Perchlorate Deserts

There are principally three sources of oxygen on Mars. The most obvious one is oxygenic photosynthesis, performed by fractarians, some spongisporians and various microorganisms. This source is limited, as the high aridity on Mars prevents the growth of much vegetation and during the exceedingly long winters, some of the flora enter long phases of dormancy in which photosynthesis may be completely halted.

The second source is photolysis. The lack of a strong magnetosphere allows the solar rays to more easily penetrate the thin Martian air, in the process breaking up the molecules inside water vapor into oxygen and hydrogen. In the past, this process may have significantly contributed to the oxygenation of the atmosphere (Sivgin 2345), but the loss of much atmospheric mass and the extreme aridity of the planet have consequently made this a minor factor.

The third source comes from an unlikely location. In some regions on Mars, both in the southern and northern hemisphere, a special type of salt flat desert has formed. Under intense UV-radiation, sodium chlorides, left behind by an ocean that has long since evaporated, react with silicate minerals in the soil and form vast plains of perchlorate salts. Perchlorates occur naturally on Earth only in places like the Atacama Desert or Death Valley and are otherwise mainly produced and used in rocket- and pyrotechnics industries. They are highly toxic to almost all life as we know it, and cause thyroid and lung damage, as well as anemia in humans. The perchlorate deserts of Mars are possibly the most lifeless places on a planet that is already pretty dead. This is despite the perchlorate deserts close to the polar regions also happening to be some of the wetter parts of the Martian surface, as perchlorate acts as an excellent antifreeze. Small streams and pools, sometimes even oases of brine lakes are sprinkled throughout these deserts, but they are so toxic that no lifeform can survive in them.

Almost no lifeform.

On Mars, as on Earth, extremophile microorganisms exist, which can actually metabolize perchlorate salts. These so-called halophilic Perchloareonta, which may dwell in almost every one of these brines, can reduce perchlorate back into harmless chloride. The amazing thing about this reaction is that it generates free oxygen as a waste product. The perchlorate deserts, as lifeless as they may seem, are downright infused with these thriving organisms and are thus one of the major oxygen-providers of the Martian surface (and potentially sub-surface). And the best thing about them is that they “operate” almost year-round.

The Horus Operations have been quick to understand the potential of these deserts. While the perchlorates have been used for a quite a while already as oxidizers for rocket-fuels, recent experiments have begun constructing bio vats in which these organisms are raised and fed in order to more easily generate oxygen for our own habitats.

References:

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

Hekubus

I lied earlier, when I said that one of the major Martian phyla are the Brachiostoma. This is because, technically, there is no such thing as Brachiostoma. It is a waste-basket-taxon for all vaguely worm-like lifeforms found so far on Mars and much of our recent work indicates that it needs to be broken up into multiple distinct phyla. Some, such as the subglacial tentacle-worm, which gave the group its name, are radially symmetrical and possess tendrils around the mouth, making them very similar to Mollizoa and perhaps Trichordata, whereas others seem to be more closely related to bilaterally symmetrical phyla, such as Onychognatha and Spiriferia.

Then there are ones which are something else onto themselves. The Cavesea Hekubus seen here is a member of the Diplognatha, a class inside the wider phylum Circulata. Circulates generally resemble annelids, in that they have a limbless, segmented body supported by a hydroskeleton consisting of multiple rings. The majority of Circulata consists of entirely soft-bodied animals, but Diplognatha stand out for having a single, endoskeletal hard part: A skull and jaw made of calcite. The configuration of this head is rather unusual, by Earth-standards. The inflexible and bow-shaped cranium is what also forms the lower jaw, while the upper jaw is split into two mandibles which can be moved independently from each other. In some forms the mandibles bear teeth used for predation, in others they have setae to filter-feed. Setae can also be found along the body of many species, where they aid in swimming.

The Hekubus is a rather harmless and small filter-feeder, fitting into a human hand. Using its setae, it filters the flooded caverns of many Martian cave systems for smaller lifeforms and food particles. During the long winters, where even in the caves most primary producers lie dormant, it buries itself into cavities and hibernates. During this phase, where most of its environment becomes anoxic, it survives through its secondary methanogenic metabolism.

Unusual for a troglodyte, the Hekubus still has eyes, four of them to be exact. This is because many aquatic cave dwellers on Mars are capable of bioluminescence. The Hekubus uses colorful markings on its body and tail fluke to attract smaller planktonic organisms, like a lamp does with moths. These markings are also used to communicate with other worms, mainly to attract mates. Larger, predatory cave dwellers, such as neckfisk, have apparently learned to exploit this and attract unwary hekubi by mimicking their mating-lights.

The Hekubus reproduces in a basic manner by laying simple eggs that are externally fertilized. Since they are hermaphrodites, it is interesting that during spawning, the mates can apparently coordinate who releases the eggs and who the sperm. In Martian animals which practice internal fertilization, this is usually determined through fights, but hekubi do apparently not resort to such behaviour, making it mysterious how they do it. Like in most arezoans, if a suitable mate cannot be found, hekubi can also practice self-fertilization. This reduces genetic diversity for the next generation, but is helpful to straddle times of low population density.  

Cave Systems

The caves of Mars are the environment about which the least is known, due to their low accessibility. Most underground chambers lie deep beneath the ice pockets and permafrost, where insulation and geothermal heat allow sections of ice to melt up to form hollow, flooded caverns. Most of these have so far only been explored with robotic probes, with meagre results. More accessible are hollow lava tubes. Lava tubes form during eruptions, when rivers of molten rock eat their way through the mountain slopes and plains. The surface of these rivers can harden before the the rest does, forming a solid roof underneath while the still liquid lava flows away, leaving behind a hollow tube. Once cooled down, water and life may seep into these caverns. The roofs of ancient lava tubes are prone to collapsing and thus have many openings to the outside world, making for adequate shelters for mountainous life. 

Fig. 2: Outside view of a massive sinkhole into a cave system formed by lava tubes. Unfortunately, giant sandworms have not been found on Mars (yet).

Life in the underground comes with advantages and disadvantages. It is usually warmer and wetter than on the surface and one is greatly shielded from dust storms and radiation. On the flipside, the lack of sunlight forces animals to adapt their sensory organs to unusual degrees and limits the energy available to the primary producers to anaerobic pathways.

Despite the lack of photosynthesis, oxygen-levels comparable to those on the surface have been found in a handful of quite deep caverns. These oxygenated caves are usually close to perchlorate deserts, so there is a possibility that the oxygen in them stems from perchloratovorous areonts.

Image Sources:

• Fig. 2: HiRISE

Equatorial Shrublands

The shrublands are among the more liveable areas on the surface of Mars. Their mean annual temperature is above freezing point, in some areas dipping below only for 67 sols of the Martian year (though in most areas the winters are more prolonged), while, unlike the deserts, they receive enough moisture from the thawing highlands to support a sizeable flora.

That does not mean life is easy here. Most of the flora, coming in form of fractarians and spongisporians, consists of low-growing species, which are adapted to long periods of dryness, as the shrublands know only two seasons: inundation and drought. During southern winter, when most of the ice masses in the highlands remain frozen, life in the shrublands enters brumation, wherein most flora and fauna becomes static and lowers down metabolic activity, trying to survive the hardship. Conditions worsen significantly once dust storms from the North reach down. In some regions, winters may even shut down photosynthesis completely, creating anoxic conditions, which most dormant animals survive by switching to their secondary methanogenic metabolism. Once southern summer melts the glacier-fringes, the meltwater flows down the slopes and canyons and passes through the equator, bringing the shrublands back to life. In some areas the resulting ponds and streams might even accumulate into larger bodies, forming watering holes for various animals. In some of these might even gather amphibious animals and flora, which lied dormant underground throughout the dry season.

The mysterious canals of Mars are most often found in these regions.

Shellubim and Zhor

The phylum Antitremata is a peculiar one. The basic bodyplan of these organisms consists of a bilaterally symmetric mantle shielded by a dorsal and a ventral valve, both made of apatite, with a stalk growing out of the back of one of the valves. In the most basal members, living in underground bodies of water and subglacial lakes, the stalk is covered in a tunicine (a material closely related to cellulose) cuticle and the mantle bears multiple lophophores supported by an arm-skeleton, made for filtering food particles out of water.

The Shellubim and the Zhor represent unique experiments of these clam-like organisms towards life on land. Shellubim are sessile as adults, like their aquatic ancestors, and are best described as “planimals”. The tunicine stalk is hardened into a wood-like stem, with many roots permanently anchoring it into the Martian soil. Extensions of the circulatory system allow these roots to take up water and minerals from the ground while also excreting waste. When shellubim open their shell, large, wing-like organs extend out of the mantle, supported by bony rods, likely evolved from former lophophores. These wings possess tissues of photosynthesizing cells, likely in the form of areont endosymbionts, which allows the organism to fuel its resting metabolism. In addition to this, one or more lophophores may extend out of the mantle, studded by setae, which are used to filter aeroplankton down to their base, where the mouth lies. Interestingly, these tentacles sometimes possess a single, simple eye at the tip, somewhat similar to what is seen in Trichordata. Apart from these eyes, shellubim possess statocysts and possibly also air-pressure-sensors. These help the organisms detect the approaching of potential predators and weather-changes which might lead to dust storms. When such danger approaches, the wings are folded in, the lophophores are retracted and the shell is firmly shut. In this state, respiration mainly occurs through caeca, microscopic canals inside the shell.

Shellubim reproduce similarly to plants by shooting gametes into the air and letting those be carried away by the wind, until they land inside the lophophores of a potential partner, which redirects them into the cloaca. Does a shellubim not receive any gametes during breeding-season, it can engage in self-fertilization. Usually, the fertilized eggs are incubated inside the body until hatching. The insect-sized hatchlings are quite different from the adults. They are mobile, the stalk is soft and underdeveloped, the lophophores shorter and the shell is unmineralized, while the wings are more strongly developed and actually used for flight! After hatching, the parent spits the hatchlings out of the cloaca into the air, where they spend most of their juvenile life as aeroplankton. It is likely during this phase that shellubim acquire their endosymbionts by feeding on phytoareonta and incorporating them into their tissues. When a larva eventually grows too big to fly, it settles down on a suitable spot. The stalk elongates, digs into the ground and hardens into the vegetative stem, while the shell mineralizes. To facilitate much of this development, the now useless higher nervous system and much of the wing-musculature are disintegrated to reallocate the resources.

By comparison, the Zhor seems like a more conventional adaptation towards land life. It completely lacks the ancestral stalk and its ventral shell is shaped into an elongated cone, which holds most of the mantle. On its rims grow two simple, exoskeletal, tunicine legs, which drag the body forward. The dorsal shell acts both as a lid for the body, when the mantle retracts back into itself as danger approaches, but also as a suspension for the Zhor’s feeding organ, which is an extendable, bony proboscis with a mandible at the end, somewhat similar to the mouthparts of a dragonfly nymph. The Zhor is largely herbivorous, feeding mainly on fractarian flora and shellubim roots, but has on occasion also been seen ambushing smaller animals scuttling by. The Zhor possesses a large gut in its conic shell, used for digesting tough materials. The digestive tract begins at the base of the mouth and winds its way almost to the tip of the shell, before doing a U-turn back towards the front of the body, where it exits as a cloaca right underneath the base of the proboscis. All members of this phylum have such a U-shaped gut with opposing ends, not too dissimilar from what is seen in Earth’s phoronids, and this is where the name Antitremata comes from.