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PLANET, dwarf planet or the little world that could? One thing鈥檚 for sure: it鈥檚 the Frankenstein鈥檚 monster of the outer solar system, because Pluto looks like bits of different worlds stitched together.

Mountains like Earth鈥檚, 3 kilometres high and made of frozen water. A comet鈥檚 tail of escaping gas pulled back by the solar wind. Smooth surfaces like the icy veneer of Neptune鈥檚 moon Triton next to cracked terrain that resembles the highlands of Mars.

鈥淭he team is cataloguing the dwarf planet鈥檚 many mismatched oddities and trying to find the joins鈥�

This variation stunned researchers when NASA鈥檚 New Horizons spacecraft beamed back the first close-ups of Pluto and its moons last week. The dwarf planet is unlike any other world we鈥檝e ever visited. So is its largest moon, Charon.

That surprise has turned even the most seasoned Pluto experts into wide-eyed surveyors, cataloguing the dwarf planet鈥檚 many mismatched oddities and trying to find the joins. The maps from New Horizons, which will be downloaded and analysed over the coming year, are already proving hard to interpret 鈥� and harder still to explain.

Smooth regions on the surface of both Pluto and Charon were the biggest shock in the data we got last week. Prior to the probe鈥檚 arrival, researchers expected to see a pair of heavily cratered worlds, bombarded since their formation in the early days of the solar system. Counting craters provides a way to date a planet鈥檚 surface because impacts build up at known rates over time. Now we see that while both Pluto and Charon have their share of asteroid scars, the existence of unmarked terrain means they must have been geologically active fairly recently, at least on planetary timescales.

鈥淲e now have an isolated small planet that is showing activity after four and a half billion years,鈥� said mission leader Alan Stern at a press conference last week, as he revealed that the first high-resolution image of Pluto鈥檚 surface seemed to have no craters at all. 鈥淚 think that鈥檚 going to send a lot of geophysicists back to the drawing board,鈥� he said.

Terra incognita

Perhaps the most perplexing site of geologic activity is a region called Sputnik Planum, where surface ice is cut into polygonal chunks around 20 kilometres wide. 鈥淲hen I saw this image for the first time I decided I was going to call it 鈥榥ot-easy-to-explain鈥� terrain,鈥� says team geologist of the NASA Ames Research Center in California. It might be that the surface is cracking like mud as it contracts, or that convection within Pluto is heating and melting the surface like a bubbling pot of porridge.

The hunt is on for the mechanism driving that activity, providing enough energy to smooth away craters after they form. These smooth surfaces are also seen on Neptune鈥檚 moon Triton, another small icy body that until last week was thought to resemble Pluto. But Neptune鈥檚 gravity warms and remoulds Triton through a process called tidal heating.

That same process can鈥檛 still be active on Pluto and Charon, because they are tidally locked so always show the same face to each other. 鈥淲e know that process is over now, but when was it over?鈥� says Wheaton College in Norton, Massachusetts. Pre-New Horizons, it was thought that a collision between 4.4 and 4.5 billion years ago between a proto-Pluto and a proto-Charon created the binary system we see today.

What if the smash-up was more recent, allowing the two worlds to retain energy later into the history of the solar system?

Pluto鈥檚 insides could also be warmed by the slow breakdown of radioactive elements left over from the formation of our solar system, team geologists say. Alternatively, the gradual freezing of a buried ocean could be helping the dwarf planet hold on to the last glow of heat left over from its birth. More data is needed to draw any firm conclusions.

The jagged mountains dotted across the high-resolution image of Pluto鈥檚 surface, thought to be made of frozen water, also hint at unexplained activity below. 鈥淭he traditional way of growing a mountain is you smoosh stuff together and it pops up, or you pull them apart and you get a volcano. These things didn鈥檛 look like any of that,鈥� says team member of the Lowell Observatory in Flagstaff, Arizona.

Collins thinks these mountains could rule out some kinds of activity. To him, they resemble nunataks, which on Earth are the tips of rocky mountains sticking out of glaciers. Nunataks on Pluto could be water ice sticking out of other ices, possibly frozen methane, which have snowed down from Pluto鈥檚 atmosphere and smothered the craters. It鈥檚 hard to tell because we don鈥檛 know enough about how different ices interact at very low temperatures.

It isn鈥檛 just Pluto鈥檚 geology that hints at surprising vitality. When New Horizons was five days out from Pluto, it saw ionised nitrogen escaping from the atmosphere, a far stronger signal than the team expected. The latest results show that the gas seems to be forming a tail behind the Pluto system, blowing in the charged particles of the solar wind. That confirms a long-held suspicion that Pluto is undergoing a process also found in the distant past of other worlds, providing us with a window backwards in time.

We鈥檝e never been able to study gas escaping on this scale before because the process is more or less complete on other planets. New Horizons saw it happening in real time, both spying the glow of ionised nitrogen and catching the particles directly with on-board sensors.

Earth鈥檚 first atmosphere, a poisonous brew of hydrogen and helium, is thought to have escaped in much the same way.These lighter gas molecules were the only ones able to break away from our planet鈥檚 stronger gravity. On Pluto, nitrogen, which is heavier and makes up most of both Earth and Pluto鈥檚 present-day atmosphere, can also escape.

The same thing probably also happened on early Mars, wicking water vapour out of the atmosphere and drying out the planet. 鈥淓xploring Pluto is testing our ideas about this very escape on Mars,鈥� says team member of the University of Colorado in Boulder.

So far, it looks as if about 500 tonnes of nitrogen gas leave Pluto鈥檚 atmosphere every hour, Bagenal says. Nailing down the exact rate of escape is a priority for the mission 鈥� it will help the team make comparisons with theories of similar behaviour in the solar system鈥檚 ancient past. And since New Horizons discovered that the atmosphere is thinner than expected, that gas is probably coming from another reservoir 鈥� within the dwarf planet itself.

That could mean plumes of nitrogen are erupting from the surface just as the Voyager probe saw on Triton, team geologists say. Smudges deposited near dark shapes could be indirect evidence of plumes, although the team thinks these features look more like streaks where winds as fast as a few metres per second have blown material around surface obstacles.

The images taken as New Horizons looked back on a rapidly shrinking Pluto could reveal such plumes, lit up in the distant sunlight. That explanation would invoke the same kind of underground heat thought to be driving Pluto鈥檚 mountains and smoothing the surfaces. Alternatively, a thick layer of nitrogen ice over Pluto鈥檚 surface could be turning to gas and floating up into the atmosphere.

And there鈥檚 so much more to learn from the Pluto system. Its largest moon, Charon, is far more varied than the team expected, and hosts a mountain within a moat that has researchers baffled. Hydra, one of the four smaller moons, is lumpy like a peanut and probably covered in dirty water ice. We鈥檝e also had a brief look at another, Nix, and will see the final two in the coming months (see 鈥�Where next for New Horizons?鈥�).

These worlds will slowly reveal themselves as data trickles down, and planetary scientists shape all these disparate parts into one cohesive whole. Researchers are as fascinated as they are confused by what they鈥檙e seeing, and the findings at Pluto show it is every bit as complex as the other planets we鈥檝e explored. 鈥淚 think the solar system saved the best for last,鈥� said Stern.

This article appeared in print under the headline 鈥淧luto: the world that鈥檚 everything at once鈥�