Probing the mystery of Pluto’s glaciers
Glaciers seen on the dwarf planet appear to violate physics, prompting a search for explanation. Richard A Lovett reports.
When the first photos came back from NASA’s New Horizons flyby of Pluto in July 2015, one of the biggest surprises was that the dwarf planet has mountains. And not just any old mountains: Pluto’s peaks are as tall as the New Zealand Alps.
Even more surprisingly, like the New Zealand Alps, they show signs of glaciation.
In some places the glaciation takes the form of valleys radiating outward from a ridge crest, like the bones of a fish — a pattern Orkan Umurhan, a mathematical physicist at NASA Ames Research Centre in California, US, calls “classic alpine valley glaciation”.
Elsewhere, vanished glaciers have cut deep gashes into the edges of high plateaus. “That’s exactly what you see in Yosemite,” Umurhan says.
But there are also conundrums.
Some of the glacial valleys are old, in terrain that has existed long enough to have been heavily cratered by eons of interplanetary bombardments. Some look as though glaciation went on for only a short period of time then stopped. Others can’t be more than 100 million years old.
But where did the glaciers come from that carved these valleys? Pluto’s current glaciers are in lowlands, far below the mountains where the valleys are now seen.
Also, they aren’t formed of water ice, like those on Earth. With surface temperatures hovering around minus-229 degrees Celsius, they are made primarily of frozen nitrogen. But water ice does constitute the “rock” into which they cut. Given the prevailing temperatures, it has much the same consistency of granite.
That, Umurhan says, produces a couple of problems. One is that Pluto’s super-cold water ice bedrock is hard. Combine that with the dwarf planet’s low gravity, and it would take glaciers at least a kilometre thick to rip it apart, he says.
To us on Earth, glaciers like that don’t seem unreasonable. After all, Antarctica and Greenland have ice sheets that are several times that depth.
But it turns out that nitrogen glaciers flow a lot faster than Earthly ones – fast enough, in fact, that Umurhan can’t figure out how they could ever build up to depths that could carve the deep valleys New Horizons found. The ice would have flowed away long before achieving the result, he says.
What’s needed, he adds, is something to stiffen the glacial ice, just as carbon stiffens iron to form steel.
The most likely candidate, he says, is methane, which is known to be present as snow-like frost on the tops of Pluto’s highest peaks and was recently discovered to form dunes near the base of some of its mountains. What’s lacking, Umurhan adds, are lab tests to see how its inclusion into nitrogen glaciers affects flow, and whether it stiffens them to the degree needed to produce the glacial valleys seen by New Horizons.
Meanwhile, he suggests, there may have been two processes at work forming Pluto’s glacial valleys.
One involves Sputnik Planitia, the giant lowland in which Pluto’s nitrogen glaciers are presently concentrated.
Most likely formed billions of years ago by a massive impact, Sputnik Planitia is a roughly circular basin, about 900 kilometres across, filled to a depth of several kilometres with nitrogen ice.
“All that ice probably meandered its way down [from above],” Umurhan says.
But some of it may not always stay there. Rather, there may be a cycle that periodically evaporates some of the nitrogen out of the basin and distributes it to the nearby uplands.
If that redistribution also involves traces of methane — and if methane is the mystery ingredient that strengthens nitrogen glaciers enough to produce noticeable erosion — then this process may be the one that produced the youngest of the alpine valleys seen by NASA.
But planetary formation models also suggest the young Pluto should have had a lot more methane than it has today, Umurhan notes. Among other things, he suggests, that might have provided methane to stiffen nitrogen glaciers all around the planet, allowing them to carve deep into bedrock when the planet was young.
Later, most of the methane was lost to space, until only trace amounts remained. “It’s speculation,” Umurhan says, “[but] there’s some numbers that support it.”
Francis Nimmo, a planetary scientist at the University of California, Santa Cruz, agrees that Pluto may once have had more methane.
“The situation might be a bit like that on Mars, where we see ancient evidence for flowing water,” he says, “although Mars has by now lost much of its water.”
Meanwhile, Umurhan finds it fascinating that Pluto has glaciers and climate cycles, even though it’s so distant from the sun that it should be vastly different from Earth.
“It’s fascinating to see analogous processes happening with completely different materials, under completely different conditions, but eerily similar to what’s happening on Earth,” he observes.
He presented his results recently at the 2018 scientific assembly of the Committee on Space Research of the International Council for Science (COSPAR), in Pasadena, California.