Do we have a clear understanding of how much deposition is possible during the Hadean winter? I know the atmosphere is slight, but if the cold sinks are pulling out faster than the sunlit side can provide, the pressure could stay low even with decent amounts of mass flow. I'm liking my idea of an equatorial icecap (maybe not complete, just Tombaugh Regio) that grows north via deposition and glacial processes during the northern winter, and then melts back and retreats while growing south in the summer.

The brown boundary above and left of Pluto's head/ears would be the limit of how far the glaciers could creep before summer hits and they start melting back.

Explaining how the dirty areas gets on the regio would require some handwaving, over millenia it should have all been pushed to the edges. Maybe the actual flow rate from the center to the edge is slow enough that it doesn't get very far before the ices sublimate away. Or the white ices form on top of the dirty material, like frost, but the dirty material is actually lighter and so once the depth is high enough they start floating to the surface, like garden rocks.

Or it could be warmer material coming up from underneath, forming giant convection cells. This seems to be the prevailing theory, but the thing that keeps bothering me is that the two icy bodies where we are fairly certain warmer material did exactly that - Europan chaotic terrian, and Enceladean tiger stripes - don't look anything like this. Is that because those processes are all water ice while Pluto has a mix of water and nitrogen ice?

Can't wait to see the IR images, that should tell us quite a bit more about what is really going on - what's warm, and what's not. I'm sticking with my "frost is added to the bottom of valleys/craters until the frost mounds meet at the ridges and peaks" hypothesis.

I tried using the images from the approach mosaic and high-resolution mosaic of Tombaugh Regio to create a stereo view of the chaotic region. Unfortunately the change in viewing angle is small enough that there's not a lot of depth in the stereo pair.

Cross-eye version


Parallel version

Has freezing the top-layer of e.g. liquid nitrogen been studied in the lab? Solid water is denser than air, yet air can be covered by solid water as the solid water manages to lift itself up by sticking to the material around it. This can be seen in frozen mud pits during winter.

Fractures in the ice sheet could allow segments of the ice to sink, but liquid nitrogen coming up from below could freeze and act as a glue before the segment properly manages to sink; holding it in place within the ice sheet (this would be one way to interpret the cell-like structure of parts of Sputnik: it's where liquid nitrogen wells up and freezes in place).

This would all depend on the exact properties of liquid and frozen nitrogen, which I know next-to-nothing about; but some lab studies would be enlightening.

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Edit: somewhat unrelated, look how shoreline-like this area is:



I believe this area will be photographed at 250 m/px by MVIC during the P_PHOTSCAN sequence.

As indicated in a few previous posts, I am in the upwelling/convecting-material camp.

I'm visualizing something that is roughly a cross between a lava dome complex and a glacier, i.e., a vast amount of some plastic substance (nearly solid nitrogen or a nitrogen/something mixture) that is slowly being extruded from the interior.

Because the material is (assumed to be) structurally weak, it would flow in the manner of a glacier once it reached a certain level instead of building up a cryovolcanic edifice. This isostatic adjustment, as well as variability in the material movement below, would cause the surface 'glacier' to ebb and flow and eventually change course as isostatic equilibrium is maintained. Sublimation of the ice would further contribute to the modification of the plain's profile.

Many, many times - you can find videos all over Youtube, e.g.:

https://youtu.be/b7K-6zEhtYw?t=53

Fluffy N2 from the freezing process could indeed float - you can see how porous it is. How long it would persist in that state though, I have no idea. If it quickly saturates with LN2, it won't be on the surface long.

I'm not saying that you're wrong, by the way - just playing devil's advocate here. I really have no clue whether actual liquids come into this picture anywhere or not (I see more of a challenge from the low temperature than anything else). But what we do see is, whether liquids are involved or not, is flowing glaciation that appears to be lofting water ice mountains about like playthings. And that in itself is pretty darned spectacular.

Well, when I said study, I really meant it. We would have to be careful in order to be sure we are replicating the environment found on e.g. Pluto. Sucking the air out produces a lot of disturbance, and probably several other factors that are not typical of Pluto (the fluffiness might indeed be one of them).

I found this video, where we can see clearly that most of the solid nitrogen stays on top (t=2:10):



At the end, they make it sink. But it's not clear whether that happened on its own, or whether they e.g. shook the thing a little. I imagine the adhesive forces between the nitrogen ice and the smooth walls of the tube aren't the best, regardless (the tube walls themselves may also propagate heat that could melt some of the ice that was supposed to stick to them, making the entire mass of ice slip). [supercool bonus of a flake of solid CO2 propelling itself over water at the end of that video; complete with its own miniature jets on top]

As for the heat - if I understand you correctly - I am envisioning Sputnik as a cryovolcanic feature. It would be akin to geothermal springs on Earth: the heat is provided by Pluto's interior.

Not yet.
Early estimates were that there should be enough insolation to evaporate the ice caps and re-deposit them every orbit.
Pre-encounter, a paper posed three ideas about where and when the ice caps would move around.
They concluded that with enough volatives, a polar cap could last year round on the northern pole.



Leslie Young's model for Pluto's year-round climate produced three different kinds of behavior.
Top row: with lots of nitrogen and larger thermal inertia, a north polar cap persists all year round.
Middle and bottom row: with less volatiles and lower thermal inertia, caps migrate from north to south and
air pressure reaches maxima at southern summer solstice and also between perihelion equinox and northern summer solstice.
Two caps persist for most of the year when there's a larger volatile inventory.
With less nitrogen available, the northern polar cap vanishes quickly after perihelion.




Of course, what we see, an equitorial ice cap and a polar cap, isn't one of the anticipated results-
But, we'll probably have a better idea of themal inertia and available volatiles...

Pluto's internal heating should be really quite weak. It's the reason why so many people expected to see a dead world. But, with clearly active resurfacing processes, there's clearly some energy at play, and there's a couple plausible sources - sublimation, thermal cycling across Pluto's orbit, convective redistribution of water and nitrogen ices closer to an equilibrium state, etc. Whether that ever correlates to sufficient "hot spots", that's in the realm of speculation at this point unless anyone ever sights convincing evidence - but it certainly would be interesting.

The ice in your screen grab may be supported by buoyancy, or it may be simply by holding onto the walls. You're right that it's hard to say more accurately how liquid nitrogen would freeze on Pluto or progress over time without more specific, controlled study (though regardless, I have difficulty conceiving of it persisting in that state over geological timeperiods, just sitting loosely over a liquid without compacting - certainly sea ice (water ice) on Earth loses its porosity with time, and snow on water quickly saturates into slush). As for studies aiming for an exact recreation of the environment on Pluto, I'm not finding anything to that effect. I did find this, concerning nitrogen circulation on Pluto in general:

https://fallmeeting.agu.org/2015/abstract/c...-ices-on-pluto/

But it's not been published, unfortunately :Þ

It seems like experiments like these would be ideal to conduct on the ISS, where the effective gravity and temperature could be controlled precisely, with space providing a "free" vacuum.

It takes a long time to bring an experiment to the ISS. Each element has to go through tons of review to make sure it won't compromise the platform. Environments can be simulated fine enough on the ground at much lower cost.

Some models give the possibility of Pluto having liquid water outside a hypothesised rocky core. The temperature requirements for subsurface liquid nitrogen are of course much less, so relative to that, it is not a very radical suggestion. Given Charon's relative youth - which would be difficult to explain with volatiles, AFAIK - the case for a relatively warm Pluto interior should be strengthened.



A more compact ice could also be stronger. With Pluto's low gravity, a ton of nitrogen ice would weigh just 60-70 kg. The low gravity should make it harder for a sheet of ice to collapse and/or fracture under its own weight (unless solid nitrogen has a fondness for cracking, or something to that effect).



Edit: The other paper Geology before Pluto: Pre-encounter considerations mentioned there is published, but I couldn't find much relevant searching for "nitrogen".

The next ship to Pluto won't be leaving for a while yet methinks.....

WRT to the Glaciers: If N2 is only liquid at depth, and freezes as the pressure drops, would the idea of am ocean that varies in consitency from near solid to fairly mobile slush, topped with a protective layer of hard ice, fit the known facts to date? Slush would be closer in density to the solid ice layer, and so the solid stuff wouldn't sink much even when the slush is mobile - plus the viscocity of the slush would be much higher than liquid. 'Slush' might be a relative term anyway - on earth solid ice retainsa liquid componentdown to about -90 deg c I think, so a N2 slush might only be a little less solid than the hard ice.

Yes, that sounds right to me. The apparent size sorting within the al-Idrisi Montes, with the biggest chunks in the south, smaller to the north and smallest even further north and between the biggest chunks and the shore, could be due to their being washed onto a sloping shore from the south. So the biggest pieces would go aground first and smaller pieces would tend to be pushed a little further north before hitting the bottom. The largest pieces are all slabs about 5 km thick, indicating that they are all fragments from a 5 km thick crust. So all the large fragments tend to hit the sloping surface underneath at a similar distance from the shore. Pieces smaller than 5 km could precede the big slabs into the shallows or flow around them after they were grounded.

The slabs of al-Idrisi Montes would make an awesome sight close up. A 5 km thick slab flipped on its edge and towering 10 km over your head would be impressive. The 5 km thick slabs that are not standing on their edges, at the northern end of al-Idrisi Montes, make up a giant raft. It's likely that only the slabs closest to the shore are aground. Those further out are likely still floating on Sputnik Planum's soft ice. A few hundred kilometers further north along the coast, near Farinella Crater, Sputnik Planum ice appears to be flowing onto the land. All of the soft ice in the north of Sputnik Planum appears to be slowly flowing north. The raft of al-Idrisi slabs is also likely to be on the move, being swept north by the general flow.

Link to blog post about al-Idrisi Montes

The only logical origin I can see for all these giant slabs is a crust that once covered the volatile ices of Sputnik Planum.

The mountain blocks do appear to be chunks of Sputnik 'shoreline' that have broken off. Do the (possible) fractures marked in yellow indicate the next 'raft' to go?:



(The 'solid' Sputnik 'coast', uncertain in places, is in red; possible ice flow directions are in blue.)

If this isn't the case, then where did the slabs/blocks originate?

That's an interesting theory, I could picture that. Pressure from beneath could destabilize parts of the shoreline, for example.

The other possibility I see for where the blocks could have come from is "from beneath". Picture a forming Pluto. Traditional "rock" (silicates and the like) become solid first. Then water. At some point you have a water ice surface with a liquid nitrogen ocean over it - water ice being heaver than liquid nitrogen, so no problem there. But then it cools further and the nitrogen solidifies. Now you have a heavier outer layer (nitrogen ice) over a lighter inner layer (water ice). Nitrogen ices have rather low viscosity and flow easily. And there's a powerful flow mechanism here - the solar wind unevenly baking it off. So you have the potential for the flowing nitrogen ices to entrain water ice from below and let it float to the surface. More surface ices can come from collisions with icy bodies. Due to the constant nitrogen-ice mantle relaxation, there would also be tectonic forces across the planet - although probably strongest near Sputnik where the ices are exposed.

The ability of ice from beneath to float to the surface would probably not be limitless, of course; the properties of nitrogen and water ices will change at depth due to pressure and one can envision a variety of factors that could become "limiting". But as the nitrogen on the surface boils off, the pressure drops beneath, so if there is a "zone of instability", it'll shift downward and allow new chunks of deep ice to break off and flow up, emerging at the centers of the convection cells, drifting to the boundaries between cells, and eventually migrating ashore.

Of course, that too is just speculation on possibilities that might fit the data.