1 Elliot crater
Pockmark clue to the solar system’s history
Near the eastern edge of a dark highland region of Pluto known as Cthulhu Regio, a canyon and a crater meet. Elliot crater, a majestic feature 85 kilometres across (in the lower left quadrant of the above photograph), is a relative rarity in these parts of the solar system. Across Pluto and its large moon, Charon, there is a relative shortage of large craters. That may be telling us something profound about how planets form.
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Pluto’s home, the Kuiper belt, is made up of small bodies left over from planet formation, known as planetesimals. New Horizons is now plotting course for one, 2014 MU69.
According to the traditional picture, planetesimals grew in the early days of the solar system as little rocks gradually came together to make bigger rocks. This process should produce a lot of objects a few kilometres in diameter, and far fewer objects tens or hundreds of kilometres across.
Planetesimals of all available sizes should hit Pluto and Charon from time to time, forming craters. So the relative lack of smallish craters on Pluto seems to paint a decidedly non-traditional picture. It might support an alternative model called pebble accretion, in which large planetesimals form almost instantly when swarms of little pebbles immersed in gas suddenly collapse – a vital stage, perhaps, in building not just little icy worlds like Pluto, but also the cores of gas giants and warm rocky planets such as Earth.
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2 Winter haze
Signature of an atmosphere surprisingly like Earth’s
Pluto’s nitrogen-dominated atmosphere is cold and thin, with a pressure at ground level equal to that 80 km above Earth. Here sunset over the Norgay mountains lights up its hazy layers, streaked, as on Earth, with fine aerosol particles.
“Nitrogen sublimates from Pluto’s surface like water evaporating from Earth’s oceans”
These layers stretch up to 200 km above Pluto’s surface, 10 times the height expected before the arrival of New Horizons. The lower-level hazes may be photochemical smog caused by the action of sunlight on methane and other gases. The higher layers must be created by some other process, perhaps the action of free electrons in Pluto’s ionosphere.
Further up still, the upper atmosphere is cooler than expected, perhaps chilled by some unknown chemical agent. That also means it is less puffed up than expected, so it is not blowing away into space as earlier models predicted.
Pluto’s weather seems to be surprisingly like ours, but with a nitrogen rather than a water cycle. Nitrogen sublimates from the ices of Sputnik Planum (see the photo above), like water evaporating from Earth’s oceans. It then falls as snow or freezes out as frost on the eastern highlands, finally flowing back down to the plain in glaciers. There are even signs of nitrogen fog in places – and perhaps, in images not yet officially released, even clouds.
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3 Sputnik Planum
A churning plain of frozen nitrogen
Surely one of the strangest terrains anywhere in the solar system, this 1000-km plain is divided up into rough polygons a few tens of kilometres across. These are almost certainly the mark of convection. Similar patterns appear on the sun’s surface and can sometimes be seen in a gently simmering saucepan.
The substance churning here is nitrogen ice. Land a probe on its tarry surface and it would slowly sink, becoming fully submerged in a matter of decades, says Orkan Umurhan of NASA’s Ames Research Center. It might keep going down a long way: Umurhan’s models hint that the ice may be between 5 and 7 km deep.
While this slo-mo maelstrom is certainly bizarre and unexpected, it is not as hard to explain as we first thought. Nitrogen ice is not only soft, but also an excellent thermal insulator, meaning even a feeble heat source from below can build up the temperature, kicking off convection. The little heat left over from Pluto’s turbulent formation, supplemented by heat from the decay of radioactive trace elements within its core, is expected to add up to about 4 milliwatts per square metre – enough to drive the churning of Sputnik Planum.
The strange pockmarks peppered across this living, shifting landscape are possibly caused by sublimation of the nitrogen ice, while the blocks gathered at the junctions of some convection cells – visible in the picture above – are probably water ice hills floating on the denser nitrogen. They could be miniature versions of the mountain ranges that may be floating around the edges of Sputnik Planum (see below).
4 The al-Idrisi range
Floating mountains of granite-like ice
Pluto’s bedrock is water ice, a substance as hard as granite at Pluto’s surface temperature of -240 °C. On the north-western flank of Sputnik Planum this ice-rock forms the jumbled peaks of the al-Idrisi range.
These kilometre-high mountains are possibly floating on denser nitrogen ice below – or may once have floated, only to become beached. “It looks like you took a surface, cracked it up, and shoved the pieces up into the corner of Sputnik Planum,” says Umurhan. What could have done that is a mystery.
5 Wright Mons
Ice volcano 4 kilometres high
Wright Mons is unlike most of the jagged mountains that punctuate Pluto’s surface, such as the Norgay range on the right of this image. Slightly left of centre in the image, this hummocky mass with a huge central pit looks suspiciously like a volcano.
If that is indeed what 4-km Wright Mons and its even taller neighbour Piccard Mons are, they would not have erupted molten rock, but instead some chillier fluid – probably water mixed with another substance that lowers its melting point.
Ice volcanoes would amount to unseemly heat and activity for Pluto – a tiny, cold world that by rights should have frozen solid long ago. Yet Wright Mons is no relic from the dwarf planet’s early days. Its sides bear hardly any visible impact craters, so can’t have been exposed for too long to the rain of space debris evidenced elsewhere on Pluto’s surface (see “Elliot crater“). It is probably much younger than a billion years, says Kelsi Singer of the Southwest Research Institute in Boulder, Colorado, and perhaps only a few million. So is it extinct or merely dormant?
The New Horizons team is now looking more closely at the hummocks on the flanks of Wright Mons to try to work out what they are – perhaps individual volcanic domes? They will also use data from the infrared spectrometer LEISA to work out the chemical composition of the mountain’s ice. That may clarify whether Wright Mons is indeed a volcano – and perhaps tell us something about how chilly little Pluto can be so hot as to spout watery lava.
This article appeared in print under the headline “Pluto: A whole new world”
