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The New Horizons mission to the Outer Solar System flew over Pluto in July 2015, grazing its surface only 12,500 kilometers after traveling 5 billion kilometers and returning incredible images and data of the tiny, icy world.
The mission was conceived, engineered, built and launched in record time because scientists wanted to reach Pluto as quickly as possible. Why? Because Pluto orbits the Sun on a decently elliptical path, and around the year 1989, that was its closest point to the Sun. After that, as it slowly receded into its 248-year orbit, it would slowly become colder. Pluto has an incredibly thin nitrogen atmosphere, only 1 / 100,000e as thick as the Earth, but it’s there. However, as the temperature dropped, scientists feared it would freeze by the time a probe went there.
Turns out they did it just in time. The atmosphere, as it is, was still doing its job when New Horizons came by. In 2018, however, there was evidence that the atmosphere was starting to condense.
On August 15, 2018, seen from Earth, Pluto passed directly in front of the star UCAC4 341-187633, which is only a little brighter than Pluto itself. An event like this is called a occultation, and these can be extremely useful to astronomers; sometimes giving the size and even the shape of the occlusive object.
For an occultation, timing and location are everything. It makes a path through the Earth, like a solar eclipse, and inside that path you see it while outside you don’t. For Pluto’s occultation in 2018, the southern limit was Belize and Guatemala in Central America, and the northern limit crossed the northern United States. The center line, where the star would cross the middle of Pluto, crossed Mexico, the Gulf Coast, and then the eastern Atlantic states.
Leave a few observers were able to measure the brightness of the star. If Pluto did not have an atmosphere, the star’s luminosity would drop very quickly as the solid body of the world passed in front of it. But the atmosphere is there and actually reaches quite high above the surface that the star has gradually faded and increased in brightness again over time. This can inform scientists about the vertical structure of the atmosphere, which is extremely useful in characterizing it.
The plot shown in the diagram here also shows a sharp increase in brightness right in the middle of the event. The air bends the light, just like a bent spoon in a glass of water. This is called refraction. When the star was exactly on the other side of Pluto, directly behind the center of Pluto, the air all around the world bent the starlight towards the observer, causing what is called a central flash. If you had looked through a telescope large enough, it would have seemed that Pluto was surrounded by a brilliant ring of light. This is commonly observed in occultations where the object of occlusion has an atmosphere; it happened with Neptune’s Triton moon for example. This flash can also tell astronomers a lot about the object’s atmosphere.
So what were the results? When New Horizons overtook Pluto in 2015, the atmosphere was actually getting thicker, doubling the surface pressure every decade. The spacecraft measured a surface pressure of about 11.5 microbars (11.5 millionths of Earth’s sea level pressure). Given the trend, the 2018 measurement had to be greater than 14 microbars.
It was not: the occultation gave a pressure of 11.4 microbars, almost exactly the same as what New Horizons saw. This indicates that the trend of increasing pressure has finally stopped.
Pluto’s atmosphere is not like that of Earth, where an equilibrium has been reached and the pressure remains relatively constant*. Pluto’s atmosphere is generated by nitrogen ice on the surface which heats up and sublimate, turning into gas. The rate at which this happens is highly dependent on temperature. Pluto’s orbit brings it 4.5 to 7.5 billion kilometers from the Sun, so it receives nearly three times as much sunlight when it is away from the Sun. perihelion (closest to the Sun) then aphelia (when it’s furthest). This means that when it starts to move away, the temperature drops enough for the nitrogen to stop subliming and the process to reverse, with the nitrogen gas in the air starting to freeze.
But why now? After all, Pluto was closest to the Sun almost 30 years before the occultation!
The reason is thermal inertia. Pluto was receiving the maximum amount of sunlight in 1989, but even as it moved away (very slowly) from the Sun, it still received sunlight and warmed up faster than it could radiate that heat (well, – 230 ° C is not exactly heat but you know what I mean). So it took a while before it could actually start cooling again. It must have started shortly after New Horizons arrived.
Nice timing. I note that an occultation in 2015, just before the flyby of New Horizons, showed that the atmosphere was not collapse, and the results were released in 2020 (and released in early 2021). So it’s funny for me that at the time of analyzing these observations, the 2018 occultation already showed that the air was collapsing! Sometimes science takes time and the results can intersect.
If the 2018 occultation results showing that the atmosphere is starting to freeze are correct, then Pluto is finally starting to cool down after getting as close to the Sun as possible. Its orbit will move it away over the decades, and when it reaches aphelion, around the year 2213, it will be about as cold as it gets. The atmosphere will be largely frozen and likely remain so for another century before slowly starting to swell again as the weak sunlight warms the surface.
Hopefully we will have sent more probes (maybe even people) well before then.
*The are some similarities; both are mainly nitrogen, and both can create blue skies. In addition, the air on Earth freezes a bit; this is how we get snow. But this happens in winter, which is due to the axial tilt of the Earth and not so much to its distance from the Sun.
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