The cliché is that a picture is worth a thousand words. But if you pay attention to all the information that scientists have extracted from New Horizons' photos of Pluto, then you'd be forgiven for thinking that's a gross under-estimate. Items large and small, from massive nitrogen seas to the mountain-sized icebergs that float above them, all provide hints of the geology of the alien world. And, even as more images were being sent back to Earth, planetary scientists pored over the ones we had, cataloging fault lines and pondering chemical traces.

Through their work, a picture of a dynamic world has gradually emerged. And now, two research teams have described the results of a careful look at Sputnik Planitia (formerly Sputnik Planum), Pluto's largest feature. They both find that a complex interaction between gravitational mechanics and Pluto's atmosphere likely resulted in Sputnik Planitia dragging the entire dwarf planet's axis of rotation around. And the fact that this happened reinforces earlier hints of an under-surface ocean filled with liquid water.

Sputnik Planitia is a giant basin filled with nitrogen ice. At the conditions on Pluto's surface, this ice is even more dense than water, allowing huge blocks of water ice to "float" on the surface. Yet it's also ductile enough that the entire basin is probably slowly mixing, driven by only the heat released by radioactive decay in Pluto's core. It's also expected that the basin is fed by a nitrogen cycle, as the gas sometimes sublimates off or re-condenses (there are also nitrogen glaciers feeding in from the nearby mountains).

All that may seem bizarre, but for people studying orbital mechanics, there's something else odd about Sputnik Planitia: it really shouldn't be where it is. Pluto is tidally locked with its largest moon, Charon, always keeping the same side facing it. Sputnik Planitia is nearly exactly opposite this face. If you drew a line from Charon through Pluto's core, it would exit the dwarf planet through Sputnik Planitia. The odds of it forming there by chance are quite small.

But we have a good understanding of the gravitational mechanics of systems like this. When mass is distributed unevenly in a planet, it can influence both the rate of rotation and the axis of rotation. When there's a second body nearby, like Charon, tidal interactions can change which side of the planet ends up facing that body. The result is what's called "true polar wander," where the axis of rotation shifts to a different location.

To figure out how this plays out in the Pluto-Charon system, a Japanese-US team built a simulation that allowed them to plug in various values for the properties of Sputnik Planitia, such as where it started and how its mass differs from the rest of Pluto. They find that, if Sputnik Planitia is less dense than the rest of Pluto, then it would likely have ended up at the dwarf planet's north pole, much as the largest impact basin on the Moon has ended up at its south pole.

But if Sputnik Planitia is more dense than the rest of Pluto, it would shift to roughly the area it's in now, dragging the rest of the planet along with it. In fact, scientists find that the axis of rotation can shift by as much as 60 degrees as the gravitational torques do their work. These models also limit the original location of Sputnik Planitia, indicating it must have formed in the northern half of the anti-Charon side of Pluto.

This gravitational torquing would create stresses on the rest of Pluto's surface, creating faults with specific orientations. The team did an analysis that shows that many (but not all) of the faults seen on the dwarf planet are consistent with the expected stresses.

All of that makes sense, but there's a problem, one also recognized by a second team (a large team that includes the entire New Horizons Geology, Geophysics & Imaging Theme Team). While nitrogen ice in these conditions is more dense than pure water ice, there's not enough of it to make Sputnik Planitia heavy enough to drag Pluto around. Both teams calculate that this would require the nitrogen to fill a basin that's more than 50km deep; Sputnik Planitia is thought to be less than five kilometers deep.

So, while the nitrogen ice may contribute to matters, it's not enough to put Sputnik Planitia where it is. So what could? Here's where the possibility of a subsurface ocean comes in. If one existed, then the impact that created this basin would have thinned the crust in the region. That would have allowed water to invade Pluto's icy shell from below. And, as liquid water is more dense than the ice it replaced, this could add significantly to the mass of this region. The end result is enough of a mass difference for gravity to start twisting the planet around.

But that doesn't mean the nitrogen ice's small contribution is irrelevant. Both teams note that the amount of nitrogen present in Sputnik Planitia is going to be influenced by the local temperature and the amount of sunlight it receives. And both of these will be influenced by how close Sputnik Planitia is to the plutonian equator. This creates the prospect of complex feedback loops between orbital forces and the amount of ice in the basin, causing it to shift Pluto's rotation dynamically. "If volatiles [like nitrogen] migrate into and out of Sputnik Planitia on seasonal timescales, then Pluto may experience small-amplitude wobbles akin to Earth’s annual, atmospheric-pressure-driven wobble," one paper concludes.

Beyond the new information these papers fill in about Pluto's past, it's worth stepping back to appreciate how quickly scientists went from being stunned by New Horizons' images to providing detailed explanations for some of the dwarf planet's features.

Nature, 2016. DOI: 10.1038/nature20120, 10.1038/nature20148 (About DOIs).