The Last Inhabitants of Mars

[lpetrich]

If Mars was once inhabited by organisms, even very primitive ones, organisms that are now extinct, what were the last ones like?

The Last Possible Outposts for Life on Mars | Astrobiology

Attempts to reconstruct a plausible sequence of retreats as Mars becomes more and more hostile.

The authors note that there are some surprisingly early colonists of land: some prokaryotes named Terrabacteria on account of that: the cyanobacteria, the Gram-positive phyla, Chloroflexi / Chloroflexota, and Deinococcus-Thermus / Deinococcota. They diverged as early as 3.5 billion years ago.

The Gram-positive phyla -- Firmicutes / Bacillota, Actinobacteria / Actinomycetota -- are named after their response to a stain that detects their thick cell walls. These cell walls make it easier for these organisms to survive dryness.

Deinococcus radiodurans (ray-enduring fearsome grain) can survive huge doses of ionizing radiation, doses that would kill most other organisms, doses that its ancestors had never experienced. It does so by hyperactive gene repair, repair of damage caused by ionizing radiation, and also dryness, a well-known outcome of being outside of water. So that feature is an adaptation to dryness.

"Representatives of this group possess specific adaptations to land environments such as resistance to desiccation, high salinity, and high UV radiation." There is some possible fossil evidence of land ecosystems around 1.2–1 billion years ago (Gya) and 2.7-2.6 Gya, and freshwater ones around 2.9-2.7 Gya.

"Perhaps not surprisingly, the cyanobacteria, a deeply rooted taxonomic group often found in aquatic environments, include some of the most desiccation-resistant organisms known, which are adapted to survive in the driest regions on Earth (Friedmann, 1980; Friedmann and Ocampo-Friedmann, 1995; Warren-Rhodes et al., 2006; Pointing and Belnap, 2012; Wierzchos et al., 2012b)."

So similar organisms could have inhabited Mars for a long time as the planet dried up.

 

[lpetrich]

•  Geological history of Mars
• ESA Science & Technology - The Ages of Mars
• Noachian, Hesperian, and Amazonian, oh my!… | The Planetary Society

• Pre-Noachian: 4.5 - 4.1 Gya
• Noachian: 4.1 - 3.7 Gya
  • Early: 4.1 - 3.95 Gya
  • Mid: 3.95 - 3.85 Gya
  • Late: 3.85 - 3.7 Gya
• Hesperian: 3.7 - 3.0 Gya
  • Early: 3.7 - 3.4 Gya
  • Late: 3.4 - 3.0 Gya
• Amazonian: 3.0 Gya - present

On Mars, the established existence of aquatic habitats during the late Noachian about 3.8 billion years ago (Grotzinger et al., 2014) and the inferred existence of aquatic environments at the end of the Hesperian ca. 3 billion years ago (McKay and Davis, 1991), together with the cumulative evidence of episodic aquatic environments during the Early Amazonian (Fairén et al., 2009; Rodríguez et al., 2014, 2015), suggest that an early martian biosphere could have had sufficient time to colonize land, assuming a similar pace of colonization as life on Earth. But contrary to Earth, most of the surface of Mars was emerged land even when liquid water was abundant (Carr and Head, 2015). The inferred existence of tens of thousands of crater lakes in the martian southern highlands, and possibly a large northern ocean, implies a potentially large net shoreline area with shallow aquatic environments that could have propelled the emergence of land microorganisms. The absence of a magnetic field since the late Noachian (Acuna et al., 1998; Carr and Head, 2010) along with the continued decline in atmospheric pressure in the first 2 billion years (Lammer et al., 2013) would have exerted strong selective pressures on martian organisms to adapt to desiccation, UV radiation, and high salinity, thus paving the way for the colonization of land. After the colonization of land, long-term habitability on Mars would no longer have been tied to the existence of aquatic environments but to atmospheric sources of water. This could have significantly expanded the martian habitability window beyond the Hesperian period.

 

[lpetrich]

Then discussing the  Aridity index using AI = (precipitation) / (potential evapotranspiration) = (how much water comes in) / (how much water can go out).

Hyperarid .../ AI = 0.05 ... Arid ... AI = 0.20 ... Semiarid AI = 0.50

The Earth has a lot of full-time or part-time arid environments: some 35% of its land area. "Research conducted over the past 40 years suggests that there is a predictable pathway of ecological change with increasing dryness, driven by the need of organisms to maximize exposure to liquid water during sporadic wet events."

Many desert organisms can dry up without dying, and in plain arid regions, a biological soil crust (BSC) can form, covering as much as 70% of the soil surface. They have a turnover of organic carbon (OC) over decades to several centuries.

But going from arid to hyperarid conditions makes an ecological collapse and the BSC becomes patchy, with fewer individual organisms and species of organisms. The OC turnover time becomes tens of thousands of years.

The remaining organisms mostly live on rocks (epilithic), inside of rocks (endolithic) or underneath rocks (hypolithic). The latter two environments can retain water, and also, "The protected lithic microenvironment also provides shelter against erosion and wind, amelioration of extreme temperature changes, and shielding against UV radiation (Friedmann, 1980)."

"Yet there are also trends of extinction within lithic habitats with increasing dryness." Hypolithic colonization of translucent rocks is typically 100% in semiarid regions, <50% in plain arid ones, and <1% in hyperarid ones, except when fog forms every now and then.

At the driest end of hyperaridity, where soils and most lithic substrates can no longer sustain active biology (e.g., Warren-Rhodes et al., 2006; Ewing et al., 2008; Crits-Christoph et al., 2013), relatively complex and abundant communities of autotrophic and heterotrophic bacteria and archaea (de los Ríos et al., 2010; Robinson et al., 2015) are still found inside porous, hygroscopic salt crusts (Wierzchos et al., 2006) (Fig. 1D). The communities inside the salt nodules use liquid brines that form in the interior of the salt substrate from the vapor phase via deliquescence and capillary condensation (Davila et al., 2008, 2013; Wierzchos et al., 2012a). Because the salt substrate retains water efficiently, cyanobacteria inside the salt nodules can fix CO2 for days after a wetting event (Davila et al., 2015), and the community is capable of carbon cycling rates in timescales of decades (Ziolkowski et al., 2013) even in the absence of atmospheric precipitation. Such deliquescent substrates provide a minimalistic habitat for survival under extremely dry conditions, and in the driest place on Earth they likely represent the last available habitat for life.

 

[lpetrich]

The authors then propose this sequence for Martian organisms:

• Aquatic (in water): Noachian - early Hesperian
• Edaphic (in soil): early Hesperian - early Amazonian
• Lithic (in rocks): mid-Amazonian
• Hygroscopic salts: late Amazonian - present

"If hygroscopic habitats such as those found in the Atacama Desert represent the last possible outposts for life under extremely dry conditions, then we can use them to approximate the last time when Mars might have been a habitable planet."

The authors then try to make some estimates.

Hence, as a first approximation the water abundance in the present-day martian atmosphere is lower than in the driest regions on Earth by a factor of 10–100, but during high obliquity it is comparable to levels measured in the hyperarid core of the Atacama Desert. Assuming—rather arbitrarily—that a PWV of 1000 pr-μm represents the minimum atmospheric water abundance that can sustain deliquescence-driven microbial ecosystems, the abundance of water on a comparison of the martian surface to the driest deserts on Earth suggests that the habitability window in this type of substrate might have closed relatively recently, perhaps during the late Amazonian, or it may possibly still be open.

Deliquescence - absorbing water until it dissolves

So the last organisms on Mars would likely have lived on salt crusts, like in the  Atacama Desert in western South America, one of the driest places on our planet.