(Newbie here.) This track is not an image artefact, is it? The lower right corner indicates its location.

(Beautiful images, thank you all!)

The following is speculation from a newbie...

I have been trying to create a mental image of a temperature map superimposed on the images of Tombaugh Regio, and trying to imagine how the temperature changes with time in this region, as Pluto's year progresses. It seems to me that the upper part of the heart is more fluid than the lower part which is more frozen, which seems natural given that, as I understand it, it has been receiving more sunlight on average (it's closer to the center of the iluminated hemisphere). Making an analogy with the Earth and thinking that the exterior of the heart is "land" and that the heart is "ocean" (viscuous/solid/freezing), and supposing that the heat capacity is different ("land" heats/cools more than "ocean"), it could help explaining that the little peaks that appear in the heart ("islands") would be hotter and that cell boundaries would tend to be located there, freezing later than the central parts of the cells. I am assuming that boundaries are locally warmer, freezing later. It would then seem natural that boundaries seem to irradiate from these peaks/islands, tending to join other islands. In this view, darker cells would tend to be shallower/warmer regions (where "ocean" would be closer to hotter "land" - but note that there are other factors too, such as proximity to the regions receiving more sunlight, etc) and white cells would be more frozen. In that case, if Pluto is approaching a sort of "Winter", the upper part of the heart would also freeze and become whiter as time passes.

The small islands in the heart, and also mountains close to the "ocean" seem to be eroded, possibly because the ocean ciclically expands and contracts, raising and lowering its average level, causing friction in the islands (which also creates heat, contributing to a slightly higher temperature near the islands/shore/cell boundaries).

(of course, this is a huge simplification... another look near the upper shore of the heart made me wonder at all the slow movement that may be occuring along the submerged valleys/hills... there should be more movement in regions far from from the static shore, so it's much more complicated...)

This section of episode 4 of BBC's "Frozen Planet" (minute 16 - video starts there after some ads) describes the freezing of an ocean. The similarities with the cells in Tombaugh Regio is striking (and even with the mountains that seem to appear at the cell boundaries)... The scale is completely different of course, but suddenly Pluto seemed a bit less alien...

BBC's Frozen Planet, Episode 4 (Autumn)

(resemblances with Pluto's small black spots also appear at minute 43:50 )

Hi, I hope I am not adding too much noise to the forum (I am a newbie and an outsider in this field), but I though that you could eventually be interested in this way of visualizing the LORRI images. Each row spans one period of Pluto's rotation. Oldest (most recent) images are at the bottom left (top right).

Pluto:


Charon:


I don't know much about the LORRI images - some images are rotated and I am not compensating for that. I used a power of 2 to enhance the darker regions. Ideally, it would be nice to see the same angular position vertically and the rotation phases horizontally, correctly aligned...