Thanks for sharing. If I'm not mistaken, the dunes and ropey terrain are one and the same thing. There are also a couple of major features worth including. In blue I've circled a region of swirls where there is likely to be a second frozen lake. In red is Pluto's version of Valles Marineris.

EDIT: I've added a green rectangle covering another possible frozen lake. This one appears to have drainage channels leading from the north and west into a series of interconnecting(?) basins, bounded on the south and east sides by flow fronts from widespread resurfacing events dating from soon after Pluto's formation. Interestingly, the channels on the north side all seem to emerge from holes in the ground!

Come to think of it, it doesn't even require an impact to disrupt tidal lock and melt the ice at Sputnik Planum. The passage of any large body near the system will change the orbital period of Charon, without changing rotation periods. There are likely to be hundreds of bodies as large as Sedna scattered to great distances that pass through the Kuiper Belt at perihelion.

Bear in mind there's a major rift valley on the left going all the way to the north pole that could contribute to the impression of a double-ring system on that side.

Coming to Sputnik Planum, it could be a frozen lake that melts whenever tidal lock has been disrupted by a large enough impact somewhere in the Pluto-Charon system. If I'm not mistaken, it only takes something like a 30C increase to melt N2 and CO near the surface. During these rare periods, a denser atmosphere would develop. The rise and fall of tides erode the mountains, especially so when there is any ice floating on the surface to act as an abrasive. There's a temperature range between 63 and 68K where liquid nitrogen can have CO ice floating on top. Presumably, methane has been depleted in this area, or it would be a factor too.

The alignment of that impressive chain of eroded mountains at Norgay Montes is consistent with it being part of the rim of an ancient elongated basin enclosing Sputnik Planum (or is it Lacus) I proposed on July 14 as being caused by the grazing impact of a large body (such as Charon's precursor). Here's an updated version of the image I posted, with ellipse extended to fit Norgay Montes more accurately.

The curving scarps I circled would be the basin's outer rim. It is equivalent in size relative to Pluto that Mare Imbrium is to the Moon. Mare Imbrium also has an incomplete rim and no trace of inner rings, after being flooded with lava when the floor rebounded. Moderate-sized craters on Pluto can be made out despite having been blanketed under a kilometre or so of ice deposits, but there are none inside Tombaugh Regio. However, there appears to be craters west of that area that haven't been blanketed, particularly on the upper half of the whale's head. If Pluto ever had a thick atmosphere, there could be signs of its effects on the surface in high-resolution images of that region.

It might not be a coincidence that Tombaugh Regio lies inside what l think is an impact basin roughly 800km wide.

Pluto is differentiated and likely to have layers of different volatiles at various depths. Here the impact penetrated deeper than anywhere else in the equatorial zone. Liquids filled the void, flowed south and carved a shoreline. Some of the nearby ropey terrain was eroded, leaving behind those craggy water-ice mountains surrounded by blocky debris. It's anyone's guess though, whether the surface CO deposits were laid down at that time or more recently.

We can rule out another TNO, given it is not moving in the same general retrograde direction as Pluto. MBAs and NEOs can also be ruled out for similar reasons. Knowing how thorough Tombaugh was with his survey, he wouldn't have overlooked something of that magnitude. The non-symmetrical appearance in both images suggests they were specks of dirt on the plates when they were eventually scanned.

One clue is the smooth terrain in the equatorial region of Charon that could indicate a period of melting when tidal forces were significant. Curiously, the shoreline is tilted in latitude. That could be evidence of polar motion prior to the two bodies becoming tidally locked.

Could this be a large slightly-elongated impact basin with icy ejecta spreading toward the lower right?

Charon had to have bounced off somewhere before going into orbit!

I've modified a capture from the approach animation to show a gibbous-phase Pluto illuminated by Charon alone during the solar eclipse. The only other possible light source will be a ring of light scattered forward by the exosphere at the limb, so that might be the best opportunity to see the Charon-facing side again, in addition to the otherwise hidden southern hemisphere.

Regarding the trajectory design, the solar and radio occultations for both bodies was no doubt a primary objective and probably ruled out a close approach pass on the Charon-facing side.

Thanks Explorer1 and JohnVV for advice on the tidal effects at present being insignificant. That leads me to believe the lake-like basins were formed before Pluto and Charon's rotations became synchronised. There must have been a long period when the equatorial region was heated enough by the tidal forces to potentially melt the ice, form a thick atmosphere and instigate a hydrological cycle. With Charon's proximity, tides could have been so extreme that fluid sloshed back and forth to gouge immense cliffs at lake margins. When the energy source ran out, Pluto began to lose its atmosphere and the lakes dried up and froze.

If that interpretation is correct, Pluto will be amazing when we see it in high resolution. I wonder if any sample images or subframes thereof are scheduled to be sent to Earth in the early stages of the data transmission?

Are you saying the barycentre stays EXACTLY in the same relative position to Pluto and Charon, despite other bodies in the system spending most of their time at different position angles?

It seems those regions resembling lakes on Titan are bordered by cliffs at least 10km high, perhaps resulting from erosion by fluids, at a time in the distant past when the atmosphere might have been more substantial than now.

Correct me if I'm wrong but shouldn't the position of the barycentre relative to Pluto be constantly changing, given this is a sextuple-body system. Wouldn't that be responsible for generating tidal forces in the equatorial zone?

As I recall, that material is actually regarded as having originated in solution deep underground, prior to deposition on the surface via a spring or geyser - exactly what I've been arguing for to explain the spots on Ceres.

The time any recently-separated components of a rubble-pile asteroid spend in close proximity with one another, would be an extremely small fraction of their total lifespan, and the odds of colliding with another asteroid during that interval would be equally astronomical!

The central spot looks nothing like an impact crater. The debate should be resolved once and for all when their chemical composition is announced. I'm expecting ice and salt at spots inside Occator crater and salt at all the others. Also looking forward to colour images to see if any spots display a brown hue indicative of radiation-darkened salt.

You're comparing apples with oranges. There's no atmosphere on Ceres to disrupt rubble piles. Its gravitational field is too feeble and the Roche limit too close, to result in anything other than single impact craters.

Quite apart from that, there's no evidence on steep slopes of the existence of a bright layer of material anywhere near the surface.

I get the impression the dark equatorial band represents thin (and younger) ice relative to the pole. The implications are that there's a subterranean liquid layer at low latitudes, and any surviving impact craters will be limited to high latitudes only.

The possibility of a sea at low latitudes suggests tidal heating is at play. The obvious candidates are Charon (if its orbit isn't perfectly circular) and that of Nix and Hydra, especially when aligned in orbital longitude.

You don't have to look far to find smaller versions of the mountain - every spot should have them. I've arrowed the largest in a field of about a dozen. It's situated on the mandatory fault line - the sharpest crack I've yet seen on the surface of Ceres.

ZLD's morph also gives the impression the spot sits in an irregular depression with promontories. But rather than promontories, they are more likely to be large blocks of crust displaced during past explosions as part of a debris field surrounding the spot. That goes a long way to explaining the fan-shaped blast patterns of dark material. For instance, the nearest obstruction lies only 2.5km west of the spot's centre, and there's no spray pattern in that direction. This correlation implies the site was a source of at least some of that far-flung dark material and that there have been multiple explosions.

I compared the reflectivity of the core of that spot to the normal surface in PIA19584 and found it to be as bright as snow. My opinion on the secondary spots is that they represent the fallout of minor geysers located on fault lines. The fact that they diminish in brightness smoothly from their centres, suggests material has been distributed onto the ground after being lofted, but their small size in some cases equates to an exit velocity comparable with that of a lawn sprinkler!

I applied a used-defined transfer function to display the full range of brightness levels in a single image.

The bright spot looks like a hill on a crack to me. If it lies in a depression, it's an irregular one -- more like those in central pit craters than a circular impact structure.

For what it's worth, my interpretation of the spots is that they begin with impacts that pressurise the putative ocean and cause water to erupt out of weak points elsewhere in the crust, particularly at fault junctions. Eruptions could therefore recur at different spots simultaneously after long intervals.

In the intervals, some spots slowly deposit salt via geysers, due to the squeezing effect on the ocean by the gradually-thickening crust as it freezes. Spot-5 could be a recently-active example of that.

At the next impact-triggered eruption, mud is deposited and salt may be excavated and displaced. Evidently, eruptions at spot-1 have been so great as to overflow the crater, deposit blocks of crust on its flanks and broaden the base. Steepness of the inner wall may be evidence of erosion by a transient lake, before it drained or froze and sublimated away.

That's unlikely, given it is sitting on a crack that is part of a deep and extensive network of cracks.

Even Spot 1 crater has cracks but they're hard to trace under the deposits and regolith. A mound with a possible depression at its centre is located at an intersection of cracks, some of which I've highlighted.

Just as I suspected, there's a hill at the centre of Spot 5, rather than a crater. Bear in mind it is illuminated from below (south) in the original image. The secondary spot to its east also shows signs of positive relief.

The mound left of centre in this crater could be the source of some of the potentially erupted material. Is that a pit I see at its summit?

This is a section of the floor of the western of the two southern basins, including light and dark markings seen since January. North is at top.


As mentioned earlier, I suspect both light and dark markings represent volcanic deposits in regions where the crust is weak. That's why light and dark markings are often found on fault lines and in proximity with one another.

My suspicion is that light-toned spots represent salt gradually deposited over a period, and therefore should show varying degrees of positive relief, depending on how long each site was active. I'm yet to see any spots displaying definite signs of impact-related excavation (the consensus explanation in this forum since day 1). The light-toned area left of centre appears to include a ridge on a fault line. If I'm not being fooled by the lightng angle, two thirds of the way along it is a steep cone-shaped mountain. Judging by other large Cererean basins, that mountain looks too small and steep to be its original central peak. The ridge itself could represent a piece of crust that has been broken and tilted prior to formation of the mountain on top.

And I suspect dark markings result from impacts elsewhere, forcing water to explode through those weak points in the crust, spraying and flooding the area with mud -- the basin floor right of centre has clearly ruptured, just where it is darkest. By the way, just at the top of that rupture is a curious mound with two depressions in it that seems to have formed there sometime after the groove it is sitting on.