Sunday, 31 January 2021

The Many Mysteries of Darwin IV - Part 2

For part one go here

After a long absence I am finally back to deliver part 2 of this series. In the meantime the reprint of Expedition, the occasion which has inspired these posts, has finally come out and you should definitely check it out if you have not already. After the last part, in which we have examined the orbital mechanics and physical properties of the planet and its parent stars, we will now take a look at the atmosphere and geology of the planet Darwin IV.

My copy of the reprint has finally arrived!


Little is said about the air of the planet in detail, other than that it is denser than Earth’s and that it is very rich in oxygen, both of these characteristics being the main reasons why Darwin IV is capable of hosting not only very large, but also floating organisms. The book however gives us pretty much no figures or hard numbers, such as exact atmospheric mass or composition. The best I could find is again that DVD-brochure for the documentary, which I showed in part 1. It says that the atmosphere has twice the density of that on Earth. If we take this at face value, we may calculate some interesting things. The density of Earth’s air at sea level and at 15 degrees Celsius is 1.255 kg/m3. If we take twice the amount of that, 2.45 kg/m3, and multiply it by the universal gas constant and by 288.15 Kelvin (15 degrees C), the air pressure at Darwin IV’s sea level (whatever that might be since the planet has no oceans) would have a value of about 202’654 Pascal. Note however that this assumes dry air, the calculation for humid air would be more complicated and require data we do not have, though there are a lot of signs that the air on Darwin IV is indeed very dry compared to Earth. I also assumed that Darwin IV has on average temperatures like modern Earth, though we do not get much information on that other than that the climate is “temperate” (Barlowe 1990, p. 17). The air-pressure we calculated equates to 2 atmospheric units or 2.07 bar at sea level. This may at first not sound like a lot, but on Earth this would be the pressure you would feel if you were to dive to a depth of 10.3 meters underwater, twice the depth of your average pool. While this is a joke compared to what one would experience on Venus, roaming the surface of Darwin IV might nonetheless feel more akin to swimming or wading than walking. This, together with the low gravity, certainly does give credence to the sizes and locomotion of the aliens. But again, all of this is based off a DVD brochure, so take these values with a grain of salt.

While Darwin IV has a denser atmosphere than Earth, its surface area is more similar to that of Mars, so calculating the total mass of the atmosphere was a bit tricky. What I did is open Universe Sandbox 2 and create a mock-Darwin IV in the program with all the geophysical properties discussed in part 1. I raised this planet’s air-pressure up to 2 atm and this resulted in an atmospheric mass about 90% that of Earth’s. That is quite a lot for such a small planet, something you should keep in mind. What this atmosphere is made out of is another question. Again, the book is unclear on this, other than that there is a high concentration of oxygen. The documentary barely goes into more depth, but for a few seconds it shows us this graph, which I photographed below from my TV screen:

Unfortunately, the axes are not labelled, so I am unsure what is actually being depicted here. Perhaps the Y-axis is meant to be the height above ground in kilometers, but what is the X-axis then? It is not even said if this is actually meant to be a graph they made up for Darwin IV or if this just some stock image. If you can make more sense of this than I do, please tell me. Without definite values to deduce from this, we have to guesstimate the atmospheric composition by observation. Given how life on the planet’s surface seems to be working off biochemistry not unlike our own, the overall composition cannot be too different from Earth. The majority is therefore likely made up of nitrogen, the rest of oxygen and then a small amount of trace and greenhouse gases. One of the perhaps oddest things we can observe about the climate of Darwin IV is that it rarely rains. This is emphasized in the documentary, where it is addressed that the probes never encounter rain on their entire journey. It does rain once in the book (Barlowe 1990, p. 110), but above the Amoebic Sea, which is (presumably) the lowest area on the planet and is the largest biological structure that would transpire water to the atmosphere. If we also look at the skies of Barlowe’s paintings, we can see that most structures that look like clouds are actually swarms of aerial plankton. We can therefore conclude that the air on Darwin IV holds very little water vapor, which has implications for its atmospheric evolution and its greenhouse effect. This is corroborated by the fact that most of the flora is of the succulent type (Barlowe 1990, p. 15). If this dryness really is the case, it does however raise the question of where all the aeroplankton and floating aliens get their water to live from. Perhaps the air does have decent amounts of water, but like the water on the ground, it is locked up in the aerial biosphere itself, meaning the majority of the water is encased in all the aeroplankton and other organisms and simply gets recycled through the food-chain. However, on one of the title pages (Barlowe 1990, p. 5) we also see some typical large fliers simply landing next to a waterhole and drinking from it, so perhaps many of the other aerial creatures, even if they do not appear to have landing-organs, do have the ability to simply ground themselves and lap up some water from local oases.

The Geology and Geography of Darwin IV

 

Fig 3: The map of Darwin IV I pieced together with my lousy scanner. The black bar indicates where features got distorted by the spine.


The most obvious thing about the surface of the planet is that there are no oceans, save perhaps for the Amoebic Sea, which one might argue cannot even be called a sea. It is however heavily implied for this to not always have been the case, with the current savannahs likely once having been seabeds. The current surface of the planet seems to be relatively young, as Darwin IV has few craters on its surface. This is a sign that the planet is still geologically active, as the surface gets continually renewed through erosion and endogenic processes, like on Earth or Venus. Objects with very little geologic activity, such as Mars, Mercury or the Moon, tend to be plastered in craters they have amassed over the eons and never got rid of. One of the most conspicuous features that rarely gets talked about (perhaps because it was not prominently featured in the documentary) is the large mountain chain which is centered around the equator. I find this very weird, but unfortunately this placement is never elaborated upon. How did it come into that position? One of the first things Darwin IV’s mountains reminds me of is the real-life moon Iapetus. The third-largest moon of Saturn has a somewhat similar mountain chain (or really ridge), which rises 13 km above the surface and almost perfectly girdles the moon’s equator. Unfortunately, the reason for this ridge’s existence remains controversial. One hypothesis is that the ice moon used to rotate faster and was more plastic in its past and as it slowed and cooled down, the ice-sludge that centrifugal forces brought to the equator began to freeze in place. Since Iapetus is an ice moon and Darwin IV is a rocky planet we can obviously not use this as an analogy however. Another hypothesis is that Iapetus’ ridge is an actual mountain chain created by convective forces under the surface and that these pushed towards the equator due to coriolis forces. This might be more applicable to Darwin IV. The text does at least insinuate something to that extend:

The mountains of Darwin IV are relatively young, sharp-edged and jagged, though not terribly high. Very few peaks remain sheathed in snow through the year. Looking all the more forbidding for their bareness, they form a planet-girdling band, evidence of Darwin IV’s active sub-continental shield regions.” (Barlowe 1990, p. 129).

On that note it is interesting to ask whether or not Darwin IV has plate tectonics like we know them and if these “sub-continental shield regions” are actual tectonic plates. As mentioned in part 1, the internal structure of the planet is, based off its density, likely considerably different from Earth’s. Looking at the map of Darwin IV it is also hard to recognize any features one would expect from plate tectonics, especially if you compare it with maps of Earth where the oceans are digitally removed. There is a large valley on Sinus Columbus, which seems to be splitting the mountain chain in half, however that is not how a typical diverging plate boundary would look, as those usually form volcanic ridges in their midline, the Mid-Atlantic Ridge being the most prominent example. Perhaps it is just the dried-up bed of an ancient river. One can see other ridges and rifts, but these are curiously arranged perpendicular to or bisecting the mountain chain. I we assumed that the mountains formed through plate-collisions, these features should instead be arranged in parallel to the mountain chains.

Fig. 4: Image of equatorial ridge of Saturn’s moon Iapetus, taken by the Cassini space probe.

Perhaps more similar to Darwin IV’s mountains than the Iapetus Ridge are instead the highland regions of Venus, such as Ishtar Terra or Aphrodite Terra. While Venus does show signs of obvious tectonic deformation on its surface, it is a debated question whether or not it actually has plate tectonics like Earth, as, like Darwin IV, it has no obvious signs of spreading ridges or subduction zones. It is thought by some that Venus has no real asthenosphere and that its surface tectonics are caused by direct mantle upwellings which can either compress, spread thin or delaminate the overlying crust from underneath. Another, though related hypothesis is that Venus does have a form of plate tectonics, but that its plates are all micro-sized and paper-thin and/or that all the action that happens is buried underground, with the topmost surface layer being too buoyant and stiff, due to its dryness and heat, to move with the plates underneath, therefore getting deformed in weird ways through delamination. This does sound a lot like the “sub-continental shield regions” Barlowe writes about. Like Venus, Darwin IV may currently not have Earth-like plate tectonics due to the lack of water on its surface. On Earth the abundant presence of water in the crust not only “lubricates” the movement of the plates, but also creates different densities when new crust is formed. These density-differences are what allows oceanic crust to be subducted by continental crust in the first place. Applying similar models and what we learned from Iapetus to Darwin IV, we might speculate that centrifugal forces create either an upwelling or downwelling region under the equator of the planet, which causes convective currents to delaminate the crust at the polar regions and compress and pile up material at the equator. Local hot spots then may also compress and spread out the other plains, valley and rift regions we see, such as Sinus Columbus.

Fig. 5: Topographic map of Aphrodite Terra on Venus. There is something oddly scorpion-like about it.

Applying Venusian geology to Darwin IV may however be problematic. The global conveyor belt of Earth’s tectonic plates allows for the heat caused by radioactive decay in the planet’s interior to escape safely and gradually. On Venus however, the heat just keeps building up underneath the crust, to the point where it cannot be stored anymore and violently erupts in what is called a global resurfacing event. The last time this happened on Venus was 800 to 200 million years ago and it is thought that during this event the majority of the planet’s surface was covered by streams of lava as deep as 5 to 10 kilometers (Grinspoon 1997, p. 262). Could Darwin IV have suffered from a similar episode? The land surrounding the mountain chain does look suspiciously smooth, but that may be explained through a whole lot of other reasons. More importantly, it is highly doubtful if any higher lifeform on the planet would have been capable of surviving such an event (unless maybe all the terrestrial organisms we see descend from air-dwellers). If Darwin IV ever had such a resurfacing event, it must have been in the very distant past, long before complex life evolved and the oceans vanished. More likely than that, it never experienced a resurfacing episode, for the simple fact that it is much smaller than Venus. Darwin IV has only about 16% the mass of Earth and a volume and surface size similar to Mars. That means it has a lot less radioactive material that can heat the core and a higher surface area compared to its volume from which heat can escape. In other words, the planet most likely is never able to produce enough internal heat for a resurfacing event to take place or cooled down before the threshold for such an event was reached. This in turn does however raise the question of how Darwin IV is still geologically very active and has not cooled down to the same degree as Mars has. Keep this in mind too.

Lastly one might ask whether or not Darwin IV has a magnetosphere. The strong magnetosphere of the Earth is caused through an inner dynamo, produced by temperature differences and rotation in the liquid outer layer of the core. As established in part 1, the closest analogy for Darwin IV’s interior structure might be Mercury, which does have a magnetosphere caused by such a dynamo, though only 1.1% the strength of Earth’s. Venus, which may match Darwin IV in geophysics as well, has a negligible magnetosphere produced by the ionosphere’s interaction with solar wind, not by an internal dynamo. The cause of this is thought to be the internal build-up of heat, which shuts down most convective currents at the core. Mars, which is similar in size to Darwin IV, has currently no real magnetosphere to speak of. For all these reasons it seems likely that Darwin IV has at best only a very weak magnetosphere. “Wouldn’t then everything on the surface die from radiation?”, you may ask, to which I would answer: Probably not. The idea that it is the magnetosphere which primarily protects us from cosmic radiation is common, but far from being a fact. As James Kasting points out in his critique of the Rare Earth hypothesis, a sufficiently thick atmosphere is already enough to do that job (Kasting 2001). Our own magnetosphere significantly weakens or may even disappear entirely during geomagnetic reversals, which happen on average every 450’000 years, yet there is no real evidence that these events have ever caused mass extinctions, likely for the aforementioned reason. With an atmosphere of nearly the same mass but twice the density of Earth’s, life on Darwin IV is likely sufficiently protected from cosmic radiation. Nonetheless, the lack of a strong magnetosphere does still have important implications for the atmospheric evolution of the planet.

Piecing together the Past

Not much is said or known about the planet’s past, except for the expedition’s speculation that the grasslands perhaps once used to be sea-floor (with emphasis on speculation), this bit of information:

Evidence in the form of countless fossilized tree stumps scattered throughout the plains has led our chief botanist, Dr. Dorothea Kay, to postulate that Darwin IV was once a far warmer and more humid planet.” (Barlowe 1990, p. 12).

And this speculation on why almost all of the animal life relies on sonar and lacks camera-eyes:

It is the expedition’s best guess that animals who had developed these sensory organs proved too formidable for those creatures with rudimentary optical abilities struggling for life and dominance in Darwin IV’s thick, primordial mists. Now the mists are gone, but so are the optically sighted animals.

We can therefore assume that Darwin IV was not only warmer and wetter in its past, but also used to have a fairly opaque atmosphere. This is somewhat reminiscent of theories about Mars’ early atmosphere. Due to the faintness of the young sun, it is thought that early Mars must have been warmed not only by a strong cocktail of greenhouse gases but also by layers of CO2 ice-clouds. As Forget and Costard describe:

We can envisage a dense atmosphere, rich in CO2, with a tendency to condense into clouds of CO2 ice. These clouds would have been able to reflect and effectively trap thermal radiation emitted from the surface, thereby considerably warming the atmosphere. We might therefore picture a youthful Mars swathed in dense clouds, below which rivers flowed in perpetual semi-darkness.” (Costard 2006, p. 70).

Applying similar conditions to Darwin IV would explain why all the animals seem to descend from ancestors adapted to low light conditions. However, drawing parallels between the atmosphere of Mars and Darwin IV may not neatly work out. Like discussed in part 1, Darwin IV is likely placed more favourably in its star’s habitable zone than Mars, making such atmospheric chemistry unnecessary. Unlike Mars, the alien planet also still has a very dense atmosphere and hospitable temperatures and humidity on its surface. This is likely because Darwin IV has a way higher gravity than Mars (60% that of Earth’s in fact) and was therefore better able to retain its air. A Mars-like model for Darwin IV would therefore not sufficiently explain where its water went and also not why this dark ice-cloud climate stopped at some point. Photosynthetic life would also likely not develop well under such darkened conditions. Perhaps the mists Barlowe writes about are meant to be taken literally and the air used to be filled with tons of water vapor. This might actually make more sense based on what we learned so far and could also explain where all the water went afterwards. Due to the known processes of planetary formation, Venus is thought to once have had large bodies of water, perhaps even oceans, on its surface. Due to its proximity to the sun a lot of that water became vapor in the atmosphere, creating a strong greenhouse effect, as water vapor is a powerful greenhouse gas even stronger than CO2 and methane. The planet heated up to a point where more and more of the water evaporated. This in turn stopped the subduction processes of any of the tectonic plates that may have been present on Venus. Surface crust became saturated with carbon without being recycled, causing CO2 to amass in the atmosphere, worsening the greenhouse effect until all the seas evaporated, creating the runaway greenhouse effect that is responsible for the hellish conditions we see on the planet’s surface today. Even if Darwin IV is not as close to its parent stars as Venus is, we might apply a similar process to the alien planet if we consider the possibility that it was not all that wet to begin with. Based on the map, it does not seem like the majority of the surface was covered by continuously connected oceans, like on Earth, but rather by singular large seas/lakes of similar size to the Amoebic Sea (similar conditions are in fact speculated for primordial Venus). While that would be considerably wetter than the current Darwin IV, it would have made the crust still too dry for subduction to take place, halting carbon recycling, increasing the atmospheric mass through volcanic outgassing and starting a runaway greenhouse effect that evaporated much of the surface water. The planet would have then been dominated by a warm, humid, but also misty hothouse climate exactly as described. Now of course, the first objection one might raise to this scenario is that Darwin IV cooled down again after this happened, while Venus went on to become an infernal hellscape. The reason for this might again be the difference in mass and gravity. Darwin IV and Venus both have wimpy magnetospheres, causing molecules in the upper atmosphere to be easily disintegrated into their components by radiation, but the alien planet additionally also has a lot lower gravity, making it easier for gases to escape the atmosphere. Not to mention that Darwin IV orbits an F-type star, which have a stronger energy output than our own sun and would therefore erode planetary atmospheres even stronger through solar wind. All these factors would help counteract the extreme conditions of a Venus-like greenhouse effect, while at the same time literally bleeding the planet dry. The water in the atmosphere may have been split up by radiation into oxygen and hydrogen, with the hydrogen being carried off to space by solar wind, at about the same rate that it evaporated from the surface. The majority of Darwin IV’s currently high oxygen content may have been produced this way, as, apart from the aeroplankton, the vegetation does not seem extensive enough to account for it. The same process of atmospheric loss may have also happened with the CO2, if it were not already being kept in check by photosynthetic life on the planet. At some point the atmosphere would have simply run out of enough vapor to keep up its greenhouse climate, with what was left being locked up in the biosphere. With most of its surface water and forests gone, the surface now also began reflecting more radiation than it was previously absorbing, increasing albedo and cooling down the planet. Simultaneously the planet’s interior may have significantly cooled down, slowing geologic activity and in turn volcanic outgassing. In time the planet cooled down enough that most of the water that was left became concentrated into polar ice caps, increasing the albedo even further. The planet now went from a humid hothouse to an arid ice age climate. The aliens found themselves now in a world without primordial mists but also without much rain or forests. 

Fig. 6: Even the highest mountain-peaks on Darwin IV often lack snow, a strong sign of lacking humidity in the air.


The last major question one might ask here is how old the planet is and when approximately these events may have taken place. Since the world seems to already be past its prime one would assume that it is already quite old by our standards. The age of the planet is unfortunately given neither in the book nor the documentary. The only hard number we ever get is again from the DVD brochure, which states the planet is 2 billion years old. For us humans, who live on a 4.56-billion-year-old planet, this does seem surprisingly young. Should we therefore simply dismiss this as non-canon? Do not forget that Darwin IV orbits an F-type main sequence star. Our own sun, a G-type star, may have a total lifespan of about 10 billion years, with the Earth becoming uninhabitable long before that however. By comparison, F-type stars remain stable for only 2 to 4 billion years (“a candle that burns twice as bright burns half as long”). Assuming the latter end-date for Darwin A, Darwin IV being only around 2 billion years old places its age in the middle of its sun’s lifespan, just like Earth, making this actually a quite plausible age. It would also explain how the planet is still geologically active despite its Mars-like size, as it simply has not run out of energy yet. Obviously, you may object to this by saying that the life we see on Darwin IV is way too advanced for such a young planet. The planet is arguably in its “dinosaur-stage” of development, something stated by the author himself, or even its stone age if you take the Eosapiens into account. On Earth nearly the first 4 billion years were dominated by unicellular life, with multicellular organisms only taking over in the last 500 to 600 million years and the first terrestrial megafauna arguably appearing with the Late Permian therapsids 260 million years ago. But who is to say that the long tyranny of microbes will be a universal constant on other planets as well? Consider for example the Francevillian biota, a group of fossil organisms I talked about before. These fossils from Gabon appear to be multicellular, eukaryotic organisms which lived 2.1 billion years ago, or in other words when the Earth was just 2.4 billion years old. The oldest geochemical evidence for eukaryotic cells in the form of steranes is 2.7 billion years old (Knoll 2003, p. 94), meaning these organisms evolved “just” 600 million years after the first complex cells showed up. More astounding is the find of a multicellular algae fossil from the Russian Kola Peninsula, which was found in rocks dating to 2.45 billion years ago (Rozanov et al. 2013). This halves the gap between the oldest known eukaryotes and the appearance of multicellular organisms to 300 million years. Unlike these algae, the Francevillian biota seems to have gone extinct shortly after it evolved due to a global anoxic event. If that had not happened, there is nothing that says that these organisms could not have diversified further and begun their own version of the Cambrian radiation, all the way back in the Paleoproterozoic era. Findings like these show that complex life may have had the chance to develop all throughout the Precambrian eons, but simply got unlucky. This perhaps shows that Earth may actually be less hospitable to complex life than other planets. The appearance of eukaryote-type cells themselves is generally thought to be an endosymbiotic chance event and may happen at random times on different planets. On Earth they appeared around 1.8 billion years after the planet’s formation, on Darwin IV (which like Mars may have cooled down and become habitable quicker than Earth), they could have appeared just 1 billion years after its formation. Assuming an optimistic scenario we gleaned from our own fossil record, first multicellular life could have then appeared just 300 – 600 million years afterwards and if it were lucky enough to not get completely wiped out it would have had an additional 400 - 700 million years to evolve and diversify into the forms we now see on Darwin IV. That is of course assuming that cellular life on the alien planet works by the same prokaryotic-eukaryotic divide as ours, but it may very well not. Perhaps biochemistry on Darwin IV was weird enough that even the oldest cell-types had sufficient pre-adaptations for evolving multicellularity, making the time of appearance of eukaryote-type cells a moot point. One might also consider Darwin IV’s weak magnetosphere. While not enough to cause mass extinction, the biosphere on average might still receive more radiation than it does on Earth. This would heighten the mutation rate of the lifeforms, which in turn would accelerate the rate of evolution, further shortening the time between major evolutionary events. But for simplicity’s sake let us work with the aforementioned scenario. Assuming that the mass-diversification of multicellular organisms corresponds with oxygenation events, this rough timeline may help us constrict the times of atmospheric evolution. Like mentioned before, most of the oxygen in Darwin IV’s atmosphere may not come from drawn out photosynthetic processes, like it did on Earth, but instead from the splitting of water vapor in the upper atmosphere during its phase of intense greenhouse climate. The first appearance of multicellular life may therefore very well correspond with the beginning of this intense greenhouse climate. As this was shortly followed by the evaporation of most seas, it would have forced the first animals and plants to quickly adapt to life on the misty land (which is not problematic, seeing as how some of the oldest terrestrial trace fossils on Earth date as far back as the Cambrian). When exactly the change from hothouse to coldhouse climate happened is hard to say, though the hothouse climate seems to have persisted some considerable time after the seas had already evaporated, seeing how there are fossilized forests at the bottom of former seabeds. The vanishing of the mists also seems like it could not have been that long ago, as all the animals, except for the Rimerunner, work with sensory organs still adapted to that environment.

Based on everything we have discussed I have made this very basic timeline:



Now that we have most of the basics down, in the next part we can finally analyze the life on Darwin IV in more detail. Thank you for reading and see you until then.

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Literary Sources:

Papers:

Image Sources:

  • Fig. 3: Barlowe 1990, p. 2 – 3.
  • Fig. 4.: Wikimedia
  • Fig. 5: Wikimedia
  • Fig. 6: Barlowe 1990, p. 118.