Glacial Floods Push Down Pluto’s Crust

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The ice tsunami

In the north of Sputnik Planum ice appears to be flowing onto ancient terrain, along with its irregular polygon shapes.

ice tsunami over northern Sputnik Plnum

[Note that it’s not yet clear whether the ice of Sputnik Planum is predominantly frozen methane or nitrogen].

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Processed closeup image of ice flows in Sputnik Planum(Ron Baalke)

At the northern shore the ice is flowing around obstacles. Here the shapes of convection cells have become because the obstacles are smaller than the 30 km size scale of the convection cells.

It would be interesting to have observations of this flow over time to know just how fast this ice is engulfing the land here.

The thin crust model of Pluto’s surface (1) offers an explanation of these flows without the need for giant ice waves as in the thick crust model (2). If the crust is somewhat flexible it might be pulled down allowing Sputnik Planum’s soft ice to flow over it, while the ice itself maintains an almost perfectly flat surface (or at least one that follows an equipotential surface).

A tsunami of ice flows glacially over Pluto's surface
Stage 5. The intact edge of the region of broken crust is pulled down. Ice continues to evaporate from the surface of Sputnik Planum releasing vast amounts of gas into Pluto’s atmosphere.

It is in the nature of Rayleigh-Benard convection that cells form up to the edge of the intact crust (as in stage 4) so cold ice will always be sinking right along this edge.  So there is a natural tendency for the icy solid state convection to pull the edge of the crust down. If if the crust can be pulled down and some ice does get on top of it the extra cooling area of this ice flood will increase flow of colder descending ice over the edge of the crust, dragging it down even more strongly. Of course the 3-dimensional pattern of warmer ice flowing onto the crust to replace the cold ice falling off it, must be much more complicated than can be represented in this schematic cross-section. The flow of warm ice into the flooded area will be a combination of the bulk flow of ice onto the crust and the convective circulation, where warm ice is drawn in to replace the chilled ice that pours off the edge of the crust.

Any chunks of fractured crust floating on the ice will be carried with it when it flows over a sunken section of crust. The chunks will go aground and when the sagging crust eventually springs back to the surface they will be mountains. The ranges of mountains composed of big angular chunks along the western side of Sptunik Planum, including Norgay Montes, Hillary Montes, Zheng He Montes and al-ldrisi Montes, were all likely formed this way. The fact that these chunky mountain ranges all lay along the western side of Sputnik Planum implies that this was the first place where surrounding intact crust was drawn down after the crust shattering impact.

jumbled blocks in the upper left(Glastoner)The slab-like nature of these angular mountains is particularly obvious for the al-ldrisi Montes

cellular terrain outlined in yellow

 

The extent of the al-ldrisi Montes is outlined in yellow (thanks to Glastoner)

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The Shadow of Sunken Crust

The northern third of Sputnik Planum is darker that the rest. There is a clear line of demarcation that arcs across the plain separating the zones. This line likely marks the edge of the Sputnik Planum dirt division north upintact but submerged crust. South and east of the line the ice is whiter and cleaner. Here ice convecting to the surface likely comes from down deep. To the north and west of the line the ice is darker. Here the convection would be shallower. Warm ice likely picks up dark solids from the surface of the submerged crust, before carrying it to the surface. The surface of the ice is even darker near the northern edge of Sputnik Planum. This is where the ice flood proceeding right now. Here the ice is shallow and rolling over crust that may not have been inundated before, so may have had a lot of loose regolith for the convective motions to dredge up, similar to the way that a tsunami churns up debris.

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