Landslides on Pluto Happened Quickly

If Pluto is really 4.6 billion years old, something happened almost instantaneously out there some time ago.

In planetary science, where experts are accustomed to thinking everything is billions of years old, sudden unique events present problems. Almost everything should have settled down by now into a routine. One can certainly argue for exceptions, where tipping points were reached and dramatic changes occurred, or impacts fundamentally changed a surface feature, but in general, sustainable conditions are preferred to sudden changes. Things that look “young” pose exceptional concerns to explain.

In 2015, the New Horizons spacecraft gave the world a first-ever chance to see the most distant planet in our solar system (granted that Pluto’s status as a planet is a matter of debate). The images were astonishing – not at all what planetary scientists expected for such a distant, cold, isolated body. Instead of an old surface pockmarked by ancient craters, the planet (and its large moon Charon) exhibited features that looked young! These included glaciers of frozen nitrogen, sharp jagged peaks and smooth plains devoid of craters, with sand dunes clearly evident.

Dunes, glaciers, and convection cells in Pluto’s Sputnik Planitia

Now, the planetary journal Icarus calls attention another group of features that apparently formed by rapid, catastrophic events – huge landslides lubricated by liquid nitrogen. O’Hara and Dombard write about “downhill sledding” that moved mountains. The highlights include these bullet points:

• Pluto’s mountain block ranges could have formed by gravity-driven basal sliding.
• Basal lubrication from liquid nitrogen is required, implying global climate change.
• Emplacement times by this mechanism are very short, as low as hundreds of years.
• Blocks could also be carried by glaciers if they were sufficiently submerged.

So Pluto has climate change, too, but certainly not caused by fossil fuels. When did the landslides occur? What triggered the movement of mountain block ranges at some point in Pluto’s history? In a scheme involving billions of years, events taking hundreds of years are instantaneous, relatively speaking. On a 46-foot rope stretched across a stage, symbolizing Pluto’s timeline, a thousand years is such a tiny sliver it would be nearly invisible to a viewer sitting in the audience. Even a 100-million-year segment would be represented by just one foot on such a rope. A millennium would be one 100,000th of that!

If the landslides occurred all at once, the planetary scientists might have to imagine this: a billion years, nothing happens …. another billion years, nothing happens …. BANG! Landslides! …. a billion years, nothing happens …. a billion years, nothing happens ….

Based on their description of the action, it’s not even clear that a single century would be required to account for the landslides. On Earth, landslides happen quickly once the critical point is reached, and end quickly, too.

We present a force-balance analysis of the mobilization and emplacement of the large mountain block chains observed on the western rim of Pluto’s Sputnik Planitia basin. These mountain blocks are likely disrupted pieces of Pluto’s water ice crust that are grounded in the nitrogen ice that fills the basin. After fracturing from the basin rim, they could have slid downslope into the basin under their own weight on timescales as short as hundreds to thousands of years, collecting into the observed ranges where the basin slope shallows below a critical value. This movement would require a lubricating fluid, most likely nitrogen, to be present at the base of the blocks.

Mountains on Pluto from New Horizons, July 14, 2015

Landslides, furthermore, were also inferred on the moon Charon (8 Nov 2016) a few years ago. The Pluto system is not looking like a cold, dead pair of worlds in steady state that was anticipated. Things have happened since Pluto formed, and could be happening now (see 16 July 2018). Recall the mission scientists’ surprise when the first images came in, 17 Sept 2015. The authors of the current paper say,

If the blocks did emplace by basal sliding, the requirement that liquid nitrogen be present to provide lubrication provides a key to Pluto’s past climate history. This work adds to the body of evidence that erosional features and other surface modification could have been driven by the surface or near-surface flow of liquid nitrogen resulting from global climate change on Pluto.

It could be argued that Pluto undergoes cyclic climate changes due to the high eccentricity and inclination of its elliptical orbit. That would bring it closer to the sun and then back out each 248-year period. But how long can these nitrogen cycles be repeated? Wouldn’t much of the nitrogen have sublimated to space after billions of years? Wouldn’t all the mountain block chains have sledded into position long ago? Wouldn’t all the erosional features have stabilized by now? If these events had occurred soon after Pluto’s formation, they should have been obscured by space weathering, further erosion and cratering, it would seem.

The conundrum is reminiscent of the high activity seen at Jupiter’s moon Io. So much activity is observed on that volcanic moon, the entire moon would have been recycled through its volcanoes 45 times over in 4.6 billion years.