Christopher P. McKay, “Prospects in the Search for Life on Mars" - MA Space Grant Consortium Public Lecture at MIT (4/2/1997)

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MODERATOR: The Massachusetts Space Grant Consortium is a NASA-funded program whereby we're trying to distribute government funds and industry funds to worthy students and faculty members throughout the Commonwealth in an attempt to keep the spirit alive of the importance of space exploration and spacecraft technology and make sure that the next generation is able to have some good fun that those of us who were in the origins of the space program has had.

The members of the Space Consortium in this state led by MIT and through the Boston University, the Draper Laboratory, Harvard, Tufts, University of Massachusetts, Wellesley College in Worcester Polytechnic Institute, newly added this year, the Woods Hole Oceanographic Institute, and starting this summer, the Five College astronomy department [INAUDIBLE] in the Pioneer Valley. We have programs that involve summer internships, undergraduate research opportunities, and fellowships. And those who would like to find out more can find us on the web.

The speaker this afternoon, Christopher McKay, follows in a line of distinguished earlier speakers beginning with Bill Lenoir in 1990, and most recently Bob Stevens last year. Chris's subject, prospects in the search for life on Mars, is an exciting one to all of us. I was reminded that our department head in Aeronatuics and Astronautics here at MIT, Ed Crowley, said this fall what a wonderful summer it had been because now following the Allan Hills Satellite-- I'm sorry, the Allan Hills meteorite finding, the possibility of life on Mars. Ed says, isn't this wonderful? Now you go to cocktail parties and everybody wants to talk to you.

[LAUGHTER]

So we're-- certainly we're grateful, not only for the [? finds ?] but for the favorable public awareness that is associated with the exciting prospect of life on other planets.

Chris McKay is uniquely qualified to discuss the prospects for us, for exploration of Mars, with us. And I heard Chris talk about this on previous occasions, and I assure you, you're in for a treat. He received his PhD in astrogeophysics at the University of Colorado, went to NASA's Ames Research Center, where he has been ever since. He's a research scientist in the Space Sciences Division, the recipient of many prestigious awards, including the [INAUDIBLE] [? Urey ?] Prize by the Division of Planetary Sciences in astronomy.

In addition to his interest in geophysics, he has a strong interest in life sciences, both on this planet and what it tells you about exobiology, or life outside this planet. This has led him to work in such cold places as Antarctica, Siberia, and now Boston.

[LAUGHTER]

Chris, I'm glad you made it through the storm. We welcome you to the Massachusetts Space Bank Consortium.

MCKAY: Thanks, Larry.

[APPLAUSE]

Hey. [? Thanks, Larry. ?] [? What up? ?] As Larry was telling, I'm going to talk about today is Mars and the prospects for understanding whether there was life on Mars. Great, thanks.

Let me start by getting the right end of the view graphs. First, with just a one word chart. It's only word chart I use, I promise, but since I'm from NASA I'm required to use at least one word chart. I want to motivate what I do, what I'm interested in.

This is a goals of planetary sciences put together by some committee a while ago, and it's the standard stuff. Determine the origin and evolution the solar system, understand the Earth, survey the resources. But the one that I like is one in the box, and in some sense that's what I take as my job description, to understand the relationship between the chemical and physical evolution of the solar system, the appearance of life. And this is what got me into planetary science, and this is what I really do.

And just so you know, just last week, when I was doing my taxes, there's a little slot this says occupation, I started writing, to understand the relationship between--

[LAUGHTER]

I've never been audited, you know?

[LAUGHTER]

I think they're afraid of what they'll find when they come and look. Well, suppose that was your job, to understand the relationship between the chemical and physical evolution of the solar system and the origin of life. Obviously, most of your work would be on Earth. There's good samples. There's a good planet here. It's easy to get to. A lot of lifeforms. But other than the Earth, the planet that's the next best is Mars.

And so I'm going to talk about how Mars compares to Earth, how it represents a second example, possibly, of life, and our understanding of it as a planet and how we might investigate, and in particular, where we would go. And I'll use places on Earth as analogs, and our study in places like Earth, very cold, dry places. Like Larry said, Antarctica, Siberia, and now here at-- in Boston. Today-- as models.

Let's start with this image of Mars, which is a ground-based image. And if you go out, Mars is in the night sky now. You can look through a small telescope and see the polar caps and watch them grow and shrink and dark features, moving north and south. It's reasonable to suspect that this would've been water, and this vegetation, and I think that's what led to the notion of Mars as the planet most likely to have life, maybe even civilizations, because of this comparison with seasonal change on Earth.

And partly for that reason, in '76, Viking went to Mars, a little more than 20 years ago. And its mission was to search for life. As you may remember, it had a robotic arm to reach out, scooped up some dirt, and looked to see if there is any microbial life in the soil. We knew enough by then to know that there was no widespread biosphere on Mars, no civilization, no forests, anything like that. But there's still a possibility that there was microbial life.

What did Viking find? I won't go into detail, but basically, it found that the soil had some sort of reactivity, some kind of oxygen, bleach or something. It found that there was no organics in the soil at all. So the reactivity is-- that's why the activity was thought to be caused by some kind of chemical reaction. So it's not very interesting from a bio-- a life point of view. The results of Viking really were, it's dead, Jim. There's nothing there.

Why is Mars so dead? I think the answers-- where is the answer? Well, let's look at the atmosphere. Just to remind you what the atmosphere of Mars is like, it's not [? in ?] the atmosphere. This is the atmosphere you'd expect, mostly carbon dioxide, nitrogen, argon. Very similar to the atmosphere of Venus, very similar to the atmosphere that Earth had, early in its history. In fact, this is the kind of atmosphere you'd expect the Earth to have. It's only recently that it's been polluted by the presence of oxygen, only a billion or two years. Life has been producing this pollutant oxygen, replacing the CO2. And of course, human beings, in their wisdom, are getting rid of this polluting oxygen and bringing back the CO2--

[LAUGHTER]

--restoring the Earth to its environmentally-correct condition, which is this kind of atmosphere, which is what Earth-like planets should have. So the atmosphere of Mars is not surprising, and you can see that all the elements needed for life are here. CO2, nitrogen, water. And in fact, as we talk later about humans going to Mars, imagine taking this CO2 and taking the C off. You have oxygen, nitrogen, and those are the components of breathable air, and here's water. Basic requirements needed to sustain humans on Mars are right there in the atmosphere, ready to get to.

So it's not the atmosphere, per se, that's surprising on Mars. What's surprising is how thin that atmosphere is. And let me just show you the Viking results for three Mars years, six Earth years. This is a pressure at the two Viking sites, slightly different elevation. And the pressure's in units of millibars. As you know on Earth, sea level pressure is a thousand in these units. So the pressure on Mars is a hundred times thinner than the pressure on Earth. It's equivalent to the pressure about 20 miles high on Earth.

This is important in two respects. First, for humans, the vapor pressure of water at body temperature, 30 centigrade skin temperature, is about 60 millibars, which means that fluids exposed to Mars environment at body temperature would start boiling. It's not recommended, which is why if you go to Mars, we'll need spacesuits. From a point of view of humans, it might as well be the vacuum of space.

Also, the triple point pressure of water is here at 6 millibars. So it says that water is not going to exist very readily as a liquid on Mars. On Mars, water is going to behave the way carbon dioxide does on Earth. As you know, dry ice, if you take a chunk of dry ice and warm it up, it goes directly from the solid to the vapor without forming a liquid phase, which is why it's called dry ice. On Mars, the pressure is low enough that water does the same thing.

And then fundamentally, I think that is the answer to the question-- or to the double question, why is Mars so dead and red? I think it's related. And the question really is that at the present epoch, there is no liquid water at any place, at any time. This is a profound observation, because if we look at life on Earth, life on Earth is really, essentially, bags of liquid water. That's what all life forms on Earth are. There's some dormant stages where they can live without water, but all life in its growing, reproductive phases, is bags of liquid water. No liquid water, it's not surprising that it's dead.

Doesn't mean there's no water on Mars. Here's my favorite picture from Viking, showing the surface of the planet. See these white patches? This is water frost. But as I said earlier, as that ground warms up, it's going to go directly into vapor. There's no point is there liquid.

Well, if that was the whole story in terms of biology on Mars, it would be pretty dull. But one of the things that Viking did find-- I'm sure you're all familiar with these images showing fluvial features on Mars, evidence that at one time, Mars had lots of water. This-- the scale on this feature is some 300 kilometers across, showing fluvial features carving through the ancient crater terrain. You can see the heavy-- heavily the evidence of craters indicating that this terrain dates back to the late bombardment, some 3.8 billion years ago.

This is, essentially, the-- I think these are the most interesting pictures we have of anything beyond the Earth, from the point of view of life. We don't have any evidence directly of life. We have the next best thing, which is liquid water. We don't even have liquid water, but we have pictures of what used to be liquid water. So we're several steps away, but still, these are the most exciting pictures that we have from a biological point of view once we leave the Earth.

Now, just as a word of caution, here's another interesting feature, picture, that looks like a fluvial feature, nice [? oxbows ?] and whatnot. But this is Venus, and the temperatures were some 600 degrees when this fluid flowed. So one has to be careful. How do we really know that these features are water?

I won't go into a lot of detail, but the morphology of the feature. This is down flow this way, so it's a tributary system, not a distributary system, so it's unlikely that it's anything but a distributed fluid. Candidates, CO2, water, whatever. Water is the logical one. It's the only explanation that really holds water, so to speak.

So we'll take this as evidence that at one time, there was lots of water on Mars. We have other indications that Mars was more active in the past, for example, Olympus Mons. This is volcanoes, large volcanoes on Mars. This one's 27 kilometers high, 500 kilometers across. All indications are that Mars was an active world at one time. We have not seen any eruptions at the present time. We have no direct evidence of current volcanism on Mars, and there's a lot of evidence of past volcanism.

So it's interesting. Everything we know about Mars from the images from Viking would tend to indicate that it was at one time an active planet and has become inactive. What was it like when it was active? Well, I put up this picture to bring to mind the notion that early in Mars history when it had water, it might have looked like this.

Now, this picture is actually drawn based on taking the estimates of the amount of water required to carve the channels. Look at the channels. We know the size of the channels. Calculate how much water that is. Extrapolate to the planet. We deduce a layer of water from that geomorphological argument of 500 meters thick. If you take 500 meters of water spread over the whole globe, sort of pour it onto the present topography, sort of like that Sherwin-Williams ad where they're pouring paint over the globe, just let it relax into the present topography. This is the correct image that one gets. The northern lowlands are flooded. Valles Marineris is flooded, and then the southern highlands [? or ?] [? Tarsus ?] is above ground.

This is a realistic picture. This is a picture-- it's an artist conception of what Mars was like 3 and 1/2 billion years ago, might have been like. Sometimes I say, well, this picture is taken by Hubble, because it can look backwards in time--

[LAUGHTER]

--and this is a photograph of Mars, 3 and 1/2-- but you guys know better than that, so I won't even try to convince you of that.

Now, when you look at this picture, I'm sure it must remind you of the Earth seen from space. The famous-- what's become essentially a visual icon for the space age, the picture of the Blue Marble Earth seen from space, Apollo 17 picture. And that's really the essential point in the whole discussion of Mars and what motivates the interest of Mars, that at one time, there might have been two blue marbles, Earth and Mars. That's very interesting. It's also interesting that somewhere along the line we lost one of our marbles, and something went wrong with it.

But to me, the most interesting thing is not that there was another Earth, per se, but that Earth and Mars were similar at a time. When early Earth and early Mars were similar at a time when early Earth had life. We look at early Earth. We know that it had water, CO2, atmosphere, volcanism. We know that early Mars had water, CO2, atmosphere, volcanism. But we know that the time they were similar was a time when we first see evidence for life on Earth.

And let me go into this in a little more detail, because I think it's a very fascinating story, the early history of the planets. Here is the history of the Earth and Mars compared. Let's start with the Earth forming 4 and 1/2 billion years ago. The moon forming event somewhere around here, for [? 4.6 ?] or something, but the Earth is really forming, the surface is stabilizing. The end of the late bombardment, 3.8 billion years ago. So I take that as a birthday of the Earth, April 1st, 3.8 billion years ago.

[LAUGHTER]

That's when the Earth yesterday-- was its birthday. 3.8 billion years ago. And what's interesting from a point of view of biology is that we have evidence for life at 3.8, 3.9 in that form of chemical fossils. Life selectively takes carbon-12 over carbon-13 by about a few percent, 2%. And we see that isotopic signature in sedimentary record. It's not really hard evidence. It probably wouldn't stand up in a criminal trial, but you might get by with in a civil trial--

[LAUGHTER]

--in terms of evidence for life, okay? But-- and that's why I call it possible life here. And it's virtually instantaneous, if not before the end of the [? end ?] [? late ?] bombardment. Where 3.5, we have definite evidence. This is beyond a shadow of a doubt, beyond reasonable doubt, direct evidence for life at 3.5 in terms of fossil algae. And I'll show you those images from those kind of rocks.

So this is amazing in that it shows that Earth forms, and then essentially, possibly instantly, geologically speaking, where an instant is 10 million years, life appears on the planet. We can't reproduce this in the lab, by the way, and we don't have a consensus theory for how it happened. We just have the evidence that it happened really quickly on Earth.

And then what happens, just to complete the story, as it's pretty dull for a long time, we have microbial life, and then eventually oxygen builds up, multicellular life, and then very recently, animals, and of course-- just to keep perspective, here is the dinosaur extinctions, just there. And I tried to fit in some more events like the election last year in there, but it just doesn't fit in the scale.

So it's really-- this first half billion years that's the time period that's of interest in terms of life on planets. And in fact, our current understanding of the biology would suggest that by 3.5 for sure, all major metabolic pathways were in place. We had DNA life, DNA-based life like we have today, with only two major pathways not yet realized, and that is aerobic respiration, which we all depend on, and the ability to metabolize silicon parts like diatoms do. Other than that, the story is written by 3.5.

I call these years, the first half billion years, the Wonder Bread years. These are the years-- you remember those old commercials? Certainly dates in terms of what I watched on TV, but it's-- this were the years, the formative years, when Earth and life are learning to work and live together. By 3.5, really, it's all details after that.

Well, let's go to Mars. This is the same period, 4-- 3.5 to 4, when we think there was water on Mars. And why do we think that? It's because the fluvial channels interlace the ancient crater terrain. It's very convenient that the surfaces in the solar system are time-stamped. There was a big giant time stamping a 3.8 billion years ago called the ancient crater-- the end of the heavy bombardment.

We look at the lunar highlands. We're seeing material-- we're seeing that timestamp. We're seeing a surface that dates back to 3.8 billion years. When we look at Mercury, when we look at the ancient crater tray on Mars, we see that timestamp. On Earth, that timestamp has been erased by the very activity that's kept Earth alive, but we see it on Mars, and we see that [? while ?] fluvial features predominantly are associated in that ancient terrain. So that tells us that water flowed during this point, some time before and after. And that's the time when life is found on Earth.

That is, I think, a very compelling argument that if Earth and Mars were indeed similar, and similarity really means only liquid water. From a biological point of view, the only thing that matters is liquid water. And I'll show you arguments for that. Then-- and Earth-- life appears quickly on Earth, then could it have appeared on Mars? That's the essential scientific question.

Let me answer a related question, which is Earth and Mars started off so similar. Why did they take such different trajectories? Why does-- why did Mars go bad? Why do planets go bad? Is it bad schools or bad neighborhoods? Federal programs? What's the problem?

We have some understanding of that. We have some theories for why Earth stayed good and why Mars didn't, and they have to do with the carbon cycle. Imagine the early Mars, early Earth as well, with thick CO2 atmospheres. These atmospheres are required to keep the planets warm. Now, interestingly, we can't consistently explain Mars' early climate with a CO2 atmosphere, but nonetheless, let's take it that Mars had a thick CO2 atmosphere. The CO2 is not stable in the presence of water, represented here by these clouds. The water will dissolve the CO2 to form a weak acid, which will weather silicate rocks, and that result will be the formation of calcium carbonate.

Now, on Earth, organisms have made use of this saturation of calcium carbonate to make shells and other things. But even the absence of life, if left alone, the CO2 would turn into solid material on ocean and lake basin floors, chalk and limestone deposits on the bottom of ocean and lakes.

Well, on Earth, fortunately, that material is carried by subduction to depths where temperatures of around 1,100 degrees centigrade, the carbonate turns back to CO2 and comes bubbling up in arc volcanoes. This is exactly what's happening, say, at Cascade Range. The Juan de Fuca oceanic plate is subducting under the North American continental plate. Those sediments volatilize, and they come bubbling up, and that's the gases that are emanating from the Cascade Range. So the gases coming out of Mount St. Helens are literally the recycling of carbon that was deposited in the Pacific Ocean. So while those gases and ashes were an inconvenience to the people living near Mount St. Helens, in terms of the health of the biosphere, it's an important recycling mechanism.

Well, on Mars, there there's no evidence that this occurs. There's no evidence of plate tectonics. We don't see any rifting, the spreading centers, I mean. We don't see any subduction areas and we don't have anything that looks like arc volcanism. So Mars being a one-plate planet, there's no way to recycle the carbonate.

So in some simple sense, we think what happened is Mars started off with a thick CO2 atmosphere. It formed carbonate deposits. And there's no way to recycle, and that was all she wrote. A hundred million years later, everything got cold and everybody died.

So in some simple sense, we think this explains the differences between the two planets. That early period of Mars' history, this period of half a billion years between 4 and 3 and 1/2 billion, that's the period that we're interested. That's when Viking should have landed on Mars. Viking was 3 and 1/2 billion years late. It wasn't NASA's fault, but we get the blame anyway. 3 and 1/2 billion years late.

We have other evidence that suggests that Mars was very different early in its history as well, and this is the meteorite evidence. I want to talk briefly about it. We have meteorites on Earth that came from Mars. How did they get here? Well, it's represented in this picture. What we think happens is stuff slams into the planets. A good example is look at the night sky and there's comet Hale-Bopp, racing through the solar system. Fortunately, it's missing all the planets. It's missing Earth in particular.

But if it wasn't missing them-- there's always a finite chance it would hit-- it could slam with enough force to knock material in-- with velocities exceeding escape velocity, and the stuff would never fall back down onto the planet that was hit. And it could go into a solar orbit, and millions of orbits later, by some random chance, it could hit another planet come slamming into the atmosphere. And it would be found. In this particular case, we find them in Antarctica.

Well, it's not the meteorites have some sort of perverse attraction for Antarctica the way tornadoes do for trailer parks or anything like that.

[LAUGHTER]

It's just that in Antarctica, it's very easy to recognize a meteorite because a big white sheet, and the only way a black rock can get on that sheet is literally falling down from the sky. Well, so we have on Earth 12 rocks, a dozen rocks, that we think came from Mars. This is a very nice diagram by Jim Gooding that shows how these rocks form four classes of rocks. There's the S, N, and C rocks. 11 of the 12 rocks fall into these three classes. And what I'm plotting here-- what Jim plotted here is the age-- the crystallization age of the rock, when the rock formed, versus a estimate of the depth below the surface at which it formed.

So you can see that the S, N, and C types, which is 11 of the 12, are all fairly young, 200 million years for the S type and only about a billion years for the N and C type. These are young rocks. They formed recently on Mars, and were then kicked into orbit and came to Earth. And then there's one rock, a famous rock now, that was found in Antarctica, that formed 4 and 1/2 billion years ago on Mars.

Well, these 11 are all consistent with this planet that's cold, with oxidizing water, and no organics stable. These are all consistent with the Mars we see today, and we're telling us that it looks like Mars has been the way it is today for at least a billion years, a cold, dry desert world, probably lifeless on the surface, at least.

But this one is not consistent with that kind of environment, and it's consistent with a warm, reducing conditions with organic compounds, stable, again indicating Mars went through some very deep change in its environmental conditions from an environment that was much more conducive to life.

[? When ?] [? is ?] [? it? ?] Let me just do sort of a parenthetical discussion here about how do we know these rocks came from Mars? Because it's often an interesting point. And I'll just go over the evidence briefly on that. This is-- the logic takes two steps. One is that all these rocks, 12 rocks, came from the same place. And that's based on-- primarily on oxygen isotopes, and I just illustrate that here with a diagram that shows the oxygen isotopes of different meteorites. And they cluster in certain classes and the SNCs form a distinct class right there, as you can see. This is oxygen-17 to 16, oxygen-18 to 16, and ocean water would be 0, 0 on this plot, which would be right around about there.

So you could see these form-- each different meteorites tend to form a distinct class or line on this plot. And there's a distinct cluster there, the SNC, that tells us that these all came from the same parent body. They all came from the same place. [? This ?] [? doesn't ?] told us that they've come from Mars. It just tells us they come from the same place.

But what's interesting, and the way we figure out that they came from Mars is one of these types, and particularly this one, the Elephant Hills 79001 was analyzed by a variety of people, including Bob [? Pepin ?] in Minnesota. And they found in that meteorite bubbles of air, bubbles of gas. And those bubbles of gas are exactly the same as the gas on Mars as analyzed by Viking.

This is the Mars air in a log plot, and this is the gas in the S-type meteorite. And the identity function would be the solid line. And here are the points, and the size and the circles of the error bar. So you can see that over a range of nine orders of magnitude, the gas in this bubble is exactly like the gas in this meteorite on Mars, which formed 200 million years ago. And this is not like the gas anywhere else. It couldn't be. This is not Earth air, Venus air, Pluto air, or anything else. This is definitely a signature that these things came from Mars, and particularly the argon-36 to 40 ratio is different than anywhere else. And it would've been different earlier in Mars' history.

So what it tells us is two things. One is that Mars' atmosphere hasn't changed much, at least over 200 million years, and that this rock came from Mars, and therefore all 12 of them came from Mars. So that's the basic logic of why we think they came from Mars.

Now, of course, the famous one is this Allen Hills 85001, in which interesting features were found that look like that, little, tiny, nondescript ovals. And the possibility was raised that these are direct evidence of fossils. Now, I want to make a distinction between the geochemical evidence we had from the meteorites, this meteorite in particular, that Mars was a warmer, wetter place, and then this evidence that there might have been fossils. This evidence is, I think, weak. It certainly can't be proven wrong readily, but it's not very convincing evidence of past life. But the evidence of the meteorite that it was warm and wet is much more robust. So they're not necessarily the same.

Why is this evidence weak? I think partly because these fossils are so ambiguous in terms of evidence for life. They're very small. Typical scale is about 10 nanometers across, which is very small compared to microbial life on Earth. And they're just nondescript, little oval shapes.

Let's compare that to the evidence we have on Earth for fossils. The earliest, oldest evidence for life on Earth is-- this is a picture of that. This is rocks that are 3 and 1/2 billion years old. This is the oldest evidence we have for life on this planet. And these are from Australia. This is from work of [? Bill ?] [? Scharf ?] and his colleagues.

And this is a rock. It's actually picture of a rock. This shows sort of a lasagna-like shape feature. What it is a microbial mat, organisms living in a mat on a shoreline or an ancient lake, and then successive waves or years, mat builds up, and it forms a layered structure, and then it's solidified and lithified and it forms this characteristic shape, stromatolitic shape.

And within these structures, we can find micro fossils. This is a direct picture A and panel A, and this is a line representation of it. And we can see, we can lay down a modern filament of cyanobacteria, blue green algae, and show that they're very similar. This is very good evidence for biological processes, and in fact for photosynthesis. Not just for biology, but for photosynthesis, for complete microbial mat communities on Earth at 3.5 billion years ago. And we can even make a plausible argument that these are cyanobacterial mats. So it's much better argument than we have for life on Mars.

So the one question I want to now turn to is, if we want to go to Mars, if we want to get better evidence, how do we do it? How would we do it? Well, I think the answer is we're going to have to go to Mars. It's sort of a standard sales pitch. The first rock is free. The second one costs $500 million.

[LAUGHTER]

So how-- where are we going to go on Mars to buy this $500 million rock? Well, the first thing you might say is we'll go to the same place where this Allan Hills rock came from and get another one, get a better one.

Well, people have identified-- Nadine Barlow at Central Florida has identified a place where this rock could have come from. She thinks it comes from this crater right here, based on the fact that here is a spot in the ancient crater terrain. The scale here is about 200 kilometers across. So these are large craters, old craters. But there's one here, a fairly young crater, only about 10, 15 million years old, consistent with the age of ejection for the Allan Hills rock. It's slightly oval, indicating a side or a glancing impact, which is consistent with a higher efficiency for ejecting material.

So this could be it. Maybe the thing to do is to direct our sample return mission to land right in that crater, drill down, and pull up, and bring back another rock just like the one we got. Well, maybe that's not a good idea, because we might find just like the one we've got, only more. We haven't resolved the question any better.

Are there better places to search for life than this type of material? I think there could be. And so I want to develop a logic for a place where I think would be the best place to search for evidence of past life on Mars. And this logic is based on going on places on Earth which are very Mars-like. So what is the most Mars-like place on Earth? It's the dry valleys of the Antarctic.

Let me show you. This is a Landsat photograph looking down in the Antarctic. The scale here is about 300 miles across. This is Ross Island, this island here. This is Mount Erebus, the active volcano in Antarctica. This is the main United States station, McMurdo station. And you can see this icebreaker track coming in carrying in Cheerios and milk and other important supplies for the base there.

But the reason we're-- the region we're interested in is this region here, the dry valleys. This is the coldest, driest place on Earth. The mean annual temperature is minus 20 centigrade, and the precipitation here is tiny compared to the amount of snow we have outside. It averages only snow, only snow, and it averages about 2 centimeters equivalent water a year, which is-- which is, for example, 20 times drier than the Gobi Desert. This is an extremely dry, extremely cold environment. Needless to say, in this environment, there's hardly anything alive. In fact, looking down in these valleys, it's lifeless. No birds, no insects, no anything. It's dead as a doornail. I always take a doornail with me just to be sure. Dead as a doornail.

But on the floor of these valleys, there's a big sheet of ice. Here's [? it, ?] about 5 kilometers across. A big sheet of ice. That's not surprising. This is-- I mean, the annual temperature, minus 20, ice is not surprising. But what's surprising is that underneath that sheet of ice, there's water.

So what we do is we go down there and we melt a big hole in that ice. It turns out it's about a little bit less thick than this room. It varies, but we melt a hole through the ice, about that big a hole, by heating up some ethylene glycol and pumping it through some copper coils. As it melts down, we've got a [INAUDIBLE] sump pump that just sits on it and sprays the water out on the ice surface. We get the hole through, and then we take some nonessential federal employees--

[LAUGHTER]

--which we have a lot of. Tie them to a rope and lower them down. Say, what do you find down there?

Well, what we find down there in the water underneath this ice is that there's algae there. There's life there, the same sort of cyanobacterial mats that we think populated the early Earth. So here in the most Mars-like environment on Earth we find liquid water that's persisting all year round. Some of these lakes can be 30 meters deep or deeper underneath, now, relatively thin ice covers, persisting all year round with life in it.

Well, this is pretty amazing. And the first question that we tried to address was, well, how can there be liquid water environment where the mean annual temperature is minus 20? And how can the life survive there? And then what does this tell us about Mars? So we tried to develop models of this. And I won't bore you with the details, but I'll just quickly go through, as sort of a fun sophomore physics exercise in thermodynamics.

There's energy sources. There's sunlight coming in. There could be geothermal heat. It's not very important. Turns out the geothermal heat in this area is the global average, some 70 milliwatts per square meter, nothing to write home about. Sunlight comes through, enough to photosynthesize, about 1% transmission. 10 to the minus 2, 10 to the minus 3, but not enough to be appreciable heat source. It turns out the heat source is the summer melt. What happens is that, indeed, the mean annual temperature is minus 20. Let me show you a plot of the temperature out-- throughout the year at a site like this.

This is a summer, winter, summer. Summer, the sun's shining. January, it's quite warm. Temperatures can be a few degrees above freezing, which for Antarctica is very warm. And then when winter comes, the sun goes away. It gets very cold. Indeed, the average is minus 20. But what's important here is that in summer, for a couple days each year, the temperature climbs above freezing. When that happens, go back to this picture. You might have noticed it. When that happens, the water, the ice in these glaciers surrounding the valleys melts and the water flows down as liquid water into-- underneath the ice cover.

And what is it-- when water is melted, it has the latent heat of fusion, which for water is enormous, 80 calories per gram. And in order for the-- that water to freeze, it has to release that latent heat, and that latent heat has to be conducted through the ice cover. And it's that light and heat that forms the energy source that keeps the lake liquid.

So what's essential for understanding this environment is that the temperature, not that the mean annual is below freezing, but that the temperature gets above freezing for a little bit each year. The mean annual temperature determines the thickness of the ice, of course, because that determines the barrier.

What it can do to the thermodynamics is very simple, because the ice-water interface is at 0 degrees, by definition. The mean annual is minus 20. Heat is being lost by conduction. Energy comes in, sunlight. That's negligible. Geothermal heat, negligible. This is the latent heat flux. And you can very simply integrate that and soften the thickness of the ice. And sure enough, it works in Antarctica.

We can then model it on Mars and say, how long could lakes like this persist on Mars? And indeed, we could show that long after the rest of the planet is dead, frozen, cold like Antarctica, there could be ice covered lakes like the Antarctic, down to mean annual temperatures on Mars, down to minus 35. We've done some simple climate models that suggest that this time period could be on the order of half a billion years. So life could have existed in ice-covered lakes on Mars, like in Antarctica, half a billion years after the rest of the planet would have been dead.

Are there lakes on Mars? Can we find spots where there would have been lakes? Well, let me show you my favorite spot on Mars. This is a canyon, Hebes Chasma in the Valles Marineris canyon system near the equator. Hebe was one of the Greek pantheon. And this is her canyon. And in the middle of the canyon, it's a-- you see the scale bar here. It's several hundred kilometers long on the order of a hundred kilometers wide. It's got-- in the middle of the canyon, there's a plateau, and if you look carefully, you can see that the plateau has laminations on the side.

We think that this canyon was full of water and that this plateau is sediments that were deposited, possibly carbonate sediments, that were deposited when that canyon was full of water. There's evidence that suggests that this whole region experienced extensive groundwater flow, as water drained from Tharsis through this region down onto the northern plains. So there's reason to think there is extensive groundwater flow in here, would have been filled with a lake. This deep canyon would have been filled [? with. ?] The lake was a relatively thin ice cover, even as temperatures on Mars got down below minus 20, down to minus 35. There might have been life in this lake.

What we'd like to do is land in the middle of this plateau, drill down to see if there's any fossils. And there's two reasons here. One is a lake is a good place to live. As I showed you in the Antarctic, long after the rest of the environment became too dry and cold for life, there might have been life in a lake. A lake, an ice covered-lake, would've been a good place to live on a cold planet.

But even more important, a lake is a good place to die because if something dies in the lake, it can get buried and preserved in the sediments. Those river features I showed you might have had water, and it might have been great places to live, but it's not clear that they're great places to find fossils.

And I often drive this point home by telling the story of going to Los Angeles, which I don't recommend. But if you do go to Los Angeles, to visit the La Brea Tar Pits. Here's this big tar pits, and there's all sorts of fossils of saber tooth tigers and wolves. And you think, why did they live in tar pits? Well, they didn't live in tar pits. It's just that tar pits is where evidence of them were preserved.

The same challenge is going to be on Mars. It's not enough to find a place where there is life. We have to find a place where the evidence of that life is preserved. And in this case, nature has been kind to us in that a lake bed, I think, provides both examples. It provides an environment which would have supported life long after the average, and it also provides an excellent environment for preserving evidence of that life as fossils.

Unfortunately, they won't let us land anywhere near this lake. You look at, for example, the [? aero-lift ?] on Pathfinder. It's 160 kilometers. That's about like that, you've got a reasonable chance of hitting a 5 kilometer cliff and rolling down. And the project manager doesn't like that. So they want us to find a lake bed that's easier to get to, one that's near an airport or something like that.

[LAUGHTER]

So we've been trying to find that, and we think we've got one. And that's this one right here. This is our practical site. This is a lake, Gusev Crater, in the southern ancient cratered highlands. You can see from the scale bar it's about 100 kilometers across. There's a river, Ma'adim Valles, that flowed in. And we think these sediments are off-- basically deposited when this canyon-- crater was full of water. We think this was a crater lake, with deep sediments deposited when it was full of water.

And it's much smoother and flatter. No cliffs here. Easy target. Land in the middle of the Rover. Drive around, head down, cross the shoreline, go up the creek. We could be up the creek on Mars and find what we're looking for. So this is our current best example.

And now I want to-- so this will be-- I want to emphasize a point that in both cases, what we're searching for is fossils. This is what was in the Allan Hills meteorite, if we believe it. This is what we find on rocks on Earth, 3 and 1/2 billion years ago. And this is what we'd be searching for in these ancient lake. That's fossils, basically footprints. We don't actually find the organism itself. We find the footprints.

So I want to make the case now that that's not enough. That would be interesting, and it would make my day. It would make my decade if we found really good fossil evidence of life on Mars, but it's not enough, and it's not the end of the story in that we want to be able to find something that we can compare to life on Earth. And I want to illustrate this with a very interesting graphic that was published in the Journal of Irreproducible Results, which is a comparison of apples and oranges. People say you can't compare apples and oranges--

[LAUGHTER]

--but in fact, you have to compare apples and oranges. If you want to understand the broader category of fruit, you have to compare apples and oranges. In fact, you'd also want to compare bananas, and even tomatoes, which, legally, are vegetables, but biologically, they're fruits. You want to compare them all.

And this is the point. On Earth, we only have apples in terms of life, or oranges. Well, we only have one. If you look at life on Earth, you see that it's all the same stuff. It's all genetically related. We have one example of life on Earth. This is the 16S ribosomal RNA tree of life from Carl Woese showing the main families of life. These are the eukaryotes, animals, plants, mushrooms, and all those sorts of things are up here in the green branch. And then the bacteria form two branches, the eubacteria and the archaeobacteria.

All life on Earth is related. There's no reason, a priori, why this had to be. We could have mapped out the tree of life and found that there are two distinct trees, two separate genesis of life on Earth that have learned to live together. Ha, ha, right? But it's not. It's only one. If there was somebody else, we ate them. They're gone.

[LAUGHTER]

All that's left is one example of life. And this is a profound-- it's what's called the unity of biochemistry. All life on Earth has the same DNA, RNA code, the same 20 left-handed amino acids. It's all the same.

Well, this is what I mean by one data point. If we want to understand life as a general phenomenon, we've got to be able to get life from somewhere else and sequence its ribosomal RNA, if it's even got it, look at its amino acids. Fossils are not enough. We need actual bodies. We need to be able-- something that's alive or something that's dead, but still preserved organically.

Well, there's a possibility that the life could be survive-- still survive on Mars. Not on the surface. It's way too dry. But maybe under-- deep under the surface, there might be a magma source associated with ground ice forming liquid water and providing an energy source. For example, hydrogen and CO2, for which methanogens could be the basis of an ecosystem. And we're now finding these on Earth, interestingly.

So this diagram actually was published about six years ago. And since then, people are now finding these kind of systems completely isolated from the surface of the Earth, subsurface microbial ecosystems. Many microbial ecosystems on Earth that we think of as being isolated are not. For example, the deep sea vents, because they rely on oxygen from the upper atmosphere-- from the surface. But we're now finding ones that are truly anaerobic, chemoautotropic systems in the subsurface on Earth. There's been at least one.