[X] My Assistant

[Nafnlaus]

One possible trigger for the formation of Sputnik Planum could have been an impact at a location where Pluto's crust happened to be relatively thin. Some areas of Pluto do have quite big craters. The crust must be thicker there. You can find a schematic illustration of an impact formation scenario HERE and how Sputnik Planum could have grown to it's present size HERE.
Judging by the slabs deposited in al-Idrisi Montes the thickness of Sputnik Planum's previous crust has turned out to be ~5 km, rather than the ~1 km guess-timate in the schematic.

Rather than an impact, the crust under Sputnik Planum might have cracked up through entirely internal geological processes due to a slow build up of heat under an insulating crust. This would be something like the scenario that has been proposed for Venus, where large sections of its crust get recycled in massive episodes of volcanism after a long periods of relative quiescence during which internal heat builds up. Because it doesn't need a lucky impact this sort of slow cycle scenario might be a more plausible explanation for Pluto's activity.

Do you think it's just a coincidence that Sputnik is exactly opposite Charon?

[JRehling]

The Moon settled into a tidally locked orientation where the thinnest crust was exactly opposite the Earth; consequently, the overwhelming majority of maria are on the Earth-facing side, clustered around the sub-Earth point. As I understand it, this could equally well have turned out the opposite, with the thinnest crust centered at the anti-Earth point, for reasons similar to those why we have two high tides each day on Earth, with one facing the Moon and one opposite.

Consequently, the thinnest part of Pluto's crust "should" be either at the sub-Charon point or the anti-Charon point. So it's a pretty good conjecture that Tombaugh and Sputnik are at the anti-Charon point because that's where the thinnest crust settled. But it's still a conjecture.

[Charles Astro]

Do you think it's just a coincidence that Sputnik is exactly opposite Charon?

I agree with JRehling, the location of Sputnik Planum is probably no coincidence. Though that may not help to decide between impact or internal volcanism triggered formation of Sputnik Planum.

If the shape of Pluto were prolate, tidal forces should cause the long axis to align with Charon. The same thing happens if Pluto’s crust is more massive (or surface layers) on one side because, if I understand correctly, that would cause an offset between Pluto’s center of mass and the center of mass of its core, which effectively gives it a long axis.

At first sight, the location of Sputnik Planum opposite Charon suggests an internal volcanic origin because the crust there was likely to have been thinnest and therefore most likely to give way to a slow build up of internal stresses. On the other hand, if Sputnik Planun was originally covered with crust of fairly common thickness which was hit and broken up due to a cosmic impact, when the broken crust was swept away and ice came flooding up from underneath the mass distribution of Pluto’s surface layer would have changed. Tidal forces would then tend to move Sputnik Planum to a point on the Pluto-Charon axis, no matter what latitude and longitude it started at.

If Pluto has a planet-wide ocean of soft ice under its solid crust this would make the tidal reorientation all the easier since the crust could easily slide with respect to the core and energy could easily be dissipated, so that Pluto could quickly stop rocking side to side after the reorientation.

The since the crust appears to float on an ocean of Sputnik Planum ice this means that the Sputnik Planum ice is denser than the crust. So might that mean that Sputnik Planum is now a mass concentration on Pluto’s surface. But then again, the elevation of Sputnik Planum seems to be lower than the surrounding crust so density differences might balance out. Exactly how the soft ice of a global ocean under a thin crust would redistribute itself after Sputnik Planum was opened up seems quite complicated. Perhaps it’s a good topic for some numerical simulations.

[dvandorn]

I will bring this over here, since it's rather speculative. I've made a mention of the concept in the primary thread, but this gets even more speculative.

I'm thinking that we're seeing actual plate tectonics on Pluto. The plates appear to be smaller and more irregular than we see here on Earth, but it seems that the whole Tombaugh Regio may be the equivalent of seafloor spread, pushing plates of water ice away from it. This would mean that all of those fold-edge chasms we see are actually subduction trenches, where the water ice crust is being forced back underneath the surface, into a "molten" (or at least plastic) mantle composed of similar materials to what we see spreading out onto the surface at, say, Sputnik Planum. The plate movement, and plate collisions, could also account for the raising of the water ice mountains.

The snakeskin terrain could reflect surficial folding from the pressure of plates being shoved along and into each other.

What do you think?

-the other Doug

[alan]

Interesting idea, perhaps the wrinkles to the SE of Norgay Montes have a smaller scale because the crust is thinner there.

On another note, I've been wondering if this area was once another version of Sputnik Planum, with its own chaos terrain at the east west edge.

[Nafnlaus]

I will bring this over here, since it's rather speculative. I've made a mention of the concept in the primary thread, but this gets even more speculative.

I'm thinking that we're seeing actual plate tectonics on Pluto. The plates appear to be smaller and more irregular than we see here on Earth, but it seems that the whole Tombaugh Regio may be the equivalent of seafloor spread, pushing plates of water ice away from it. This would mean that all of those fold-edge chasms we see are actually subduction trenches, where the water ice crust is being forced back underneath the surface, into a "molten" (or at least plastic) mantle composed of similar materials to what we see spreading out onto the surface at, say, Sputnik Planum. The plate movement, and plate collisions, could also account for the raising of the water ice mountains.

The snakeskin terrain could reflect surficial folding from the pressure of plates being shoved along and into each other.

What do you think?

-the other Doug

Why would water ice subduct on Pluto into nitrogen ice? "Cold", rocky crust on Earth can be denser than the mantle underneath it (when under the same pressure), so subduction makes sense. How does it make sense on Pluto? Why would it go down? Any water ice that reaches the surface should never leave the surface.

Not disagreeing about plate tectonics, just about subduction zones. Trenches have many explanations, the most obvious being that they're grabens.

[dvandorn]

My understanding, from reading a variety of speculations about Pluto's internal structure, is that water ice is lighter than some nitrogen ices, heavier than others, and heavier than liquid nitrogen. So perhaps water ice is subducting into zones where it is indeed heavier than the mantling material. And, of course, the mantling material may not be pure nitrogen ices or pure liquid nitrogen, there may be other solid/liquid/slushy gasses mixed in which could affect the density of a given mantling zone. Remember that some light granitic "float" material on Earth ends up being subducted in some places, along with the heavier, highly hydrated seafloor material. It's not a black-and-white kind of thing.

Also, crustal spread requires either crustal pile-up at plate boundaries or sliding some crustal plates underneath other crustal plates. Some subduction, in terms of plates sliding on top of one another, will occur from mechanical forces alone.

-the other Doug

[Nafnlaus]

What form of nitrogen ice are you envisioning that is less dense than water ice (whether we're talking Ic, Ih, or XI - probably the latter)?

While there seems to be a general consensus that the material we can see exposed at Sputnik is far from "pure" nitrogen ice - it's contaminated by both methane and carbon monoxide ice - there also seems to be a general consensus that it's predominantly nitrogen ice. It wouldn't make sense if it wasn't, anyway - we'd surely see relevant quantities of those contaminants in Pluto's atmosphere and condensed on the surface, as they sublimate quite readily as well (there are of course different rates of loss to escape and photochemical reactions between different chemicals).

It should also be noted that carbon monoxide ice is also heavier than water ice.

Faults leading to canyons can occur due to contraction or expansion. They do not require subduction, and subduction doesn't require the movement of water ice to below nitrogen ice (only water ice under water ice). Tension-induced faulting doesn't even require any sort of "pile-up" - the planet can literally change size (due to thermal expansion/contraction or loss of material to space). In terms of contraction, Pluto loses somewhere on the order of 130kg/s of nitrogen to space - equating to several kilometers of depth over geological timeperiods.

[Nafnlaus]

Thoughts on the Charon "mountain in a moat". First, a few observations: while it's the most pronounced one, there's actually quite a few such shapes on Charon - some with multiple mountains per moat. These exist in (according to the NASA flyover video) in a relatively flat plains (Charon's south), at least with contrast to the north. The south is clearly old-ish, given by the cratering, but seems to lack any large, deep craters - either never having been hit, or flowing in and hiding them well. It also has that randomly-trenched look similar to what one finds on other solar system bodies involving a relatively thin water ice crust over a material that can flow.

Now, for the hypothesis: what we're seeing on Pluto around Sputnik could have been carried out in Charon's past - the potential of mountain-sized water ice chunks "floating" on a nitrogen mantle-sea. Being a smaller body, it "aged" quicker, managing to more quickly ice over its entire surface with water ice. This froze any "floating mountains" in place. The nitrogen mantle-sea however would still exist under the ice, and some nitrogen would still seep out through cracks, requiring settling of the ice crust. What would happen? The ice over it wouldn't settle uniformly - rather, anywhere that the ice is thicker and heavier would sink more than where it's lighter. Aka, where there's a big mountain-sized chunk of it. The nitrogen-ice mantle would flow to redistribute the pressure as needed.

It's somewhat akin to taking a stick of room-temperature butter, putting a sheet of paper over it, and setting a marble on the sheet. The paper "crust" doesn't want to shear all the way through (it's clearly thick, at least at present - no punchthroughs, Charon's gravity is weak, and cryogenic water ice is very hard), but over time it will deform or give via numerous small fractures. The butter "mantle" beneath redistributes the stresses, and the marble "mountain" sinks into its own moat. Charon's northern hemisphere, which its apparently thicker crust, is more like setting a piece of cardboard on the butter. It's just too much resistance to allow localized sagging.

Anyway, just a possibility.

Another variant would be that Charon fully lost a nitrogen ice mantle, draping the ice crust over the next layer down. If, for example, in the past an impact had ripped a crater all the way down through the nitrogen ice mantle to the next layer down, the surface ice crust would be draped across this gash when the nitrogen ice disappeared, causing a localized sag. This would occur regardless of what the shape of the surface ice was.

[HSchirmer]

Thoughts on the Charon "mountain in a moat".
...

Now, for the hypothesis: ...
- the potential of mountain-sized water ice chunks "floating" on a nitrogen mantle-sea.

Well, if Charon has an occasional layer of N2 ices, (IIRC P&C warm and cool over a processional cycle of several million years) then Charon could be something like Mars, glaciers buried under an insulating layer, H20 "dust" covering N2 glaciers. If the mountain is sticking up through the glacier, over time the mountain heats up
and drives off the buried glacier.

Also, "mountain in a moat" is a form we see in melting snowpacks on earth.
A lone column of H2O ice standing in a crater of H2O snow.
http://snobear.colorado.edu/Markw/Research/rills.pdf

On earth, a bit of dark material on snow can trigger ice under the dark material, not melting.
Heat transfer in a freezing/thawing snowpack creates a column of ice in a depression in the snow.
The ice has high thermal inertial and good heat conduction to the cold ground below.
The surrounding snow has low thermal inertia and poor conduction to the cold ground.
The snow melts, and feeds the ice tower.

On Charon, I suspect we are seeing a similar heat flow generated feature.
A crystalline H20 mountain which is in contact with "bedrock" and conducting internal heat up
through a less conductive ice layer.

Attached thumbnail(s)

 

[HSchirmer]

It almost looks like this island broke apart from the mainland, was carried off and then listed in the Great Melt of the southern hemisphere.
Note how some of the parts fit, similar to South America and Africa.

Not sure it's a clean fit, and it could also be a fissure where both edges erode back from the newly exposed surface.
Thinking about "new surface" it helps to understand just how much mass transfer could occur on P&C.

Pluto and Charon flip poles on a 3 to 3.7 Myr cycle.
Solar System Ices: Based on Reviews Presented at the International Symposium

Published estimates suggest that the ~3 Myr cycle could drive 1.5 km of ice from pole to pole on Pluto. That suggests that it is possible to move a substantial amount of mass from pole to pole on Charon as well.

[Nafnlaus]

Pluto and Charon flip poles on a 3 to 3.7 Myr cycle.
Solar System Ices: Based on Reviews Presented at the International Symposium

That's based on the concept of 1,5km of polar ice cap building up and changing hemispheres. Pluto clearly has no polar ice cap.

I suspect that it is not so much a fracture and a drift as it is a landslide feature.

Has anyone yet done any sort of photogrammetry work to get a sense of what the heights involved are? E.g. if both are roughly equal height then that would strongly suggest drift, while if the southern piece is significantly lower than the northern than that would strongly suggest landslide. It seems to me altitude change is really the only significant distinction between the two - even if it's slid downward, to remain so intact would suggest some sort of effectively fluidized layer underneath it. And even if it was drift, that doesn't mean "drifting on a liquid", objects can drift on flowing solids as well.

[Nafnlaus]

I'm surprised that nobody has commented about how the land just north of this area appears to be a sort of horst-graben terrain, as if many chunks had attempted to shear off but not separated completely. So within the context of the terrain, does the "landslide" hypothesis make sense for all of them? As horst-graben terrain is created by tensile forces drawing the land apart, would it not stand to reason that whatever was dragging it apart was also dragging the "island"? Could Charon's weak gravity actually impart such tensile forces over such a broad area just from a mere height difference?

Something built very dramatic terrain on the surface of Charon - a highly rugged northern hemisphere, and a strangely flat southern hemisphere interrupted by undulating fossae. Assuming a gravity-induced slide suggests that this is part of a post-tectonic relaxation of Charon's surface toward gravitational equipotential. But can we assume that this occurred only after the terrain-building processes, and not as part of it? Certainly there exist craters in the gap between the "island" and the "mainland", at least in its northern portion, so it can't be too young. The broad, flat southern plains also bear small ridges along the same ridging direction, but fractured at a much smaller scale.

[ngunn]

If I'm reading the source right the precession of the rotation axis happens anyway due to the effect of the Sun's gravity. Long term migration of volatiles is mentioned merely as a possible consequence of ithat precession provided suitable materials are present.

[HSchirmer]

That's based on the concept of 1,5km of polar ice cap building up and changing hemispheres. Pluto clearly has no polar ice cap.

Well, clear is the operative word

“We just learned that in the north polar cap, methane ice is diluted in a thick, transparent slab of nitrogen ice resulting in strong absorption of infrared light,” said New Horizons co-investigator Will Grundy, Lowell Observatory, Flagstaff, Arizona.