Astrobiology Underground

by Paul Gilster on January 24, 2014

I’m a great believer in what I might call the ‘conventional’ habitable zone; i.e., a habitable zone defined by the possibility of liquid water on the surface. The definition is offered not to exclude exotic possibilities like micro-organisms floating in the clouds of Venus or aquatic life deep inside an ice-covered moon like Europa. Rather, it acknowledges that finding life is hard enough without losing our focus. In terms of exoplanets and feasible near-term study, a warm planet with liquid water — the kind we live on — would command our immediate attention.

But as we look at much broader issues of how life forms, we may indeed learn that our kind of life is but one component of a vast continuum, as recent work out of the University of Aberdeen reminds us. In a new paper published in Planetary and Space Science, researchers tackle the question of life living deep underground. Now the habitable zone starts to broaden, because things get warmer as we go deep.

We know of life here on Earth that exists more than five kilometers below the surface, and given the difficulty of probing these regions, we probably have fragmentary knowledge at best of what’s down there at deeper levels. So if we’re talking underground microorganisms, maybe a place like Gliese 581 d, evidently just past the outer edge of its star’s habitable zone in terms of liquid water, would still qualify. The Aberdeen team thinks conditions less than two kilometers below the surface there could be clement.


Image: New studies are examining habitable conditions below planetary surfaces, where liquid water might exist. Internal heat might keep even a ‘rogue’ planet moving without a parent star capable of hosting some kind of life underground. This image is an artist’s impression of the rocky super-Earth HD 85512 b. Credit: European Southern Observatory.

The Aberdeen work revolves around a computer model that estimates the temperature below the surface of planets of varying sizes at varying distances from their star. Doctoral student Sean McMahon comments on the implications in this Aberdeen news release:

“Using our computer model we discovered that the habitable zone for an Earth-like planet orbiting a sun-like star is about three times bigger if we include the top five kilometres below the planet surface. The model shows that liquid water, and as such life, could survive 5km below the Earth’s surface even if the Earth was three times further away from the sun than it is just now. The results suggest life may occur much more commonly deep within planets and moons than on their surfaces. If we go deeper, and consider the top 10 km below the Earth’s surface, then the habitable zone for an Earth-like planet is 14 times wider.”

An extension of the habitable zone indeed, taking us well past the orbit of Saturn in our own Solar System. And ponder the case of a rogue ‘super-Earth,’ a planet four or five times more massive than the Earth that has for whatever reason been ejected into the interstellar deep. We’ve talked about such places before and their prospects for generating enough internal heat to make some kind of life possible even in the absence of a parent star. And when you start extending habitable zones this far, a cosmos teeming with at least primitive life seems possible.

Since we’re pushing habitable zones in new directions, let me also mention Craig Stark (University of St. Andrews), whose work has focused on alien atmospheres extending from exoplanets to brown dwarfs. Stark argues that prebiotic molecules can form in these environments, saying “The atmospheres around exoplanets and brown dwarfs form exotic clouds that, instead of being composed of water droplets, are made of dust particles made of minerals.” Add a dose of lightning and you have charged particles and interesting possibilities:

“These charged gases are called plasmas – like those found in fluorescent lights and plasma televisions. The dust can find itself immersed in the charged gases and the charged particles stick to the dust making the dust charged. The charged dust attracts onto its surface other charges from the surrounding plasma helping grow molecules on the dust surface.”

The precursors to life in the atmosphere of a brown dwarf? Now we’re really pushing the definition of ‘habitable zone.’ The Stark paper is “Electrostatic activation of prebiotic chemistry in substellar atmospheres,” accepted at The International Journal of Astrobiology (preprint). The McMahon paper is “Circumstellar habitable zones for deep terrestrial biospheres,” Planetary and Space Science Vol. 85 (September, 2013), pp. 312-318 (abstract).


{ 21 comments… read them below or add one }

Michael January 24, 2014 at 15:34

“Using our computer model we discovered that the habitable zone for an Earth-like planet orbiting a sun-like star is about three times bigger if we include the top five kilometres below the planet surface.’

Lower the surface gravity of a planet so an atmosphere is less massive or non existent and you could go in past mercury towards the sun as well. Mercury has potentially ‘ice’ at the poles and surprisingly it is frozen solid below several meters of the surface.

“The atmospheres around exoplanets and brown dwarfs form exotic clouds that, instead of being composed of water droplets, are made of dust particles made of minerals.” Add a dose of lightning and you have charged particles and interesting possibilities:’

I am not so sure about these types of worlds been suitable for life, they have intense gravities and enormous amounts of heat which would destroy life. Maybe (unlikely) later on in the their lives when they have calmed down a bit it could be possible.

Alex Tolley January 24, 2014 at 16:29

I think we need to be a bit careful about the implied meaning of “Habitable Zone”. The assumption is that with a suitable environment that includes liquid water, life as we know it can arise. While the deep biosphere supports microbial life, AFAIK, life cannot originate there. We’re not clear how it got there, but it seems reasonable that it was carried there by primarily by geologic processes. Therefore, without agency, an HZ that depends on lithosphere conditions is most likely sterile.

Mars is interesting because it was probably once warm and wet and therefore life may have migrated and continued to survive in the lithosphere. However I don’t know if the subsurface temperature would permit liquid water in the rock. Last time I looked, it was just theoretical calculations, and there didn’t seem to be any direct measurements.

Similarly, the excitement over Europa is predicated on the possible analogs to Terrestrial oceanic vents which we know support life, but not that life forms there. Depending on your views of how life forms, or estimates for panspermia rates, life may be there or not. I tend to think Europa is sterile, but it is clearly worth looking for.

Even if we did know that life on Earth is indigenous and the mechanism of its creation understood, that may not help with determining how common life is elsewhere, even Earth-like life. However, if biosignatures work, a Kepler type experiment looking for biosignatures to build a statistical picture of the prevalence (or not) of life would be immensely interesting, even if the biosignatures could only work for Earth-like or water worlds.

In our local system, if Curiosity discovered unequivocal signs of fossil life, whether traces like iron bands in rocks, or macro fossils, I would have thought that it would be worth checking for remaining internal heat and then drilling for subsurface life.

Tulse January 24, 2014 at 17:28

“I think we need to be a bit careful about the implied meaning of “Habitable Zone”. The assumption is that with a suitable environment that includes liquid water, life as we know it can arise. While the deep biosphere supports microbial life, AFAIK, life cannot originate there”

Exactly. Consider that, for humans, LEO is “habitable” with sufficient support, but we could never has evolved there. If we’re interested in where life might be in the universe, we have to consider what conditions are necessary for abiogenesis, and not just for habitability.

Tarmen January 24, 2014 at 17:37

I imagine that there are all sorts of tubes, shafts and tunnels that climb from the deeps to the surface vents and percolate with heat and chemistry beyond our ken. To me it seems likely that biology will have found niches and eventually empires in those old dark sub-surface realms. Nearby Europa may be rotton with biochemistry.

NS January 24, 2014 at 17:52

This paper [pdf] tries to assess planet habitability for both existing life and abiogenesis:

Like Alex Tolley above, it suggests the possibility that there are habitable planets that are uninhabited because the conditions for the origin of life never arose.

Eniac January 24, 2014 at 21:09

Alex Tolley:

While the deep biosphere supports microbial life, AFAIK, life cannot originate there.

How do you know that? There are many theories on the origin of life, some involving minerals. I am pretty sure we do not know enough about where life can or cannot evolve, or even where our form of life, in particular, originated. Near the surface, in the deep ocean, volcanic vents, underground, aren’t they all still in play?

Enzo January 24, 2014 at 23:37

Paper “Habitable zones around mainsequence stars : new estimate” :

And related article :

Claims that the HZ can be as close as 0.95 AU. Even a warmer sun with 500 ppm of CO2 would result in average global temperature raising from 15 to 40 C but stable, no runaway greenhouse.

Tom Mazanec January 25, 2014 at 7:00

I recently came across the idea of “superhabitability”, the concept that a larger planet orbiting a smaller star would be optimum for life, and that Earth is only “marginally habitable”.

tesh January 25, 2014 at 9:40

“Using our computer model we discovered that the habitable zone for an Earth-like planet orbiting a sun-like star is about three times bigger if we include the top five kilometres below the planet surface.”

Not sure that you can say you’ve discovered anything using computer modelling. I guess you can say that our models predict “x” or “y”. Discovered sound too much like they went out there and planted a flag on it!

Norman Wells January 25, 2014 at 12:43

I can’t except the opinion that Earth is only marginally habitable. Surely the life which has emerged on Earth is optimised for the climatic environment in which it exists by reason of its evolution in that environment. Because conditions are not uniform across the planet the life that has emerged covers a large range of species with varieties able to exist within practically the whole environmental range experienced . But conditions on Earth seem to have been optimal for the evolution of life since hereon it has developed to what one could presume is its peak in the shape of Homo Sapiens . On other planets ,provided that life similar to the form it takes on earth has struck root so to speak then it could have evolved in regions having environmental conditions similar to those on Earth ,and may well have reached a similar level of complexity . One can imagine too that similar life forms could also have evolved under considerable environmental differences than our own ,eg with different levels of gravitational force . Despite the environmental differences .they might not appear outwardly to be significantly different. Whether life can exists in totally chemical configurations is yet to be determined

ljk January 25, 2014 at 18:59

Earth Won’t Die as Soon as Thought


2014-01-22 14:30

Take a deep breath—Earth is not going to die as soon as scientists believed. Two new modeling studies find that the gradually brightening sun won’t vaporize our planet’s water for at least another 1 billion to 1.5 billion years—hundreds of millions of years later than a slightly older model had forecast. The findings won’t change your retirement plans but could imply that habitable, Earth-like alien worlds are more common than scientists thought.

Humans are warming the planet by emitting heat-trapping gases like carbon dioxide. But behind the scenes, a far slower, deadlier warming process is unfolding. The sun is getting brighter and hotter over time. As it does, more water evaporates from Earth’s surface into the atmosphere, where it traps additional heat from the planet. This water-driven greenhouse effect will keep going long after people have stopped burning fossil fuels that now add CO2 to the atmosphere. Eventually, Earth’s greenhouse effect will spin out of control, vaporizing all of our planet’s water and ending life as we know it.

How long does Earth have? Climate modelers disagree. In one recent study, planetary scientist Ravi Kopparapu of Pennsylvania State University (Penn State), University Park, and colleagues used computers to model how Earth would respond to increasing solar radiation. Just 6% more sunlight was enough to send the greenhouse effect into overdrive and vaporize Earth’s water, the researchers found. At the current rate of solar brightening—just over 1% every 100 million years—Earth would suffer this “runaway greenhouse” in 600 million to 700 million years.

Earth will suffer some preliminary effects leading up to that, too. After just 150 million years, the researchers found, the stratosphere will warm enough to let some water vapor reach high in the sky, where solar radiation will break it down into molecules that can escape to space. In this “moist greenhouse,” the planet would be too hot for complex surface life, but a few hardy marine organisms and microbes could soldier on.

But not so fast, says Eric Wolf, a doctoral student at the University of Colorado, Boulder. Kopparapu’s model is pretty rudimentary, Wolf says: It analyzes what happens in one dimension—altitude. As a result, the model excludes clouds and wrongly assumes that climate factors like humidity are the same everywhere on Earth. Wolf and his Boulder colleague, Owen Brian Toon, simulated Earth’s future using a more realistic 3D climate model from the National Center for Atmospheric Research. Their model included clouds, and a host of other details such as regional differences in moisture, Wolf says. It also assumed that atmospheric CO2 levels would start at 500 parts per million—25% higher than today—and stay there indefinitely.

Full article here:

Alex Tolley January 25, 2014 at 21:36

@Eniac –
Today, life at depth is very sparse, existing rather tenuously, compared to what we see in watery habitats. It is important in so far that the volume of the lithosphere where life has been found is very large compared to the surface biosphere. If life evolved at depth, we might expect a large number of forms to have evolved there, but that is not what we see from the boreholes done so far.

For abiogenesis, there needs to be a way for pockets of precursor chemicals to form, with an energy supply (assuming that is the basic mechanism). However, I think that primitive selective will be needed beyond just forming amino acids and proteins or RNA, but I don’t see that unless we are talking fairly shallow rocks. [Can a geologist chime in to correct me?]

My guess is that lithospheric life is just trapped and surviving, rather than actively exploiting its environment.

So while “cannot evolve” might be too strong, “extremely unlikely” may be a better phrase.

Eniac January 28, 2014 at 21:43

Alex: We do not know the mechanism of abiogenesis, even less do we know the optimal environment for it. The much larger volume of the lithosphere would favor the chances of it having happened there. I do not see how the current density and distribution of life can be any indication, since it is driven primarily by photosynthesis and oxygenation, neither of which played a role in abiogenesis.

Alex Tolley January 28, 2014 at 23:13

Here are my thought experiment predictions if abiogenesis does not happen in the lithosphere:

1. Living species will be more diverse and abundant in sedimentary rock than igneous.
2. I would predict that an igneous intrusion into sedimentary rock would should a distinct decline in diversity and numbers the deeper into the intrusion you drill, i.e. distance from the sedimentary source.
3. The species found will correlate quite well with those in ocean sediments. One might even be able to test origin using DNA sequencing.

If data refutes these findings, then I would certainly consider a a very different hypothesis.

Eniac January 29, 2014 at 23:35

Alex: I am sure your hypothesis has merit and is likely to bear out. However, it only tells you about the distribution of current forms of life. It says nothing about where the earliest forms lived. If you take away high energy respiration and medium energy photosynthesis, the chemical energy resources in the lithosphere do not look so bad in comparison to whatever else you might come up with. Life will be where its resources are. Certainly, over billions of years, it will have covered the planet thousands of times over and retain no memory whatsoever of its initial location.

Eniac January 29, 2014 at 23:44

Oh, and I think you alluded to this: Before the evolution of cell membranes there would have to have been another, inorganic, form of containment. A mostly solid environment seems like a plausible way to provide this.

Alex Tolley January 30, 2014 at 11:33

@Eniac – cell membranes are almost trivial to make. Phospholipids spontaneously form vesicles in water.

As regards current vs historic distributions of bacteria. Bear in mind that the bacteria can barely move between rock grains. Their metabolism is so slow, that they might take millenia to reproduce (instead of hours). We can find rocks billions of years old. As you say this does not rule out abiogenesis in the lithosphere, with subsequent migration to the surface to evolve rapidly and then be trapped again for millions of years in sediments. However, the slow metabolism and trapped nature of the bacteria suggests to me that any other abiogenesis in more energy favorable environments is more likely to occur and dominate. What I would want to see is evidence that the lithosphere in some way contributes to abiogenesis in a way that surface rocks cannot.

I recommend Robert Hazen’s Genesis: The Scientific Quest for Life’s Origins as a good primer on theories of abogenesis.

Eniac January 30, 2014 at 21:50

Phospholipids spontaneously form vesicles in water. Alex:

True, but the phospholipids themselves do not. It takes metabolism to make them, metabolism that has to evolve first.

As for “more energy favorable environments”, I agree. But what are they? By lithospheric I do not mean to exclude surface rocks, or undersea rocks, as long as they are rocks. For all I know, when you exclude photosynthesis, rocks are easily be the most energy rich environment. What is the alternative? Ocean water, I suppose? Not much chemical energy there, at all. Or is there?

Alex Tolley January 31, 2014 at 11:11

By lithospheric I do not mean to exclude surface rocks, or undersea rocks, as long as they are rocks.

Then I agree with you. But here’s the thing. The lithosphere is deep, and we do find living organisms at kilometer+ depths, which was once never considered. For some worlds, like Mars, where the surface is [now] inhospitable to life, a lithosphere that is still warm enough for liquid water may harbor life at depth. Worlds that are too small to ever hold atmospheres, yet are warm enough to still contain some liquid water at depth could also harbor life if abiogenesis is possible at depth. I hold out a little hope for Mars as life could have retreated to the lithosphere, but I don’t think the latter case of life forming at depth is possible.

Eniac February 7, 2014 at 0:15

I don’t think the latter case of life forming at depth is possible.

I am not sure I understand your reasoning, though. Do you claim there is too little chemical energy at depth compared with the surface? After excluding light and oxygen? What makes you think that?

Alex Tolley February 7, 2014 at 17:36

Energy must flow to be useful. Where are the sources and sinks going to be in rocks? Note that it was recently found that bacteria create their own electron sinks in seafloor surface sediment to handle this flow.

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