July 20th, 2018
Hogenboom
On page 95 I presented some results from a Hogenboom et al., paper where experiments were performed on H2O and NH3 at various concentrations, temperatures and pressures designed to mimic conditions found in moons and planets like Triton and Pluto. I had to read Hogenboom's paper multiple times just to begin to understand what he's trying to convey. Hogenboom's info is not easy to visualize but is crucial to understanding Pluto's potential interior structure.
Quoted papers on this page
Quoted papers on this page
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| thermal_evolution_of_pluto_and_implications_for_surface_tectonics_and_a_sub-surface_ocean_guillaume_pluto.pdf |
| reorientation_of_sputnik_planitia_implies_a_subsurface_ocean_on_pluto_nimmo_hamilton_mckinnon |
Some definitions
Pa = a pascal is a unit of pressure - MPa is a million pascals. The Marianas Trench is 11 km (6.8 mi) deep and 110 MPa of pressure
Eutectic = an evenly dissolved liquid
Peritectic = pressure/temperature point where a liquid and solid combine to form a new solid while cooling
Incongruent = irregular inconsistent
Congruent = consistent regular
Polymorphs = two compounds combined can melt at a lower temperature than either one individually NH3 in H2O = antifreeze.
Monohydrate = one H2O plus one NH3 frozen as a solid lattice structured compound
Dihydrate = two H2O plus one NH3 frozen as a solid lattice structured compound
Pa = a pascal is a unit of pressure - MPa is a million pascals. The Marianas Trench is 11 km (6.8 mi) deep and 110 MPa of pressure
Eutectic = an evenly dissolved liquid
Peritectic = pressure/temperature point where a liquid and solid combine to form a new solid while cooling
Incongruent = irregular inconsistent
Congruent = consistent regular
Polymorphs = two compounds combined can melt at a lower temperature than either one individually NH3 in H2O = antifreeze.
Monohydrate = one H2O plus one NH3 frozen as a solid lattice structured compound
Dihydrate = two H2O plus one NH3 frozen as a solid lattice structured compound
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Croft et al., produced this phase diagram of ammonia di and mono hydrates as well as pure ammonia.
The dashed line represents the density of water ice 1h but appears to be off slightly as it reflects an H2O ice density of 0.93 not 0.917 g/cm^3 (or more likely there's something I don't understand about it). The important thing to glean from this is what Hogenboom says about it. Liquid NH3-2H2O is less dense than solid NH3-2H2O and both liquid and solid NH3-2H2O are more dense than frozen 1h water ice. Hogenboom Our data confirm the result of Croft et al. (1988) that the NH3–H2O peritectic liquid has a density at low pressures (0.946 g cm^3 at 0.1 MPa) that is less than the density of solid ammonia dihydrate I (0.965 g cm^3) but greater than that of water ice I (0.917 g cm^3). Hence, NH3–H2O liquid can be extruded from a rock-rich icy mantle and through a thick ammonia hydrate crust, but it cannot be extruded readily through a thick mantle or crust of water ice I. These density relations extend to 160 MPa, above which the liquid is denser than ammonia dihydrate I up to >300 MPa, |
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In a nutshell Hogenboom is saying this;
The rocky core is more dense than the solid ice ammonia hydrates so the solid NH3-H2O is extruded (squeezed) from the rock core. Solid frozen ammonia hydrates are more dense than its liquid counterpart so liquid NH3-H2O is extruded from the solid hydrate. Water ice 1h is less dense than the liquid hydrates so it is extruded or floats on top of all these denser materials. Its like a layered cake of rock, solid NH3, liquid NH3 then H2O ice 1h. Less dense stuff layers or stratifies on top of denser stuff. |
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The experimental layering results get a little more complicated as two types of water ice and ammonia infused ice form at greater pressures but for the most part if Pluto's core is rock then the highly compressed H2O water ice II and ammonia hydrate II don't exist above this rock barrier. If Pluto is only partially differentiated then it is possible for more complex stratification of the ices.
A eutectic (homogeneously dissolved liquid) ammonia–water ocean is possible on icy satellites at pressures between 160 and 300 MPa. From 20 to 300 MPa ammonia dehydrate and ice melt at a eutectic to form water-rich liquids; at lower and higher pressures, ammonia dihydrate melts incongruently to (inconsistently, irregularly, lumpy clumpy) ammonia-rich liquids.
In other words, at pressures less than 20 MPa and greater than 300 MPa the H2O-NH3 liquid that can form is ammonia rich (30% to 50% NH3) but is formed in inconsistent blotches or irregular (incongruent) patches or blobs, whereas, liquids that form between 20 to 300 MPa are much more water rich (80% to 99%) and the ammonia is dispersed into the liquid more evenly (congruently).
A eutectic ammonia–water ocean is possible on icy satellites at pressures between 160 and 300 MPa (an important point that deserves repeating).
If ammonia is organized into blotchy patches near the surface (0.1-20 km) and mixed with nitrogen which can reach its explosive triple point then perhaps my concept of Pluto splatter painting blotches or blobs onto Charon and the small satellites with ammonia is correct (page 69).
Between 160 to 300 MPa a water based ammonia ocean can form but it will be an ocean rich in water and low in ammonia which matches the 5% to 10% suggested by Desch et al., The formation of these liquids are of course temperature dependent. H2O-NH3 liquids do not form below 120 K regardless of pressure or NH3 concentrations.
The disappearance of the peritectic at 20 MPa is at a pressure reached inside all the principal icy satellites except Mimas, Enceladus, and Miranda (Kargel 1990); however, the high-pressure phases of the ammonia hydrates occur only above 300 MPa, i.e., pressures attained in the deep interiors of Ganymede, Callisto, Titan, Triton, and Pluto, but not in other icy moons (Kargel 1990). If Triton and Pluto have a dense rock/iron core, the icy mantle above their cores would not include the high-pressure polymorphs (ammonia hydrates).
At Pluto temperatures (distance from the Sun), ammonia is a crystallized solid but when combined with crystallized H2O ice under pressure along with some additional heat both the NH3 and H2O solid ices can become a liquid at lower temperatures than either could do by themselves alone. This is polymorphism when these two crystalline solids morph into something else called ammonia monohydrates (one water molecule combined with one NH3) and ammonia dihydrate (two H2O molecules combined with one NH3). The high pressure polymorphs form at pressures greater than 300 MPa. Hogenboom is saying that if Pluto is differentiated it then doesn't have enough pressure from the above ices to create the high-pressure polymorphs.
I'll explain this a little clearer below.
In other words, at pressures less than 20 MPa and greater than 300 MPa the H2O-NH3 liquid that can form is ammonia rich (30% to 50% NH3) but is formed in inconsistent blotches or irregular (incongruent) patches or blobs, whereas, liquids that form between 20 to 300 MPa are much more water rich (80% to 99%) and the ammonia is dispersed into the liquid more evenly (congruently).
A eutectic ammonia–water ocean is possible on icy satellites at pressures between 160 and 300 MPa (an important point that deserves repeating).
If ammonia is organized into blotchy patches near the surface (0.1-20 km) and mixed with nitrogen which can reach its explosive triple point then perhaps my concept of Pluto splatter painting blotches or blobs onto Charon and the small satellites with ammonia is correct (page 69).
Between 160 to 300 MPa a water based ammonia ocean can form but it will be an ocean rich in water and low in ammonia which matches the 5% to 10% suggested by Desch et al., The formation of these liquids are of course temperature dependent. H2O-NH3 liquids do not form below 120 K regardless of pressure or NH3 concentrations.
The disappearance of the peritectic at 20 MPa is at a pressure reached inside all the principal icy satellites except Mimas, Enceladus, and Miranda (Kargel 1990); however, the high-pressure phases of the ammonia hydrates occur only above 300 MPa, i.e., pressures attained in the deep interiors of Ganymede, Callisto, Titan, Triton, and Pluto, but not in other icy moons (Kargel 1990). If Triton and Pluto have a dense rock/iron core, the icy mantle above their cores would not include the high-pressure polymorphs (ammonia hydrates).
At Pluto temperatures (distance from the Sun), ammonia is a crystallized solid but when combined with crystallized H2O ice under pressure along with some additional heat both the NH3 and H2O solid ices can become a liquid at lower temperatures than either could do by themselves alone. This is polymorphism when these two crystalline solids morph into something else called ammonia monohydrates (one water molecule combined with one NH3) and ammonia dihydrate (two H2O molecules combined with one NH3). The high pressure polymorphs form at pressures greater than 300 MPa. Hogenboom is saying that if Pluto is differentiated it then doesn't have enough pressure from the above ices to create the high-pressure polymorphs.
I'll explain this a little clearer below.
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Look at the size of the "Theoretical Rocky Core" (gray) in the above drawing. It extends outward to 850 km. The pressure that pure ice (density of 0.917 g/cm^3) would place on this rocky core is slightly less than 200 MPa (base of the pink band).
High pressure polymorphs develop at pressures above 300 MPa. Low pressure polymorphs exist between 50 and 100 MPa. Basically there are high and low pressure ammonia hydrates just as there are low and high pressure water ices (Ice I & Ice II). Low pressure ice is crystalline with a strong lattice similar to a rigid scaffold whereas high pressure ice is amorphous (the lattice structure collapses into a disorganization form).
F. Nimmo et al., state the temperature must be from 180-250 K for the ice shells to last 4 byr.
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Nimmo et al., ocean dome concept. Red text temperatures added
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They present the below graph (Fig 3) to support the idea that an ice shell on Pluto could survive for 4 byr.
Nimmo et al graph/chart
I placed red vertical lines at 62.5 (their thin ice shell size) and 150 km (their thick ice shell size) which correspond to temperatures of 201 K and 184 K respectively.
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The above Nimmo et al., chart indicates that to maintain a thin shell of 62.5 km for 4 byr the temperature would need to be 201 degrees Kelvin.
The thin shell directly below Sputnik Planitia (SP) must have a temperature of 201 K. On the other hand, to maintain a thick shell of 150 km for 4 byr, the temperature of the ammonia infused ocean water would need to be 184 K. The warmer the temperature the thinner the ice shell. Do you see the problem? The temperature is rising as the ocean gets farther from its heat source. |
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The temperature to maintain a thin shell is hotter than the temperature to maintain a thicker shell but the thicker shell is closer to the supposed radioactive core heat source. How is the top of the ocean dome (62 km = 201 K) hotter than the deeper shell (150 km = 184 K) when the heat creating the ocean is closest to the base of the 150 km deep shell?
5-10% NH3 Ocean
Guillaume and Nimmo paper says
The concentration of NH3 relative to H2O in Pluto is probably 1-5 wt% according to Desch et al. (2009), although if complete nebular concentration of nitrogen occurred, this value could reach 15 wt%.
The concentration of NH3 relative to H2O in Pluto is probably 1-5 wt% according to Desch et al. so lets take a look at some info related to water infused with 5 and 10 percent concentrations of ammonia.
The concentration of NH3 relative to H2O in Pluto is probably 1-5 wt% according to Desch et al. (2009), although if complete nebular concentration of nitrogen occurred, this value could reach 15 wt%.
The concentration of NH3 relative to H2O in Pluto is probably 1-5 wt% according to Desch et al. so lets take a look at some info related to water infused with 5 and 10 percent concentrations of ammonia.
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According to Hogenboom's experiments, a 5% (red lines) concentration of NH3 dissolved into water at 100 MPa of pressure (175 km deep) needs a temperature greater than 259 K for the solution to be liquid.
At 200 MPa (352 km deep) this 5% concentration of NH3 must be warmer than 246 K to remain a liquid but that's a mute point as we bounce into a rocky core at 338 km. With a higher concentration of 10% ammonia (blue lines) and at a depth of 175 km (100 MPa) the temperature must be greater than 250 K to keep the solution liquid. At this 250 K temperature Pluto's ice shell according to Nimmo's graph, cannot last 4 byr. |
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At a depth of 352 km (200 MPa) a 10% ammonia rich solution needs to be warmer than 238 K to remain a liquid, this would make Pluto's outer ice shell about 10 km thick after 4 byr according to Nimmo's chart but this thin outer shell would not support the 2.5 mile high mountains at Norgay Montes. As the pressure decreases the temperature must increase in order to keep the solution in a liquid state. To keep the top of Nimmo's ocean dome liquid with a concentration of 10% ammonia, the temperature at the dome roof would need to be around 260 K (green lines). At this temperature there would be no dome or crustal shell according to Nimmo's graph.
I took the Hogenboom pressure zones where NH3-H2O transitions and laid it side by side (to scale) with the interior model theory presented in the Nimmo et al., paper where an ocean dome is postulated along with a positive gravity anomaly and a fully differentiated radiogenically hot core which is generating about 250 degrees Kelvin heat. If the Nimmo ocean itself is any hotter than 250 degrees then the ice shell above can't survive for 4 byr so 250 K becomes an upper limit on the ocean/core temperature.
I took the Hogenboom pressure zones where NH3-H2O transitions and laid it side by side (to scale) with the interior model theory presented in the Nimmo et al., paper where an ocean dome is postulated along with a positive gravity anomaly and a fully differentiated radiogenically hot core which is generating about 250 degrees Kelvin heat. If the Nimmo ocean itself is any hotter than 250 degrees then the ice shell above can't survive for 4 byr so 250 K becomes an upper limit on the ocean/core temperature.
A eutectic (homogeneously dissolved liquid) ammonia–water ocean is possible on icy satellites at pressures between 160 and 300 MPa. At 281 km deep or 160 MPa an ocean of NH3-H2O can form but then the rocky core is only 57 km further down. this restricts the theoretical ocean size to about a third of what is being suggested by mainstream scientists. But to maintain a shell 281 km thick the temperature would have to be lower than 176 K according to Nimmo's chart.
Hogenboom
0-20 MPa = incongruent clumpy blotches of liquid NH3-H2O assuming high enough temperatures
20 MPa = transition of incongruent to congruent melting of ammonia rich hydrates
20-300 MPa = water rich liquid (minimum temperature is required, it changes with pressure)
> 200 MPa high pressure ice II forms and solution goes NH3 rich > 50% ammonia to water (doesn't apply on Pluto if core is diff)
> 300 MPa = incongruent = or > 32.1% NH3 rich clumpy high pressure ammonia hydrates II (doesn't apply on Pluto if core is diff)
Hogenboom
0-20 MPa = incongruent clumpy blotches of liquid NH3-H2O assuming high enough temperatures
20 MPa = transition of incongruent to congruent melting of ammonia rich hydrates
20-300 MPa = water rich liquid (minimum temperature is required, it changes with pressure)
> 200 MPa high pressure ice II forms and solution goes NH3 rich > 50% ammonia to water (doesn't apply on Pluto if core is diff)
> 300 MPa = incongruent = or > 32.1% NH3 rich clumpy high pressure ammonia hydrates II (doesn't apply on Pluto if core is diff)
Surface Expansion Fractures
Among the most important unresolved problems in the geology of the icy moons is the origin of global extensional features on some but not all of these moons; these fractures indicate a net increase of surface area of several percent. We can now completely rule out earlier speculation (Consolmagno 1985) that these extensional features could be caused by the melting and refreezing of NH3-rich liquids; it is now clear that the volume change is too small to account for the inferred expansion.
This is an interesting statement by Hogenboom which appears to indicate that extension fractures are not created by melted and refreezing ammonia rich oceans of water as the volume change is too small to account for the expansion fractures. This apparently takes place because ammonia infused water does not expand as much as water alone. Liquid H2O (1 g/cm3) becomes less dense and expands its volume when it freezes (0.917 g/cm3) into a crystalline structure. NH3-H2O liquid (0.946 g/cm3) density increases 4 percent by volume to 0.965 g/cm3 as it becomes a solid ammonia dihydrate. According to Hogenboom this four percent increase in volume is too small to account for expansion fractures on the surfaces of icy moons or (in Pluto's case) icy planets.
Nimmo's title to his paper "Reorientation_of_sputnik_planitia_implies_a_subsurface_ocean_in_Pluto" infers that because Pluto's skin is slipping there is therefore a subsurface ocean he then goes on to assume that, that ocean is enormous 150-266 km. Pluto's skin could slip if there is simply a lubricant sandwiched between two shells. This lubricating layer could be deep or shallow. Since I can see evidence that nitrogen has been in a liquid state and there is evidence of its volcanic pressurized nature it seems reasonable to first and foremost accept nitrogen as a near surface lubricant.
Its far more difficult to get water and ammonia warm enough (176 K) to become a liquid than nitrogen (63 K) but it is potentially possible there is a potential 57 km layer of ammonia infused liquid water sandwiched between a core with a radius of 850 km and an ice shell at 907 km. Based on Hogenboom's experiments it doesn't seem possible that there is a 150 km ocean or a dome or a positive gravity anomaly.
But then again perhaps I've misinterpreted something.
The behavior of the system H2O–NH3 in many ways follows that of MgO–SiO2 , and the roles of ammonia–water in icy satellite evolution may parallel those of magnesium silicates in Earth’s structure, volcanism, and deep mantle tectonism. Pressure-related effects, including a pressure influence on the ammonia content of cryomagmas, might be significant in determining some potentially observable aspects of cryovolcanic morphologies, surface compositions, and interior structures of icy satellites...
By extension, we predict that NH3-rich icy satellites may have ammonia monohydrate-saturated lavas formed either at high pressures (>300 MPa, such as deep in Titan, Kargel 1990) or very low pressures (<20 MPa, as throughout Enceladus) and water-ice-saturated lavas formed at intermediate pressures (20–300 MPa, as throughout Triton’s icy mantle).
Based on his experiments Hogenboom expected we would find evidence of ammonia saturated lavas on the surface of bodies like Triton and Pluto while their interiors should have water-ice saturated lavas (not oceans). Do we consider Earth's molten rock mantle to be like an ocean?
On Pluto we see surface cryovolcanism in the form of nitrogen based volcanoes (no ammonia) and possibly venting of tholin-water-ice with elevated water-ice mountains formed from a thick shell. On Triton we see sulfur (mineral) rich black plumes (no ammonia) ejected onto the surface with a thin outer shell, very low (no mountains and few impact craters) smooth land features. Triton and Pluto are nearly identical in size and surface composition yet these two worlds display very different surface features indicating completely different internal energy levels taking place.
Hogenboom admits comparing a simple experiment of pressure, temperature, water and ammonia to real icy worlds is a weak representation of the potential complexity of the processes taking place in the solar system but its the best we currently have for understanding the potential interiors of these fascinating places.
Baby steps.
This is an interesting statement by Hogenboom which appears to indicate that extension fractures are not created by melted and refreezing ammonia rich oceans of water as the volume change is too small to account for the expansion fractures. This apparently takes place because ammonia infused water does not expand as much as water alone. Liquid H2O (1 g/cm3) becomes less dense and expands its volume when it freezes (0.917 g/cm3) into a crystalline structure. NH3-H2O liquid (0.946 g/cm3) density increases 4 percent by volume to 0.965 g/cm3 as it becomes a solid ammonia dihydrate. According to Hogenboom this four percent increase in volume is too small to account for expansion fractures on the surfaces of icy moons or (in Pluto's case) icy planets.
Nimmo's title to his paper "Reorientation_of_sputnik_planitia_implies_a_subsurface_ocean_in_Pluto" infers that because Pluto's skin is slipping there is therefore a subsurface ocean he then goes on to assume that, that ocean is enormous 150-266 km. Pluto's skin could slip if there is simply a lubricant sandwiched between two shells. This lubricating layer could be deep or shallow. Since I can see evidence that nitrogen has been in a liquid state and there is evidence of its volcanic pressurized nature it seems reasonable to first and foremost accept nitrogen as a near surface lubricant.
Its far more difficult to get water and ammonia warm enough (176 K) to become a liquid than nitrogen (63 K) but it is potentially possible there is a potential 57 km layer of ammonia infused liquid water sandwiched between a core with a radius of 850 km and an ice shell at 907 km. Based on Hogenboom's experiments it doesn't seem possible that there is a 150 km ocean or a dome or a positive gravity anomaly.
But then again perhaps I've misinterpreted something.
The behavior of the system H2O–NH3 in many ways follows that of MgO–SiO2 , and the roles of ammonia–water in icy satellite evolution may parallel those of magnesium silicates in Earth’s structure, volcanism, and deep mantle tectonism. Pressure-related effects, including a pressure influence on the ammonia content of cryomagmas, might be significant in determining some potentially observable aspects of cryovolcanic morphologies, surface compositions, and interior structures of icy satellites...
By extension, we predict that NH3-rich icy satellites may have ammonia monohydrate-saturated lavas formed either at high pressures (>300 MPa, such as deep in Titan, Kargel 1990) or very low pressures (<20 MPa, as throughout Enceladus) and water-ice-saturated lavas formed at intermediate pressures (20–300 MPa, as throughout Triton’s icy mantle).
Based on his experiments Hogenboom expected we would find evidence of ammonia saturated lavas on the surface of bodies like Triton and Pluto while their interiors should have water-ice saturated lavas (not oceans). Do we consider Earth's molten rock mantle to be like an ocean?
On Pluto we see surface cryovolcanism in the form of nitrogen based volcanoes (no ammonia) and possibly venting of tholin-water-ice with elevated water-ice mountains formed from a thick shell. On Triton we see sulfur (mineral) rich black plumes (no ammonia) ejected onto the surface with a thin outer shell, very low (no mountains and few impact craters) smooth land features. Triton and Pluto are nearly identical in size and surface composition yet these two worlds display very different surface features indicating completely different internal energy levels taking place.
Hogenboom admits comparing a simple experiment of pressure, temperature, water and ammonia to real icy worlds is a weak representation of the potential complexity of the processes taking place in the solar system but its the best we currently have for understanding the potential interiors of these fascinating places.
Baby steps.