I recently posted a question on Reddit under the New Horizons category.
Here's the question along with some dialogue about the theoretical positive gravity anomaly or as I would call it the negative gravity anomaly at Sputnik Planitia. Reddit conversation It's being suggested that Sputnik Planitia (SP) is a positive gravity anomaly (is more dense has more mass) because of a theoretical raised subsurface ocean from a theoretical impact. |
|
|
This positive gravity anomaly, in turn, is supposed to be why SP is aligned directly opposite Charon. Roll an unevenly weighted ball on the floor and it will always come to rest with the heaviest side facing the center of earth identical to how our moon is tidally locked with the densest side facing earth. |
Francis Nimmo has said that for a core to still be potentially radioactively hot after 4 byr it's radius must be at least 1000 km.
Bill McKinnon says that Pluto's silicate rocky core material is at most 67% of Pluto by mass but only 40% by volume. Pluto's maximum core radius would then be 850 km which is too small to be hot (according to Nimmo). |
Francis Nimmo quote
For a silicate core >1000 km in radius the heat diffusion timescale is longer than the age of the solar system, so large silicate cores provide a long-term reservoir of energy which can potentially maintain a subsurface ocean. Conversely, for bodies with small silicate core radii like Enceladus or Tethys, (I include Pluto's core at 875 km) ancient heat cannot be stored in this manner. |
But if you throw in the actual information from Robbin Canup's collision model, only about 40% of Pluto heated enough (about 5% at 175 - 250 degrees Kelvin and about 35% at 85 - 150 K) to potentially cause differentiation. Charon remained at 30 K during Canup's impact model. If you then take Bill's 40% volume (rocky material) and take Robin's 40% of that (heated enough to differentiate) times the 1,188 km radius it would suggest Pluto's core is only 350 km (partially differentiated) far too small to be hot and far too small to make a subsurface ocean. Water freezes at 273 K add 5% ammonia and it drops to 176 K. During Canup's collision model theory only about 5% of Pluto could have gotten warm enough to soften water ice to a point allowing for separation and differentiation. Quote from Robin's collision model paper In moon-producing impacts, the satellite material experienced little heating (Temp = 30 K), because it had for the most part avoided direct impact with the planet, whereas the target was heated more substantially (Fig. 2). |
According to F. Nimmo, the original accretion process and small impacts would play a minor to negligible roll in contributing to differentiation. |
Quote from the Francis Nimmo paper
5.2 Heat Production For the icy satellites, there are three main sources of heat: accretion, radioactive decay, and tidal heating. Even for Ganymede-size satellites, the gravitational energy released during accretion is rather modest, so that initial differentiation is not guaranteed [Barr and Canup, 2008]. (Ganymede is more than twice Pluto's size) If accretion happens sufficiently rapidly, some melting will take place [e.g., Lunine and Stevenson, 1982], but the overall contribution to the existence of present-day oceans is minor to negligible. |
Robbin's model only worked if the two planets that theoretically collided were uniform not differentiated prior to impact.
Differentiated pre-collision planets couldn't work in her model they merged into one ball. For the collision model to work both planets were uniform at collision and weren't heated enough to fully differentiate. Robin Canup's failed pre-differentiated collision model. Fig. 1. Time series of a potential Pluto-Charon–forming impact yielding a planet-disk system (run 70 in table S1 with N 0 120,000 particles). Results are shown looking down onto the plane of the impact at times t - 1.3, 3.2, 7.5, 11.8, 14.5, and 24.6 hours; units shown are distance in 103 km. Color indicates material type (blue, water ice; orange, dunite; red, iron), with all of the particles in the 3D simulation overplotted in order of increasing density. The impacting objects are identical—both are predifferentiated into 40% ice mantles and 60% rock cores by mass with initial surface temperatures set to 150 K, increasing with depth (7) to a central temperature ,800 K. After an initially oblique impact in the counterclockwise sense (A), the two objects separate (B and C) before recolliding. After the second collision, the denser cores migrate toward the center, as a bar-type mode (36) forms in the rapidly rotating merged objects (D). From each end of the bar emanate spiral structures (D and E), whose self-gravity acts to transport angular momentum from inner to outer portions. The arms wrap up on themselves and finally disperse to yield a ring (36) of material (whose differential motion would on a longer time scale produce a disk), together with the central planet (F). |
I never did adddress synthfish's barycenter point.
Barycenter The barycenter (or barycentre; from the Ancient Greek βαρύς heavy + κέντρον centre) is the center of mass of two or more bodies that are orbiting each other, or the point around which they both orbit. The barycenter is the center of Pluto and Charon's mass it is the little dot within the little circle in this image. This is their collective center of mass. Imagine, if you will, Earth is the large outer circle, You are in the house (top right) rolling an unevenly weighted ball on your floor. The ball always comes to rest with the densest side facing toward the center of mass. This is what happened with our Moon, as well as, Pluto and Charon, no exceptions. Sputnik Planitia is aligned opposite Pluto and Charon's center of mass. It is not a positive gravity anomaly it is a negative gravity anomaly. The densest side of Pluto is facing towards it's shared center of mass with Charon. |
This subsurface ocean of water hypothesis depends on three things temperature, pressure and ammonia.
Even if Pluto was fully differentiated (unlikely) its core is too small to still be hot making water ice into a liquid. But Nitrogen flows at a much lower temperature 64 K. This temperature can be reached and along with axial wobble nitrogen can reach its triple point turning into a liquid on its surface. Place that nitrogen underground (increased pressure) and it can relatively easily reach its liquid phase. |
In order to have a subsurface water ocean you need temperatures greater than 175 K.
NASA scientists have long expected Pluto to deliver certain results. They predetermined Pluto/Charon collided, created a model that only supported partial differentiation but then ignore that fact in favor of full differentiation. |
I've made plenty of my own interpretive errors on this web site but its been because of educational ignorance. I've tried to point out my mistakes and/or correct them. Its not as though I don't make mistakes too. I've tried to learn and grow my knowledge to reduce my errors because I don't want to mislead anyone.
I suppose this is what separates us, I know so little my eyes are wide open while some scientists' know so much they need not see more. I have a tremendous amount of respect for what these scientists have accomplished, I just wish they would look, with open minds, at what Pluto/Charon has to say and stop pushing their expectations onto the scene. Just learned something new about volcanism and atmospheres which means something I said previously is not completely correct. Learning can be frustrating as well as exciting but its becoming tedious to have to keep correcting myself. |