James Tuttle Keane
The cliché is that a picture is worth a thousand words. But if you pay attention to all the information scientists have gleaned from New Horizons’ images of Pluto, you’d be forgiven for thinking that’s a gross underestimation. Objects large and small, from vast seas of nitrogen to the mountain-sized icebergs floating above, all provide hints of the geology of the alien world. And as more images were sent back to Earth, planetary scientists pored over the images we had, cataloging fault lines and pondering chemical traces.
Through their work, an image 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 discover that a complex interaction between the gravitational mechanics and Pluto’s atmosphere likely led to Sputnik Planitia dragging the dwarf planet’s axis of rotation. And the fact that this happened reinforces earlier hints of a subsurface ocean filled with liquid water.
Sputnik Planitia is a giant basin filled with nitrogen ice. Under the conditions on Pluto’s surface, this ice is even denser than water, allowing huge blocks of water ice to “float” on the surface. But it’s also ductile enough that the whole basin is probably slowly mixing, driven only by the heat released by radioactive decay in Pluto’s core. The basin is also expected to be fed by a nitrogen cycle, as the gas is sometimes sublimated or re-condensed (there are also nitrogen glaciers flowing in from nearby mountains).
That may all seem bizarre, but to people who study orbital mechanics, there’s another strange thing about Sputnik Planitia: It really shouldn’t be where it is. Pluto is tidally locked with its largest moon, Charon, always facing the same side. Sputnik Planitia is almost exactly opposite this face. If you drew a line from Charon through Pluto’s core, it would leave the dwarf planet through Sputnik Planitia. The chance that it arises there by chance is quite small.
But we have a good understanding of the gravitational mechanics of these types of systems. When mass is unevenly distributed across a planet, it can affect both its rotational speed and rotational axis. When a second body is nearby, such as Charon, tidal interactions can change which side of the planet ultimately faces that body. The result is what’s called “true polar wander,” where the axis of rotation shifts to a different location.
To find out how this plays out in the Pluto-Charon system, a Japanese-American team built a simulation that allowed them to input different values for Sputnik Planitia’s properties, such as where it started and how its mass differs from its rest of Pluto. They discover that, if Sputnik Planitia is less dense than the rest of Pluto, it probably would have ended up at the dwarf planet’s north pole, just as the largest impact basin on the moon ended up at the south pole.
But if Sputnik Planitia is denser than the rest of Pluto, it would shift to about the area it is in now, dragging the rest of the planet with it. Scientists are even discovering that the axis of rotation can shift as much as 60 degrees as the gravitational couples do their job. These models also constrain the original location of Sputnik Planitia, indicating that it must have formed in the northern half of Pluto’s anti-Charon side.
This gravitational torsion would create stresses on the rest of Pluto’s surface, creating errors with specific orientations. The team performed an analysis showing that many (but not all) faults on the dwarf planet are consistent with the expected stresses.
That all makes sense, but there is a problem, one that is also recognized by a second team (a large team that includes the entire New Horizons Geology, Geophysics & Imaging Theme Team). While nitrogen ice is denser than pure water ice under these conditions, there isn’t enough of it to make Sputnik Planitia heavy enough to drag Pluto around. Both teams calculate that this would require nitrogen to fill a basin more than 50 km deep; Sputnik Planitia would be less than three miles deep.
So while the nitrogen ice may add to things, it’s not enough to put Sputnik Planitia where it is. So what could? This is where the possibility of a subsurface ocean comes into play. If one existed, the impact this basin created would have thinned the crust in the region. That would allow water to enter Pluto’s icy shell from below. And since liquid water is more dense than the ice it replaced, this could significantly increase the mass of this region. The end result is enough mass difference for gravity to spin the planet.
But that does not mean that the small contribution of the nitrogen ice is irrelevant. Both teams note that the amount of nitrogen in Sputnik Planitia will be affected by the local temperature and the amount of sunlight it receives. And both are affected by how close Sputnik Planitia is to the plutonic equator. This creates the prospect of complex feedback loops between orbital forces and the amount of ice in the basin, causing it to dynamically shift Pluto’s rotation. “If fleeting [like nitrogen] migrating in and out of Sputnik Planitia on seasonal timescales, then Pluto may experience small-amplitude wobbles similar to Earth’s annual atmospheric pressure-driven oscillation,” a paper concludes.
Leaving aside the new information these papers contain about Pluto’s past, it’s worth taking a step back to appreciate how quickly scientists went from being baffled by the New Horizons images to providing detailed explanations for some characteristics of the dwarf planet.
Nature2016. DOI: 10.1038/nature20120, 10.1038/nature20148 (About DOIs).
List image by NASA