Now narrated by Dagoth Ur

Welcome dear reader, friend or traitor, come. Contrary to popular belief, I, Dagoth Ur, was not slain by Nerevar at the end of Morrowind's Tribunal period. At least not in spirit, for I am a god and I was reborn in this new dimension as the creature which you call the Sivgin. While the loss of Akulakhan and the Heart of Lorkhan has struck me dearly, I have been able to take up the more peaceful pasttime of "speculative evolution". After all, having created the corprus and blight diseases, I already had an interest and experience in manipulating the biological. However, in this life I have become especially infatuated with the potential of the planet which you mortals call "Mars". I hope you have enjoyed my scriptures on it so far.

After having to bear multiple so-called "memes", where my voice and likeness was bastardized by the mongrel dogs of Youtube, I have decided to retaliate and release my own original content to show who the real Dagoth Ur is. For who is better suited to narrate my own writings than myself? Omnipotent, omniscient, sovereign, immutable, how sweet it is to be a god!

Come, Moon and Star. Come and look upon the channel and the playlists. And bring upvotes, I have need of it.

Happy Holidays from Mars!

A good time is coming, I wish it were here

The very best time in the whole of the year.

I’m counting each day on my tendrils and stumps,

The weeks that must pass before Mars’ approaching comes.

Then, when the first green mists begin to come down,

And the heat ray burns sharp and the sky is torn down,

I’ll not mind the screams, though my ear it numbs,

For it brings the time nearer when our invasion comes.

Drain them we shall, blood will stain their filthy soil

While in our breeding facilities their children toil.

Fractarian trees our red weed will generate,

Which with their skulls we shall decorate.

These primitive cattle will cry their last hurrah,

When they hear us scream only:

Taxonomy on Mars

Taken at face value, Martians are not all that different from Earthlings. They are carbon-based, need liquid water and also require classic elements like nitrogen, hydrogen, oxygen, sulphur and phosphorus to function. But it is the combinations of these elements that makes them different. For storing information and replicating, Martians use a polymer derived from glycol nucleic acid, although billions of years of evolution made it more complex and perfected it so much that it does not resemble anything we were yet able to synthesize in our laboratories. Functionally it works almost the same as DNA, though the majority of Martian lifeforms have five rather than four nucleotide bases like we do. They also have a simpler, even less well understood, polymer that most likely serves as an analogue to RNA. Genetic studies comparing the heredity of the extant lifeforms to each other have only started in the last few decades. Until then, the reconstruction of the Martian tree of life was largely reliant on anatomic and fossil analysis, which is not always reliable as we have seen in the case of Earth. Two lifeforms looking similar to each other does not automatically mean that they are closely related, due to convergent evolution. Studies comparing embryological development has also been difficult, as we have struggled with raising these extraterrestrial lifeforms in captivity.

Important Kingdoms and Phyla

For the majority of this work, we will be using cladistics, a system that classifies organisms based on their ancestry that uses almost infinitely stackable clades, ranked by increasing exclusiveness. But for a start we will still use the classic taxonomic ranks of Linnaeus, such as kingdoms and phyla, as they serve as an easy and illustrative introduction into the basic parts of the Martian biosphere (although not without problems, as we shall see). They are here listed by increasing complexity, but this does not represent a line of descent.

The Tree of Life on Mars (click to enlarge). It is always in flux as our scientific understanding about these organisms grows.

“Domain” Xenovira: If there was ever an example of a perfect waste-basket taxon, Xenovira would probably be it. A waste-basket taxon is usually an unnatural taxon into which various unrelated species are lumped in due to misleading similarities and lazy taxonomists. In this case, Xenovira has become a catch-all term for any extraterrestrial, biologic molecular structure that, as the name implies, even slightly resembles a virus. And by any I do mean any, as even similar structures found in Jupiter’s moon Europa and Venus’ atmosphere have been called Xenovira. They are all characterised by being too simple to be, by most definitions, considered life, but still showing enough reproductive and darwinistic capabilities to be in some kind of pre-biotic limbo. Another defining trait is that all of them are exclusively parasitic, preying on and abusing the metabolisms of actual lifeforms. But that is really where the similarities end. Some resemble classic viruses from Earth, by being genome-strands encased in membranes or protein-shells, while others are just free-floating strands of biopolymers or even just self-replicating proteins, similar to prions like mad cow disease. Some may be survivors of an ancient pre-biotic primordial soup that evolved to prey on later lifeforms, while others may be the descendants of actual life that lost complexity over time as an adaptation to parasitism. Due to this heterogeneity, many astrobiologists have tried breaking up Xenovira into multiple natural groupings. The “true viruses” of Mars, which possess genetic material similar to the other Martian lifeforms and membranes/shells, are called Euxenovira or Areovira. They are predictably the most common pathogen in the Martian biosphere, though they are thankfully not capable of infecting humans, as our DNA and cellular systems are incompatible with theirs. The same is true for any other Martian pathogens, as well as terrestrial ones in reverse, but handling foreign microbes or introducing our own is still considered dangerous for both parties, as, even if we cannot infect each other, some microorganisms might still release waste products that are unexpectedly toxic to other lifeforms.

Domain Nanobacilli: Informally also known as Nanobacteria. This taxon has no real equivalent to Earth-life. Originally grouped among the Xenovira under the name Nanoglobuli, they were long thought to be too simple and primitive to be considered life as we know it, but this quickly changed as we discovered that they met all criteria to be considered lifeforms, as they, unlike viruses, are capable of independent reproduction and metabolism, despite most of them being smaller than 200 nanometers. This is rather challenging, as the existence of “bacteria” in this size-range was previously deemed impossible, since the bodies of such organisms were considered too small to hold all the necessary molecular machinery for independent replication. This may have been true for terrestrial biochemistry, but apparently things on Mars work a little differently. By some poorly understood process the Nanobacilli seem to have adapted an internal machinery that consists of protein-based alternatives to the genome used by most other Martian life, allowing them a rudimentary metabolism and reproduction without having classic genetic material. Yet, they are all encased within membrane structures closely matching those of areonts, making it possible that they descend from more complex cells that have simplified to an extreme degree. In a sense they are a prion trying to be a bacterium or, rather, the other way around. Nanobacilli can live in a variety of lifestyles and habitats, though autotrophy is relatively rare. Commensalism with more complex organisms is instead the most common way of life for these organisms.

Domain Areonta: Areonta are the closest analogues on Mars to earthly prokaryotes (Bacteria and Archaea). Although still single-celled, they are usually far larger and more complex than Nanobacilli. They either have heterotrophic or autotrophic lifestyles, reproduce asexually, are one of the major backbones of every ecosystem, make up the majority of Martian biomass and have probably ruled this planet for over four billion years. Just your average “bacteria”. In an interesting contrast to terrestrial prokaryotes, areont cells have a significantly higher propensity to evolve intercellular membranes, meaning walls and tendrils that extend into the cytoplasm. These internal membrane-extensions function more similarly to the endoplasmatic reticuli of  Earth’s eukaryotes and often serve as “work-spaces” for enzymes and proteins. Why Martian prokaryotes have a higher tendency to evolve such intercellular membranes than Terran prokaryotes is not known. This might simply be a consequence of differing biochemistries. All higher lifeforms on Mars and likely even the Nanobacilli are descendants of the Areonta. The Kingdom could therefore be considered a paraphyletic and therefore unnatural clade, as it does not include all of its descendants. Important phyla include the Phytoareonta (photosynthetic cells similar to cyanobacteria), the Methanoareonta (methane-producers) the Perchloareonta (perchlorate consumers).

Kingdom Macroareonta: Macroareonta are what one could call a multicellular prokaryote, as they are not just simple cell-colonies, but complex organisms made up of differentiated cells, but their individual cells are still structurally areonts. A “multicellular bacterium” does indeed sound strange, but there are similar lifeforms even on Earth, such as the prokaryotic Myxobacteria and several species of Cyanobacteria that have multicellular life stages, complete with cell-differentiation and fruiting bodies. Macroareonta are similar to these, although far more widespread on the red planet and more complex, some resembling microscopic versions of plants, fungi and even animals. Despite this complexity, none of the known species grow larger than 15 centimeters. Some larger fossil relatives possibly show that this did not use to be the absolute size-limit. If this is true, modern members instead seem to be adapted for smaller niches due to competition with Rhodokarya. Rhodokarya have on average much larger cells than Macroareonta, meaning that a macroareont of the same size as a multicellular rhodokaryote is at a significant disadvantage, since its body is made up of many more cells, making it more prone to cancer and similar issues. Macroareonta seems to be polyphyletic, as apparently not all of its members have the same common ancestor, rather several lineages of Areonta evolved multicellularity independently from each other. The Macroareonta were long thought to be transitional forms between Areonta and Rhodokarya, but this seems increasingly unlikely, as Macroareonta lack cammaculae and the ancestral rhodokaryotes seem to have been unicellular.  Some extant photosynthetic species build shells made of silicon, similar to diatoms. Entire sediments made of these silicon shells have been found in marine fossil assemblages, showing that Macroareonta once constituted a major part in ancient planktonic communities, making them important for biostratigraphy. The clade of the filulithophores are today also among the most important nitrogen fixers.

Domain Rhodokarya: Rhodokaryotes are characterized by their cells having something akin to mitochondria (in their case called cammaculae) that engage in oxygenic respiration, as well as other organelles, which is why many declare them the Martian analogue to Earth’s eukaryotes. They encompass many taxonomic kingdoms that come in unicellular, multicellular and intermediate forms and constitute the majority of the macroscopic biosphere, though they have lost a lot of their former glory. It was originally thought that their organelles evolved out of Gigananobacilli, the largest known Nanobacteria, originating through an endosymbiotic event, where an areont cell was permanently inhabited by a nanobacillus, either through symbiosis or perhaps even through parasitism. The parallels of this idea to Earth’s eukaryogenesis, where an archaean cell was entered by a bacterium, is obvious (and might reflect some form of chauvinism by earlier researchers), but recent research has cast significant doubt on this hypothesis. The cammaculae bear no resemblance to Nanobacilli, do not float freely inside the cytoplasm, have no genetic material of their own and are not shared between cells during reproduction. Instead, they develop directly out of the inside of the cell membrane after mitosis. The more likely scenario is that, as Mars gradually built up more oxygen in its atmosphere, the ancestral hydrogenotrophic areont cells began to protect their anaerobic methanogen-metabolism by folding their intercellular membranes onto themselves to create a separate chamber inside the cell. The protein-studded walls of the resulting chamber could then effectively filter out free oxygen to create a controlled anoxic environment inside the chamber while the rest of the cell and the environment were oxygenated. Afterwards, aerobic-respiration may have originally evolved to more effectively eliminate free oxygen from the cell, before this gradually became the main metabolism. What corroborates this are recent genetic studies which show that rhodokaryotes which only engage in hydrogen respiration are actually more ancient than oxygen respiring ones (and not the other way around, as originally thought by analogy to Earth). The fact that life on Mars evolved the “complex cell” completely autonomously and not through an “accident” like Earth’s eukaryotes seems to have had a tremendous effect on the story of the biosphere. Biogeochemical signs of rhodokaryotes show up in remarkably ancient rocks 3.4 billion years ago (Sivgin 2345), far older than the first signs of eukaryotes on Earth, and as fossils show, this seems to have facilitated the evolution of complex multicellular life only about four hundred million years later (interestingly, the time of appearance between the first rhodokaryotes and their first multicellular members is similar to that between the first eukaryote cell and the Francevillian biota in Gabon).

Superkingdom Proteroareozoa: Try saying that three times fast. Proteroareozoa is technically another paraphyletic taxon that encompasses several kingdoms of unicellular rhodokaryotes, far too many to mention them all here. The many lineages of Proteroareozoa can be distinguished from each other by their modes of life, cell-structure and the way they move using their flagella. All rhodokaryotes ancestrally reproduce sexually, although asexual reproduction has independently evolved in multiple lineages. A prominent group of proteroareozoans are the flechtoids, a semi-multicellular group which resembles a mix between slime molds and lichen.

Kingdom Arephyta: Arephyta is the kingdom that includes large, multicellular, photosynthetic rhodokaryotes that share one common ancestor. They are in some ways the Martian analogue to plants and algae, by having cells with cell-walls and being immobile autotrophs. However, they are not actually the dominant constituents of the global flora. They originally seem to have been, during the Thermozoic Era (Sivgin 2345), but have gradually declined in favour of Spongisporia and Fractaria. A plausible explanation for this might be their metabolism. Arephyta do not engage in oxygenic photosynthesis like Earth-plants, but instead use sunlight to react hydrogen sulphide with carbon dioxide to generate sugar and, as a waste product, elemental sulphur. This is a form of photosynthesis only used on Earth by some of the most archaic organisms. Arephytes are of a principally two-layered build, with an external germ-layer consisting of oxygen-respiring cells, which protect an inner layer of anaerobic cells that are specialized for this mode of photosynthesis. Such a metabolism seems to have been viable even on land in the earlier periods of Mars, as constant volcanic outgassing continuously enriched the atmosphere with sulphuric gases. But as volcanic activity died down, the arephytes lost much of their livelihood and the increasing oxygenation likely made it even more difficult to maintain their metabolism. Today, Arephyta without special adaptations, such as the divisions (the botanic equivalent to a phylum) Porphyta, Cochleophyta and Arechlorotia, are solely found close to the hangs of active volcanoes or along geothermal hot springs. More derived groups have gained a more flexible range by living in endosymbiosis with sulphur-reducing areonts, which recycle the waste sulphur back into hydrogen sulfide. Part of this group are the Arthrophyta, which are notable for possessing a segmented body with both internal and external bilateral symmetry, something not seen in any living plant on Earth. Lastly there are also the Pennatophyta, possible offshoots of the Arthrophyta, which have lost their autotrophic lifestyle and have instead become heterotroph detrivores, fulfilling a similar ecological role on Mars as fungi do on Earth. Unlike Earth-plants, Arephyta have no alternating generations.

Kingdom Spongisporia: Like the Arephyta, spongisporians possess cell walls (except for the dedicated filtering cells), though composed of different polysaccharids than in arephytes. Spongisporia have no true differentiated tissues and they are immobile their whole life, but since they are heterotrophic, they are not reliant on sunlight or chemosynthesis. These traits combined with their rhizome networks makes it easy to draw parallels between these lifeforms and Earth’s fungi, but the similarities are superficial. Fungi can exist both in unicellular (think yeast) and multicellular states and are mostly made up of loosely connected hyphal networks without a central body (mushrooms are just spore-bearing fruiting-bodies that grow out of the mycelium for mating). Spongisporia on the other hand are exclusively multicellular and have a more defined morphology. They have a central, often tube-like trunk and a root-like network of hyphae underneath. The earliest members of this group seem to have lived on the seafloor (Sivgin 2345), had porous walls with a hollow central-cavity, a sclerite-skeleton made of silicon dioxide and ate by filtering the seawater with cilia. This made the earliest spongisporians, as the name implies, more similar to Earth’s sponges and indeed many extant members still have the characteristic porous body-cavities (although they now serve a different function) and sclerite-skeletons.  Consequently, the majority of spongisporians live as filter-feeders, not just underwater in cave systems and glacial lakes, but also on land, where the low gravity allows for the existence of an aeroplanktonic community. Some forms also live in symbiosis with photosynthetic areonts, essentially being giant sponge-lichen. Spongisporia reproduce through spores and have haploid and diploid generations, which often differ greatly from each other.

Kingdom Fractaria: These are a lot more complex than Spongisporia and Arephyta, with clearly visible symmetrical builds and differentiated tissues, but they are still too “primitive” to be comparable to animals. The majority lack a gut, mouths or any digestive organs, as well as the cell walls and hyphal roots of the spongisporians. While they ancestrally were filter-feeders like the Spongisporia, many have also become autotrophs by living in symbiosis with colonies of Areonta that live in their tissues. These forms live off osmotrophy, chemotrophy and even photosynthesis. Because they are relatively easy to keep in captivity, their biology has been well researched. A Fractarian starts out life as an immobile propagule (either by budding off the stolon of a parent, as a free-floating spore or in more derived species from a capsule sometimes called an egg or a seed) which grows into a base not unlike that of a spongisporian embryo. Out of this, a fern-like frond develops with a simple internal system of interconnected filtering-chambers. This frond gives rise to another frond, which also gives rise to another one, until the whole organism ends up with the characteristic glide-symmetry that the group derives its name from. Such basic fronds seem to be the simple building block of this kingdom, with most members having found some way of iterating in remarkable ways on this template, for example by evolving multiple vanes or increasingly more fractally growing fronds. The majority of the “higher” Fractarians however reach their complexity instead through colonial living. Like corals, seapens or sessile versions of the Portuguese Man O’War, many clones growing from a single ancestor grow into larger, plant-like superorganisms. An important group among these colonial forms are the Hylozoa, which have evolved outer walls made of cellulose, similar to wood, which allowed them to colonize terrestrial habitats and through symbiosis with Phytoareonta have become a major part of Martian flora, in the past even growing into forests (Sivgin 2345). Fractaria are generally not classified as Martian animals, since at no point in their lives do they have a mobile stage. The sole exception to this, as has only recently turned out, is the superphylum Pseudarticulata. Originally classified as primitive Arezoans (Martian animals proper), we now know that these are actually fractarians, which evolved from solitary fronds that found a way to use the filtering cilia on their skin to lie flat on the ancient seafloor and crawl about the microbial mats like giant amoeba. Out of these primitive forms eventually derived descendants, which, independently from Martian animals proper, evolved their own versions of nervous systems, muscles and sense organs. Pseudarticulata, with their fake-segmented body, today largely resemble flattened worms and maybe arthropods. Their most successful and famous clade are the Chirorbita, sometimes nicknamed “Martian trilobites”, which have evolved a tunicine exoskeleton, camera-eyes, and move about with hydraulic tubefeet similar to starfish.

Kingdom Arezoa: In older literature also called Zoomimia/Zoomimida, the “animal mimics” of Mars (technically an alien creature cannot be called an animal, since an animal can only be a member of Animalia, a taxonomic kingdom from Earth. However, for convenience, Arezoa will still be called animals here). These naturally make up the bulk of what can be called the planet’s fauna. They have differentiated tissues, lack cell walls, are heterotrophic and are all capable of free movement at least at some point in their life cycle. Interestingly though, the majority of them possess metabolic pathways that are capable of recycling much nitrogen from their waste, which makes them more similar to Earth-plants than to Earth-animals. The exact number of arezoan-phyla that exist or have existed on Mars is a debated question. Therefore, we will only list the most important phyla here:

• Mollizoa
• Brachiostoma
• Aspiderma
• Spiriferia
• Trichordata
• Onychognatha
• Antitremata

Instead of discussing each in detail here, you will get to know them by example in the upcoming pages. The exact origin of the Arezoa is a controversial issue as we lack a lot of fossil or embryological data. Anatomical comparisons predict that the Fractaria are their closest relatives, but researchers have also noted similarities to the Spongisporia. It is interesting to note that, unlike most animals on Earth, the arezoans are predominantly hermaphroditic, with only a few known species having something akin to differentiated sexes. During mating either both partners get pregnant or, what more often happens in the more derived groups, the partners first fight each other to determine who gets to impregnate the other. Many of the simpler forms also practice self-fertilization, similar to Earth-plants. Why this difference exists is unknown. There are hypotheses whereby the difficult inheritance of mitochondria between cells is what drove sex-differentiation in Earth’s eukaryotes. Since rhodokaryotes do not have true mitochondria, the true need for sexual differentiation may have never arisen on Mars.

References:

• Sivgin, T.K. : Life on a Dead Planet. The first 3 billion years of Evolution on Mars, Zürich 2345.

Introduction II: Brave New Mars

[Author’s note: Some of the things mentioned here about modern day Mars are very much true, but others are very much not for the sake of making this scenario work. Some things, such as the atmosphere and temperatures, are extrapolated either from pre-Mariner-era papers (such as Salisbury 1962), current work about prehistoric Mars billions of years ago (Ramirez 2017 and especially Palumbo & Head 2018) or are completely made up by me. For immersion’s sake, I did not bother to mark which information is which, you should therefore never use this page as an actual reference for real life current day Mars and instead do your own research.]

Welcome to Mars, ladies and gentlemen. My name is Ogilvy, retired lead-astrobiologist on the Horus-2 Operation. After Horus-1 had already successfully brought the first humans to Mars and made contact with native life, the goal of Horus-2 was to resupply the Horus-1 teams, establish permanent research settlements and to further study the fauna and flora of the planet, which I believe we did a great job at. Horus-2 consisted of five orbital stations powered by nuclear-electric propulsion, each coming with smaller landing crafts attached. The stations were assembled in Earth-orbit and possess each a nuclear reactor at the bottom.

Fig. 1: Horus-22, my old ship, drawn from memory.

Apart from providing heat, the reactor evaporated silicon oils into steam, which then rises through a central shaft to then power a turbine, which generates electricity. The steam that has already passed through the turbine is funnelled into a giant, circular cooler, where the frost of space can condense it again to flow back to the reactor. The main method of propulsion of the stations are ion-thrust chambers, where the generated electricity is used to charge a platinum grid. Through this grid is then blown evaporated caesium. The charge ionizes the caesium-atoms, which are then blasted out into space at a rate of billions per second, thrusting the spaceship forward at incredible speeds. At the top of the ships, opposite the reactor and shielded from it by the cooler, were the living modules that could house teams of up to twenty astronauts and generated artificial gravity through centrifugal rotation. Atop the modules sat the auxiliary landing gears. After circling Earth for four months, enough speed was generated to escape its gravity and slingshot towards Mars for seven months. Once its gravitational field was entered, two more months of deceleration were needed to sink into a lower, stable orbit around the red planet. Once settled, the landing gear was manned and each ship’s crew landed in a different region of Mars. My team, the crew of the ship Horus-22, landed in Arabia Terra to successfully make contact with the previous crew of Horus-14. Despite minor setbacks, the Horus-2 Operation was an outstanding success and laid the most crucial groundwork for all future Horus missions.

The Basics of Mars

Mars is the fourth planet from the sun and the outermost of the rocky planets. Compared to Earth, it has a significantly more elliptical orbit, with Mars being 1.38 astronomical units close to the sun at perihelion to then retract as far as 1.67 AE at aphelion. A year on Mars lasts the equivalent of 687 days (1.88 Earth-years). The day-length on Mars is on the other hand close to that of Earth, lasting 24 hours and 38 minutes (meaning a year actually lasts 668 Mars days). What is more difficult to adjust to for astronauts than the daylength is the low gravity. Mars is only about half the diameter of Earth and thus has only about 38% of our homeworld’s gravity or 0.38 g. Mars has no large moon like Earth, instead only two smaller satellites called Phobos and Deimos, which might either be captured asteroids or the remnants of proto-planetary collisions. The magnetosphere of Mars is weak and partially regionalized by magnetised rocks.

Geography and Geology

Modern Mars lacks any oceans or comparable large bodies of water. Today, the Martian surface consists largely of layers of volcanic rock, lain over by thin sheets of sand and dust. These are what mainly gives Mars its red coloration, as they consist of eroded iron oxides. In general, the Martian crust is much richer in iron than that of Earth, likely because the lower gravity did not pull most of the iron atoms towards the planet’s core during its creation. Under the volcanic layer are often deep pockets of ice and, in warm enough regions, groundwater. Under the ice usually lies primordial regolith. In the more poleward regions, the water lies much closer to the surface in the form of permafrost, creating tundraic environments. Sedimentary rocks are mostly common in the northern hemisphere for reasons soon discussed.

Fig. 2: Topographic map of Mars, showing the distinct difference between the northern and the southern hemisphere, as well as the extreme height of the Tharsis Plateau and the depths of the Hellas Basin.

As Mars lacks oceans by which to cartograph coastlines, the planet’s geography is best explained by use of a topographical map. As can be clearly seen, there is a significant dichotomy between the low-lying and almost smooth-surfaced northern hemisphere and the taller, much more cratered and ancient southern hemisphere. If this difference evolved through massive impact events in the north or through internal tectonic processes is not known. Ironically, the highest point on Mars, the titanic shield volcano Olympus Mons, lies in the northern hemisphere, while the lowest point, the Hellas Basin, lies in the south. There is good reason to believe that most of the lower northern hemisphere was once the basin of an ancient ocean, hence the many sedimentary layers found here and the fossil deltas and estuaries that can be found at the north-south boundary. The remnants of this ocean now lie beneath the surface in the form of massive fields of permafrost ice around the polar circles.

Fig. 3: Mars Odyssey’s map of sub-surface water and ice on the red planet.

Mars has stopped having plate tectonics very early in its history, with features like Valles Marineris being the last remnants of such processes. What mostly shapes the surface of the planet are thus impact and dust storm events, the formation and eruption of massive shield volcanoes, erosion and deposition by seasonal streams of water and especially glaciers, as well as the actions of lithotrophic organisms. Geosyncline forces created by the shrinking of the cooling planet may also be an important, though understudied factor. While the crust is mostly inactive, the core remains molten and many of the volcanoes still show signs of fairly recent activity.

Atmosphere and Biosphere interactions

The average air pressure felt on Mars at mean global altitude is 0.51 bar, which is the pressure you would feel on Earth if you were standing on the highest points of the Alps. One may question how Mars has a relatively complex ecosystem with such low pressures while mountaintops on Earth are deserted, but the latter case is usually more due to the topography than the air pressure. Flat plateaus of similar altitudes in the Andes mountains are stable enough to give vegetation room to grow and animals to roam and such is also the case on the wide plains of Mars.

The air on Mars consists mainly of 75% carbon dioxide, 10% hydrogen, 9-10% oxygen, 3% nitrogen and the remaining 2% consisting of noble and volcanic/biogenic greenhouse gases. This is in stark contrast to Earth’s, where nitrogen and oxygen are the main constituents. While the pressure is survivable enough without a spacesuit, under current conditions, an astronaut with a broken breathing gear will suffocate on Mars, as has infamously and unfortunately happened to a crewmember of the Horus-4 Operation that came after us.

The native life is however perfectly in tune with the planet’s nature. The vast majority of lifeforms on Mars, including the multicellular flora and fauna, are either obligate or facultative hydrogenotrophic methanogens. This means they can react the CO₂ of the atmosphere with H₂ to generate the energy they need for living. The waste products of this metabolism are water and methane, which rise into the air as potent greenhouse gases, before the sun breaks them up into their constituents, which are then breathed in again by the lifeforms. Through this cycle, life on Mars has apparently been able to keep conditions for itself habitable for a long time, though it has not been able to completely stave off the loss of the atmosphere and much of the water vapor to the solar winds.

The large amount of hydrogen in Mars’ atmosphere poses a great mystery. While likely to have been a major part of its primordial atmosphere, like on Hadean Earth, hydrogen is the lightest known element in the universe and the planet’s low gravity would therefore dictate that almost all of it should have quickly escaped into space ever since. That this did not happen suggests either that some atmospheric effect is preventing escape into space or that there is a constant source of free hydrogen resupplying the atmosphere. Neither potential factors have yet to be clearly identified. For the production factor, it has been proposed that some geochemical processes deep inside Mars may be performing natural versions of otherwise industrial hydrogen-generators, such as steam reforming, where heated methane is reacted with steam. Various organisms on Earth are also known to produce hydrogen through anaerobic metabolisms and the same is most likely also true on Mars, though it could be occurring on a much vaster scale deep underground. More radical ideas have also been proposed, such as all of Mars’ current hydrogen coming from a geologically recent event that has been called the “hydrolysis-catastrophe”, where somehow most of the water on the surface was instantly or in multiple phases split up into its constituents. What could have caused this has never been sufficiently explained and it seems highly unlikely that such an event ever occurred given our current evidence. As for the prevention factor, it may be, as mentioned above, that the biosphere recycles the hydrogen before it can escape to space, though this would only work close to the ground. It has been proposed that in the past, when Mars was more habitable, the air was filled by vast swarms of aeroplankton, which would have helped recycle the gas more evenly through the atmospheric column. If true, today’s lack of aeroplankton on most of Mars will have a negative influence on its hydrogen balance in the future, fuelling a self-propelled cycle of worsening habitability. Without hydrogen, many lifeforms will lose a metabolic resource, while the planet will also get drier. While not itself a greenhouse gas, hydrogen’s presence in the atmosphere also lengthens the longevity of methane and ozone, so this trend will also lead to even colder temperatures and worsening UV-radiation.

 Oxygen appeared in the Martian atmosphere only gradually and later in its history and is used only secondarily by its biosphere. Its concentration in the atmosphere is just high enough that macroscopic, multicellular organisms can use it for respiration and also just low enough that anaerobic organisms are not too impeded by its presence. Many of the Martian “animals” possess a complex lung system that can utilize both forms of respiration, though to differing degrees. There are mainly three pathways by which oxygen is produced on Mars and we will discuss these on another page. 

A rather bizarre factor about the atmosphere, and a major challenge to our engines and reactors, is that it is potentially explosive, as hydrogen and oxygen can react violently with each other in the so-called Knallgas-reaction. The autoignition temperature of this reaction is under normal Earth-conditions 500 degrees Celsius, however, and even higher on Mars with its low atmospheric pressure. As most of the planet is very cold and there is not much vegetation to cause wildfires, this reaction does not occur naturally outside of maybe lightning strikes. But lightning is, counterintuitively, not a regular feature of the Martian atmosphere, as it requires a difference in charge between the cloud and the ground separated by a gap of  normally non-conducting medium. Mars lacks this characteristic, as the ever-present dust clouds can fill up the whole atmosphere and act as an electrically conducting medium, leaving no chance for charge-differences between ground and sky to build up (Morden 2021). In the very rare cases where lightning strikes do occur, they are truly devastating. Acts of God is perhaps the only apt description. Thankfully, the fires caused by these explosions are short-lived, as the product of the Knallgas-reaction is water, which douses the flames.

The most major factor that actually impedes life on Mars are the low concentrations of nitrogen (though even on Earth, lifeforms are able to only utilize a very small amount of the nitrogen in the air, with most heterotrophs gaining it through ingestion of other creatures). This has however led to some remarkable innovations by nitrogen-fixing lifeforms.

Weather, Climate, Biomes and Climate Change

Fig. 4: Top: Rough temperature zones on Mars, polar circles excluded. Compare and contrast with the topographical map. White areas encased by a solid black line mark regions of permanent ice. The solid red line marks the 273K-isotherm (0 degrees Celsius). The red dotted line marks the 283K-isotherm (10° C). The black-dotted areas mark regions where the mean annual temperature (MAT) is below freezing point and are therefore dominated by tundra or snow wastes. The blue area between the two isotherms is where the MAT rises above freezing and where meltwater from the highland regions humidifies the ground enough for shrublands to exist, though the actual extent of these biomes is far smaller than the blue area due to the lack of rainfall. In red dotted areas the MAT is above 10°, but the low precipitation only permits steppes, cold deserts and sometimes hot deserts. Only in the Hellas Basin does the MAT reach above 20° and the large amounts of meltwater around the crater create a high groundwater-table. This allows for an environment analogous to a savannah. Bottom: The same map, but with more emphasized color-coding for better convenience.

Mars is generally a very cold place. While the mean annual temperature on Earth is around 14 degrees Celsius, the long distance from the sun and the thinner atmosphere conspire to make the MAT on Mars only about 2° C, only a little bit above the melting point of water. Due to the varied geography, this temperature is however not equally distributed. The southern hemisphere is usually much colder than the northern one, with the geographical delineation between the Northern Lowlands and the Southern Highlands also being the 273 K (0° C) isotherm. Most of the south therefore has an MAT below freezing point and the region is mostly covered in year-long ice all the way up to latitudes 60 degrees south. Temperatures above freezing point occur in most of the highlands only in the warmest parts of summer for only 100 Mars days or less. The exception to this cold southern climate is the quite warm Hellas Basin, thanks to its depth generating higher air pressures and therefore greenhouse effects. Another especially cold region is the very tall Tharsis Plateau, whose volcanic summits stay frozen the whole year. Most of the northern hemisphere meanwhile enjoys mean temperatures that can reach from 10 to 20 degrees. Some of the warmest summer days may even peak above 40. On Earth, such temperatures would be enough to permit trees to grow, however, such vegetation is almost completely absent on Mars due to the aridity. Precipitation in the form of rain occurs only on 0.8% of the planet’s surface and, ironically, only in the Southern Highlands, as the altitude causes adiabatic cooling of the clouds. The majority of precipitation, occurring on about 31% of the surface, instead comes in the form of snow. The north is therefore arid because most of the water that evaporates from it travels south and becomes trapped as snow or ice. Most complex flora is therefore dependent on meltwater coming from the polar ice caps and tundras or groundwater welling up from oases. In terms of the biomes of Mars, as one generally travels from the South to North Pole and ignores special regions, one begins with eternal ice, followed by vast hyperalpine tundra, then thin bands of taiga, shrubland and cold, craggy deserts, flat hot deserts and flat cold deserts or steppes, followed by a thinner strip of flat tundra and then again eternal ice.

Fig. 5: Mars during one of the worser global dust storms. The southern polar cap and the Hellas Basin are barely visible through the veil.

The aridity and lack of deep-rooted vegetation coupled with erosion creates large amounts of dust on Mars, which through summer heat becomes highly activated and can form into gigantic dust storms. These can sometimes envelop the whole planet for a few months. Most organisms have learned to cope with the ever-present dust and some even thrive off it.

The rather bizarre climate dichotomy of Mars’ hemispheres is explained by its seasons. A year on Mars lasts almost twice as long as on Earth and its seasons are caused by its axial tilt. Currently, this tilt is at around 25 degrees, which is not actually that far off from Earth’s value. However, Mars’ orbit around the Sun is far more eccentric than on Earth. These conditions cause the northern hemisphere to be closest to the Sun during its winter and farthest during its summer, creating mild seasons. As a consequence, though, the southern hemisphere is farthest from the Sun during its winter and closest during its summer, making for more extreme seasons, especially combined with the long length of a year. Northern Spring/Southern Autumn lasts 193.3 Mars days/sols, Northern Summer/Southern Winter lasts 178.6 sols, Northern Autumn/Southern Spring lasts 142.7 sols and Northern Winter/Southern Summer 154 sols.

There are plenty of signs that it has not always been like this. Mars, like the Earth, goes through Milankovitch cycles, wherein values like the axial tilt wander between 15 to 35 degrees every 124’000 years, the precession changes every 171’000 years, and the orbit can vary from elliptical like today to nearly circular like Earth every 100’000 years. Sometimes these cycles reinforce each other, sometimes they cancel each other out. There is now strong evidence that between 2 million and 400’000 years ago, a stronger axial tilt and a more circular orbit made Mars go through a warm age, in which the MAT was higher and precipitation by rainfall was more common. Possible fossilized forests in Isidis Planitia may attest to this. Currently Mars seems to be going through a colder phase, with conditions possibly getting even colder in the future, though some models also predict another warm age. As one goes even deeper into the past, changes become more drastic, as evidence amounts that up to two billion years ago an actual ocean had existed on most of the northern hemisphere, the atmosphere was much thicker and global temperatures may have reached 20 degrees on average. Fossils of megafauna and likely ancestors of modern lifeforms are known from this time. We are not currently at the capacity to discuss or interpret these finds in detail, though future volumes such as Sivgin 2345 are currently being worked on and I kindly refer the reader to those. The reason for why Mars has stopped being this Earth-like is likely its lower gravity making space escape of volatile gases in the atmosphere easier. Photolysis by the Sun has broken up much of the former water vapor into its constituents, with the much lighter hydrogen escaping into space and leaving oxygen behind. Though biosphere interactions and the build-up of an ozone layer have partially mitigated this, billions of years of solar bombardment have nonetheless taken their toll.

References:

• Heldmann, Jennifer et al.: FOLLOW THE WATER: APPLYING A MARS EXPLORATION STRATEGY TO THE ARKAROOLA ANALOG REGION, SOUTH AUSTRALIA, in: American Astronomical Society, 6, 2006, p. 71 – 92.
• Morden, Simon: The Red Planet. A Natural History of Mars, London 2021.
  
• Palumbo, Ashley & Head, James: Early Mars Climate History. Characterizing a “Warm and Wet” Martian Climate with a 3-D Global Climate Model and Testing Geological Predications, in: Geophysical Research Letters, 45, 2018, p. 10249 -10258.
• Ramirez, Ramses Mario: A warmer and wetter solution for early Mars and the challenges with transient warming, in: Icarus, 297, 2017, p. 71 – 82.
• Salisbury, Frank: Martian Biology. Accumulating evidence favors the theory of life on Mars, but we can expect surprises, in: Science, 136, 1962, p. 17 – 26.
• Sivgin, T.K.: Life on a Dead Planet. The first 3 billion years of Evolution on Mars, Zürich 2345.

Image Sources:

• Fig. 2: Goddard Space Flight Center
• Fig. 3: Heldmann et al. 2006.
• Fig. 5: JPL photojournal