A Brutal Winter Dawns on Pluto
The story of Pluto's seasons is far more complex than we could ever have imagined
Clyde Tombaugh, rather fittingly, discovered Pluto in the midst of winter. Two photographs were taken: the first on January 23, 1930, the second a week later. Between them was a slight change; a speck of light that seemed to have drifted across the star strewn image. Confirmation came when Tombaugh dug out an even earlier photo, one that proved the light was indeed slowly moving across the sky.
That faint light, he reasoned, was a planet; a distant world drifting far from our Sun. More than that, though, was hard to say. The telescopes of the time lacked the power to see Pluto as anything more than a distant pinprick of light. Astronomers could not tell how big it was – noting merely that it must be smaller than the Earth – nor say much about how it looked.
Only in 1978 did we realise that Pluto has a large moon, Charon. Another twelve years passed before the Hubble Space Telescope was able to clearly photograph the two as separate bodies. Yet even with all this extra telescopic power, astronomers still regarded Pluto as a mysterious world.
Some things, however, seemed clear. Since the Sun – billions of miles distant – shone only with a feeble heat, Pluto must be incredibly cold. No liquids could survive on the surface and even substances like nitrogen – nearly always a gas on Earth - should freeze solid. Its orbit, too, was well known, if rather odd, and seemed to imply a complex pattern of seasons danced slowly across its surface.
Unlike the other planets in our Solar System, which stay roughly the same distance from the Sun throughout their whole orbits, Pluto swings between two extremes. In 1930, when Tombaugh spotted it, Pluto was roughly six billion kilometres away – just under four billion miles. But by 1990, when Hubble photographed it, Pluto was only four and a half billion kilometres distant, almost a third closer.
That, astronomers calculated, should mean Pluto experiences extreme decade-long seasons. As it moves inwards towards the Sun – as it was doing between 1866 and 1989 – it gradually warms up. Its surface, made up of frozen nitrogen, methane and carbon monoxide, turns to gas, slowly thickening its atmosphere.
In this Pluto behaves somewhat like a comet: distant objects that gradually melt as they approach the Sun, their newly formed atmospheres streaming behind them in a long tail. When, however, Pluto moves away from the Sun – as it will do until the 22nd Century – it cools.
The atmosphere it gained over the previous century fades away, freezing back onto the surface. By the time Pluto reaches the furthest point in its orbit, the heat of the Sun has fallen by two thirds. Its temperatures will then fall to just thirty-three degrees above absolute zero.
That, at least, was the theory. But until 2015 – when the New Horizons probe flew just eight thousand miles from Pluto – we still knew very little about the distant world. Hubble’s cameras gave us a vague sense of its mottled and varied surface and careful measurements of distant starlight filtering through its atmosphere revealed vital clues about its composition. But these were small details, hardly enough to build up a picture of a faraway dwarf.
New Horizons radically improved our view of Pluto. For the first time we had clear and stunning images of its surface. The probe was able to measure its atmosphere in vivid detail. It told us, for example, that Pluto’s atmosphere is far larger than thought, stretching hundreds of kilometres above the surface. It was hazy – probably a result of interactions between the Sun’s light and methane – and it may, some tantalising hints suggest, have clouds.
By then, Pluto was gradually spinning further away from the Sun, and should - as the Sun's heat faded -have started to cool. The reality, as always, was more complicated. Like Earth, the climate on Pluto is shaped by its surface, by its mountains and basins, by its plains of nitrogen, and perhaps even by an underground ocean.
At the centre of speculation is Sputnik Planitia, part of the famous heart-shaped region revealed by New Horizons. This, scientists believe, is a deep basin excavated by a long ago asteroid impact. Water may then have gushed up from the interior, rapidly freezing and cauterizing the wound. Over the aeons nitrogen, condensing from the atmosphere, filled the rest of the basin.
Today, Sputnik Planitia controls Pluto’s weather. When day dawns, and the distant sun rises over the horizon, the nitrogen contained within warms and starts to flow into the atmosphere. As night falls, the atmosphere and surface cool, and the nitrogen freezes once again. This daily cycle creates a pump, pulsing nitrogen in and out of the atmosphere; and sending winds flowing across the surface, sculpting mountains and dunes of ice and methane crystals.
On the other side of Pluto, almost directly opposite Sputnik Planitia, is a fractured landscape of splintered terrain. This, some researchers argue, is evidence of a long ago asteroid impact. Shockwaves from the collision, amplified, perhaps, by an internal ocean, raced across the small world: meeting and amplifying at the exact antipode of the impact.
A massive crack, too, seems to stretch across the entire small planet. This, planetary scientists speculate, could also hint at the presence of an underground ocean. Cycles of freezing and melting may have splintered the surface, tearing the dwarf planet apart, only for water to rush into the chasms and freeze.
Radioactive decay in Pluto’s interior could still heat that ocean, keeping it, theoretically anyway, liquid. When it bursts through the surface, as it sometimes must, it would create cryovolcanoes: a kind of volcano where water, not lava, pours out, forming mountains of ice. Detailed inspection of Pluto’s surface indeed found two possible such volcanoes, shaped in ways seen nowhere else in the Solar System.
These mountains and glaciers of ice could even absorb much of Pluto’s heat, keeping it warm and delaying the onset of winter by a few years. That seems to be happening: despite spending thirty years heading away from the Sun, Pluto’s atmosphere only recently started to thin.
Whatever residual heat remained on Pluto must now be fading fast. Nitrogen, and other gases, should be freezing back onto the surface. Pluto’s long drift into darkness means its atmosphere will likely decline for decades more and Sputnik Planitia, the heart of Pluto’s climate, will continue to thicken and expand. A long, dark winter looms for the dwarf at the edge of our Solar System.
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