lpetrich
Contributor
The Last Possible Outposts for Life on Mars. - PubMed - NCBI noting The Last Possible Outposts for Life on Mars | Astrobiology
It extrapolates from Earth deserts and Mars's geological history to get an idea of where Martian organisms may have lived, especially the last ones if they are now extinct. It does not cover Mars-interior ones, only Mars-surface and Mars-near-surface ones. But those are the most accessible ones, at least if they have survived.
One can quantify how much of a desert that a desert is by using an aridity index (AI), defined as (rate of water precipitation) / (rate of water evaporation). Semiarid: 0.20 < AI < 0.50, arid: 0.05 < AI < 0.20, and hyperarid: AI < 0.05.
In ordinary arid regions, there is plenty of vegetation and plenty of soil microbes, and much of the soil is covered with Biological Soil Crusts. Its turnover time of organic carbon is typically decades to centuries.
But this ecosystem collapses as one goes to hyperaridity. Vegetation becomes confined to dry streambeds, and soil microbes and BSC's become very patchy. The land ends up looking much like the visited parts of Mars itself. The organic-carbon turnover time goes up to tens of thousands of years. Organisms move to rock surfaces, and even those eventually fade away. The remaining ones live in hygroscopic salt crusts, crusts that absorb water vapor.
Looking at the history of Earth life, molecular-phylogeny techniques have made it possible to go back to the Last Universal Common Ancestor (LUCA) of all known present-day cellular organisms. Along the way, such techniques have clarified many previously-known or previously-suspected relationships, and also uncovered many others.
Among them is the Terrabacteria, a clade of prokaryotes that include many with adaptation for tolerating dryness. The Terrabacteria include Cyanobacteria, Chloroflexi, Firmicutes, Actinobacteria, and Deinococcus-Thermus. Its ancestors split from other bacteria around 3 billion years ago, meaning that the Earth's landmasses have been inhabited by microbes long before they were inhabited by multicellular organisms. The Firmicutes (Latin, "strong skins") have thick cell walls that are good for keeping water in. Many of these organisms also survive dry conditions by making spores called endospores. Some cyanobacteria make spores called akinetes. Deinococcus radiodurans can stay active in low-water conditions with hyperactive genome repair. This repair also makes the organism able to survive doses of ionizing radiation that would kill most other active organisms.
Mars has abundant evidence of liquid water from its early history, its first billion years or so. Much of that water is now gone, evaporated into outer space, and most of the remaining surface water is in the polar icecaps. The paper's authors then propose this sequence of organism habitats as the planet got drier and drier:
It extrapolates from Earth deserts and Mars's geological history to get an idea of where Martian organisms may have lived, especially the last ones if they are now extinct. It does not cover Mars-interior ones, only Mars-surface and Mars-near-surface ones. But those are the most accessible ones, at least if they have survived.
One can quantify how much of a desert that a desert is by using an aridity index (AI), defined as (rate of water precipitation) / (rate of water evaporation). Semiarid: 0.20 < AI < 0.50, arid: 0.05 < AI < 0.20, and hyperarid: AI < 0.05.
In ordinary arid regions, there is plenty of vegetation and plenty of soil microbes, and much of the soil is covered with Biological Soil Crusts. Its turnover time of organic carbon is typically decades to centuries.
But this ecosystem collapses as one goes to hyperaridity. Vegetation becomes confined to dry streambeds, and soil microbes and BSC's become very patchy. The land ends up looking much like the visited parts of Mars itself. The organic-carbon turnover time goes up to tens of thousands of years. Organisms move to rock surfaces, and even those eventually fade away. The remaining ones live in hygroscopic salt crusts, crusts that absorb water vapor.
Looking at the history of Earth life, molecular-phylogeny techniques have made it possible to go back to the Last Universal Common Ancestor (LUCA) of all known present-day cellular organisms. Along the way, such techniques have clarified many previously-known or previously-suspected relationships, and also uncovered many others.
Among them is the Terrabacteria, a clade of prokaryotes that include many with adaptation for tolerating dryness. The Terrabacteria include Cyanobacteria, Chloroflexi, Firmicutes, Actinobacteria, and Deinococcus-Thermus. Its ancestors split from other bacteria around 3 billion years ago, meaning that the Earth's landmasses have been inhabited by microbes long before they were inhabited by multicellular organisms. The Firmicutes (Latin, "strong skins") have thick cell walls that are good for keeping water in. Many of these organisms also survive dry conditions by making spores called endospores. Some cyanobacteria make spores called akinetes. Deinococcus radiodurans can stay active in low-water conditions with hyperactive genome repair. This repair also makes the organism able to survive doses of ionizing radiation that would kill most other active organisms.
Mars has abundant evidence of liquid water from its early history, its first billion years or so. Much of that water is now gone, evaporated into outer space, and most of the remaining surface water is in the polar icecaps. The paper's authors then propose this sequence of organism habitats as the planet got drier and drier:
- Water
- Soil
- Rock surfaces
- Hygroscopic salts
