The Judgement of Jupiter is a story written around 1495 in Germany and published under the pen name of Paulus Niavis. It tells of a case in the court of law of Jupiter. The accusation is parricide (destruction of the environment). The accused is a mine worker; the victim is Mother Earth. Mercury is the lawyer and the witnesses are a variety of deities. Bachus complains about the destruction of his vineyards by mining. Ceres (god of agriculture) says that his fields have been devastated. Faunus, god of forests, protests against the cutting of his trees for the coal burners, used in the silver and gold mining. Among the witnesses are Pluto and Charon. Pluto, god of the underworld, complains that because of the noise of the mining, sleep has become impossible and he is barely able to live in his realm. Charon, as the ferryman carrying people across the river Styx to the land of the dead, says that the underground water is all being used and diverted, and soon the Styx will be too low for him to deliver the deceased souls to the land of Pluto. The miner defends himself vigorously: he argues that Mother Earth is not a true mother but a hostile stepmother, who hides her treasures from their rightful heirs and owners. It is his right to explore and to recover those for his own use. The Judgement of Jupiter describes a very modern conflict. (The modern case of coal mining in the Hunter wine valley in Australia comes to mind.)
Jupiter refers the case to another judge, Lady Fortuna, who finds that humanity had no choice but to dig for resources, but predicts that if humanity continues in its exploitation, it will suffocate in the bad air, be poisoned by the wine, plagued by hunger, and swallowed by the Earth.
In our age of modernity, Pluto and Charon no longer rule our own world but have been exiled to the outer Solar System, far from human interference. But their peace didn’t last. On July 4, 2015 New Horizons visited the dwarf planet and its moon. Pluto’s realm has again been entered.
The images from New Horizons showed us the surface of Pluto’s realm. The highest resolution mosaic is shown at the top of this post: click for the full image if you dare. For those of us who were familiar with the crater-pocked, indistinct surfaces of places such as Mercury, the Moon, and Ceres, it was a revelation. Every part of the dwarf planet seems different. There are mountainous regions, with mountains up to 4 km tall, craters, long canyons or rift valleys, and even an ocean. But nothing here is what it seems; we are after all in Pluto’s dark realm. The structures may look familiar but they are made from different substance, like Shakespeare’s dark tale of Macbeth wonderfully transposed to ancient Japanese culture by Akira Kurosawa: the same story but build on a different background and characters. Pluto’s ocean cannot be water; in fact it is an ocean of nitrogen ice. The mountains aren’t rocks: they are made of water ice which at these temperatures is rock hard. Pluto’s mountains are true ice bergs. The true rock is deep below, in the core of the dwarf planet.
Craters and canyons
There are fewer craters than may be expected, but they can be seen. The image shows a region with a variety of different looking craters. The ones on the left side are bowl-shaped, surrounded by rings, as you might get when throwing stones into wet sand. The ones in the centre have broken edges, like weak sandstone, with the appearance of a central peak: they seem to have exposed layers of different material. On the right hand side, the craters have much clearer central peaks. Vaguely visible, there is a large impact crater in the centre of the field, with a flat bottom, looking similar to the Moon’s maria. The appearance suggests that this crater was filled in with other material later. It is dubbed Burnley basin, after the school girl who named Pluto. The appearance of many of the craters suggest that the ground incorporates material that flows a bit on impact: nitrogen and methane ices fit the bill.
And what about the rifts and valleys? They haven’t received much attention because they were outperformed by an even deeper crack on Charon, but they are evidence that Pluto has geology. A good example is shown here. Obviously they can’t be rivers – can they? Perhaps they are more like grabens: depressions due to extension of the crust.
Secrets of the heart
The biggest surprise, and the most visually appealing, was the white heart. The pictures are mostly of the western part of the original heart; it has been called Sputnik Planum. (Names on Pluto are provisional: the final names will be assigned by the International Astronomical Union, and they may or may not follow the ones given by the New Horizons team. Sputnik Planum will probably survive the naming battle, but some other names may not.) The flat, smooth surface of Sputnik Planum was quickly identified as the expected nitrogen ice cap, although it being near Pluto’s equator was a bit counter-intuitive. The southern edge is on the equator; the northernmost part extends to 45 degrees latitude. It has survived the northern summer: it is a permanent, 1000-km glacier, one of the largest of the solar system. There is a complete lack of impact craters on the Planum: this means that the ice surface cannot be old, and either flows easily or reforms its surface perhaps every few million years. Pluto’s geology is alive and kicking! The eastern side of the heart was out of sight during New Horizon’s closest approach, but more distant images look like snow-covered terrain. Perhaps the snow is coming of the Sputnik Planum glacier.
In the eastern part of Sputnik Planum we see floating debris on top of the glacier. What are these? They are icebergs! Nitrogen ice is denser than water ice. The mountains are made of water ice: if a bit of mountain breaks off, and falls on the nitrogen glacier, it will float. The icebergs are floating mountains, carried to the heart on the ice and bumbling along on top, subject to the whim of the glacier. There may be a few hundred of these, all in a fairly small part of Sputnik Planum. Why only here? Perhaps there was a Pluto quake here which left debris, later collected by a rising glacier. But that is speculation. The time scales are long: the icebergs may form at a rate of one per million year. Once formed, they can exist forever. The larger ones are over 10 kilometer across.
Sputnik Planum shows a cell-like pattern on its surface. The centres of the cells are smooth, and the surface becomes more rough towards the edges. The icebergs are located along the edges, between the cells. What is this? Models suggest it is a convection pattern. The ice slowly rises from below, cools at the surface, and sinks, like an old-fashioned lava lamp. The edges are where it sinks. This explains the roughness near the edges (the surface is older here) and the location of the icebergs along these edges: they are carried by the flow but cannot sink in the denser nitrogen. The edges collect floating debris like an extraterrestrial Saragosso sea. The convection flow is exceedingly slow: centimeters per year. The turn-over takes a million year. Still, that is fast on geological time scales. The closest analogue on Earth would be the convection in our mantle, which also is a slowly flowing solid.
The pitted surfaces seen in some places are caused by sublimation: you may get the same effect on old snow on Earth. The nitrogen slowly sublimes into the atmosphere, freezes out as new ice elsewhere, and flows back to the glacier. It is a weather cycle of nitrogen. On Pluto, ice behaves like rock and nitrogen like water.
Nitrogen is not the only substance on Pluto that creates weather. Methane behaves similar. The image below shows a mountain range with the tops covered in a white substance. This is believe to be methane snow. At plutonian temperatures, nitrogen and methane ice mix only little, and each form their own layers. The mountain range involved, provisionally called Cthulhu (a name less likely to survive), is stretched out near the equator. A thick layer of methane ice also seems to exist on Pluto’s north pole.
So what is Sputnik Planum? The convection models suggest the nitrogen glacier may be ten kilometer deep or more. That puts the bottom well below the rest of Pluto’s surface. The vaguely circular shape suggests that it may have started its life as an impact crater, which slowly filled up with the ice. (Against this, the southern part does not look even remotely circular so this suggestion should be taken with some caution.) Now it is the closest Pluto has to an oceanic basin, filled with the densest flowing material it has. On other planets, it might have filled with sand. Here, it is an ocean of frozen nitrogen, surrounded by mountains of ice.
Pluto’s atmosphere is mainly nitrogen, mixed with 0.25 per cent methane. The surface pressure is minute, 10 microbar, but that is enough to get weather. Surface winds may be a few m/s: some dark trails on Sputnik Planum are believed to be wind-blown. Similar to Earth, mountains force the winds to go up and over them, and clouds may form. New Horizons saw many different layers of haze which may have formed in this way. The air is colder higher up (albeit only by 0.5 K per km) and so can contain less methane: this may cause methane condensation, hence the methane snows on Cthulhu. The coldest surface temperature was seen at Sputnik Planum, 37 +-3 K, probably cooled by sublimation of nitrogen from this glacier.
Above 5 km, there is a temperature inversion: temperatures begin to rise again. That is caused by absorption of sunlight (as happens on Earth, in the ozone layer) and ineffective cooling. It reaches 70-100 K at 10 km altitude. The low gravity makes the atmosphere very extended: it is detectable up to 1000 km (!) altitude. Before New Horizons, it was thought that from this altitude, some nitrogen could escape altogether, when molecules reach the escape velocity. But at 70 K, that doesn’t happen. However, methane, which is lighter than N2, moves a bit faster and this can still escape. Pluto is therefore expected to have a wind, losing a bit of methane to space. Over its life time, 28 meter of methane ice has been lost in this
way. A little bit of weight loss, but nothing dramatic even for something as small as Pluto.
New Horizons, looking back while the Sun was behind Pluto, saw as many 20 separate haze layers in the atmosphere. The layers are separated by about 10 km and extend to 200 km height: the most prominent are at 10, 30, 90 and 190 km altitude. They seem to be a mixture of very small and larger particles, with the larger particles mainly at lower altitudes. The small particles are small enough to have the same effect that gives Earth’s sky its colour. Pluto’s sky is also blue – although you would need good eyes to see it. The hazes are probably due to chemistry caused by UV radiation on methane – a photochemical smog. Humanity does not have a monopoly on producing pollution!
The high resolution images of Pluto show a number of mountains. That is interesting in itself since mountains don’t form out of thin air. They need a push from below; their presence tells you that at some time in its past, Pluto was geologically active – things were moving underground. Two of those mountain in particular captured people’s interest. Provisionally called Wright Mons and Piccard Mons, they are 3 to 5 km tall and 150 km wide, and adjacent to each other. So far nothing special, but both have large depressions at their summits; a hole on top of a mountain is normally a clear volcanic feature. Are they volcanoes? People are unsure.
On Earth (and Mars), large depressions on top of mountains means calderas, and they form by the emptying of the magma reservoir below. Bardarbunga in 2014 comes to mind. Can that have happened on Pluto? You can’t have proper magma chambers where there is no rock to melt. The ‘rock’ is ice, and any volcanoes must be based on water ice. (Ice volcanoes are also called cryo volcanoes.) But water ice doesn’t erupt easily, because when you melt ice to fill the ‘magma’ chamber with water (think swimming pool), it has higher density than the ice around it. So it tries to sink rather than erupt. This will make an excellent caldera, but how can sinking water make a mountain?
One can speculate. These two mountains are close to the nitrogen glacier and nitrogen ice is denser than ice or water. If the surface was cracked, nitrogen may have crept inside, and pushed up the ice mountain. Or you invert the volcano. No, not that way. I mean start with liquid water underground, and freeze it. The freezing expands the ice, and raises the ground above – there you have your mountain. Keep a central chamber liquid (the water magma chamber) and let it drain – now you have your central caldera. But you will get little or no surface eruption.
That may be a problem for this explanation, as the surface of the mountains seem to be devoid of impact craters and is probably young. The texture looks like something that flowed and solidified, a highly viscous material. A partial melt, perhaps, that crept down the mountain? Perhaps we are barking up the wrong tree (if you can say that on Pluto). If you put nitrogen underground, could this erupt? Overall, there isn’t enough nitrogen within Pluto to drive eruptions, but it might be different near the white heart glacier. The only other type of volcanism driven by a trace element era the hydrothermal explosions on Earth (there are nitrogen geysers on Triton but not nitrogen-volcanoes). Perhaps Pluto does nitro-thermal. For now, Pluto’s volcanoes remain mysterious.
Oceans of the deep
How can water be liquid at plutonian temperatures? It can’t. The phase diagram of water shows that the liquid water does not exist below 240 K. Pluto is FAR colder, and so can’t have liquid water. Adding anti-freeze may help. The most effective one is ammonia, and that does exist on Pluto. Adding ammonia to water reduces the freezing temperature to as low as 140 K (at Pluto’s air pressure). This is still 100 K warmer than the actual temperature, but perhaps on occasion this difference is surmountable? But even if you warm the surface by this amount, you’ll just boil away the mountain.
Underground, things are more promising. The pressure goes up with depth, by about 6 millibar per meter. Liquid water can exist at a pressure above 10 millibar, so in theory it could exist within meters of the surface. All we need to do is add heat! Heat is conveniently provided by radioactivity. The heat this generates is estimated to be 6 milli-Watt per square meter (this number is very uncertain). For comparison, on Earth it is about 90 mW/m2 on average (but rather higher in Iceland). On Earth, as you dig down the temperature goes up by 25C per kilometer. On Pluto, with ten times less heat to be conducted, you expect 2.5C (or 2.5K) per kilometer. (This assumes that rock and ice have similar thermal conductivity. It ignores many other complications but lets stick to this for now). You need a temperature of 200-300 K warmer than the surface. That is reached roughly 100 km down. The pressure here is 6 bar – we are in the liquid water range! In reality, it is marginal. Deep liquid water may or may not exist. But if it does, you wouldn’t get a few drops. Remember this region is mostly ice and little rock. Melt it and you’d get a massive underground ocean.
Radioactivity is a decaying process. Over time, it diminishes. That affected New Horizons itself, which receives its power from nuclear decay, and now lives of 20% less power than when it was launched. The radioactivity inside Pluto decays much more slowly, but it still would have been stronger when Pluto was young, and Pluto’s interior was warmer at that time. Thus, even if an ocean is not there now, there is little doubt that Pluto would have had a mantle ocean in the past. Imagine Captain Nemo traversing an ocean the size of the Atlantic and 100 kilometer deep.
When (or if) this ocean froze, it expanded. This would have increased Pluto’s diameter by 10 to 20 km, because ice is less dense than water and therefore needs more space. Such an expansion could explain the rifts and canyons on Pluto! They would be true extensional features, plutonian rift valleys. The rifts look relatively little eroded and could be geologically young, meaning that the deep ocean would have frozen fairly recently, or is still in the process of freezing.
But not all ice is the same. At pressures above 2 to 20 kbar (for temperatures of 50 to 250 Kelvin), it becomes ice II, which has a different structure. This kind of ice is denser than water, by about 15 per cent. A pressure of 20 kilo-bar (for 250 K) is found at 300 kilometer depth. If the ocean goes deeper than this, its freezing would have caused the water to contract, not expand, and Pluto would have contracted rather than expanded. Pluto’s surface shows no evidence of features caused by crustal contraction. This suggest the ocean was deep but not superdeep. Recent models, assuming 5% ammonia, suggest that ice-II would indeed form, and since crustal contraction is not seen, the ocean must still be partially liquid. Bring on the Nautilus.
if the mantle of Pluto is an ocean, the crust floats and can shift around. On a spinning planet, the crust would orient itself in such a way that the most massive regions would be on the equator. That appears to have happened on Mars, where the massive Tharsis bulge is precisely on the equator. But the maps of Pluto show no particular effect. (We do not have detailed images of the other side, and crucial evidence may be hiding there.) But this may be an argument against the underground ocean.
The ocean’s to-be-or-not-to-be remains an open question. But if there is still liquid water down there, the possible cryo-volcanoes and the apparent geological activity would be easier to explain.
New Horizons did not get as close to Charon as it did to Pluto. (Charon is stuck on the far side of Pluto compared to Sputnik Planum, and do New Horizons could get close to one or the other, but not both.) The images do not show as much detail. They reveal a more cratered landscape, lacking the white ices of Pluto. But the craters are still few and the surface may not be that old. Charon as a whole is darker than Pluto, because of the lack of nitrogen ice. It also lacks an atmosphere.
There are three notable features. At the north pole, there is a dark region. This seems to be a thin layer of hydrocarbons. How did they get there? Charon is close enough to Pluto to capture some of its escaping methane and products of the photochemical smog. This may have ended up on the north polar region: the dark colour can be the organic molecules from the smog. This also explains the lack of white ice regions, as nitrogen does not escape from Pluto, and thus does not get to Charon. However, why would the dark stuff collect only on the north pole? The higher resolution image shows the appearance of a large impact structure here: perhaps the dark debris is instead from the impact of a comet.
The second notable feature is the deep cliff on the edge of our images. It is as much as 9 km deep. We don’t understand how it formed and sadly we may only see part of the full structure, but it indicates that Charon is an active body.
The most enlightening is the third feature, a series of cliffs and troughs arranged along the equator, possibly extending around the entire moon. This is exactly what one might expect if Charon has contracted over time. It may be from thermal contraction, when the interior of Charon slowly cooled over time – being smaller than Pluto, it would have lost its heat faster. But the relatively young surface suggests another explanation. Perhaps Charon too had a deep liquid ocean, which kept its surface active but froze some time ago. But being colder than Pluto (smaller bodies can’t keep their heat as well), it froze into ice-II and Charon contracted. This contraction caused wrinkles on the surface, in particular the long cliff, which oriented itself along the equator in the same way that Pluto didn’t. If this is correct, the presence of a relic ocean in Charon strengthens the case for an ocean in Pluto’s depth considerably. Charon, the ferryman of the Styx, may again have run out of underground water.
So many reasons to go back.. We really need an orbiter around Pluto but this is hellishly difficult because of the amount of fuel that is needed to slow down on arrival. Aero-braking may help, having the mission dive into Pluto’s tenuous atmosphere, but in the end it is all about the fuel. It would need to be a minimalistic mission: low weight, basic instrumentation only, and using several gravity assists to get on the way. The travel time could be decades. But why not? Pluto can wait. It can be our gift of discovery to today’s children.
Exploring the underworld
The Judgement of Jupiter is a story about a conflict of philosophy. During the Middle Ages and before, the world’s riches were seen as belonging to Mother Earth, and we explored and used them at our peril. They were not ours but belonged to another realm. The new philosophy saw the riches as meant for us; we have every right to take them for our use. The natural world (as we now call Mother Earth) has no intrinsic value. This conflict of world views continues today. We could not live without mining Pluto’s old realm, yet we are damaging the world we live in by using these resources. The story still rings true, including its ambivalent conclusion. In Pluto’s new realm, things are so much clearer. Its resources are beyond our use and are safe from us: we can only look, study and learn – and admire. Out there, we are only observers, and see a world beyond our powers, at the limit of our understanding. Jupiter would have approved.