The possibility that Charon was born from Pluto by fission was raised soon after Charon’s discovery. Now that the New Horizons mission has given us a close look at their surfaces, global surface features on both bodies appear to confirm the fission hypothesis.
Pluto’s surface features are underlain by global parallel bands that I interpret as the result of its evolution from a previously highly elongated shape to its present near sphericity. While on Charon one hemisphere of this world is wrapped in a rugged piece of Pluto’s pre-fission crust.
This rough sketch shows my interpretation of how Pluto’s present banded structure could have resulted from its early state of extreme rotation and subsequent fission. The sketch is similar to the one included in my last post by D. U. Wise illustrating his lunar origin scenario, except that I have sliced through Pluto and Charon to show how their internal structures could have given rise to the surface features we now see (please excuse the roughness).
(a) Early proto-Pluto: – for big chunk of their history (hundreds of millions or perhaps billions of years) Pluto and Charon were both part of a single body. It was spinning fast enough that its shape was distinctly oblong. Just how this proto-Pluto came to be spinning so fast is another question, but we do know that it is not uncommon for large trans-Neptunian objects to spin very fast because we see Haumea, Pluto’s fellow dwarf planet, and Varuna are in that state right now.
Proto-Pluto was composed mostly of water ice and rocky minerals. At the stage shown in figure (a) the water ice and rock have only partially differentiated although much more volatile minor constituents, primarily frozen methane, nitrogen and carbon monoxide, have long before percolated up to the surface. There they initially formed a snow white planet girdling ocean of soft ice. Soon after proto-Pluto’s formation its surface would have been like Sputnik Planum, but planet-wide. In that initial stage, evaporation from this vast sea of volatile ice would have given proto-Pluto an atmosphere a lot denser than the present one.
A tar-like skin formed on top of the soft ice, something like skin forming on top of hot milk. This was due to a combination of photo-chemical reactions and radiation from space creating complex organic molecules in the atmosphere that then settle onto the surface, where further interactions occur. This ‘skin’ grew thick enough to seal off the ocean of volatile ices and insulated it from the cold of space. After that most of the initial atmosphere would have been lost to space.
Heat released deep in Pluto’s interior, due to radioactive decay and gradual differentiation, warmed the covered ices until, at a few spots vapors burst through the covering skin and erupted onto the surface as geysers, like those still active on Triton. This is the origin of the dormant ‘volcanos’ that New Horizons found just south-west of Sputnik Planum. Stage (a) shows proto-Pluto at a time when gases venting from these volcanos/geysers resupplied a thin atmosphere. Ongoing chemical reactions and precipitation of tarry particles from that atmosphere continued to gradually thicken the skin/crust.
(b) As denser rock gradually sinks to the core of proto-Pluto the planet’s moment of inertia drops causing it to spin faster. This in turn leads to further flattening and elongation of proto-Pluto into a more ellipsoidal shape. The elongation tears the outer skin/crust exposing the ocean of volatile ices once more. This is the first step in the creation of Pluto’s present global surface banding.
Rather than sliding over the underlying water ice core the crust wants to stay where it is relative to the core. This results in tearing of the crust perpendicular to the axis of extension. The distance between tears and so the width of the resulting bands depended on the strength of the crust relative to its friction with the underlying water ice core. Lower friction would mean more sliding at the edges of a band as proto-Pluto stretched, resulting in wider bands.
In places the process of tearing apart proto-Pluto’s crust was apparently not very neat. For example, the banding north of the center of Tombaugh Regio’s heart is particularly ragged. Here a system of cliffs running generally south to north over the northern hemisphere is visible even though it looks like a very ragged coastline occasionally interrupted by craters on smaller (~100 km) scales. The global banding is only obvious on a global scale. It in not clear on a local scale, until you start to notice alignments of linear features in widely separated locations. (If it was obvious somebody other than me might have reported it by now).
The banding is perhaps clearest in the region around the north pole (directly north of the western edge of Sputnik Planum). The north-south oriented cliffs and valleys are most easily seen in black and white images because climate induced color variations tend to distract from topography. The aligned linear features exist on all sides of Sputnik Planum.
(c) Ongoing differentiation and growth of Pluto’s rocky core leads to greater elongation and more circumferential ripping. The gaps between the tears are filled with newly formed skin/crust. Different tears have different ages so the crust filling the gaps has a variety of thicknesses.
Some initial asymmetry leads to one end of the rapidly spinning proto-Pluto stretching more than the other. Charon’s birth begins. One side of Charon carries an old chunk of proto-Pluto’s original crust. Since Charon’s diameter is smaller than proto-Pluto’s we should see some deformation or faulting of the crust cap around its edge where it has been forced to conform to the smaller sphere.
(d) Charon necks off from Pluto quite rapidly so little time passes from (c) to (d). This is when proto-Pluto is at its most elongated. Centrifugal force at the extreme of Charon exceeds gravity. This may have led to a chunk of Charon’s crust cap being flung off into space, giving rise to the depression that has been called Mordor. Any remnants of proto-Pluto’s ocean of volatile ices carried away by Charon under the cap are likely to have flowed out through the Mordor hole following the ‘lid’ into space. This explains the duality of Charon’s surface and why we see naked water ice surface on the smoother part of Charon while water ice is hidden on Pluto, despite being Pluto’s major constituent. Charon’s fission from material near the surface of proto-Pluto also explains why it is rock poor and less dense than Pluto.
At this stage a similar cap of crust to the ‘lid of Mordor’ may have also been flung into space from the opposite end of proto-Pluto. If so it would have gone missing from the zone of chaotic landforms to the east of Sputnik Planum, at what I called the Western Pole in my previous post. Unfortunately New Horizons’ images of this area are tantalisingly low resolution.
(e) Charon breaks free and Pluto and Charon start orbiting each other. Blobs of ice (mostly water ice) from the neck also find themselves in orbit around Pluto. The orbits of these tiny ‘neck’ moons are unstable (the classic 3-body problem) and may impact either Pluto or Charon or more likely be perturbed out from between Pluto and Charon by Charon.
Pluto itself, having given up a lot of angular momentum to Charon’s orbit, rotates more slowly and so gradually relaxes into a less elongated shape. As the shape of Pluto changes the bands of terrain that still constitute a crust over the layer of volatile ices tend to stay in place at the position on Pluto’s surface that matches their diameter. Thus at their edges the bands are forced together. They slide over each other, like the telescoping bands the folding cup mentioned in my last post.
The fact neighbouring bands of crust have different thicknesses due to the process of Pluto’s gradual elongation and staggered tearing makes it possible for the bands to slide over each other rather than butting up against each other and crumpling. The shallow angle of view makes the results of this sliding together of neighbouring bands of crust clearly visible in photographs of the area just south-west of Sputnik Planum.
(f) Tidal interaction between Pluto and Charon over a long time scale pushes Charon’s orbit out and slows Pluto’s rotation until the pair are tidally locked. Through a process of gravitational perturbation, as the orbit of Pluto and Charon evolved, some chunks of crust that flew free and debris from neck have been shepherded into stable orbits beyond Charon, forming the satellite system we see today.
Pluto having reached its present near spherical shape the process of squeezing together neighbouring bands of crust is complete. The ocean of volatile ices (methane, nitrogen and carbon monoxide) has flowed over the bare water ice body of Pluto at the launching site of Charon. Once again the volatile ices skinned over, to form new albeit thin crust. This area can be clearly seen as a circular and rather smooth cap of terrain to the west of Sputnik Planum, near the limb of Pluto in New Horizons’ photos.
Diametrically opposite Charon’s launching place is another cap of crust that never broke up into bands when Pluto elongated. So when Pluto became spherical, instead of telescoping in like the bands, it was forced to crumple. This is the origin of the chaotic terrain to the east of Sputnik Planum and of the large undulating system of ridges underlying the ‘snakeskin terrain’. Even though the crumpling of the terrain east of Sputnik Planum must have been quite messy, the ridges and troughs of the snakeskin region lie nearly perpendicular to the original long axis of proto-Pluto.
After Charon’s birth geysers came back to life once the surface was re-sealed by the formation of new crust allowing pressure build up under the crust once again.
Since Charon’s birth Pluto has reoriented itself with respect to the birth axis of the pair. Of course the pair moved out of that alignment as soon as Charon was born since Pluto’s faster spin and Charon’s shorter orbital period were uncorrelated. Charon would have rose and set when seen from Pluto. The present alignment is the end result of the long process of tidal interaction that bought them into synchronous orbit. It does imply that Pluto is more massive across the overlapping bands of crust than it is along the original long axis with its thin ‘west’ cap and crumpled ‘east’ cap.
The present phenomenon of solid state convection and surface flooding around Sputnik Planum is evidence that Pluto has a thin crust floating on an ocean of soft methane, nitrogen and carbon monoxide (etc) ice. Underneath this is a core of water ice and rock. As the bulk of Pluto in its rock and water ice core becomes less elongated and more spherical the floating crust has to rearrange itself to fit on the surface. Floating bands of the thin crust might slide over each other to maintain a constant diameter. This would be a little like a collapsible travel cup folding up, or at least partially folding up.