Sat. May 21st, 2022

Pluto's Atmosphere: View of a silhouette 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 jumped away from the dwarf planet to a deeper space. Image via NASA / Johns Hopkins / SwRI.

Pluto’s atmosphere disappears

Remove Pluto has a very elliptical orbit around the sun. The shape of its orbit brings Pluto closer to the sun than Neptune for about 20 years in each plutonic “year” (248 Earth years). Pluto was last closer than Neptune from 1979 to 1999. And while it was still relatively close to Earth, scientists hurried to send a spaceship to Pluto. In 2015, as the New Horizons spacecraft made its dramatic sweep past Pluto, data it returned to Earth suggested that Pluto’s atmosphere doubled in density each decade. But recently, during an occultation of a star in 2018 by Pluto – an event that backlit Pluto’s atmosphere – data suggest that Pluto’s atmosphere began to decline and eventually disappear.

The researchers presented these results on October 4, 2021 at the 53rd annual DPS meeting. They said their new work confirms the idea that when Pluto gets further away from the sun in its very elliptical orbit, the atmosphere freezes and falls back to the surface. Eliot Young of the Southwest Research Institute (SwRI) commented:

New Horizons mission accomplished [excellent data on density of Pluto’s atmosphere] from its flyby in 2015, in line with Pluto’s bulk atmosphere, which doubles every decade. But our observations from 2018 do not show that the trend continues 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 mankind considered Pluto as the ninth planet in our solar system. Since 2006, it has been categorized as a dwarf planet. During the occultation in 1988, the star’s light was gradually dimmed just before it disappeared behind Pluto. The attenuation demonstrated the presence of Pluto’s thin, highly dispersed atmosphere. New Horizons was then able to analyze this atmosphere from a close distance when it swept past in 2015.

From 1988 until today, astronomers have been monitoring Pluto’s atmosphere via rare occultations of Pluto stars seen from Earth. On the evening of August 15, 2018, they were ready for yet another occultation of a distant background star of Pluto. They knew that Pluto would pass in front of the star 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 move across the Earth’s surface. The center line of this shadow trail 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 fell as it passed behind Pluto’s atmosphere and then rose again as the star appeared on the other side of Pluto. You can see the light curve in the inside of the image below. But why is the curve W-shaped? What is the central peak?

Map with the path to Pluto occultation over Mexico, Louisiana and the northeast, with a graph inserted.
On August 15, 2018, scientists deployed telescopes near the center line for an occultation of a star at Pluto. The effort shows the resulting W-shaped light curve, which helped astronomers confirm that Pluto’s atmosphere freezes to the surface as it moves further away from the sun. Image via SwRI / NASA.

Reads Pluto’s central flash

Scientists can analyze the light curve that occurs when Pluto occludes a star, to obtain information about the density of Pluto’s atmosphere. But note that the light curve on top of the image above is W-shaped. It has what astronomers call a central flash. This central flash appears only to these observers on the precise centerline of the occultation. It happens when the star is most directly behind Pluto, when Pluto’s atmosphere allows light from the occult star to refract or bend into a point in the center of Pluto’s shadow. Seeing this central flash gives astronomers confidence that they are in the right place to observe the occultation. And it lets them know that 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 anyone has ever seen in a Pluto occultation. The central flash gives us very accurate knowledge of Pluto’s shadow orbit on Earth.

All this attention to fine detail is important as you stare at billions of miles of space in the faint (and temporary) atmosphere of a world as small as Pluto. This small dwarf planet is less than 1,500 miles across (2,400 km) as opposed to Earth’s 8,000 miles (13,000 km). That is why the appearance of a central flash was so important to these scientists. It helped give them confidence to claim that yes, Pluto’s atmosphere is disappearing. And now the question may be – if Pluto was closest to the sun between 1979 and 1999, and if New Horizons saw the atmosphere still rise in 2015, why did we see it start falling in 2018? Why did it not start to fall before?

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

A “delay” or thermal inertia

Pluto’s surface is icy. And its largely nitrogen atmosphere is supported by the vapor pressure of its surface ice. It is the tendency of the ice to change to a gaseous state when the temperature rises. So when Pluto came closest to the sun between 1979 and 1999, the temperature of its surface ice warmed, and Pluto’s atmosphere began to rise from its surface. As Pluto now continues to shift farther and farther from the sun in its orbit, the temperature of the ice drops to the surface. But it does not fall immediately. This is because ice has thermal inertia. That shops a little warm. 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 sand on a beach. Sunlight is most intense at high noon, but the sand then continues to soak up the heat during the afternoon, so it is warmest in the late afternoon. The continued persistence of Pluto’s atmosphere suggests that nitrogen ice reservoirs on Pluto’s surface were kept warm by heat stored below the surface. The new data suggest that they have begun to cool.

So new observations of Pluto during the 2018 occultation help scientists understand not only the atmosphere of the dwarf planet, but also how Pluto stores and releases heat.

Diagram of the solar system showing orbits, including Pluto's elliptical.
This map of the solar system 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 more icy territory as it orbits further out on its elliptical orbit. Image via Cybersky.

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

Via Southwest Research Institute


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