Are We Alone? An Interview with Dr. Norman Sleep

The following is a transcript of an interview conducted by Charles H. Lineweaver with Norman Sleep, on 14 January 2020 in Professor Sleep's office at Stanford University, Palo Alto, California, USA. The verbatim transcript has been edited for clarity and brevity. A video of the full interview can be found at https://youtu.be/gWfUNgtnNCU

Charley Lineweaver: Could you give us a brief introduction of yourself?

Norm Sleep: Sure, my name is Norm Sleep, and I am a professor of geophysics at Stanford.

Charley Lineweaver: Norm, you are primarily known as an Earth scientist. Can you tell me about your journey? Where did you grow up, and how did you become an Earth scientist?

Norm Sleep: I grew up in Kalamazoo, Michigan. It's a glaciated area. Outcrops are extremely rare, but the glacier brought everything down from the north, so fossils from the coral reefs are present in Michigan from several hundred million years ago, and billion-year-old rocks from the Canadian Shield, such as granites, were very common.

Charley Lineweaver: Is that how you got interested in rocks?

Norm Sleep: I had the good fortune to be in the Boy Scouts, where we had a trip to the western United States. We arrived at Yellowstone right after the 1959 earthquake. Geysers that hadn't gone off in the last 50 years were going off. Crystal Pool was full of mud. I suddenly realized that the Earth was active, not just something that had sat there, and that piqued my interest more.

At that time, I was living in the Bible Belt of Michigan. So we had no evolution education at all in 9th-grade biology. In 12th-grade biology, they had a very thin chapter on evolution with no connection to any other biology. The teacher announced at the beginning of this that she had no opinion of evolution whatsoever.

Charley Lineweaver: What year was this?

Norm Sleep: This would be 1962—it's after Darwin! So I started to press her, but then I stopped suddenly as I realized that she'd given a ringing endorsement of Darwin. Anything more positive about evolution, and her job would have been over by nightfall.

Charley Lineweaver: How did this shape your interest in astrobiology? I would have thought that would have put a damper on it.

Norm Sleep: It temporarily put a damper on any interest I had in biology. Up to that moment, I had no idea I'd be any good at research science. My ambition went to being a high school math or science teacher, not beyond that. Suddenly, I was driven to avoid the fate of having to put up with all this duplicity.

So I went to Michigan State University. Biology, at that time, was kind of a gatekeeper class for the premeds. A lot of memory, very little science. I had interest in geology. I realized very soon that a lot was known about what had happened in geological time, but very little was known on how it happened as physics.

At Michigan State, continental drift was treated as a testable hypothesis rather than an abomination. And I knew I had to get the basic math, physics, and chemistry, which I did. I enrolled in MIT, and then plate tectonics suddenly fell in my lap.

Charley Lineweaver: MIT played a big role in this?

Norm Sleep: Yes. The senior faculty were off working on the Moon program, so the younger people had to kind of run the scientific revolution, in part.

We had a conference at Woods Hole in September, just before classes started. There had been a big research program in the Atlantic to see if seafloor spreading actually occurred. By the third talk, it was evident that seafloor spreading was basically correct. And by the end of the conference, it was very evident to me that the physics were unclear as to why things were occurring. The plate tectonics came a couple years later.

Charley Lineweaver: But, wait, you're talking about plate tectonics. What does that have to do with astrobiology?

Norm Sleep: It gets into the physics of how planets work. If you understand how the Earth works, you can export it to the early Earth. You can hopefully export it to other planets.

Charley Lineweaver: I think the first time I came across a paper of yours was back in 1989. You wrote a paper (Sleep et al., 1989) on something like how big does an object have to be, if it hits the Earth at some normal velocity, to kill all life? Such an interesting question. So, how big does an object have to be?

Norm Sleep: Somewhere between 300 and 500 km in diameter. A 300 km object would boil off the ocean or leave the brine in the ocean so hot that it couldn't support life.

Charley Lineweaver: Okay. Moving on, how long have you been at Stanford?

Norm Sleep: Since 1979.

Charley Lineweaver: And do you teach any courses that are related to astrobiology?

Norm Sleep: I taught a nonmajor astrobiology class. We have seminars where things come up, but we don't have a formal class now in astrobiology taught by me.

Charley Lineweaver: What is the closest thing to astrobiology at Stanford?

Norm Sleep: It is scattered, unfortunately, between many departments. They're starting to set up a space science, astrobiology consortium with people in different departments. But most of the people are doing the physics and chemistry of other planets rather than directly astrobiology.

Charley Lineweaver: So, here we are in the Earth Science Department, and I noticed that by talking to Earth scientists, some of them are interested in the effects of life on the Earth, and others just aren't. Now, you, of course, are interested in astrobiology and in the effects of life on the Earth.

Norm Sleep: You pick up a rock. Say, this is a lava that's come up from the interior of the Earth. If we analyze this rock very carefully, analyze isotopes of rare elements, we will probably see the effects of life.

And pieces of this material are incorporated in very trace amounts, but we can see the effects. If we take a normal mid-ocean ridge basalt, we can see the biological effects of uranium isotopes on what gets subducted in the mantle and what stays up in the crust.

Charley Lineweaver: The mantle plumes, they're very deep. Would we see evidence of life in them, as you described?

Norm Sleep: Yes, in Hawaii you can see the evidence of subducted manganese nodules, which are only produced if there is life—they're only produced if the ocean is oxic. So they had to be produced in the last 1.85 billion years.

Charley Lineweaver: Have you been trying to get Earth scientists more interested in the effect of life on Earth and therefore closer to astrobiology?

Norm Sleep: Definitely. We published a paper that was provocatively entitled “Paleontology of Earth's Mantle” (Sleep et al., 2012).

The typical geologist is trained to study a hard rock. It is not the typical geologist that's trained to study a fossil. So the biological effects that you see in the mantle get treated as oddities. They do not get treated as evidence of life on the early Earth.

Charley Lineweaver: Right. Now, I know people are looking for fossils, and they go back 3.5 [billion years]. But you're talking about isotopic evidence of life. Does that usually show up earlier than the fossil evidence?

Norm Sleep: We see diamonds with nitrogen isotopes that are appropriate for organic matter that was subducted before the Great Oxidation Event. We know this because the 14-to-15 ratio gets changed by life. Because of the way the fractionation works, the sign of the fractionation differs before, compared to after, the Great Oxidation Event.

Charley Lineweaver: Let us suppose that there was no life on Earth. What type of place would it be?

Norm Sleep: One of the biggest effects may be that life causes weathering. This [hand sample of] shale is composed of basically fine-grained silicon dioxide, but the silicon dioxide and the clays in shale—all that comes from weathering of rocks on land. Life greatly speeds up the weathering, so we would have a much slower rock cycle.

Charley Lineweaver: Speeds up the weathering on land. What if there were no land?

Norm Sleep: There might be no big continents built up, because it becomes a lot harder to have the weathered rocks, like this shale that can get remelted to form a granite. And granites are kind of what hold up the continents.

Charley Lineweaver: So, you're saying that life increases erosion? I have a garden, and when there's roots, they stop the erosion.

Norm Sleep: They stop the erosion, but they cause the chemical decay. Eventually, your garden will erode over a long geological period of time. It will erode as weathered rocks rather than pieces of unweathered granite, or whatever is in your garden. Life greatly speeds up the chemical decomposition of rocks in the hydrological cycle.

Charley Lineweaver: What kind of life are we talking about? Are we talking about lichen, bacteria, archaea?

Norm Sleep: Bacteria, archaea, anything that happens to be around. During early Earth, we had sulfide photosynthesis. We also had the photosynthesis that will take ferrous iron and turn it into ferric iron, basically making rust.

In the ocean, there is a lot of ferrous iron and a lot of sulfide. The microbe gets sunlight then binds to CO₂. It doesn't have any problem making or getting the building blocks to make organic carbon.

Charley Lineweaver: If we imagine there was no life on Earth—would that rock exist?

Norm Sleep: We certainly would not have rock with concentrated carbon in it. We would not have rock with a strong sulfur cycle. We would not have a separation of iron from magnesium. We have rocks like iron formation. You have a lot of that in Australia, where there is ferric iron, and it makes it red. There's ferrous iron, but very little magnesium.

Charley Lineweaver: A lot of the things you mention are chemical, and they're hard to see remotely. But let's suppose that we are aliens, and we are on a planet around Alpha Centauri. We're looking at Earth with high-resolution telescopes. We have transit spectroscopy of light from the Sun going through Earth's atmosphere. As aliens, how are we going to detect life on Earth?

Norm Sleep: We would see the oxygen first.

Charley Lineweaver: But I've heard that oxygen can also be produced abiotically, for example, H₂O being dissociated in the upper atmosphere by UV.

Norm Sleep: That doesn't make much of it.

Charley Lineweaver: Well, if there's a lot of H₂O, and it gets up there, it would make a lot?

Norm Sleep: There is generally a cold trap at the base of the stratosphere on top of the clouds.

Charley Lineweaver: That cold trap has nothing to do with life?

Norm Sleep: Correct, nothing to do with life. If life wasn't here, we'd still have a cold trap. It becomes very hard to get water high enough in the atmosphere that it photolytically dissolves. We have lost tens of meters off the top of the ocean over geological history. This oxygen, there is very little of it, and it will very quickly react with pyrites or with ferrous iron in the basalt; it cannot build up to high quantities.

We have about 21% oxygen in the atmosphere. If we did not have any more photosynthesis, this would be used up within a million years. So this large amount of oxygen in a planet that is habitable, where it's clement, there is no good way that we know of to build it up without life.

Charley Lineweaver: Oxygenic photosynthesis is what you're talking about. But early photosynthesis was not oxygenic, right?

Norm Sleep: Right, and we would not be able to use that technique.

Charley Lineweaver: So if I am going to try to remotely detect life through its gases in the atmosphere, what do you think are the best biosignature gases that we should be looking for?

Norm Sleep: I would put oxygen first, but if we don't have oxygenic photosynthesis, we're not going to find it. I'd put methane second, because when organic matter rots anaerobically, methane is produced. And we can get a lot more methane by orders of magnitude than the methane we get from abiotic sources.

Charley Lineweaver: When you say rot, what do you mean?

Norm Sleep: You have organic matter made up of very complicated compounds, and it contains oxygen. It contains hydrogen. It contains carbon, if we're going to be very simple. The thermodynamically stable compounds are methane and CO₂.

A microbe will take the organic matter. It may just take organic matter by taking sugar, CO₂, and alcohol, but if you have different types of microbes that have different abilities, when push comes to shove, you'll eventually end up with CO₂ and methane.

Charley Lineweaver: But H₂O, NH₃, CH₄, and CO₂, these are things that are very common in comets. We don't imagine that they have been put there by life. And yet you're saying methane can be used as a biosignature. Presumably, you mean if you detect it at a much higher level than expected.

Norm Sleep: Yes, abiotically. You get a little bit of it abiotically. You get abiotic methane on the Earth.

Charley Lineweaver: Okay, but, on the other hand, methane on a rocky planet is something that comes to the surface. Let us say, I have an Earth mass full of cometary material. It will differentiate. All of the light ices will come to the surface, and at certain temperatures, methane is very prominent in the atmosphere. Like in Titan or even Jupiter, with lots of methane, and we don't think it was produced by life, right?

Norm Sleep: Agreed. But we know that Titan has a very low density, so that most of the planet is made basically out of something you could find in a comet.

Charley Lineweaver: So we're going to use the density of the planet to figure out whether it should have a lot of volatiles. If it is very high density, it shouldn't, and if yet we see it, then methane becomes a biosignature?

Norm Sleep: It becomes a biosignature, though not as good as oxygen. If we have good astronomy, we could see the change in the reflectance of the planet. If you look at light reflected from the Moon, you see the crescent of moon and you see the Earthshine. With technology available to Galileo and probably to Pliny, you can tell whether the Earthshine is created by the reflectance of light over an ocean or over the Sahara Desert. If we get a sensitive instrument and see the detailed spectra or the light reflecting off the Moon, we could see absorption and reflection bands—we can see leaves are green.

Charley Lineweaver: So here we are on Earth, do you agree that we should look at the most fundamental aspects of life on Earth and then use those as our best guesses about life elsewhere? For example, do you think endosymbiosis is something so fundamental that we should expect life elsewhere to do it?

Norm Sleep: We expect life elsewhere to do it, but we don't know whether that is going to lead to complex life. It has certainly helped the complex life here. We have very complicated, large one-celled organisms. Foraminifera and some of the larger ones reproduce sexually, just like multicellular eukaryotes.

Charley Lineweaver: Let me ask you about the carbonate silicate cycle. It is often cited as an abiotic negative feedback mechanism on the temperature of the Earth. What is your understanding of that, and do you agree with that? And how abiotic is it?

Norm Sleep: It is not really abiotic. The biology helps with the weathering, by a factor of a few, probably. The biology determines where the calcium carbonate ends up; the calcium carbonate is precipitated by shell-making organisms. Some of these are multicellular. Some are one-cellular.

Right now, on the Earth, we have pelagic organisms that live in the middle of the ocean and do photosynthesis. When they die, their calcium carbonate shells sink to the bottom of the ocean.

If you go to the ocean, near the equator, where they're particularly active, the whole seafloor is covered with recently dead calcium-carbonate-precipitating organisms. These organisms did not live on the Earth in any abundance until about 200 million years ago, and they've only been abundant in the last 100 million years or so.

Charley Lineweaver: How long has this carbonate silicate cycle been able to regulate the temperature of the Earth?

Norm Sleep: Probably in the last 2.5 billion years. We have carbonate reefs starting at about 2.9 billion; they progressively become more and more abundant.

Charley Lineweaver: Would you agree that you could not have this abiotic thermal regulation before about that time?

Norm Sleep: Well, we had biotic regulation. The calcium carbonate ends up in the basaltic rocks here, which are the black rock around the green rock in this sample. That weathers on the seafloor. And so we are still getting a cycle. We're just not getting the White Cliffs of Dover.

Charley Lineweaver: I see. Have you talked to James Lovelock or Lynn Margulis?

Norm Sleep: I never knew Lovelock. Margulis was a frequent visitor to Northwestern, so I knew her as I was there for 6 years on the faculty.

Charley Lineweaver: What did you think of her stuff?

Norm Sleep: I don't like her five kingdoms, but her endosymbiosis I found very persuasive.

Charley Lineweaver: There was a book chapter (Harding and Margulis, 2009) that she wrote with Harding about Water Gaia. The idea was that life somehow does something to keep the surface of Earth wet. Perhaps this is due to the clouds somehow, or the albedo. What do you think of this idea?

Norm Sleep: A lot of the cloud nuclei are biological, dead organisms. There are organisms that live in clouds. Not enough to make the clouds green.

Charley Lineweaver: Right, but how about enough to control the albedo of the Earth?

Norm Sleep: They affect the weathering. Start with black basalt—the Arabs of Arabia cross a field of basalt at night; Lawrence of Arabia is factually correct about this. They cross it with the camels at night because the black basalt would absorb all the sunlight, and it would be unbearable to cross during the height of the day.

If we weather basalt, it becomes progressively whiter. If we weather extensively, make clays that melt, it becomes granite. The granites weather—we get white quartzite, and we get a white sand desert, like the Sahara or the deserts in Australia, which are very reflective. They are now a thermostat for the Earth.

During the Ice Age, the Sahara Desert was covered with vast lakes, a vast forest. It was very absorptive of light. When the Ice Age ended, it progressively became drier. The lakes dried up. The civilizations pretty well vanished. The forest died off, and we ended up with this very reflective quartz sand desert.

Charley Lineweaver: Speaking of the Ice Age, does that have anything to do with life? Does life have anything to do with the Milankovitch cycles?

Norm Sleep: Life doesn't affect the Milankovitch cycles, but it enhances the effect.

Charley Lineweaver: Now, when I look at the Earth from outer space, I see about 30% of it covered with clouds. If there was no life here, but there was water, how would that affect the cloud cover?

Norm Sleep: We would probably get fewer clouds. Water gets remobilized. You take a tree: the rain falls, there is evapotranspiration. Before there were trees, you would have microbes that would do the same job less effectively. You would have microbial mats probably covering the entire continents except where it was very dry.

Charley Lineweaver: You're suggesting that life enhances the amount of clouds that a planet will have?

Norm Sleep: Yes.

Charley Lineweaver: By a factor of…

Norm Sleep: Probably not double it.

Charley Lineweaver: Is there a way to measure that?

Norm Sleep: You could keep track of oxygen and hydrogen isotopes. On the modern Earth, if you have rain, let's say, in the center of Australia now, where, while we're talking now, there are fires. If it rains there, we could record whether that moisture has come directly from the ocean or from evapotranspiration.

On the modern Earth, we get desert microbial mats. These affect the albedo. The Israeli-Egyptian border in the Sinai is easily visible from the Moon with the naked eye. The Israelis restrict goat grazing. So, this desert mat, which contains eukaryotes as well as microbes, builds up. The desert is dark. On the Egyptian side, goat grazing is not as regulated, and it all gets eaten, and we are back to a highly reflective quartz sand desert.

There were no goats in the Archean or the Proterozoic to eat the mat. There would still be microbes that would eat either the live or dead photosynthetic microbes, but we would still have this mat that would be less reflective.

Charley Lineweaver: In your research, what are the most important questions that you have answered?

Norm Sleep: Starting with my career in chronological order, the first would be that asteroid impacts could have a large effect on life. They have a negative effect. They exterminate life. The pinprick attack at the end of the Cretaceous wiped out the dinosaurs. It killed a large number of photosynthetic species, also, from fires and darkness.

So, this gave us ideas. We go back to the early Earth, when these big basins were forming on the Moon. Notably, if an impact wipes out everything, it's not interesting. The Titanic would not be an interesting story if there were no radio and no survivors. We'd just know it didn't reach port in New York, and the ship would have been found with submersibles 70 years later, akin to digging up fossils.

But we don't have fossils from that time. If we have a few biota survive that become the ancestors of what follows, the genome is effectively telling the story retold over 4 billion years. But there is still a lot of the story left—the ancestors of bacteria and archaea each appear to be a thermophilic organism, which is exactly what would survive an ocean-boiling impact down a kilometer or so where it's hot anyway, but doesn't get any hotter during the few thousand years it remains hot at the surface.

Using the CO₂ cycle, we calculated that after the Moon-forming impact, the Earth would stay in this thermophilic state for a significant time, millions of years, maybe tens of millions, if you push it. But it cannot stay in that state for the whole Hadean.

Charley Lineweaver: That was just after the Moon-forming impact?

Norm Sleep: Yes, just after the Moon-forming impact. We have to start out extremely hot—we get to clement, so we have to pass through thermophilic conditions. Basically, the CO₂ that's in the air after the impact has to be subducted down to a level of a few bar from around 100 bar to where the Earth will become clement. The Earth will be thermophilic from probably the time the pressure is 25 bar down to where it's a few bar. It takes millions of years to subduct that much CO₂.

And these big impacts, we were only getting a few of them, what you could count on your hand. There could be hundreds of millions of years between when life originates and when it gets almost exterminated by an impact. Hyperthermophilic organisms could evolve at the subsurface and then be the lucky survivors.

Then the molecular biologist has to take over, look at things in the genome, in the chemical makeup of the organism, which are complicated, and try to infer whether LUCA, the last universal common ancestor of both bacteria and archaea (which is not the first common cellular ancestor) started out cold and then we had this hot cross.

Charley Lineweaver: Right. Do you have an idea about the difference between the origin of life and LUCA, how long a period that might have been?

Norm Sleep: That could be tens of millions of years or 100 million years. It would be less than a billion years as we know that things teemed with life by 3.8 [billion years ago].

Charley Lineweaver: We were talking about important questions you've answered in your research. What else?

Norm Sleep: The concept that there were viable geochemical cycles before photosynthesis. When I was taught biology, it began with oxygen-making photosynthetic organisms. We had to have primary producers.

We know that in this rock, if water gets into it, there's energy from the reaction of the water and the rock that life could use. We have modern organisms that do not need the bounty of photosynthesis to live.

There's no way on the Earth that you can get away from the products of photosynthesis, but the organisms living in water circulating through this type of rock would be able to live perfectly well had the rock not been slightly contaminated by photosynthesis in the mantle.

The organisms don't care about the isotopic ratio of U-238 and U-235 or some subtle thing that a very skilled geochemist can measure that tells us that this rock contains the products of biology that's been subducted into the mantle.

Charley Lineweaver: Right now in this office, there's about 20% oxygen. You and I are breathing it. Do you think that aerobic respiration in animals in general was evolved in order to buffer the amount of oxygen in the atmosphere that was being produced by the oxygenic photosynthesis?

Norm Sleep: If we get a lot of oxygen in the air, rock rusts faster, and forest fires become much more prevalent. So it becomes harder for leafy plants to live, because they catch fire.

People have looked at the Permian Period, where we had insects as big as this room. Insects on the modern Earth don't have particularly efficient ways to get oxygen out of the air like we do, with big lungs. So, the idea is that the planet was maybe up to 30% or more oxygen then. Back before we had land plants, we may have been at 5%.

Charley Lineweaver: But what I'm asking is, do you think there's some type of regulation going on between the amount of aerobic respiration and the amount of oxygenic photosynthesis?

Norm Sleep: There is, but it's complex. And other than the forest fires, it's not well understood.

Charley Lineweaver: All right. What other questions have you answered in your research?

Norm Sleep: We've looked at the tectonics of Mars. Mainly other people have done that. Mars was tectonically active early in its history, probably before 3.5 billion years ago. Life could have easily got back and forth with meteorites. A modest-sized asteroid, like at the end of the Cretaceous, hits. There are no dinosaurs to wipe out then, but it ejects rocks into space. Within 8 or 10 months, these rocks are falling on the other planet.

Charley Lineweaver: Do you think that life on Earth came from Mars?

Norm Sleep: I think it had a good chance of doing that. On Mars, you could find rocks, plenty of them, that are more than 4 billion years old. The ALH meteorite that had possible biological signatures and the paleomagnetists have looked at the detailed properties of the magnetite in the rock. The rock has never been heated above 40 degrees C.

The rock fell in Antarctica. The magnetite would have been reset if it fell in Australia and lay on your desert during a hot day. This rock [in my hand] is 3.8 billion years old and has been up to 500 degrees C at least twice. We could just do the isotopes of the carbon. We would never even hope to look for DNA in it. The rock sat at the surface, so you'd find DNA, but it would be from modern biology, and people would not bother doing that.

Charley Lineweaver: Do you think that if we're looking for life elsewhere, or if we're trying to find something about the origin of life on the Earth, do you think we should look at the Moon or Mars?

Norm Sleep: The Moon is only useful for knowing the amount of asteroids that hit. We'd probably find a little bit of terrestrial life that has been ejected. We know meteors hit the Earth, so the Earth was showered with meteors at the end of the Cretaceous.

If we looked 4 billion years ago, we would probably find some. It would be a very difficult search. We'd need a lot of debris from the Moon.

Charley Lineweaver: Would it be better to look at the Moon or Mars for evidence of the origin of life on Earth from rocks on Earth that were sprayed into space?

Norm Sleep: That would be better for the Moon. Mars is just harder to hit with Earth rocks. If we were wanting to see life originate on Mars and seed the Earth, if we find a viable organism and it has 16S ribosomal RNA like a terrestrial organism, we would know very quickly that the Earth and Mars exchanged life, and we don't have an independent origin.

Charley Lineweaver: What are the most important questions that you're trying to answer now?

Norm Sleep: I've looked some at the tectonics on the icy moons—Io and Europa, mainly—how long they stay hot, how material is brought from the surface. Places where we have strong active ice tectonics now. I've looked at the physics of ice. On the Earth, a lot of it is in the camp of the molecular biologists. I use their results, but I do not attempt to do that myself.

Charley Lineweaver: Here is one question for you: Are we alone?

Norm Sleep: I think certainly not. We know now that there are at least many millions of planets in the habitable zone in our own galaxy. There are probably a trillion trillion planets—some huge number—in the observable Universe.

The reason that the aliens in science fiction and movies look like humans is it's easier to have humans disguised as them.

Charley Lineweaver: Yes. So, in the question—Are we alone?—what does the word “we” mean to you?

Norm Sleep: It can mean two things: microbial life or complex life.

Charley Lineweaver: How about viral life? That doesn't count?

Norm Sleep: Viral life counts. We don't really understand how viruses in total evolved. There may be different origins of viruses. We have RNA viruses and DNA viruses. The viruses that we see are pathogens or symbionts. They need a cellular organism to live.

Charley Lineweaver: Do you think that life can be defined by a cell wall, by cellularity?

Norm Sleep: Not necessarily. The search for acellular DNA and RNA organisms has been almost nonexistent. If we go back 30 years and look at a tree of life, almost every microbial organism on it is a pathogen. Some person or animal got sick, and the doctors and the biologists studied it.

We have things like the Asgard archaea that were not on the tree of life then because nobody had looked for them. If we start out with acellular life, there has been a proposal that acellular life originates in these white smoker chimneys; they are basically like travertine. They have microscopic pores. They're as big as a microscopic cellular organism. The organism doesn't need to make a cell wall. It's got it for free.

These chimneys only last tens, hundreds of years. The chimney field may last millennia, but that's short in geological time. That could be the seafloor spreading or something equivalent to it. If the organism is going to survive, it's going to have to disperse.

One of the ways to disperse it is it could package its RNA or DNA in a protein packet, send a lot of these out, like plants or some fish species that disperse millions of eggs. So even with modern cellular biota, sending a lot out would not likely be a problem. One of these will find a new chimney and colonize it.

So you've got something that looks a lot like a virus, but it is acting as a spore. Later on, when they get to cellular life, it finds a cellular organism and colonizes it rather than colonizing a chimney. Now we've got a traditional virus.

We don't know enough about viruses, but this is a potential path to get them, and we really don't know how the many kinds of viruses originated. There is valuable information in here, but it's in the camp of the molecular biologists. Viruses do not generally leave fossil records.

Charley Lineweaver: So let's just accept that there are billions of Earth-like planets with water on their surfaces. But that doesn't tell you that there's life there because we know so little about the origin of life. I guess you had to assume that the origin of life is easy in order to make that step.

Norm Sleep: It occurs fast on the Earth compared to the age of the Earth, which is one of the traditional arguments for it being easy. The elements life uses are very common elements on the Earth. Chemical disequilibria are common. Starlight is common. Internal energy from the interior of a planet is common.

For example, you take an ordinary zinc sulfide crystal, sphalerite, a relatively common mineral. There are microbes that can sit on it and eat the electrical current from it acting as a photoelectric cell. So the idea is about an electrotrophic origin of life, where it is ultimately photosynthetic but really electrotrophic. There are archaea and bacteria that will send biological wires between different cells that effectively act as a consortium of cells that act as a battery or a photocell, depending on how you want to think about it.

Charley Lineweaver: Would you agree with Christian de Duve when he says that life is a cosmic imperative?

Norm Sleep: I wouldn't go as far as “imperative.” I would say it is something that has to be extremely likely. If we find life on Mars that is really independent of life on Earth and not exchanged, we know life is everywhere. If we find simple life on Europa or Enceladus that is not exchanged—or exchange is difficult—we know life is everywhere.

Charley Lineweaver: Now, you're using the word “life” quite a bit here. Do you have any type of understanding of what life is?

Norm Sleep: It's somewhat like going out on a research project to find the world's smallest giant. Exactly when something becomes life or nonlife is semantic.

Charley Lineweaver: So that means there is no boundary?

Norm Sleep: There is a boundary between life and nonlife. There's something called threshold theory, that if you have nonlife and nascent life, it is almost like the nascent life is having to compete with the nonlife. The nonlife is doing the same chemical reactions that release favorable energy that the life is trying to do.

Once the life reaches a threshold where it can outcompete the nonlife, meaning it could gather energy, gather resources, and disperse, it very quickly becomes extremely abundant.

Charley Lineweaver: Why do you conceptualize that as a threshold? Why not a continuum?

Norm Sleep: It is a continuum, but once you pass a certain point in the continuum, the winner wins.

In every connected environment, you get these thresholds that produce threshold bottlenecks. The first organism that can efficiently do photosynthesis where it doesn't need molecular hydrogen, it can use ferrous iron and sulfide, that becomes extremely abundant.

Charley Lineweaver: Okay. So, thinking about astrobiology as somehow a scientific story of genesis of how we got here. It's a very big-picture thing. Do you think that studying that makes you a better person, like the examined life is worth living, the unexamined life is not?

Norm Sleep: I wrote a web book (Sleep, 2020) for astrobiology for serious liberal arts majors. Science has driven the change of worldview, and the change of worldview has driven science. Chemistry, physics, and modern biology are all things that come into astrobiology—astrobiology applies all the modern sciences. Take the question of the origin of life. Francesco Redi in Renaissance Italy showed that flies originate from other flies, eggs and maggots, rather than from rotting meat as spontaneous generation. Pasteur showed that microbes originate from other microbes, not from decaying organic matter. When we reflect on the origin of ideas, we see different aspects of astrobiology coming in at different times in human history.

Charley Lineweaver: Do you have any advice for students who are thinking about becoming astrobiologists?

Norm Sleep: My path was in other sciences with an astrobiology slant. There's a practical thing of employment—if your specialty is geochemistry, you can be in the regular department, do the astrobiology, but have the expertise in one of the things that astrobiology is derivative on.

There are places like Cal Tech, JPL [Jet Propulsion Laboratory], where you're essentially doing astrobiology full time. So there are various paths, but you need a strong background in the fundamental sciences. For example, if you're studying other planets, you need a strong background in geology, geochemistry, and tectonics. If you're going to be detecting life, a strong background in how light is absorbed in atmospheres.

There are a lot of people doing a lot of different things, and astrobiology conferences are big now. It is no longer like the Rare Earth book (Ward and Brownlee, 2003) where the authors can quote about 10 people including themselves that were actively doing it. It is a large field. Hopefully we'll have astrobiota soon.

Charley Lineweaver: Astrobiota?

Norm Sleep: Confirmed astrobiota, yes. Astrobiology as a field is scientific but has not yet conclusively found life on another planet. Even though I expect within the lifetime of young astrobiologists we will get at least fossil evidence from Mars.

Charley Lineweaver: What do you think are the biggest misconceptions that your colleagues here in Stanford, or scientists in general, or students have about astrobiology?

Norm Sleep: It is not like when I first started, where you were having to fight that you're doing astrology. We don't have to fight uniformitarianism requiring things in geology always to proceed at a pace that would be too slow for snails.

People know that asteroids have hit the Earth. Meteor crater, when that was understood, kind of shifted things. In the early Apollo program, senior scientists all thought that the craters were volcanic. By about the third or fourth mission, everybody knew they were impacts. But they did good science on the early missions, even though they had their concepts wrong. So science can proceed when you don't fully understand.

Charley Lineweaver: You are talking about misconceptions in the past. What about today? What are the misconceptions that are most common?

Norm Sleep: Among the public, that we're going to find little green men suddenly. Among scientists, the biologists are getting a better understanding. It was not that long ago, I was speaking at a conference. I had a large number of biologists, and somebody asked, “Why don't you go back and get a 4-billion-year-old rock and just look at the DNA in it?”

There is still a lot that is not understood. There are people that will do something very specialized. I was at a talk where they were doing prebiotic chemistry in the lab with ruthenium, which is a platinum-like element that's quite rare. It turned out that they had not thought about whether you could actually get ruthenium metal on the early Earth.

This is one of the things that drove me to look to see what on the early Earth could be abiotic. Now you have the oxygen around that's clearly biological. You can't have abundant oxygen in the earliest stages of a route to form life. So we went through a large number of things, including modern discoveries. I got to legitimately cite my grandfather, who studied the tallgrass prairie in the western United States. And the question there was whether the tallgrass prairie was human-managed by the Native Americans.

Over a century ago, this was a very unpopular topic, and he had relegated himself to being the starting faculty of the biology department of a small teachers' college. But that's become quite popular now: What is really wilderness? A large number of things that were thought to be wilderness by the Europeans were actively heavily managed by the Native Americans. And in this field, we have the same thing. The Earth has been so affected by biology.

Charley Lineweaver: If I gave you $100 billion and you have to spend this to help answer the question—Are we alone?—how would you spend it?

Norm Sleep: I would do Mars thoroughly. We could get a lot of rovers up there for $100 billion. If we get a few astronauts up there, we contaminate it.

Charley Lineweaver: You wouldn't invest in electron microscopes to look for nano-aliens?

Norm Sleep: Probably not. We don't know what nano-aliens are. I would spend money to look for free-living DNA and RNA.

Charley Lineweaver: What kinds of aliens would you like to find?

Norm Sleep: Peaceful ones. If we find aliens, they're likely to have been around for a much longer time than we have. And they will not be interested in us to eat. They'll be interested in our sociology, our science. They'll be mainly interested in our observational science. It would be a lot easier for them to get telemeter-detailed descriptions on how the Earth is working than it would be for them to actually send somebody here.

Charley Lineweaver: In the next 10 to 20 years, where do you think most progress will be made in astrobiology?

Norm Sleep: We'll get sample returns from Mars. Hopefully, they'll be useful. With molecular biology, which is advancing rapidly, we'll discover new groups of life—such as manganese-using photosynthesis––which is the bridgehead to get oxygenic photosynthesis. We'll likely get many more of these great new clades of organism. We'll probably get images of planets. We may find an Earth-like planet in the habitable zone and with an oxygen atmosphere.

Once these things are found, the incentive to spend billions of dollars to do better will come with it, because the public is interested. We had the rover land on Mars, and people filled the grassy areas around the Smithsonian to watch it on a large screen. People were watching it on live TV. The whole project to get the real thing cost less than one science fiction movie. The expense for nonhuman exploration is relatively small.

Mars life may very well be related to human life. We don't know if Mars life is related to Earth life, and we certainly don't know when it branched off. So we may find clades of organisms that are related to Earth organisms that, say, don't have the 16S ribosomal RNA, or it's so different we don't immediately recognize it. So we don't want to bring back a contaminated sample.

A molecular fossil would be the discovery of the century. If we find a live organism, it would be the discovery of the millennium. It would be regarded in the year 3000 as one of the major discoveries. I am optimistic and hopeful; there is a lot we don't know yet.