Hellas Savannah

Hellas Planitia is likely the most distinctive region of the red planet. It is a giant basin that spans 2300 km between its widest ends and its depth reaches over 7 km below the planet’s datum (its equivalent to a sea level). It is large enough that early astronomers could already see it from Earth with their telescopes. Billions of years ago, when the rocky planets were still in their formative phase, the basin was formed by a giant impact, one of the largest in the solar system’s history. As the young and violent Mars entered its aqueous phase, water soon accumulated and Hellas Planitia became home to a vast inland sea, as confirmed by local stratigraphy. However, this sea vanished so long ago that now, even beneath the deepest points of the basin, can be found the petrified remains of land-living animals lying in desertous sediments dating back hundreds of millions of years ago. When the Hellas Sea vanished, it left behind only a vast salt flat, which, after being bombarded by thousands of years of UV-radiation, must have become the largest perchlorate desert that ever existed in the solar system. But even this reign of the toxic sands came to an end. The continuous growing and melting of the ice shields and glaciers of the southern highlands has flooded the basin for hundreds of thousands of years with crushed stones and debris. Over the course of millennia, the salt desert has been buried by deep layers of gravel, on which eventually could grow fertile soil again.

Fig. 2: Extent of the Hellasic Savannah climate. Note that the warmer savannah region is bordered by a ring of more temperate shrubland before transitioning into tundra.

Thanks to a multitude of factors, the Hellas Basin is now the most biodiverse region of the planet and likely represents the last remnant of an otherwise lost ecology. The two main factors that facilitate this condition are the basin’s depth and geography. The region lies so deep beneath the Martian datum that at its bottom, the air pressure is 103% higher than on the average elevation of the planet, or in other words, 1.035 bar, which is actually slightly higher than the average on Earth. Such a thick carbon dioxide atmosphere results in an amplified greenhouse effect, creating temperatures that are much more comparable to those found in the great northern deserts than to those in the tundras which surround the basin. But what distinguishes Hellas from the Boreal Desert is its humidity and vegetation. While rainfall is nearly nonexistent on Mars, the Hellas Basin manages to stay well-hydrated in a quite simple way. Being surrounded on all sides by frozen tundra, the basin becomes a natural drainage area when the top layers of the permafrost thaw every spring. In the basin, the waters seep into the soil and, like in the Swiss Mittelland, swell up the deep layers of ice age gravel, becoming excellent aquifers. Below much of the basin is thus an extensive and quite high water-table that plant-like lifeforms can access throughout the inundation season. Unlike anywhere else on Mars, the Hellas Basin can thus support a savannah-like vegetation-cover. Its dry climatic condition supported by a high water-table is somewhat comparable to parts of the Gran Chaco or, perhaps more aptly, the Late Jurassic Morrison Formation.

The thick air suspended in low gravity also has other ecological influences. Only here are there still clouds of aeroplankton thick enough to blot out the sun. These consist of shellubim- and wadjet- larvae, a multitude of microflier onychognaths, spores and gametes of various sessile organisms and even algae-like macroareonts suspended on tiny balloon-organs. Many of these organisms are fascinatingly bioluminescent, making for spectacular night skies. These clouds support both the wider aerial and terrestrial ecosystem. The perhaps most distinctive type of flora in this savannah are the giant tube-trees, who are among the largest living organisms on Mars. These are spongisporians, which function like a mix between giant lichen and land sponges. In their tissues live various endosymbiotic microbes engaging in photosynthesis, fuelling the huge organism’s resting metabolism in exchange for shelter. But chiefly, these tube-trees feed by filtering the air for aeroplankton with their tube-like outgrowths. Inside the organism are vast canals and tubes manned by rows and rows of hair-like setae, whose motions produce a continuous airflow in and out of the body. Once trapped in this flow, the aeroplankton is siphoned into a cauldron-like cavity to be slowly digested by mild acidic fluids. A long and agonizing death.

The giant tube-trees themselves are important for various other organisms. Various trichordate and spiriferian spongivores feed on their squishy, porous skin, which quickly regrows.  In some areas, the tube-trees grow in dense groups, creating reef-like islands in the middle of the savannah, on which various organisms live and roost.

The other mainstay of the savannah are the scale-trees. Although they resemble coniferous plants from Earth, they are internally quite different. These are fractarian organisms, more specifically polyfractarians. Each tree is actually a clonal colony of multiple individuals, called fractophores, working together as one organism. The condition is somewhat comparable to a Portuguese man o’ war. The tree begins life as a single individual growing from a spore. This is the genophore, from whose bottom then grow multiple connected clones, who develop into rhizophores that build up a root system. From the top of the genophore then grow in an alternating pattern the dendrophores, which build the stem, branches and leaves. Once mature, the top-most dendrophore produces gonophores, whose sole task is reproduction.

Most fascinating about the scale-trees is their solution to transport. Instead of transporting water and nutrients through something akin to a xylem, almost every fractophore possesses a heart-like organ that slowly pumps the fluids through the body. Standing close to a tree, the slow heartbeats of these large organisms are actually audible. Combined with all the other trees and animals across the savannah, this makes for a truly unique soundscape:

This trait of the scale-trees is fascinating for multiple reasons. Fractaria do not ancestrally have muscle-tissues, though they do have placozoan-like precursors to such tissues, which pseudarticulates evolved into true muscles. It would be of high interest to investigate if the muscle-like tissues which power the polyfractarian hearts are homologous with similar tissues found in pseudarticulates or if it is an entirely independent development that arose out of shared building blocks. Furthermore, recent studies (Bomhoff 2339) have found that the fractophores are capable of coordinating their heart-rates as well as fluid-flow in unison across the whole tree’s body. How they are capable of doing that despite not possessing nerve-cells is unknown and demands further inquiry.

Another mystery of the scale-trees pertains to the reproduction of some species. The gonophores of most scale-trees reproduce primitively through exchanging airborne gametes, which then develop into airborne spores that grow into new genophores. However, in some rare species found across the savannah, the spore becomes encased in a woody shell over which then grows a sponge-like coating. These often coconut-sized “eggs”, as they are informally called, then fall on the ground, where many of them rot without developing into a new genophore. There is a distinct possibility that what we are looking at here is the Martian version of a fruit or nut, but there seems to be no animal large or willing enough to eat and disperse these organs, which is maybe why these egg-bearing trees are so rare.

There are other such ecological anachronisms found across the savannah. Many of the giant tube-trees have defensive spikes, derived from their spicule skeleton, growing across their whole height, which seem like good deterrents for large herbivores. Some shellubim also have defensive toxins of such high potency that they seem like overkill for most living animals that could still step on them. It seems that not too long ago, the savannah was still home to megafauna, but it has all vanished, leaving behind a dwarf fauna. This could easily be linked to the worsening conditions of the planet as a whole, but, puzzlingly, calculations done on the vegetation cover indicate that the floral biomass of the Hellas Savannah would theoretically still be capable of supporting populations of much larger animals than can be found today (Schröckert 2340) (the caveat here being that those calculations had to estimate many variables using values from earth-ecosystems, which might not accurately reflect Martian ones). The true cause for this absence therefore remains mysterious. Possibly, the savannah recently went through a harsh dip in habitability, with the flora being able to recover again to previous levels today, while the now impoverished fauna has not kept up. Or this last megafaunal extinction was caused by non-ecological factors, such as suffocation by the gigantic global duststorms, which may have only developed in the last couple of hundred thousand years. Widespread disease caused by limited space may have also been a factor. Or our calculations and observations are simply wrong. Those adaptations we think are ecological anachronisms could simply serve an entirely different purpose that we currently do not realize. Tellingly, there is little direct physical evidence of recent megafauna in Hellas Planitia. The best we currently have is a large fossil scolecodont from some kind of predatory periostracan found in Hellas Chasma, dating back to around 430’000 years ago (Sivgin 2345). From around the same time is a fossil trackway in Hamakhis Vallis, which attests to the existence of a roughly giraffe-sized nothornithe (Krätschmer 2122). Both the tooth and the ichnofossil have frustratingly never been analysed in detail again since their original discovery.

References:

• Bomhoff, Nils: Coordinated flow of coelomic fluid in Titanofractus lanali. Implications for polyfractarian physiology, in: Areobiology Magazine, 67, 2339, p. 28 – 40.
• Krätschmer, Simon: Description of a nothornithe trackway from Hamakhis Vallis, in: Strate Station Geological Journal, 460, 2122, p. 1456 – 1496.
• Schröckert, Daniel: How much can the Martian savannah support? Outline and limits of modelling extraterrestrial ecosystems, in: Astrobiology Magazine, 704, 2340, p. 11 – 23.
• Sivgin, T.K.: Life on a Dead Planet. The first 3 billion years of Evolution on Mars, Zürich 2345.