According to members of NASA’s New Horizons science team, the icy surface of Pluto’s Sputnik Planum basin is being constantly renewed by a process called convection that replace older surface ices with fresher material.
Sputnik Planum basin: this scene, which is about 250 miles (400 km) across, uses data from New Horizons’ Ralph/MVIC instrument. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.
The vast, icy basin informally named Sputnik Planum is central to Pluto’s geological activity.
Composed of exotic ices, but dominated by nitrogen ice, nearly the entire surface of this region is divided into irregular polygons.
“For the first time, we can determine what these strange welts on the icy surface of Pluto really are,” said New Horizons team member Dr. William B. McKinnon, a researcher with Washington University in St. Louis.
“We found evidence that even on a distant cold planet billions of miles from Earth, there is sufficient energy for vigorous geological activity, as long as you have ‘the right stuff,’ meaning something as soft and pliable as solid nitrogen.”
Dr. McKinnon and co-authors used state-of-the-art computer simulations to show that Sputnik Planum’s surface is covered with icy, churning, convective ‘cells’ 10 to 30 miles (16-48 km) across, and less than one million years old.
They believe the pattern of these cells stems from the slow thermal convection of the nitrogen-dominated exotic ices that fill the basin.
A reservoir that’s likely several miles deep in some places, the solid nitrogen is warmed by Pluto’s modest internal heat, becomes buoyant and rises up in great blobs before cooling off and sinking again to renew the cycle.
The computer models show that ice need only be a few miles deep for this process to occur, and that the convection cells are very broad.
The models also show that these blobs of overturning solid nitrogen can slowly evolve and merge over millions of years.
Ridges that mark where cooled nitrogen ice sinks back down can be pinched off and abandoned, resulting in Y- or X-shaped features in junctions where three or four convection cells once met.
“Sputnik Planum is one of the most amazing geological discoveries in 50-plus years of planetary exploration, and the finding that this vast area is created by current day ice convection is among the most spectacular of the New Horizons mission,” said New Horizons Principal Investigator Dr. Alan Stern, from the Southwest Research Institute.
These convective surface motions average only a few cm a year which means cells recycle their surfaces every 500,000 years or so. While slow on human clocks, it’s a fast clip on geological timescales.
“This activity probably helps support Pluto’s atmosphere by continually refreshing the surface of ‘the heart,’” Dr. McKinnon said.
“It wouldn’t surprise us to see this process on other dwarf planets in the Kuiper Belt. Hopefully, we’ll get a chance to find out someday with future exploration missions there.”
The researchers reported their results in a pair of papers (paper 1 & paper 2) in the journal Nature.
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William B. McKinnon et al. 2016. Convection in a volatile nitrogen-ice-rich layer drives Pluto’s geological vigour. Nature 534, 82-85; doi: 10.1038/nature18289
A.J. Trowbridge et al. 2016. Vigorous convection as the explanation for Pluto’s polygonal terrain. Nature 534, 79-81; doi: 10.1038/nature18016
