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How’s this for cool: some of the most distant worlds in our solar system might have surprising geological activity that gives them a bright, icy covering.

The dwarf planets Eris and Makemake, which reside beyond Neptune, are probably home to ice volcanoes that recoat their surfaces continually with nitrogen frost, said Will Grundy at Lowell Observatory and Orkan M. Umurhan at the SETI Institute in work presented at an American Astronomical Society meeting on 17 October.

These dwarf planets are the second and third largest objects in the icy Kuiper belt region. Both worlds seem much too small and cold to have the sort of inner planetary activity that can lead to volcanism.

But Grundy and Umurhan reason Eris and Makemake must have some inner activity. The proof is in their ices. When we measure reflection from the surface of both worlds, we see strong spectral lines associated with frozen methane. When this evaporates, it creates a reddish aerosol “gunk” called tholin on the tiny worlds’ surfaces. Grundy and Umurhan calculate that this gunk makes up about 10 per cent of the total ice on Eris, but more on Makemake.

Judging by the amount of methane, we would expect tholin to be a big part of the surface, making the worlds appear darker. Instead, they appear bright white, more like the regions of Pluto covered in nitrogen ice. So Grundy and Umurhan reason the dwarf planets must have volcanoes spewing nitrogen ice to cover the tholins.

Nitrogen ice doesn’t behave like water ice. It has a “stretchiness” to it when heated, which Umurhan compares to a thick tar. The surfaces of Eris and Makemake could be stretching and eventually burying the tholin. Or a process like we see in the ice plains of Pluto’s Sputnik Planitia region could contribute, where a sort of glacier of frozen nitrogen is replenished from below by convection. “We’re simply taking that [idea] and grafting this onto Makemake and Eris,” Umurhan says.

But what’s powering the ice volcanoes that make the ice? Since the dwarf planets are so far from the sun, there must be something else heating them. He says the internal heat sources on the dwarf planets may be radioactive rocks near the cores. That could result in this global geologic activity, which could power icy volcanoes that cover the ground in nitrogen ice. Or, in addition, there could be several Sputnik Planitia-like regions depositing new nitrogen ice on the surface and obscuring methane gunk.

Bill McKinnon, a project scientist from NASA’s New Horizons mission to Pluto, is interested in the conduction regions idea. “It’s a regional hypothesis, but we can’t prove it from the data alone,” says McKinnon, for that you need to know how deep the nitrogen goes. If it’s the amount seen on Pluto, that’s less than 500 metres deep – not enough to generate convective layers, and thus not enough to recover the surfaces of the worlds.

“(Grundy and Umurhan) have to do a little more work to see if the amounts are plausible,” McKinnon says.

Large glaciers of nitrogen ice could even be behaving like plate tectonics as rigid upper layers interact with squishy layers below, Umurhan says, similar to the theory that Sputnik Planitia may move due to the influence of a subsurface ocean. But there’s no evidence for this on Eris and Makemake.

Journal Reference: Paper Forthcoming, from Astrobiology and Comparative Planetology panel at 49th DPS, Control ID 2817623

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