A visitor descending towards Pluto would meet contradictions before reaching the ground.<\/p>\n
Above lies a blue atmospheric haze, even though the surface below is mottled red, cream and grey. The mountains are built from water ice, but the glaciers flowing around them are dominated by frozen nitrogen. Some broken pieces of the water-ice crust appear buoyant enough to travel across that nitrogen like icebergs. Overhead, the largest moon does not simply circle Pluto\u2019s centre. Both bodies orbit a point suspended outside Pluto, between the two worlds.<\/p>\n
Then there is the clock. Humans discovered Pluto in 1930, but Pluto needs about 248 Earth years to complete one trip around the Sun. We have changed its classification, sent a spacecraft past it and mapped named features across its surface without seeing it finish even one observed year.<\/p>\n
Each fact needs a little precision. The haze appears blue rather than the ground. The floating hills have not been filmed moving in real time. Charon is still officially a moon, despite the unusual orbital geometry. Taken together, though, the details show why the 2015 New Horizons flyby transformed Pluto from a distant point of light into one of the Solar System\u2019s most physically inventive landscapes.<\/p>\n
Pluto\u2019s blue sky is a veil of organic haze<\/p>\n
The first colour images of Pluto\u2019s atmosphere revealed a blue tint in its high-altitude haze. NASA produced the view by combining blue, red and near-infrared data from New Horizons\u2019 Multispectral Visible Imaging Camera to approximate colours a human eye would perceive.<\/p>\n
The explanation is related to, but chemically different from, Earth\u2019s blue sky. Here, tiny molecules scatter short blue wavelengths efficiently. On Pluto, scientists think sunlight initiates reactions among nitrogen and methane high in the atmosphere. Those reactions build complex, soot-like organic particles called tholins.<\/p>\n
The particles themselves are thought to be grey or red. Their size allows the suspended haze to scatter blue light, according to NASA\u2019s account of the discovery<\/a>. As the particles grow, volatile gases can frost their surfaces before they settle and contribute to Pluto\u2019s reddish ground.<\/p>\n \u201cBlue sky\u201d therefore describes the colour of sunlight scattered by a tenuous atmospheric veil. It does not describe an Earth-like atmosphere, nor a blue surface. The sunlight at Pluto is much weaker, and the gas around the dwarf planet is extraordinarily thin.<\/p>\n The same substance can be a glacier here and a mountain there<\/p>\n Pluto reverses the roles familiar from Earth. Water ice is so hard at temperatures near 38 kelvin, roughly minus 235 degrees Celsius, that it behaves as structural bedrock. It can support mountains several kilometres high. Nitrogen ice is softer and more mobile under Plutonian conditions, accompanied in places by carbon monoxide and methane ice.<\/p>\n The clearest stage for this chemistry is Sputnik Planitia, the bright western half of the heart-shaped Tombaugh Regio. The basin contains a vast sheet of nitrogen-dominated ice with glacier-like flow at its margins and polygonal cells across its interior.<\/p>\n Those cells are not cracks in an inert shell. A New Horizons analysis published in Nature<\/a> concluded that convection within the nitrogen-ice layer could continually renew the surface. Warmer material rises, cooler material descends, and the slow overturning helps explain why so few impact craters interrupt the plain.<\/p>\n Earth offers the analogy, but not the ingredients. Here, water ice creeps downhill over rock. On Pluto, nitrogen ice can flow around obstacles made from water ice, which fills the geological role of rock.<\/p>\n The \u201cicebergs\u201d are probably broken pieces of Pluto\u2019s crust<\/p>\n New Horizons photographed isolated hills scattered across Sputnik Planitia. Individual blocks measure roughly one to several kilometres across. There are chains along apparent flow paths and clusters near the borders of the plain\u2019s convective cells.<\/p>\n The leading interpretation is that these hills broke from rugged water-ice uplands and entered the nitrogen glacier. Water ice is less dense than nitrogen-dominated ice under the relevant conditions, so the fragments can be buoyant. NASA\u2019s annotated image of the floating hills<\/a> says they are believed to move over time like icebergs in Earth\u2019s Arctic Ocean.<\/p>\n Some clusters reach about 20 kilometres across. Challenger Colles, a particularly large accumulation near the northern edge of the imaged area, spans roughly 60 by 35 kilometres and may mark a place where hills became beached as the nitrogen ice grew shallower.<\/p>\n The language deserves its qualifier. New Horizons passed Pluto at high speed and did not stay to track a particular hill across years. \u201cFloating\u201d and \u201cmoving\u201d are inferences from the blocks\u2019 likely composition, relative density, position and the visible flow and convection patterns around them. They are well-motivated inferences, not a time-lapse observation.<\/p>\n Pluto and Charon dance around a point outside Pluto<\/p>\n Charon is about half Pluto\u2019s diameter and roughly one-eighth its mass, unusually large proportions for a moon. The result is a centre of mass, or barycentre, that falls beyond Pluto\u2019s surface.<\/p>\n Every 6.4 Earth days, Pluto and Charon both travel around that point in the space between them. NASA\u2019s New Horizons mission history<\/a> explains that this geometry is why the pair are sometimes described as a binary system. The term is informal in this case. The International Astronomical Union has not formalised a binary dwarf-planet category, and Charon remains classified as Pluto\u2019s moon.<\/p>\n The pairing is stranger still because the two bodies are mutually tidally locked. Charon always presents the same hemisphere to Pluto, and Pluto always presents the same hemisphere to Charon. An observer on Pluto\u2019s Charon-facing side would see the moon fixed in one region of the sky. Someone on the far side would never see it.<\/p>\n NASA\u2019s record of Charon\u2019s discovery and orbit<\/a> notes that they circle their common centre of mass every 6.387 days. The distant system is not a large body sitting still while a negligible companion loops around it. Both worlds visibly participate in the dance.<\/p>\n Humanity has watched less than two-fifths of one Pluto year<\/p>\n Clyde Tombaugh identified Pluto on photographic plates in February 1930. Pluto\u2019s orbit takes about 248 Earth years, so only about 96 of those years will have passed by the time this article appears. That is less than 40 per cent of one circuit.<\/p>\n