The ‘subterranaut’ describes how the discovery of ancient bacteria miles beneath the Earth’s surface opens the possibility of finding life on Mars. He picks five books that show how our knowledge of life deep in this planet could lead us to discover it elsewhere.

What life exists below the surface of the earth?

Previously biologists believed the only subsurface life was at the soil zone, that you go a metre down and it is inconsequential, except for in caves. But even then the people looking in caves didn’t realise the caves were being formed by sub-surface life. There were actually bacteria eating the rocks that make the caverns. They were looking at these minerals and gypsum deposits and saying “how did these things get here, how did they form?” — not realising that these enormous rooms that exist in caves are produced by a thin layer of bacteria dissolving the rock in order to get minerals. That’s an underlying theme of all subsurface life. It’s bacteria eating rocks and living off energy in the rocks and in the process dissolving the rocks and making more room.

We call these things chemolithoautotrophic microorganisms. They’re able to make organic matter from the inorganic carbon and nitrogen that exists down there. Other organisms come in and start living off the waste products of the organisms that are feeding away on the rocks, and then other organisms come in, like nematodes, that feed on the bacteria that are feeding on the waste products of the other bacteria. They start developing these complex communities.

How far down?

Potentially on our planet it can go down, depending on where you are, ten kilometres. That’s quite deep. Porosity-wise, things get quite small down there. But if you go to a planet like Mars, where the gravity field is one-third of that of the earth, the porosity at depth is quite large. So, even though Mars is a small planet, the room available for sub-surface life on Mars is as great as it is here on the Earth.

What are the implications of the discovery of subterranean life?

One of the first things I learned about bacteria is that unlike us, or trees, or even nematodes, they don’t have any finite lifespan. It’s not genetically wired into their chromosome. They can live forever. So one question is: how long can they live beneath the surface? Clearly, as long as they are getting energy, the answer is forever!

Anything could happen on the surface of our planet. We could lose our atmosphere from a nuclear holocaust or a really big meteor impact could wipe everything off the face of the planet—and a kilometre beneath the surface the bacteria would be perfectly fine. They wouldn’t even feel it. That was the immediate implication.

When I was driving home from the first talk I ever heard on subsurface bacteria it occurred to me that life could live beneath the surface of Mars. It could still be there today and alive beneath the surface. And there could be other places in the solar system where you’d also find life beneath the surface of a planet, even though the surface was completely inhospitable to life as we know it.

Can you outline the challenges scientists face in doing this kind of research?

There’s no proof that life exists on or beneath the planet Mars, just all this potential. The question is: how do you go about looking for subsurface life on a planet? One challenge is that the surface is obviously very dry and cold now and if you go beneath the surface you have to go through about two miles or five kilometres of solid ice-saturated rock to get to the liquid water that exists beneath that, where all the organisms would presumably be living. That is not going to happen any time soon.

“You’d expect alien life to be completely different to what we encounter here on Earth, and what we encounter here on Earth is pretty alien itself.”

Every now and then scientists at NASA identify a feature on Mars that looks like a spring came out of the side of a crater and splashed down a hill. Those regions, if they truly represent an aquifer breaking out of the icy crust and pouring down through a crater, would contain inhabitants of the subsurface. The problem is that they have been designated as ‘special regions’, which means that if we go to investigate we have to have a specially sterilised rover that contains no earth organisms that are hitching a ride to Mars. So we can’t go there to look for life. That’s one of the challenges.

So what are we going to do? The other things you could try to look for are the ejecta from meteorite impacts that you might find, and try to bring back some of that rock to earth and analyse it here. There are other places on Mars where you see the crust exposed where you might try to obtain a sample you could bring back to Earth that would represent the fossilised remains of subsurface life. Or maybe even relatively intact material that you could resurrect, because the surface is so frozen anything alive would be frozen but preserved for a certain period. By the time you got these samples back, which would be twenty years from now, we’d have probably developed all sorts of techniques for rejuvenating dead life.

So those are the challenges.

Read 1 Journey to the Centre of the Earth by Jules Verne Read

The first book you’ve chosen is Jules Verne’s

Journey to the Centre of the Earth. Imaginative fiction is a good place to go to get everyone excited about what life could exist below the surface of the earth.

Jules Verne is a natural in that regard. The book has just enough science that it seems real. If you read it as a kid and re-read it as a geologist, you think there are some very interesting things in there.

He plays around with certain facts. He comes up with a very interesting theory to explain that it doesn’t get hot as you go deeper underground (which was in vogue at the time), but the book imagines the preservation of prehistoric life in the subsurface and that’s something we’re still looking at. Many of the organisms that we find down there today look to be, from an evolutionary point of view, extremely primitive. The conditions we find them in are very much what the surface of the Earth used to look like, three billion years ago. There is no oxygen and three billion years ago there was very little oxygen on the surface of our planet.

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They find the subsurface through Snæfellsjökull, an ancient volcano in western Iceland. Iceland, as we now know, sits on top of a mantle plume. It’s formed of magma that ascends from the core mantle boundary, essentially the centre of the earth. If you’re going to propose going down a volcano, you would propose going down a volcano which is an ancient mantle plume volcano. So I found that interesting.

The other thing that yielded fruit for me is that he cites, in the book, the explorations of Alexander von Humboldt into a cavern in Venezuela. I started looking at Alexander von Humboldt’s work and realised that he’s the earliest published biologist of subsurface life. When he was 24 he was a mining engineer upgrading the mines of the Prussian Empire. He wrote a tome on all the fungi that he discovered in the gold mines near Freiberg. This is the first reported gold mine life, in 1790, and now, nearly 200 years later, I’m following Alexander von Humboldt’s footsteps in a South African gold mine.

Via Jules Verne.

Via Jules Verne, otherwise I would never have figured that out.

Read 2 Intelligent Life in the Universe by Carl Sagan & Iosif Shklovsky Read

The second book you chose is Iosif Shklovsky and Carl Sagan’s Intelligent Life in the Universe.

This is a great book. It was published in 1966 and I read it in 1968, the year Apollo 8 circumnavigated the moon. It was an exciting time, 2001: A Space Odyssey was released, Star Trek had been on for one year.

This is the first astrobiology book. A dozen or more astrobiology books have appeared over the last ten years, it’s become such a hot topic, but many of them repeat this book. Certainly our knowledge of the planets has increased since then. But the underlying principles are still all expounded in this book. It has everything from cosmology, the origin of the universe, all the way up to searching for intelligent life and first contact.

You won’t find another book remotely approaching this until ten years ago when the NASA Astrobiology Institute was created. More and more scientists got involved, they started teaching at university level and that’s when the books proliferated. But they still cannot beat this book. After all, it’s Sagan.

Did the ideas put forward in this book go into your field?

It was because of this book that it was quick for me to make the connection between subsurface life and life in the universe. Sagan has several chapters about Mars but he never mentions subsurface life because at that time it was not known to exist. He talks about panspermia and he talks about shielding bacteria from cosmic rays during their transport between star systems. But of course he knew nothing about subsurface life, that was the missing piece of the puzzle. So you put those two together and you get to what we’re doing today.

What is the possibility of us finding any kind of life?

The possibility is so close to us. If you look up in the night sky, you see Jupiter. With a pair of binoculars, you can see the Galilean moons flying around it, and the second moon there is Europa, which has an ocean larger than all the oceans on Earth combined. All you have to do is send a probe there—the cost of the probe is really not that much, in terms of the amount of money you spend as a society—to answer that question. It’s right there in front of us; staring at us. You don’t have to send a probe to another star system, it’s right there.

The only question really is whether or not, in the history of Europa, life could have originated beneath that icy crust. That’s the real question. We don’t know enough about the origin of our own life to answer it. We know some of the steps, but we’re just not entirely sure whether or not having photons interact with some of these pre-biotic molecules is the key step. He talks a little bit about that in his book as well.

Read 3 The Universe Below by William Broad Read

The third book on your list is William J. Broad’s The Universe Below (1998). Why did you pick it?

When I read this book I was only two years into this new field I was working in. I was impressed by three things.

One is that what stimulated this book was the discovery of deep-sea vent communities. This was back in the late 70s. Scientists sent a submersible and discovered that around these deep-sea vents you had riftia tubeworms and crabs and shrimp, and they were all feeding off of bacteria, not requiring photosynthesis. That was a big shock for the biological community. Life was never the same after that.

“Anybody with a really good telescope can go search for their own exoplanets, particularly if they’re around red dwarfs.”

Another is that he goes back in time, to the voyage of the Beagle and other epic 19th-century expeditions where people were trying to figure out what was deep beneath the ocean. The idea then was that, hidden in the really deep waters, were prehistoric beasts. There was the fishing crew that recovered a Cretaceous coelacanth and made them think there was a domain of animals down there from the Mesozoic.

The third interesting aspect is that once the Department of Defense developed submersibles designed to rescue submarine crews from disabled submarines at depth, they were turned into scientific tools. A general in the navy department manipulated the funding so that marine scientists could get access to these tools and explore the deep. It reminded me of the situation I was involved in with the Department of Energy, which was the primary supporter of research into subsurface life. It was the programme that started the investigations anew. Frank Wobber, as programme manager, kept this programme alive for ten years until finally the DoE pulled the plug.

It seemed very parallel to our own experience: first exploring a complete unknown, secondly the idea deep life was ancient, thirdly the politics behind big science. Without the Department of Energy, God knows how many decades would have passed before people started thinking subsurface life is something worth pursuing.

What are the different pressures on life surviving deep under water and life underground?

Because you’re at a higher pressure and low temperature, there are some similarities. But deep-marine ecosystems are complex mixtures of different kingdoms and bacteria, and there’s always oxygen down there and that’s why these organisms exist—because they’re breathing dissolved oxygen. When you go into the subsurface, you’ve got the pressure, you’ve got higher temperatures, and you don’t have the oxygen. So now you’re exploring a different domain of life, where predominantly bacteria live, or any type of multi-cellular animal that can live off tiny amounts of oxygen or by some other means. Those are the main differences between these two domains of life.

The initial perceptions as we were going into this field were that the life forms would always be very primitive and their level of activity would always be very slow and that, in order for them to do anything, you’d have to stimulate them to do something. Over the course of my book, you discover that, in fact, these systems are far more complex than we could have imagined and that they’re extremely active and they’re constantly churning over energy and nutrients and carbon. They’re very efficient at recycling all their waste products as well. That’s different from what you see in the deep marine systems, where you have the mass expanse of open space and water and dissolved oxygen and you’re living off whatever’s coming down from above that’s being fed by the photosphere. The subsurface is completely detached from the photosphere of our planet, so the ice ages can come and go, meteorite impacts can evaporate the oceans, but that won’t affect these guys down there, a mile or two underground.

Read 4 A Traveler's Guide to Mars by William Hartmann Read

The fourth book is William K Hartmann’s A Traveller’s Guide to Mars (2003).

Again, it’s a little bit dated. We’ve gone through a decade of amazing Mars exploration.

This book partly tells the history of the exploration of Mars. In 1976 the Viking missions landed—the first and last life detection experiments ever carried on a spacecraft—and then did not discover life (Carl Sagan was very much involved at the time). There’s a perception that, at that point, NASA or Congress lost interest in going to Mars, or the public did. So it wasn’t until twenty years later that they finally sent a tiny rover to Mars. After that a continuous campaign of missions mapped the planet in all of its beautiful geological history. This book came out when we were getting the very first high-resolution space images of Mars, fantastic photography which reveals Mars as far more complex than we thought we had left it back in 1976. So now we realise it has a whole history of climate change and recent glaciation, and that there are a lot of ice bodies close to the equator. We didn’t anticipate this, and it has lots of implications for human exploration on Mars. Essentially, this recent exploration made it much more Earth-like.

“As you read through this book you’ll come across this conundrum: where do you go to search for ancient life on Mars?”

It’s a good book, both in terms of providing the reader with a fantastic history of the exploration of Mars (even though it doesn’t have the latest missions in it) and making the geology of Mars much more exciting. And how can you go wrong? The images that you’re looking at in this book are superlative. In terms of a book that’s readily available to the lay person who’s never had any knowledge of Mars before, it’s a good way of getting an excellent introduction.

What are the implications of the discoveries of Mars’s geology?

Mars appears to have had liquid water on its surface in the past. You see these beautiful deltas that look very much like the Mississippi River delta. You have water on the surface, but, as it says in the book, how could Mars have had an ocean in its northern hemisphere three and half to four billion years ago when the Sun was actually cooler then? We have no means of explaining how Mars could have been warmer than it is today. The idea that Mars had a very thick atmosphere and greenhouse gases—sufficient to raise the temperature up to the melting point of ice in the presence of a sun which had only 70% of its current luminosity—has failed totally.

The geologists that work on Mars today are stuck in this Martian conundrum. Episodically, it may have been warm enough under a combination of situations: impact debris that somehow generated a temporary warmth that melted water which flowed across the surface. Then everything froze back up again. If you’re left with that scenario, it’s challenging you as to where to look for life. That’s really the implication of this book. In the next mission that goes to Mars from NASA, in 2020, we’re going to send a rover to collect samples for a return to Earth. Those samples are supposed to contain, hopefully, biomarkers of ancient Martian life. So where do you go? Do you go to these fluvial deposits, even if they only formed within a thousand years? Or do you go look for something which is from the subsurface and may have been in existence for a billion years? As you read through this book you’ll come across this conundrum: where do you go to search for ancient life on Mars?

And where are you in terms of solving that conundrum?

I don’t think the conundrum is ever going to be solved. I think that these deposits that look like they were formed in the presence of liquid water on Mars were actually formed in very short episodes in time. That has implications. On Mars, an ocean will not have existed for very long and it would have existed under a very thick layer of ice. What does that mean about the potential for life originating on the surface? And if it did originate on the surface it would have inhabited the subsurface. There’s no doubt about that, it will penetrate down. The fluids will go underground and life will go with it. If you’re life, you will survive longer beneath the surface of Mars than you would on the surface because that icy lake that you’re in is going to evaporate and dry up and you’re going to get exposed to radiation because there’s nothing to shield you from the radiation in the atmosphere. But if you’re down there, you’ll be doing all this stuff that we know subsurface life has the ability to do. We know from the evolutionary record that, on Earth, the most primitive organisms are those that exist in the subsurface, whereas organisms that grow by photosynthesis appeared later on. It takes time to develop photosynthetic enzymes, it could have taken a billion years to develop that on Earth. On Mars you may not have had that amount of time to do that before the radiation came and destroyed you.

That’s the battle that we’re now engaged in. It’s a hypothetical battle. Do we expect photosynthesis to have evolved on Mars? If it had, then, yes, we’d expect to see beautiful mats existing on the surface in the lakes. But if it had not, then it has to be life that exists like chemolithoautotrophes, feeding off the rocks, and where do you expect them to live for a long period of time? In the subsurface.

Read 5 The Living Cosmos by Chris Impey Read

Your last book, The Living Cosmos by Chris Impey, was published in 2011.

The thing one should look for in a book like Chris’s is all the work being done on exoplanets. That’s a field that has exploded and should be, on principle, sustainable. Anybody with a really good telescope can go search for their own exoplanets, particularly if they’re around red dwarfs.

We can expect to see more instruments and space telescopes emerge that allow us to analyse the atmospheres of these planets, especially those transiting planetary solar systems. Trappist 1, for example, a star which is just bigger than Jupiter—it’s not even a red dwarf, it’s like a brown dwarf—has seven planets around it that are, in many respects, very terrestrial. It’s more like the Galilean system around Jupiter. It’s a different class of solar system that has planets that could be habitable. Could life arise and civilisations form on those types of systems?

“It will be interesting to see how this field progresses and how many years we are away from actually discovering life. ”

Astrobiology has evolved from characterising simply whether or not life exists in a zone where liquid water is on the surface to searching for atmospheric biosignatures, or even spectroscopic signatures. This is described in Chris’s book. If you point a telescope towards an exoplanet that’s relatively close and see the infra-red spectroscopy of a sun bouncing off it and coming to Earth, you could see a signature that indicates whether you have chlorophyll on the surface. If the chlorophyll signature comes and goes with some kind of seasonal variation (you can see the seasons on the exoplanets come and go because you know the orbit), then you would say this has got to be a sign of life.

It will be interesting to see how this field progresses and how many years we are away from actually discovering life. We may find that discovery on exoplanets long before we find it on a planet like Mars or Europa.

The things you’ve found beneath the surface of the Earth are so surprising, has that changed your understanding of what we might call an alien?

Yes, definitely.

So much space is available in terms of DNA sequences or amino acid sequences. Even if you have life restricted to using just the same four nucleotides—which is probably not the case—or even the same 20 or 21 amino acids—which we know is not the case—there’s so much variation that you can never sample that variation within the entire history of the universe on all the stars that exist in the universe. It’s just impossible.

The life forms that have evolved here on this planet have followed—in the possible paths that could exist—a very, very narrow path. It’s very easy to imagine that life out there would be drastically different from anything that we’ve seen here on Earth and that it could live under different conditions to what we know of today, easily.

That comes from this exploration of the subsurface and all we’re seeing is slight variations of terrestrial life. Slight variations in how the genetic code is translated into a protein. Just normal Darwinian evolution occurring under an extreme environment leads to these strange colonies of syntrophic organisms that interact in ways that we’re still trying to figure out. They’re signalling to one another using chemical processes, actually using electrons directly. If bacteria can do that, why not entire organisms?

So yes, you’d expect alien life to be completely different to what we encounter here on Earth, and what we encounter here on Earth is pretty alien itself.