Swansong Earth: Refuges for Life

by Paul Gilster on October 31, 2012

As we begin to identify planets in the habitable zone of their stars, the larger issue becomes what fraction of stars have such planets. This is eta-Earth (ηEarth), the percentage of Sun-like stars with Earth-like planets in the habitable zone, a figure we can gradually home in on as statistical surveys like Kepler continue to churn. Right now the estimate depends on whom you talk to, with figures ranging from 1.4-2.7% (Catanzarite & Shao, 2011) to 42% for red dwarf stars (Bonfils et al., 2011). One thing I haven’t seen discussed much is the question of when planets cease being habitable, including what we can call the phases of habitability on a given world.

Jack T. O’Malley-James (University of St Andrews, UK) and colleagues have gone to work on the question in a new paper slated for publication in the International Journal of Astrobiology. The researchers note that life emerged on Earth 3.8 billion years ago and perhaps somewhat earlier. The key point is that unicellular organisms were found on our planet at least 2.5 billion years before fossil evidence of multicellular life appears. And most animal phyla do not appear as body fossils until 530 million years ago, during the period known as the ‘Cambrian explosion.’

We’re living in an era that obviously supports complex life, but the continuing evolution of the Sun will take us past that state, with microbes again being the only forms of life likely to survive in the far future. We can see that extremophiles can survive today in conditions that would be extreme to impossible for humans, existing at high temperatures, highly acidic waters and other improbable places. O’Malley-James and company argue that it is statistically likely that the habitable Earth-like planets we find will be at a stage supporting unicellular rather than multicellular life.

That means we need to understand the biosignatures such worlds might exhibit, which could be unlike those produced by the majority of life on Earth today. We don’t want to miss a planet with a living microbial biosphere because its detected biosignatures differ so widely from our own. What we will eventually need are simulations of these microbial environments under different levels of radiation and surface conditions to consider what biosignatures will be detectable, using host planets that range from Earth itself to planets around red dwarfs and in binary star systems. This paper begins that process by looking at the Earth’s next three billion years.

The factors influencing habitability include solar luminosity, greenhouse gases, orbital characteristics, hydrogen escape and the runaway greenhouse effect. Looking at the Earth, the researchers find a sharp increase in surface temperatures starting about one billion years from now, keyed to the onset of rapid ocean evaporation. In high latitude regions, unicellular life might persist for up to 2.8 billion years from the present. By assuming an upper temperature boundary of 420 K (this is an extension of today’s thermophile tolerances), they also find that life could persist about 700 million years longer at the poles than at the equator, a location that could change depending on future changes in the Earth’s axial tilt or the eccentricity of its orbit.

It’s a fascinating world even if shorn of complex life, one in which pockets of living things hold out:

Rapid ocean loss as a result of a moist greenhouse effect would likely represent the end-point of a planet’s habitable lifetime. Assuming ocean loss was not uniform across the globe due to regional temperature variations, there could potentially be pockets of liquid water that remain for a brief time before total loss of liquid surface water occurs. A source of liquid water is a prerequisite for life as we know it; hence, these last pools of water would represent the final habitable regions on a dying planet.

The locations of these last habitable regions? Ocean floor trenches for one (the Marianas Trench extends about 11 kilometers below sea level). High altitude lakes are another possibility, and there is the distinct possibility that far-future life might retreat into cave systems. We’re learning more all the time about how many microbial communities exist without solar energy, and caves with their greatest volume below their entrance could act as cold reservoirs. Moreover, a far-future Earth might be one with a greater axial tilt as the Moon recedes and Earth’s rotation slows. At this point, with obliquity values of 30 to 60 degrees, equatorial regions may be more accommodating to life than the formerly cooler polar areas.

The last pools of water on Earth will likely be warm, isolated and highly saline, which means Earth’s last lifeforms will be able to tolerate high concentrations of salt and high temperatures. The researchers think such life will be found in microbial mat communities that resemble some of the life forms we’ve found around hydrothermal vents. The low abundance of ozone in this future era will subject these organisms to elevated levels of UV radiation. We can find some analogs to this in microbiological communities in some of the highest volcanoes of Chile’s Atacama desert, which live under arid conditions with large daily temperature variations.

The biosignatures of Earth will evolve until the point at which the planet is completely uninhabitable. Until that point, the by-products of an extremophile biosphere may be tricky to find:

The microorganisms from the Atacaman volcanoes appear to use the oxidation of carbon monoxide to obtain energy…While the slowing of plate tectonics may decrease the amount of available CO, hotspot volcanoes (those caused by upwellings from the deep mantle rather than at plate boundaries) could still exist. Mars (which is not tectonically active) exhibited this kind of volcanism up to as early as 2 Myr ago…, while hotspot volcanism may still be actively ongoing on Venus…The thermohalophilic organisms listed above are all anaerobic organisms which produce hydrogen, carbon dioxide, ethanol and acetate as by-products of their metabolic processes. Acetates are not promising as remotely detectable biosignatures, only managing to escape into the atmosphere (at a very slow rate) if they are formed within very acidic (pH < 4.7) water...

One possibility is that on a planet with low carbon dioxide levels compared to today’s Earth, the biological production of CO2 would produce the kind of disequilibrium that flags the presence of life. Even so, assuming a global distribution of geothermal pools equal in size to the largest ones found on Earth today, and assuming a similar number of ice caves and active hydrothermal vents as well as a basaltic crust inhabited by microbes, the authors come up with a global biological productivity that is two orders of magnitude lower than what was found on Earth before photosynthesis occurred.

Can such a reduced biosphere produce biosignatures strong enough to be detected over interstellar distances? The question is unresolved and awaits further work on biosignatures in specific extremophile environments. What’s significant about this early analysis is that it suggests the average terrestrial world around a G-class star will most likely be populated not with complex lifeforms but microbes, based on the past and future course of life on our own planet. We’re also left to ponder the need of intelligent species to use their time wisely, aware of the window for complex life and taking steps to ensure their survival once their planet becomes uninhabitable.

The paper is O’Malley-James et al., “Swansong Biospheres: Refuges for life and novel microbial biospheres on terrestrial planets near the end of their habitable lifetimes,” accepted for publication in the International Journal of Astrobiology (preprint). Thanks to Tatiana Covington for the pointer on this one.

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Alex Tolley October 31, 2012 at 10:10

This site Deep subsurface micro-organisms has some data on what might become the last living organisms on Earth.

Now it will be interesting to see if anything like them exists on Mars, either as a 2nd genesis or an exchange with Earth, because clearly Mars is not giving us am obvious bio-signature at all.

Paul October 31, 2012 at 10:30

How powerful might humans ever become? People speculate about being able to build Dyson Spheres, co-opting the energy output of an entire star.
What if gravity were completely understood? Quantum physics? Dark energy?
Given another 10 thousand, or even 50 thousand years of fortunate survival.

If that were the case, why would we permit our planet to fry? We would adjust it’s orbit outward, or shield it from harm. Or even relocate it completely to a younger, more benign pretty yellow star. I am not pulling such ideas out of my own, puny hat. Greg Bear has a novel called “Moving Mars”. Greg is a lot smarter than I am.

I realize this misses the author’s point about the viability of life forms and where they might be found. I don’t minimize the significance of keeping a very open mind when it comes to the exiting prospect of finding, or even recognizing life on the exo-planets.

Christopher L. Bennett October 31, 2012 at 11:21

Of course, the paper is being a bit conservative. An advanced technological civilization could terraform its world or astroengineer its star in order to prolong its habitable lifespan. So it’s possible that once a world hosts sapient, technological life, it might never stop supporting macroscopic life thereafter (assuming that technological civilization doesn’t destroy itself before it colonizes space and becomes effectively immortal).

Then again, it’s possible that many technological civilizations would abandon their homeworlds to reside permanently in space, or would evolve into some postbiological form, or whatever. Naturally the paper made no predictions about such things because there’s no way of predicting them. But it’s worth keeping in mind that there are possibilities beyond the scope of the paper.

James D. Stilwell October 31, 2012 at 11:24

Are we going to suppress human reproduction or will humanity number in the trillions spread throughout the solar system, and leave the visible clue of blue green algae wherever we migrate? JDS

Tom Mazanec October 31, 2012 at 11:42

Coincidentally, I just read of such a swansong planet on the hard space opera website Orion’s Arm. The oceans 99% evaporated, creating a wet greenhouse and photodissociating. This left the planet covered in salt, raising the albedo to where the planet cooled enough to let the remaining water linger at the poles. Two somewhat different remnant biospheres are at these poles, consisting of colonial biofilms and digesters.
Did the paper you cite take into account such saline albedo raising?
http://www.orionsarm.com/eg-article/508b48d94e043

Sutpen October 31, 2012 at 14:47

We should construct planets we can inhabit. Experiments with building small worlds and seeding them with life shouldn’t be too difficult in the near future, and terraforming Mars and maybe Venus will provide practice.
Rather than taking over any already inhabited world in another system we should build a world to suit us.
We have a huge expanse of time to do this, though not an infinite one.

Wojciech October 31, 2012 at 15:21

“What if gravity were completely understood? Quantum physics? Dark energy?
Given another 10 thousand, or even 50 thousand years of fortunate survival.

If that were the case, why would we permit our planet to fry? ”
If we would have knowledge like that, we possibly would have no use for our current bodies, and could exist as something that thrives in quasars or within black holes, simulating whole universes.

IB October 31, 2012 at 18:39

“We’re also left to ponder the need of intelligent species to use their time wisely, aware of the window for complex life and taking steps to ensure their survival once their planet becomes uninhabitable.”

It’s moving how extraordinary this window is. I find myself thinking that we should appreciate it more. Although we often like to think that we will develop powerful technologies that endlessly will postpone our extinction, the fact is that our technological moment has so far been incredibly brief. Many here assume that we will have centuries and millennia more to come. But perhaps the moment will pass and the rest of our unblinking universe will never know we were ever here. Although that seems sad, it also suggests that perhaps we should invest much more effort as a species in appreciating just being here, now. From that, all sorts of positive ideas follow.

Alex Tolley October 31, 2012 at 18:40

@Paul – I have to agree. The obvious solution for technological life is to mitigate the change in their environment. A sunshade seems a relatively low cost solution solution to me. For a low tech species, covering much of the planet in reflective foils might also work..for a time. Floating in the tropical zones should increase the albedo to stave off raised insolation.

Astronist October 31, 2012 at 21:06

“We don’t want to miss a planet with a living microbial biosphere because its detected biosignatures differ so widely from our own.” — So are we ruling out the possibility of a planet with a totally subterranean microbial biosphere, such as worlds like Europa and Enceladus might be? Or such as an Earth-analog planet in a non-Earth-analog orbit, or come to that in interstellar space? Or might there exist a whole class of terrestrial worlds whose microbial life can only be detected by landing on them and drilling down to the water table?

GaryChurch October 31, 2012 at 21:54

Ed Lu has written about life evolving in solar systems that also have asteroids and comets that inevitably get sucked into gravity wells. We may be extremely fortunate in having planets like Jupiter and Saturn that absorb the majority of these impacts. Other solar systems may not be so fortunate and are periodically having their biospheres reset to the micro organism stage by impacts. One possible “great filter.” If this is so we may be able to turn these unfortunate planets into new Earth’s by deflecting impact threats and transplanting our own ecosystems. This continues the discussions in previous posts about impacts and alien invasions.

Anthony Mugan November 1, 2012 at 4:47

Others have already made the point that if a technological civilisation emerges then that creates a asymmetry in the situation with the potential for considerable technological intervention in the situation. I was also going to mention the possibility of our distant descendents taking the earth with them, inspired by the previous post around asteroid deflection strategies, thinking it was an original thought…I must catch up on my sci-fi reading!

amphiox November 1, 2012 at 14:15

Paul, at the level of power you describe, it is unclear if humans (or another similarly powerful intelligence) would even need planets, or find it worthwhile or cost-effective to bother trying to save one.

There would be a wide variety of other strategies available to them for surviving in that situation.

amphiox November 1, 2012 at 14:20

So are we ruling out the possibility of a planet with a totally subterranean microbial biosphere, such as worlds like Europa and Enceladus might be? Or such as an Earth-analog planet in a non-Earth-analog orbit, or come to that in interstellar space? Or might there exist a whole class of terrestrial worlds whose microbial life can only be detected by landing on them and drilling down to the water table?

Not so much ruling out I think, as deferring for later study, because we don’t actually yet have any way of examining such planets (we would have to get to them and land on them) in the near foreseeable future, while we do have ways of remotely investigating atmospheric signatures.

For practical considerations, that which cannot be studied is often considered beyond the purview of science, even if you suspect that one day it will be (you wait until it does become studiable before accepting it as science). This is one reason why exoplanets were considered to be the purview of science fiction and beyond the scope of real science for such a long time, and only became a serious science once techniques for detecting them were demonstrated to be feasible.

andy November 1, 2012 at 15:32

What if gravity were completely understood? Quantum physics? Dark energy?

In the context you use these terms, you may as well have said “magic pixie dust” and been roughly as scientific.

With magic pixie dust anything is possible, even reversing the cooling of the Earth’s interior and other processes that will eventually cause plate tectonics to stop even if the Sun doesn’t fry us.

Transhumanist nonsense is still nonsense, even if it is nonsense that is packaged in various sciency-sounding jargon.

xcalibur November 3, 2012 at 18:37

bigdan201 here, I use the handle xcalibur on most other sites, so I think I’ll transition to that here. my old name is based on my yahoo email, which I created in my teens.

Anyway, very interesting study. but like Paul, I’m optimistic that continually advancing human civilization will be able to preserve the earth from eventual desiccation by the aging Sun. Even if most of us live in space habitats rather than planets at that point, there will be a strong motivation to preserve the temperate conditions and complex biosphere of the earth. In the distant future, the earth may become a nature reserve, museum, and recreational park.

Stephen November 28, 2012 at 11:17

We already have the physics to move the Earth out a safe distance, to save it. The engineering looks feasible. It should only take 100 million years or so. You get an asteroid on a figure 8 orbit between Earth and Jupiter. You steal orbital energy from Jupiter and give it to the Earth. When the Sun cools, you reverse the process. We’d be stupid not to do this. Oh. I see. The Earth is toast.

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