Pluto System Speculation Options

[Nafnlaus]

I'm staring at images of Pluto right now, there clearly is no 1,5km thick north polar ice cap. There's mountains and sharply defined craters. Nitrogen ices are well known to not be able to sustain sharp topographic relief at Pluto temperature and gravity conditions (I've run into this mentioned in many different papers discussing Pluto during the leadup to the flyby - it was widely discussed that if sharp topographic relief was found at Pluto that it'd have to be water ice)

That statement was accompanying a low res Ralph image taken during the approach phase. Surely they're talking about something vastly less significant - for example Mars is said to have a seasonal north polar dry ice cap, even though it never gets any thicker than 1m - and the south polar cap 8m. But regardless, there is sharp relief, and thus there's not a 1,5km thick layer of nitrogen ice.

[HSchirmer]

That statement was accompanying a low res Ralph image taken during the approach phase.
...
But regardless, there is sharp relief, and thus there's not a 1,5km thick layer of nitrogen ice.

Ok, seems like two ideas going -
As I understand it, right now, there is N2 ice, mixed with CH4 and some CO, which is basically, all over Pluto, doesn't seem to be any information yet about Charon. In the current orbit, P&C perihelion occurs during equinox, and over the current 248 year orbit, the change in insolation at the poles should be enought to shift eh, less than a meter of N2 ice around on Pluto.

However, Pluto & Charon have a 3+ million year cycle where the poles flip, and you have very different epochs of heating. Right now, perihelion occures at equinox, so the heat is spread out by the 6 day rotation. But, there are epochs where P&C experience perihelion during solstice, and the poles get broiled for 40 years where the sun never sets. That 3 million year cycle has enough difference in insolation to potentially shift kilometers of ice from pole to pole.

[FOV]

I'm staring at images of Pluto right now, there clearly is no 1,5km thick north polar ice cap. There's mountains and sharply defined craters. Nitrogen ices are well known to not be able to sustain sharp topographic relief at Pluto temperature and gravity conditions (I've run into this mentioned in many different papers discussing Pluto during the leadup to the flyby - it was widely discussed that if sharp topographic relief was found at Pluto that it'd have to be water ice)

That statement was accompanying a low res Ralph image taken during the approach phase. Surely they're talking about something vastly less significant - for example Mars is said to have a seasonal north polar dry ice cap, even though it never gets any thicker than 1m - and the south polar cap 8m. But regardless, there is sharp relief, and thus there's not a 1,5km thick layer of nitrogen ice.

That's the thickness of the CO2 ice. Water ice thickness at Martian south pole is 3 kms. I'm looking at a pic of the Martian South Polar cap. How can you tell the thickness from just eye-balling an image? Unless you have an oblique very high res shot of cliffs or an escarpment for reference, I think it is very hard to tell what the depths of any putative ice cap is just by looking at it.

[Nafnlaus]

That's the thickness of the CO2 ice. Water ice thickness at Martian south pole is 3 kms. I'm looking at a pic of the Martian South Polar cap. How can you tell the thickness from just eye-balling an image? Unless you have an oblique very high res shot of cliffs or an escarpment for reference, I think it is very hard to tell what the depths of any putative ice cap is just by looking at it.

According to every paper I've read discussing nitrogen ice at Pluto, you *can* get a rough idea of the thickness by eyeballing it because nitrogen ice doesn't support significant topographic relief at Pluto temperatures and gravity. If you can see significant topographic relief, the landscape is built by water ice, not nitrogen ice.

I mentioned Mars's CO2 only in the context of the fact that the CO2 is still referred to as an "ice cap", even though it's transient and thin.

[Bill Harris]

Here is one of my "what-if armwaving scenarios" for a couple of conundrumish features on Charon. Just fitting puzzle-pieces and seeing what fits, so I've not gone bonkers. Yet.

There is the "Mordor" macula near the north pole. Appears to be a reddish-floored depression with some reddish deposit around it, much like an ejecta blanket made of tholins. What I'll assume is that there was an impact or a low-yield explosion which created a shallow crater or caldera. Some of the larger ejecta chunks were tossed out at low velocity and under Charon's low gravity were gently plopped down on the deposits of "Vulcan" planum, thereby creating a moat with a chunk sitting in it. Some chunks may have been more volatile and sublimated away leaving just a depression.

Charon is such a strange place that severe arm waving may be needed.

http://advrider.com/styles/advrider_smilies/y0!.gif

--Bill

[Bill Harris]

In Science Magazine:

The Pluto system: Initial results from its exploration by New Horizons
http://www.sciencemag.org/content/350/6258/aad1815.full

Page 6:

Charon mapping data...received to date reveal a complex geology characterized by numerous bright and dark spots, abundant fault scarps and darker curvilinear markings, both cratered and smooth plains, an extensive system of faults and graben, and a broad and prominent dark area centered on the north pole.

The dark polar spot, called Mordor Macula, is the most prominent albedo marking seen on Charon. This quasi-circular feature has a dark inner zone ~275 km across... Its less dark outer zone is ~450 km across and fades gradually onto higher-albedocratered plains. The inner zone of the dark spot is partly defined by a curvilinear marking that
may be either a ridge or an exposed fault, indicatingthat this feature may be due to a large impact or complex tectonic structure, and suggests the possibility of a compositionally heterogeneous substrate.

--Bill

[HSchirmer]

...
Good analogy. WaterIce-ite has an SG of 0.927 and Nitrogen an SG of 1.026 so the debris might tend to be somewhat "floaty".
...

I just realized something.

If sputnik planum DOES turn out to be an expanse of 10k deep convection cells of soft nitrogen ice- -
then sputnik planum is probably the most important sample return site in the solar system.
Not the N2 ice itself, but because that area of Pluto should be the best meteor collection site in the solar system.

It ought to be essentially similar to the Alan Hills area of antartica, where meteoroids impact on a (relatively) soft surface,
they are sealed in seasonal ice, and the conveyor belt action of the ice has an opportunity to concentrate them.

[JRehling]

Not the N2 ice itself, but because that area of Pluto should be the best meteor collection site in the solar system.

The delta-v for a sample return from Pluto in a reasonable time frame (<30 years) would be ridiculous, not even counting the requirements for sample collection. Three huge impulses (> 15 km/sec) would be required, for a total of about 50 km/sec. This is far beyond any mission yet launched.

Meteorites landing on Earth are slowed by our atmosphere first, as opposed to hitting a solid surface. A "soft" solid surface is a lot harder than air.

Collecting meteorites from Pluto is one of the least efficient and least feasible space exploration ideas I've heard of. We could collect them directly from asteroids, not to mention Antarctica, enormously faster and cheaper.

[HSchirmer]

Hmm, good points I hadn't considered, good to check on a gedankenexperiment.

Meteorites landing on Earth are slowed by our atmosphere first, as opposed to hitting a solid surface.

But they have also been accelerated by the gravity well of the sun and the local gravity well of earth. That requirement for "huge impulses" of 15 km / sec, can be a double edged sword.
The reason for that huge ▲V to get out to Pluto in the first place is climbing out a gravity well,
which is what helps impart that destructive velocity in the first place.

A "soft" solid surface is a lot harder than air.

Agreed. But is snow solid? is a box of styrofoam packing peanuts sollid?
If Sputnik is filled with convecting N2 ice, then by definition that isn't solid.
Think of a 10km deep sea of circulating nitrogen snow.

HEAT_AND_MASS_TRANSPORT_IN_NITROGEN_ICE_WITH_APPLICATION_TO_PLUTO_AND_TRITON
http://www.researchgate.net/publication/25...LUTO_AND_TRITON
Hence, the estimate for the maximum grain size of ~ 1 cm, which we have obtained for Triton (Duxbury and Brown 1997) can be applied and even reduced for Pluto. This makes the solid-state subsurface convection in perennial N2 deposits more probable on Pluto than on Triton

Sorry, I should have emphasized that "solid" is more about "solid state" than slab-solid.

Collecting meteorites from Pluto is one of the least efficient and least feasible space exploration ideas I've heard of.
We could collect them directly from asteroids, not to mention Antarctica, enormously faster and cheaper.

Sorry, I should have emphasized the benefit of collecting primordial dust, micrometeroids, and meteoroids from the Kuiper belt, oort cloud, or passing stars.
Now that you mention it, I see that it would be better to do sample analysis out in the cold.
We can't collect samples of interesting ices from earth or asteroids, because they simply boil away.
Could do that at Pluto.

I might also mention, if Sputnik IS a sea of convecting nitrogen snow crystals, then it should have
seasonal layers as well - not unlike the greenland ice cores we rely on to check for climate change.
Except that a circulating glacier would store the history of the solar system's climate-.

[JRehling]

To sample stuff from the outer solar system, I'd spend my money on intercepting a long-period comet or a comet on a hyperbolic trajectory before it enters the inner solar system. That would be a lot cheaper and faster than a round-trip to Pluto and back.

[HSchirmer]

To sample stuff from the outer solar system, I'd spend my money on intercepting a long-period comet or a comet on a hyperbolic trajectory before it enters the inner solar system. That would be a lot cheaper and faster than a round-trip to Pluto and back.

Agreed.

I guess I'm just not conveying the basic idea here. It is much easier to sample earth's atmospheric CO2 levels by using a weather balloon, compared to going to greenland and taking an ice core.
But data from a layered deposit like the greenland ice core, gives you multiple data points, which is orders of magnitude more useful than a single data point. Actually, it literally gives you dimensions; a single data point is one-dimensional, access to multiple data points that stretch through time allows you to derive multi-dimensional analysis.

If sputnik planum does experience convection, then that raises the possibility of layered deposits recording millions of years of flux of meteoroids and meteoric dust, as well as the flux of icy particles which quite simply can't exist inside the current snowline.

Pluto has a N2 atmosphere, it gets hit by cosmic rays, and by solar UV.
That means cosmic rays should create both carbon 14 and beryllium 10
while UV creates tholins which can precipitate them down to the surface.
That raises the possibility of useful radiologic dating over the range of both thousands of years using C14, and millions of years using Be10.

That is why sputnik is potentially the most important sample site in the solar system.

[HSchirmer]

So, we may have "Mare Sputnik"

Recent conferences suggest that Sputnik Planum is 3-4 KM of N2! http://www.planetary.org/blogs/emily-lakda...s-from-agu.html
"that argues for a layer of nitrogen ice within Sputnik that is about 3 or 4 kilometers thick; it's thicker at the middle (where cells are larger) than at the edges." So, 3-4 km of N2 on Pluto, is the same as 3-4 km of H20 in Earth's oceans.

But, if the N2 is 3-4 km deep, if you go deep enough, would it still be ice?

http://www.researchgate.net/publication/22...lumes_on_Triton
The Role of an Internal Heat Source for the Eruptive Plumes on Triton ICARUS · JANUARY 1997
Consequently, solid nitrogen acts as a thermal insulator for the mantle from the outer cooling. From the condition of energy conservation on the boundary between nitrogen ice and water ice, and from the estimate of P0.3 K/km for the steady-state temperature gradient in H2O ice by Smith et al. (1989), the gradient in the nitrogen layer is P15–30 K/km.

So, figure Pluto surface is around 40k, (currently) then the depths of Sputnik could be 60k to 120k warmer than the surface.
And it looks like 63K is warm enough to create liquid N2.

https://blogs.nasa.gov/pluto/2015/12/18/whe...th-meets-pluto/
In hindsight, one wonders why we were so surprised. For instance, the triple point (the location on the temperature-pressure phase diagram in which a material can coexist as solid, liquid and gas) of both carbon monoxide and molecular nitrogen is in the vicinity of 63 Kelvin (- 346 Fahrenheit), a temperature that is achievable on Pluto, given its distance from the sun.

So, eh, Pluto gets up to 63K, the triple point for N2 and CO.
On Earth, a polar ocean exists at 0 Farhenheight, the freeze-thaw point of salt water.

Very interesting, perhaps Sputnik thaws into a sea, like the lakes of Titan?

[Gladstoner]

http://www.planetary.org/blogs/emily-lakda...s-from-agu.html

This excerpt from Emily's Planetary.org post is pretty handy as it summarizes information that is useful in making sense of various Plutonian processes:

There is a huge variety of types of surfaces on Pluto. That variety relates, in part, to a variety in surface materials on Pluto. The main materials on Pluto are water ice, carbon monoxide ice, nitrogen ice, methane ice, and tholins. At the poster session, I asked geophysicist Bill McKinnon about the properties of these ices. He explained that at the 40-kelvin temperatures that prevail on Pluto, water ice is as strong and solid as rock is on Earth, but the other ices are weaker. Nitrogen and carbon monoxide ices will act very similarly on the surface (and are miscible in each other). The main difference between nitrogen and carbon monoxide on Pluto is that nitrogen is more volatile, and will turn to vapor when heated more quickly than carbon monoxide will. Both have much less strength than water ice, so should flow more readily, much as glacial ice does on Earth. Note that even though they can flow, they are still solid, and the flow we're talking about is relatively slow, in the neighborhood of several centimeters per year. Methane ice can mix into nitrogen and carbon monoxide ices as well, it's less volatile, and may be more rigid. Nitrogen and carbon monoxide ices are both denser than water ice at 40 kelvins, while methane has only half the density of water ice. Tholins are organic solids produced when solar radiation bombards these ices, and are likely the agent that produces reddish stains in various places on Pluto.

Most interesting to me are the relative densities and miscibility of the various ices.

[Gladstoner]

.... excerpt from Emily's Planetary.org post ....

Some thoughts and considerations (already stated by others, or guesses on my part -- right or wrong)....

The main materials on Pluto are water ice, carbon monoxide ice, nitrogen ice, methane ice, and tholins. .... (A)t the 40-kelvin temperatures that prevail on Pluto, water ice is as strong and solid as rock is on Earth....

Water ice is basically the 'rocky crust' of Pluto. It can intermingle with and be altered by the more volatile materials, just as granite, basalt, limestone, etc. can be altered by water in a number of ways (dissolved, dispersed as clasts, etc.).

....but the other ices are weaker. Nitrogen and carbon monoxide ices will act very similarly on the surface (and are miscible in each other). The main difference between nitrogen and carbon monoxide on Pluto is that nitrogen is more volatile, and will turn to vapor when heated more quickly than carbon monoxide will.

The mixing of one ice into another can alter their properties in a number of ways (e.g. sublimation points). I'm reminded of the properties of a cooling body of magma, but.... different (i.e. sublimation could take on a role like crystal differentiation, but 'in reverse'). And then there is the question of these properties at various depth pressures.... An igneous petrology textbook may still be handy here....

Both (N2 and CO) have much less strength than water ice, so should flow more readily, much as glacial ice does on Earth. Note that even though they can flow, they are still solid, and the flow we're talking about is relatively slow, in the neighborhood of several centimeters per year.

This could apply to diapirs and 'magmatic' bodies as well as glacial ice. Plus, how high can a pile of each ice type (or mixtures of the two) be before it flows under its own weight?

Methane ice can mix into nitrogen and carbon monoxide ices as well, it's less volatile, and may be more rigid. Nitrogen and carbon monoxide ices are both denser than water ice at 40 kelvins, while methane has only half the density of water ice.

Methane here reminds me of silica/quartz in a terrestrial magmatic system, but with a lower melting (sublimation) point. Would that make water ice the 'feldspar'?

Tholins are organic solids produced when solar radiation bombards these ices, and are likely the agent that produces reddish stains in various places on Pluto.

In a sense, tholins may 'play the role' of dust on Mars. Since tholins are a refractory residue, it could be spread across the planet via the atmosphere, and accumulate in certain spots in various ways for a number of reasons.

Again, this could be mostly (or all) wrong. I'm just trying to grasp for any kind of familiarity here. At least there are impact craters to provide some context.

[nprev]

Sputnik Planum cannot melt into a "sea" because Pluto's atmospheric pressure is far too low to support the liquid phase of N2: