Pluto might be made of a billion comets squashed together. Its chemical makeup hints that this might be its origin story, and that it may even have once had a liquid ocean.
Pluto is chock full of nitrogen. Currently, it is found in gas form in Pluto鈥檚 tenuous atmosphere, and in solid form in giant glacier made of nitrogen-rich ice, called Sputnik Planitia, which is found in the left lobe of Pluto鈥檚 heart-shaped plain.
鈥淚 would imagine if there were Plutonians, one of their primary origin questions would be where all the nitrogen comes from that drives a lot of the interesting geological activity on that world,鈥� says at the Southwest Research Institute in San Antonio, Texas. 鈥淥n Earth, the comparable question would be the origin of water.鈥�
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It鈥檚 a match
A leading hypothesis on how Pluto and the other bodies in the outer solar system formed is that they were built up from lots of smaller comets, so Glein and his colleague calculated whether Pluto could have gotten its nitrogen that way.
They used data from the New Horizons mission that flew past Pluto in 2015 to figure out how much nitrogen the small world actually has, and data on the composition of comets from the Rosetta spacecraft, which visited the comet 67P/Churyumov鈥揋erasimenko from 2014 to 2016.
They found that the abundances matched. Pluto could definitely be made of a billion comets with compositions like 67P.
Ancient reactions
The one problem is that Pluto鈥檚 surface doesn鈥檛 have much carbon monoxide at all, but comets are full of the stuff. So where did Pluto鈥檚 carbon monoxide go? Glein and Waite found that it鈥檚 possible some could be buried underneath the nitrogen ice. Or it could have been lost in chemical reactions, because it is much more reactive than nitrogen.
鈥淚f Pluto has a liquid water ocean, or even liquid water anywhere, if the carbon monoxide were to come in contact with that water, it would want to react,鈥� says Glein. Then, Pluto would have the byproducts of those reactions, but very little carbon monoxide.
There have been hints from New Horizons that Pluto might have a liquid water ocean buried under kilometres of ice, but because the spacecraft hurtled by instead of remaining in orbit, we don鈥檛 have enough data to be sure.
If Pluto is made of comets, it validates the idea that all the bodies in the outer solar system are too. 鈥淚f Pluto鈥檚 chemistry can be imagined as a giant comet, then Jupiter or Saturn might be imagined as even more giant comets shrouded in gas,鈥� says Glein.
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