While the landscape is presently being flooded with a tsunami of ice in the north of Sputnik Planum, in the south ice appears to be draining off the highlands south of the Norgay Montes and leaving behind a landscape painted in a patina of ice that shows up as a light greenish-blue in enhanced color images.
The right ventricle of the ‘heart’, to the east of Sputnik Planum, shares this greenish-blue hue of remnant ice. Although, there is a subtle difference in hue between the two lobes, with the greenish-blue of the right ventricle being slightly more greenish than the color of the southern lobe. It is hard to say whether this difference is significant due to the different lighting angle of the 2 regions, but it does suggest a subtle difference in composition, perhaps due to differing ages of the icy veneer.
The obvious interpretation of these large regions, painted with ice, extending away from Sputnik Planum is that they are the result of previous episodes of icy flooding and that they are part of a sequence of major icy inundations, ice floods. In the highest resolution image of the edge of the right ventricle, so far released, ice is apparently flowing down from terraces and high ice lakes into the Sputnik Planum but the quantities of ice flowing are tiny compared to the broad expanse of ice apparently flowing north between the Norgay Montes and Krun Macula from the southern lobe. So it seems likely that southern lobe flood occurred more recently than right ventricle (eastern lobe) flood.
The al-Idrisi Montes, Bare Montes, Zheng He Montes, Hillary Montes and Norgay Montes all lay along the western side of Sputnik Planum. An even earlier flood, the first flood after the crust shattering impact that created the Sputnik Planum, must have occurred in this direction in order to deposit the fragments of crust that make up these mountain ranges.
The greenish-blue icy veneer left behind by the right ventricle flood shows that it extended perhaps 90º around Pluto’s equator. It’s noteable that compared to this none of the fragments of crust that now make up mountain ranges lies very far from the location under Sputnik Planum where they started. Yet that initial westward flood must have been big enough in scale to sweep all the sizable fragments of crust to the west since almost none seem to be still floating around or lodged on the shores of the later floods. So it seems likely that the initial westerly flood (#1) extended far past the present western shore of Sputnik Planum but that it was not deep enough to carry the detached chunks of crust with it. The C-shaped lake of light colored ice in Elliot crater and the greyish color of the land to its north is evidence that the ice flood got at least that far.
From Bare Montes to Norgay Montes in the south, the crust chunk mountain ranges all lie up against the heavily cratered ancient terrain of Cthulhu Regio. The depth of the craters there and the height of their rims shows that the crust is thick. The strength of the crust under Cthulhu Regio must have prevented it from buckling downwards under the weight of that initial ice tsunami (#1) and saved it from flooding too.
Given the 3.5 km height of peaks within the Norgay Montes and that they are embedded at odd angles, the depth of the flood that delivered these chunks of crust to this location was probably ~3 km. Judging by the greenish-blue residue, when the right ventricle flood got as far the Sleipnir Fossa (long, narrow depression) it was not deep enough cover the highlands on either side of the Fossa, but flowed around them. This pattern of icy residue in valleys but not on highlands is further confirmation that the veneer of ice marking out the right ventricle results from flooding rather than weather.
The Nature of Pluto’s Crust
The sequential character of Pluto’s ice floods constrains the our theoretical understanding. They cannot, for example result from a huge up-welling of ice out of Sputnik Planum. If the Sputnik Planum ice had ‘broken its banks’ and flooded the surrounding surface all of the floods would have happened simultaneously. The idea that the floods could be due to waves of ice sloshing around in Sputnik Planum and breaking out in one direction and then another is implausible due the vast extent of the floods, clearly much bigger than Sputnik Planum itself in the case of the ‘right ventricle’ flood.
The only way tenable way I can think of to generate the observed series of ice floods on Pluto is if the crust itself sinks to allow ice to flow over it in a series of locations. This means that the crust must be thin and it must float on an ocean of the methane/nitrogen/carbon monoxide ice we see in Sputnik Planum. The crust must also be plastic, not too brittle, so that it can deform and return to its initial position without breaking up. Long chain organic compounds, like tar or plastic, would fit the bill if they have low enough density to float. This tarry crust could have been built up from tholins formed in Pluto’s atmosphere and raining down on the ice over long periods of time.
The flooding of the crust with ice would tend to be a runaway process, at least at first, since as warmish ice floods over the crust it increases Sputnik Planum’s surface area for cooling. More cooling means that the ice on top of the crust would become denser than that ice below the crust, tending to weigh it down and allow even more ice to flood over the crust. Perhaps it is this positive feedback that allowed the easterly flood to extend so far around the equator.
The flooding might eventually be halted when it reached a particularly thick section of crust, strong enough the resist the weight of flood ice. Or the flood might just have reached the point where it was so extended that the convection driven lateral flow supplying warm ice could not keep up with the rate of cooling. In this case, vertical convection would tend to stop where the ice flood was shallowest, particular at leading edge of the flood, but evaporation from the surface would continue. If enough ice eventually evaporated from near the front of the flood, it could lighten the load on the crust there enough for it to begin to rebound upwards. That could change the momentum of the flood so that it started to flow back towards Sputnik Planum. The final stages of such a reverse flow may be what we are now witnessing in the southernmost extension of Sputnik Planum, between the Norgay Montes and Krun Macula. Any shallow pools of CH4+N2+CO ice not flowing back into Sputnik Planum and left behind on the rising crust would rapidly evaporate, leaving only the patina of the still mysterious non-volatile impurity (H2O ?, CO2 ?) that we recognize by its greenish blue color in the false (highly exaggerated) color images from Ralph instrument on New Horizons. A significant fraction of flood ice might be dissipated through evaporation.
Crustal Healing – After the Floods
It is noteworthy that the bluish hue due to the residue of non-volatile ice left by floods to south and in the ‘right ventricle’ to the east of Sputnik planum is not visible on the site of the initial flood that created the ranges of mountains along the western edge of Sputnik Planum. This might mean that the ‘non-volatile residue’ is actually slightly volatile and does eventually evaporate or that enough time has passed for the bluish veneer of non-volatile ice to be covered by a layer of hydrocarbons raining down from the sky thick enough to mask its bluish hue.
The second possibility is more interesting because it points to a mechanism whereby the Sputnik Planum phenomenon can eventually come to an end and the surface of Pluto can be healed. If the convection in Sputnik Planum eventually stops, as it has done in the southern part of the plain, the continued evaporation of volatile ices without stirring would allow the accumulation of the bluish non-volatile residue into an insulating layer that would seal the volatile ices from the near vacuum of Pluto’s atmosphere, halting evaporation. Over time hydrocarbons raining down from Pluto’s haze layers would build up new crust over the plain.
Events such as we now see around Tombaugh Regio must have happened many times in Pluto’s past, when the conditions of a sufficient build up of internal heat and a sufficiently large impact on a sufficiently thin section of crust were met. So on Pluto today we should see the signs of other episodes where areas of shattered crust have healed over at various times in the past.