They say that the human eye is very good at detecting patterns. When the first high resolution mosaic images of the full disk of Pluto were released I got a strong impression that there were a series of diagonal but parallel lines across it. The impression is particularly strong in this collection of the individual images before being processed into a single global montage.
The impression of parallel lines arises from different kinds of feature in different parts of the disk. [Above Sputnik Planum in the picture above (which is actually to the north-west since the image is not aligned with north at the top)] In the north of (above) Sputnik Planum, where the land lies under a blanket of uniformly white polar frost it comes from rows of crater-like features, to the west (left) of Sputnik Planum the impression comes from albedo patterns, i.e. streaks, and to the south of (below) Sputnik Planum it comes from the white peaks of mountain ranges. It is as though Pluto has some underlying planet girdling structure like stripes on a ball. In the pictures above the lines run at about 48° from horizontal.
This is an overall impression, but when you look closer at individual areas of Pluto the impression often disappears, overwhelmed by local features running in some other direction or just too subtle to see.
When I first noticed this pattern on one of the New Horizons team’s global montages I thought it was probably an artifact of the way that the montage was stitched together. In fact some people who examined the montage very closely did find artifacts of the image assembly process, but they could only be seen by processing the image to enhance them and they were quite linear, clearly artificial and in another direction altogether. So whatever was giving me the impression of parallel linear features on Pluto is intrinsic to Pluto.
This earlier set of images taken when New Horizons was 446907 km from Pluto also gives a clear impression of a ‘grain’ across the face of the planet, even though they were taken with a different orientation.
Of course what appear to be straight lines only appear that way because our viewpoint is in line with their ‘equator’. They are actually circles on a sphere, like latitude lines on the Earth. If they are viewed from any angle other than their equator we would see a series of curved lines, ellipses to be precise. Since the pattern is quite subtle we may not have noticed it at all if the pictures had been taken from a different angle. This seems like a lucky coincidence, but actually since the encounter was planned to put New Horizons over Sputnik Planum, so the coincidence is that Sputnik Planum lies on the equator of the pattern of stripes.
One place where part of this linear pattern can be seen, not buried under frost or anything else, is just to the west of Sputnik Planum. These somewhat vague white lines are remarkable because they are not due cracks, channels, craters or cliffs, but persist in spite of these things suggesting that the pattern is part of the ‘bed rock’ (note: the crust is probably not rock as we know it on Earth).
The place where the banding is clearest and most clearly intrinsic to the crust is on the southern horizon of New Horizons’ images. The contrast between the bands is so clear that the whitest band looks like it is in sunlight while neighboring bands are in shade, but the difference in tone is real. It is also clear that the highest and whitest southern mountain ranges align with the clearest white line to the west of Sputnik Planum. In all likelyhood this is a single band of tough white material that extends under the ancient high and most heavily cratered part of Cthulhu Regio. This might imply that the banding is even older.
In the higher resolution color image the differing textures of the neighboring bands, their differing altitudes and the linear chasms that divide them attest to the fact that this banding is intrinsic and fundamental to the surface, at least in this region.
Rotating Pluto’s image to make the banding vertical we see that the western (left) pole of the structure corresponds to a large smooth area and the eastern (right) pole is occupied by the undulating ‘snake skin’ terrain.
It also becomes clear that the bands appear to curve slightly around the western pole. This means that in this image our viewpoint is actually a bit to the west of the banded structure’s equator.
The western pole of Pluto’s banded structure actually has the largest section of smooth surface that we can see. This smoothness implies that the surface here is relatively young and relatively thin, compared to the heavily cratered terrain of Cthulhu Regio (bottom left). Although a peninsular of rugged Cthulhu terrain containing the a crater, named ‘Oort’, does intrude into the southern side of the ‘western cap’.
At the center of the lower part of the picture a smaller smooth area intrudes into the cap. This area appears to be an earlier example of the Sputnik Planum ‘soft ice sea’ phenomenon. The ‘western cap’ is likely to be an even earlier example of an exposed sea of soft ice (primarily methane, nitrogen and carbon monoxide) where new crust was laid down once convective mixing ceased. This soft ice sea would have lapped against the shores of Cthulhu Regio around Oort Crater. Since the western cap forms part of the shoreline of the healed over soft ice sea in the lower middle of the picture the ‘western cap’ the cap itself must be older.
Rough coordinates for the center of this western cap of Pluto’s banded structure are longitude 75°±5°, latitude +15°±5°. The banding is symmetric around an axis running through Pluto so the center of the cap is the western pole of the banded structure (relative to Sputnik Planum). So the opposite pole, the eastern pole of the banding, should be at longitude 255°±5°, latitude -15°±5°. That puts it in the southern hemisphere in a poorly imaged area between Ala Macula and Balrog Macula. It lies beyond the ‘snakeskin’ terrain of the whitened eastern lobe of Tombaugh Regio. The images we have seen of this area suggest that it is as rugged, if not more rugged, than the snakeskin terrain that we can see clearly.
Due to the projection Pluto’s bands do not appear as straight lines in the map. The edge of the smooth region around the western cap can be seen as an arc of darker coloration about 30° north of the western pole. Similarly, despite the poor resolution of imaging there, the banding can be seen as a series of arcs curving around the eastern pole in the region between 0° to +60° latitude and 270° to 330° longitude. The pattern looks a little like the swirls of a poorly preserved fingerprint at a crime scene, but it is unmistakable.
The center point of this eastern pattern of arcs appears to be a little closer to Pluto’s equator than the eastern pole as I have marked it on the map, perhaps closer to 250°±5°, latitude -5°±5°. The discrepancy is not at all surprising since the pattern of stripes is purely morphological and may have suffered distortion due to tectonic over time. Furthermore, my determination of the location of the western pole is somewhat approximate. It would be more surprising if the eastern pole of the pattern did lie exactly 180° from the western pole.
The eastern pole of Pluto’s banding lies a few hundred kilometers over the horizon in the even more rugged continuation of this snakeskin terrain. So one end of Pluto has a ~60° wide cap of smooth new crust and the opposite end has rugged undulations. It is as though Pluto’s crust has slipped from west to east like a carpet that has come loose wrinkles as it bunches up against a wall, except that in this case the carpet is on a ball. This is likely exactly what happened.
It is amazing but the Sputnik Planum phenomenon has shown us that Pluto does have a relatively thin crust (4~5 km) that is floating on an internal sea of solid but soft methane, nitrogen and carbon monoxide. That the crust is light enough to float on the soft ice is demonstrated by the rafts of ‘crust-bergs’ washed up on the shore and some still floating in the soft ice of Sputnik Planum. There is also ample evidence that this thin floating crust is both flexible and tough enough to resist the stress of at least a certain amount of movement without cracking up. The snakeskin terrain is part of the eastern lobe of Tombaugh Regio. The numerous ice lakes that fill depressions, along with the light color of the lobe are evidence that the whole lobe was once submerged under Sputnik Planum ice. That soft ice is now pouring back into Sputnik Planum from the lobe via massive glaciers shows that the lobe has rebounded back to its original altitude. As well as flowing back into Sputnik Planum much of the flood ice evaporated leaving the land coated in a film of a less volatile component of the soft ice. This coating has a greenish-blue hue in the exaggerated color image and it is what gives the eastern lobe its light color, thus matching that of Sputnik Planum and forming the ‘heart’ shape.
Thus Pluto’s strong but relatively thin crust was free to slide over Pluto’s inner core, like carpet over a wooden floor, if it had a reason to do so.
So why is Pluto’s surface banded?
It’s time for some serious theorizing. The parallel banding is apparent in regions right across Pluto. This implies that it is global and that if we could remove Sputnik Planum, all the craters, deposits of frost, and accumulations of red Tholin dust (e.g. as in Cthulhu Regio) that overlie the bands, the banding would be plain to see over the whole surface. The banding has a global symmetry. It is symmetric around the east-west axis, at least roughly so. That means that whatever process created the banding must have acted globally and it must have acted symmetrically.
One candidate process is a large impact. Impacts do have the right symmetry. Impacts can create multi-ringed structures like the Orientale basin on the western limb of the Moon and Valhalla crater on Callisto. A massive impact on Asteroid Vesta gave it a set of linear ridges around its equator. The impact that created the Caloris basin on Mercury also created chaos terrain on the opposite side of the planet.
While impacts are capable of creating structures with the right global symmetry, one large enough to affect the entire surface of Pluto would have to be extremely violent. It is hard to see how such a violent event could so neatly create the large number of bands that Pluto appears to have. For example, the system of neighboring bands seen south of Cthulhu Regio (figure 8) with apparently differing compositions but clear linear boundaries is more complex and delicate than the ridges seen on Vesta or around Valhalla crater.
If the banding were the result of an impact, that impact would have had to occurred at the ‘western pole’, yet there is no sign of crater rim mountains around this area and the ‘western cap’ (figure 11) has a surprisingly straight edge (when seen edge on). This edge seems too neat to be the result of an impact violent enough to have global consequences.
An impact large enough to have global effects would likely destroy the surface rather than sculpt it into stripes. Pluto’s surface banding must have been created but a more gentle process. There is a candidate process that seems to fit: the formation of Charon by fission from Pluto.
This rough sketch illustrates the process of satellite fission. It is actually from a 1963 paper by D. U. Wise on the possibility of the Moon’s birth by fission from Earth (Link to the full text of Wise’s 1966 revisit to the topic of lunar fission). The process looks the same for the case of Pluto and Charon, except that we just need to imagine that Charon is twice as big relative to Pluto as the Moon is to Earth as shown here. In stages (1) to (3) Pluto spins faster and becomes elongated as its moment of inertia drops due to partial differentiation (i.e. as the densest material falls to center). In stages (5) to (7) Pluto’s spin slows due to tidal interaction with Charon, driving Charon away. The crucial step where Pluto’s banding would have formed is from (4) to (5) where fairly rapidly Pluto becomes more spherical.
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 layer of soft ices there is a core of harder denser ices and rock. The bulk of Pluto’s mass is in this less volatile ice ice and rock core. When the core becomes less elongated and more spherical, (4) – (7), the thin crust floating on the layer of soft ices has to rearrange itself to fit on the surface. In effect, Pluto shrinks under its crust.
During the spin-up phase, (1) – (4), Pluto’s surface would have already developed a degree of banding because as Pluto became more elongated bands of the thin outer crust would separate (mid-ocean ridge style), revealing the same soft ices we now see in Sputnik Planum. When solid state convection ceased new crust would form over the exposed strips of soft ices due to continual rain of hydrocarbons. So just prior to fission the outer thin crust of the elongated Pluto would consist of bands of varying thickness. When Pluto started spinning down and becoming spherical the thickness variations of the outer floating crust would provide natural places for it to split into bands. The floating bands of thin crust would have then slid over each other to maintain a constant diameter. On both ends of the elongated Pluto this would have been something like a collapsible travel cup folding up, or at least partially folding up. In this way bands of terrain with different characteristics could end up juxtaposed or even on top of each other, as we see south of Cthulhu Regio (figure 8).
On the end of Pluto away from Charon as Pluto became more spherical the thin crust no longer fit the shape of the planet and appears to have slowly crumpled to produce the folds of the snakeskin terrain and the chaos we see around the eastern pole (figure 14, above). Where Charon was launched a big hole was left leading the formation of the new especially thin crust we now see at the western cap (figures 11 & 12).
Charon itself might carry a chunk of crust from one end of elongated Pluto with it when it is born. It turns out that Charon does have 2 distinct types of terrain. There are even cliffs. It surprised me that such an obvious sign of fission could survive the fission process and still exist on Charon’s surface today. It seems too easy. Anyway, if it turns out that the composition of Charon’s rugged high terrain is matches Pluto’s crust that would clinch it.
. . . . . . . . Click here for more thoughts on Pluto & Charon’s fission in my next blog post . . . . .