a near distance, as it swept past in 2015.

From 1988 until the present, astronomers have been monitoring Pluto’s atmosphere via rare occultations of stars by Pluto as seen from Earth. On the evening of August 15, 2018, they were ready for yet another occultation of a distant background star by Pluto. They knew that Pluto would pass in front of the star as seen from the United States and Mexico. They knew that – as the dwarf planet and its atmosphere were backlit by the star – a faint shadow of Pluto would be moving across Earth’s surface. The center line of this shadow path ran from Baja California to Delaware. Scientists deployed telescopes along the shadow path to study Pluto during the occultation, while its atmosphere was backlit by the star.

For two minutes, the background star’s light diminished as it passed behind Pluto’s atmosphere, then increased again as the star emerged on Pluto’s other side. You can see that light curve in the inset of the image below. But why is the curve W-shaped? What is that central peak?

Map with path of Pluto occultation across Mexico, Louisiana and the Northeast, with a graph inset.
On August 15, 2018, scientists deployed telescopes near the center line of an occultation of a star by Pluto. The inset shows the resulting W-shaped light curve, which helped astronomers confirm that Pluto’s atmosphere is freezing out onto its surface, as it moves farther away from the sun. Image via SwRI/ NASA.

Reading Pluto’s central flash

Scientists can analyze the light curve that results when Pluto occults a star, to get information on the density of Pluto’s atmosphere. But notice that the light curve superimposed on the image above is W-shaped. It has what astronomers call a central flash. That central flash appears for those observers only on the precise center line of the occultation. It happens when the star is most directly behind Pluto, when Pluto’s atmosphere allows light from the occulted star to be refracted, or bent, into a point at the center of Pluto’s shadow. Seeing this central flash gives astronomers confidence they are in the exact right place to observe the occultation. And it lets them know their analysis of the event is as accurate as it can be. Elliot Young explained:

The central flash seen in 2018 was by far the strongest that anyone has ever seen in a Pluto occultation. The central flash gives us very accurate knowledge of Pluto’s shadow path on the Earth.

All this attention to fine detail is important when you’re gazing across several billion miles of space at the tenuous (and temporary) atmosphere of a world as tiny as Pluto. This little dwarf planet is less than 1,500 miles across (2,400 km) in contrast to Earth’s 8,000 miles (13,000 km). That’s why the appearance of a central flash was so important to these scientists. It helped give them the confidence to assert that, yes, Pluto’s atmosphere is disappearing. And now the question might be – if Pluto was closest to the sun between 1979 and 1999, and if New Horizons saw the atmosphere still increasing in 2015, why did we see it begin to decline in 2018? Why didn’t it begin to decline sooner?

The reason is the same physical effect that causes sand on a beach to feel warmest in late afternoon, even though the sun is highest around midday.

A “lag,” or thermal inertia

Pluto’s surface is icy. And its largely nitrogen atmosphere is supported by the vapor pressure of its surface ices. That’s a tendency of ice to change to a gaseous state as the temperature increases. So, as Pluto came nearest the sun between 1979 and 1999, the temperature of its surface ices warmed, and Pluto’s atmosphere began to rise from its surface. Now, as Pluto continues to shift farther and farther from the sun in its orbit, the temperature of the ice on the surface is dropping. But it doesn’t drop immediately. That’s because the ice has thermal inertia. It stores some heat. Leslie Young of the Southwest Research Institute is another expert on Pluto’s New Horizons mission. She commented:

An analogy to this is the way the sun heats up sand on a beach. 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.

So new observations of Pluto during the 2018 occultation is helping scientists understand not just the dwarf planet’s atmosphere, but also how Pluto stores and releases heat.

Diagram of solar system showing orbits, including Pluto's elliptical one.
This solar system map shows Pluto’s location in October 2021. Pluto was closer to the sun than Neptune from 1979 to 1999. It is now moving into even icier domain as it swings farther out on its elliptical orbit. Image via Cybersky.

Bottom line: Pluto’s atmosphere is disappearing. Scientists have seen a big change from when New Horizons visited Pluto in 2015 versus a view during an occultation in 2018. The atmosphere is freezing and falling to the surface as the planet moves farther away from the sun on its elongated orbit.

Via Southwest Research Institute

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