Some of the strange features in Sputnik Planum.
The stunning images returned from New Horizons’ flyby of Pluto revealed a wealth of information about the dwarf planet’s characteristics. That’s been followed by the long, hard work of figuring out how these features got there. One of the most striking things that needs explanation is the apparent youth of Pluto’s surface, as some areas appear to be crater-free, including the huge area called Sputnik Planum.
Now researchers offer an explanation for Sputnik Planum’s apparent youth. Two articles in this week’s edition Nature indicate that radioactivity from Pluto’s core would be sufficient to drive convection of nitrogen ice. But the sheer amount of ice involved creates yet another mystery, as it appears that nearly all of Pluto’s inventory of this element somehow ended up in Sputnik Planum.
Here on Earth, nitrogen is a gas that makes up most of our atmosphere. Those who have spent some time in a lab may be familiar with its liquid form, which is used for things that need to be cooled well below ambient temperature. But on Pluto, it’s usually so cold — about 35K — that most of the dwarf planet’s nitrogen is solid. This nitrogen ice has a number of special properties. One is that it is much denser than water ice, which would allow the equivalent of icebergs to float on the surface. The other is that because it is not held together by strong interactions between nitrogen molecules, it is relatively easy to deform.
Imaging of Sputnik Planum indicated that it consists largely of nitrogen ice, with some methane and carbon monoxide mixed in. But it also revealed some striking details: a honeycombed surface with irregularly shaped bulges, often tens of kilometers wide and up to 50 meters high. the fields. The boundaries between these polygons are lower but are sometimes filled with small jagged peaks – these are thought to be made of water ice floating on top of the nitrogen.
To figure out how these polygons formed, researchers focused on the possibility of convection in the nitrogen ice. Pluto does have a rocky core; based on the density of the planet, it is probably about 900 km across. If the core is made of material typical of other solar system bodies, it would contain some long-lived radioactive elements. The heat of their decay is minuscule: only three milliwatts for every square meter of surface. But it may be just enough to drive convection.
Both teams of researchers set up models of Sputnik Planum with a small heat source at the base and filled it with nitrogen ice. In both cases, convection produced similar polygons and occurred fast enough to alter Sputnik Planum’s surface in less than a million years. That nicely explains the lack of craters in this area. One of the two teams even suggests that there may be transitions between two phases of the nitrogen ice, with the internal heat creating a less dense phase and a more dense phase that forms and sinks at the edges.
Both also calculate that given the size and likely depth of the basin, it contains all of the nitrogen we would expect Pluto to have accumulated when it formed. The two newspapers also agree that the reason for this is a bit of a mystery. “We don’t understand at this point why most of the N2 on Pluto is concentrated in what appears to be the basin of a large ancient impact crater,” one team writes.
But they disagree about the nature of that basin. One team calculates that if the objects in the polygon boundaries are really mounds of water ice, then the nitrogen ice, in order to float, must be much deeper than we would expect from an impact basin – at least three miles deep. The other paper suggests it could be only two miles deep and still support the convection needed to reshape its surface.
In both cases, the papers paint a vivid picture of a world with a single ocean, filled with nitrogen ice. That ocean is fed by “rivers” — nitrogen glaciers falling from the mountains around Sputnik Planum — that carry the chunks of water ice we now see on the surface.
Nature2016. DOI: 10.1038/nature18289, 10.1038/nature18016 (About DOIs).