Strange fields of polygons seen during New Horizons’ visit to Pluto could be explained by million year variations in the dwarf planet’s orbit caused by its ice giant neighbors, says a team from Taiwan’s Institute of Earth Sciences. Their conclusion, which challenges existing explanations of bottom-up heating of the glacier from the warmer rocky core below, has however been questioned by those unsure that its orbital wobbles could produce sufficient temperature changes quickly enough to explain these unique surface features.
“It was very, very exciting. We had not seen anything like them before.”
In 2015 the New Horizons probe passed above Sputnik Planitia, a large glacier of nitrogen ice that runs for several hundred kilometers from Pluto’s northern hemisphere across its equator.
However, it wasn’t the size of the glacier that stunned researchers like Kenny Vilella from the Institute of Earth Sciences at Academia Sinica, Taiwan, watching the images beamed back. Instead their focus was fixed on strange fields of crazy paving style polygons — 30 km wide icy cells separated by 2-3 km wide ridges that are 50-100 m deep.
Icy ridges are not uncommon in the Solar System. Jupiter’s moon Europa is crisscrossed by them, likely formed by the influence of Jupiter’s gravity. However, Europa’s ridges are straight and randomly arranged in a jumbled mess. Pluto’s more ordered polygonal fields suggest a different mechanism. But what?
Previous work by New Horizons’ William McKinnon pointed to heating of the glacier’s base from the still warm radioisotope-containing rocky core creating a series of vertically rotating convection cells beneath the surface. However the theory didn’t sit well with Vilella.
“With thermal convection from bottom heating you get very strong upwelling. You should have much stronger positive topographies.”
Whilst bottom up thermal convection could produce the surface pattern from a bird’s eye (or space probe’s) view, Pluto’s polygons only rise to around 50 m at their highest point near their center. Vilella didn’t feel this was sufficient if heated material was rising up through the glacier from below.
“Whilst previous models had reproduced the surface features in 2D. We wanted to explain it in three dimensions.”
Vilella’s preference was volumetric heating induced convection where, like in a microwave oven, heat is evenly applied everywhere. Unlike a bottom up source, volumetric heating doesn’t produce upwelling, only downwelling when the heat source is removed.
Vilella believed this could account for a landscape where the intercell pits are deeper than the mid cell peaks are tall.
To test his hypothesis, he and Frédéric Deschamps applied the various heating mechanisms to models of nitrogen ice derived from lab testing in an attempt to reproduce the surface landscape observed by New Horizons.
Vilella’s results, published in the Journal of Geophysical Research: Planets, showed bottom-heated thermal convection was unable to produce the observed surface patterns without the missing strong positive topography. Volumetric heating, on the other hand, produced a much better approximate for the deep ridges and gently rising bowl shaped polygons.
Whilst key properties of Sputnik Planitia such as its thickness, temperatures and, in particular, viscosity, had to be estimated, observed surface patterns were produced with glacier parameters that were at least plausible, and in some cases very close to estimates from other methods.
For example, Kenny’s team’s models worked best with a 4-5 km thick glacier, a value in keeping with a number of previous figures estimated from comparisons with similar sized impact craters on other planets, and from the dimension of iceberg like features observed floating in Sputnik’s frozen nitrogen sea.
“It is when you add everything together — this is what makes the model strong.”
However, the model raises questions. What could cause this volumetric heating?
Within the Earth’s interior volumetric heating comes from radioactive isotopes. However, these are hard to find in Sputnik’s glacial ice, and there is no mechanism for sufficient tidal heating, such as that generated by the gravitational pull of Jupiter on its active moons.
One alternative option put forward by Vilella are variations in Pluto’s orbit on a scale of several million years caused by the presence of the outer giant planets.
Models have suggested Pluto spends 2 million year moving closer to the Sun, during which time heat is stored in its glacier. It then spends another 2 million years moving away as heat is evacuated and temperatures drop.
If the models are correct, we are currently in the later of these phases, resulting in a cooling Pluto.
“Vilella and Deschamps identify a serious problem,” says Orkan Umurhan, who has been studying nitrogen convection within Pluto and suggests their “viable alternative explanation” for triggering convection is certainly possible during Pluto’s long-term 2-4 million year Milankovitch cycles.
“However, even their explanation has problems,” he adds. “They can reproduce the observed patterns only for secular cooling rates that are too fast for what is typical of Pluto during the Milankovitch cycle. If you put in more realistic rates then the patterns produced in their simulations look nothing like what is observed.”
Other Pluto experts also need convincing.
“I have trouble believing the seasonally averaged surface temperature of the Sputnik Planitia ices can vary so much over multi-million year timescales,” says Bill McKinnon.
“The reason is, that despite large possible variations in ice temperature and surface pressure near perihelion, Pluto spends most of its time distant from the Sun, and it is simply quite cold most of the time, with a minimal atmosphere.”
Whilst the impact of these orbital variations would indeed be minimal Vilella believes temperatures fluctuations of only a few degrees could trigger convection to produce the shallow hexagons patterns.
Even if Vilella’s suggestions are proved correct, more work is required on a model that is good but not perfect.
The problem lies with elevation again, however rather than producing too higher ridges like bottom up heating, the volumetric heating model produces topography that is only 10 m from ridge pit bottom to polygonal cell peak, 10 times less than has been observed.
Vilella hopes a better understanding of Pluto’s changing surface temperatures during these orbital variations might make up the missing topography.
“We need to understand how Pluto’s atmosphere reacts over these timescales. It is not just about how much energy will be given by the Sun but also how the planet’s atmosphere and volatiles react.”
“There are a lot of uncertainties and this is a wide open field at the moment,” concludes Umurhan. “This means plenty of controversy to keep us all busy!”
Kenny Vilella Frédéric Deschamps. Thermal convection as a possible mechanism for the origin of polygonal structures on Pluto’s surface. Journal of Geophysical Research: Planets, published online May 27, 2017; doi: 10.1002/2016JE005215