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ABOUT THE SERIES
This 5-part debate on the Rare Earth hypothesis will run each Monday and Wednesday through July 29. It is produced in cooperation with Astrobiology Magazine, a web-based publication sponsored by the NASA astrobiology program.
Part 1: The Hostile Universe
TODAY'S PARTICIPANTS
Michael Meyer, the senior scientist for astrobiology at NASA and program scientist on the Mars Odyssey Mission.
Peter Ward, co-author of "Rare Earth," and professor of geological sciences at the University of Washington.
Christopher McKay, planetary scientist scientist with the Space Science Division at NASA's Ames Research Center.
David Grinspoon, principal scientist in the Department of Space Studies, Southwest Research Institute in Boulder, Colorado, and author of the forthcoming book "Lonely Planets: The Natural Philosophy of Alien Life"
Frank Drake, chairman of the board of trustees of the SETI Institute, and professor of astronomy and astrophysics at the University of California at Santa Cruz.
Donald Brownlee, co-author of "Rare Earth," and professor of astronomy of the University of Washington.
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When the book "Rare Earth" was published two years ago, it raised a great deal of controversy among astrobiologists. Written by Peter Ward and Donald Brownlee, the book's hypothesis suggests complex life is rare in the universe, and may even be unique to Earth. If life does occur elsewhere, the authors contend, it will only be in the form of single-celled microbial life such as bacteria.
This debate, a 5-part series beginning today, will cover a variety of topics prompted by the Rare Earth hypothesis. The moderator is Michael Meyer, the NASA senior scientist for astrobiology.
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Michael Meyer: Thank you for joining the first in what we hope will be a series of Great Debates. Before delving into the vagaries and specifics of planetary and biological evolution, and into a discussion of whether we are unique or common, it might be useful to set a baseline for at least one prerequisite for complex beings -- life itself. This leads to the first question:
Other than on Earth, is there life in our stellar neighborhood?
Peter Ward: There is a cultural assumption that there are many alien civilizations. This stems in no small way from the famous estimate by Frank Drake -- known as the "Drake Equation" -- that was later amended by Drake and Carl Sagan. They arrived at an estimate that there are perhaps a million intelligent civilizations in the Milky Way Galaxy alone.
The Drake and Sagan estimate was based on their best guess about the number of planets in the galaxy, the percentage of those that might harbor life, and the percentage of planets on which life not only could exist but could have advanced to culture. Since our galaxy is but one of hundreds of billions of galaxies in the universe, the number of intelligent alien species would be numbered in the billions.
Surely, if there are so many intelligent aliens out there, then the number of planets with life must be truly astronomical. But what if the Drake and Sagan estimates are way off? If, as could be the reality, our civilization is unique in the galaxy, does that mean that there might be much less life in general as well?
In my view, life in the form of microbes or their equivalents is very common in the universe, perhaps more common than even Drake and Sagan envisioned. However, complex life -- animals and higher plants -- is likely to be far more rare than commonly assumed. Life on Earth evolved from single celled organisms to multi-cellular creatures with tissues and organs, climaxing in animals and higher plants.
But is Earth’s particular history of life -- one of increasing complexity to an animal grade of evolution -- an inevitable result of evolution, or even a common one? Perhaps life is common, but complex life -- anything that is multi-cellular -- is not.
Chris McKay: There is no solid evidence of life elsewhere, but several factors suggest it is common. Organic material is widespread in the interstellar medium and in our own solar system. We have found planetary systems around other sun-like stars. On Earth, microbial life appeared very quickly -- probably before 3.8 billion years ago. Also, we know that microbial ecosystems can survive in a variety of environments with liquid water and a suitable chemical energy source or sunlight.
These factors suggest that microbial life -- the sort of life the dominated Earth for the first two billion years -- is widespread in the stellar neighborhood.
David Grinspoon: It is always shaky when we generalize from experiments with a sample size of one. So we have to be a bit cautious when we fill the cosmos with creatures based on the time scales of Earth history (it happened so fast here, therefore it must be easy) and the resourcefulness of Earth life (they are everywhere where there is water).
This is one history, and one example of life. When our arguments rest on such shaky grounds, balancing a house of cards on a one-card foundation, we are in danger of erecting structures formed more by our desires than the "evidence."
Frank Drake: I think this is an occasion where that old principal of good science, Occam's Razor, is helpful. Apply Occam's Razor to the question of the origin of life on Earth. We look at the Earth, and with regards to that origin, as best we know, no special or freak circumstances were required. It took water, organics, a source of energy, and a long time. Deep-sea vents are the current favorite and a reasonable place for the origin.
But even if they weren't the culprits, the chemists have found a multitude of other pathways that produce the chemistry of life.
The challenge seems to be not to find the pathway, but the one that was the quickest and most productive. The prime point is that nothing special was required. There will be a pathway that works, on Earth and on similar planets. Then, by Occam's Razor, the origin of life on Earth is nothing more than the result of normal processes on the planet. Furthermore, life should appear very frequently on other Earth-like planets. There will be microbial life nearby the solar system.
Donald Brownlee: While there is hope and even expectation of nearby extraterrestrial life, the goal of "Rare Earth" was to point out that the universe is fundamentally hostile to life. Most planets and other places in the universe clearly could not support any type of Earth-like creatures. The universe is vast, so there may be many Earth-like places, but they will be widely spaced, and if they are too widely spaced they will be isolated from each other.
What fraction of stars harbor Earth-like planets with Earth-like life? Is it one in a hundred, one in a million, or even less? Even the most optimistic have to admit Earth-like environments must be rare.
In our book "Rare Earth," we suggest that extraterrestrial life is likely to be near but that complex animal-like life is rare and will probably not be found close to us in space. A major question about life relates to the environments needed for its formation and long term evolution. Unfortunately Earth is our only successful example. Predictions of life elsewhere are problematic; presently there is no detectable life elsewhere in the solar system.
David Grinspoon: I am not convinced that the Earth’s carbon-in-water example is the only way for the universe to solve the life riddle. I am not talking about silicon, which is a bad idea, but systems of chemical complexity that we have not thought of, which may not manifest themselves at room temperature in our oxygen atmosphere. The universe is consistently more clever than we are, and we learn about complex phenomena, like life, more through exploration than by theorizing and modeling. I think there are probably forms of life out there which use different chemical bases than we, and which we will know about only when we find them, or when they find us.
An obvious rejoinder to this is, "But no one has invented another system that works as well as carbon-in-water." That is true. But to this I would answer, "We did not invent carbon-in-water!" We discovered it. I don’t believe that we are clever enough to have thought of life based on nucleic acids and proteins if we hadn’t had this example handed to us. This makes me wonder what else the universe might be using for its refined, evolving complexity elsewhere, in other conditions that seem hostile to life as we know it.
Frank Drake: All evidence of the most primitive steps in the first 700 million years of chemical evolution on Earth is apparently lost. We grope towards understanding of that profound gap in our knowledge by working backwards, hypothesizing that there once was an RNA world based on self-catalyzing RNA. But this system evolved from something else, and led to the esoteric DNA-protein world.
As David Grinspoon rightly points out, we are not remotely smart enough to hypothesize ab initio the system of the DNA-protein world, or even the RNA world. It was handed to us on a silver platter. This should be a strong warning that we are over our heads when predicting what might have taken place on other worlds.
Give us knowledge of another independent origin of life in space, and the doors to great progress in this field may open.
Part 2: Alien Proximity
TODAY'S PARTICIPANTS
Michael Meyer, the senior scientist for astrobiology at NASA and program scientist on the Mars Odyssey Mission.
Peter Ward, co-author of "Rare Earth," and professor of geological sciences at the University of Washington.
David Grinspoon, principal scientist in the Department of Space Studies, Southwest Research Institute in Boulder, Colorado, and author of the forthcoming book "Lonely Planets: The Natural Philosophy of Alien Life"
Frank Drake, chairman of the board of trustees of the SETI Institute, and professor of astronomy and astrophysics at the University of California at Santa Cruz.
Donald Brownlee, co-author of "Rare Earth," and professor of astronomy of the University of Washington.
On Monday, Part 1 wrangled with the question of whether life could originate and exist anywhere except on Earth. The general consensus was that simple (microbial) life, at least, may be common in the universe. The focus on microbial life continues today in Part 2 as the moderator asks where we can expect to find life in our solar system and beyond. The moderator is Michael Meyer, the NASA senior scientist for astrobiology.
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Michael Meyer: If there is life out there -- either microbial or complex -- where can we expect to find it?
Peter Ward: Life might have originated on Mars and Europa early in the solar system’s history (and may live there still). Many of us think that, at best, we’ll find evidence that life once existed on Mars and may or may not have started on Europa.
My guess is that the Earth is the only place in the solar system where there is existent life -- but we might expect to find a rich fossil record of extinct life on Mars.
Of all planets beyond the Earth, Mars is by far the best known. It has been poked, prodded, examined and measured by a variety of Earth- and space-borne instruments, including those many that have successfully and unsuccessfully either landed or crashed on the surface of the red planet. An enormous amount of information now suggests that early in its history, while our Earth was still a chaotic and uninhabitable world of magma oceans and unceasing asteroidal impacts, Mars may have been a benign world, of equable temperatures and almost planet-spanning oceans. It may, as well, have been a world with an atmosphere that included oxygen.
All of these factors lead to an inescapable conclusion – that the early Martian conditions would have been favorable for the development of life. Some scientists have even suggested that life arose on Mars, and then was transported to Earth.
For several hundred million years or more these benign conditions may have lasted, and in that time span evolution could have worked wonders. Perhaps the first geologists sampling Martian sedimentary rocks older than 4 billion years in age will find not only the fossil remains of bacteria, but also the remains of more complex organisms.
Perhaps the fossils of animals will be found.
What would that scene be like: the swing of a rock hammer against a Martian outcrop, splitting a piece of ancient Martian shale, and the heart-stopping joy of finding a mollusk look-alike or the bones of a fish-equivalent? Yet even if life did attain such a rapid rise in complexity on Mars, it did not last, for Mars as an environment for life died early.
Even as bacteria on Earth were readying for the rush to higher grades of life, Mars was dying or was already long dead – assuming that life originated there at all. On Mars, the oceans seeped back into the planet or were lost to space, the oxygen in the atmosphere bound itself to rocks, and life died out.
David Grinspoon: I agree with the belief that Mars is currently lifeless. My impression that Mars today is dead is derived from the stale atmosphere (no signs of biological disequilibrium yet discerned) and the lack of internally driven geological activity. I think that to support a biosphere over billions of years, a planet needs more than isolated pockets of water.
Don't get me wrong -- I am a big proponent of Mars exploration. No matter what we find there, we will learn a lot. And if Mars is lifeless, this gets us off the hook because there won’t be any difficult ethical choices about human activities there. But all opinions about life elsewhere are just that. We need to go and look.
Donald Brownlee: If no evidence for life is found on Mars, then the formation of life probably is neither easy nor common in the solar system. We already have seriously negative results from asteroidal meteorites.
There are now over 30,000 asteroidal meteorites in captivity, and none of them show compelling evidence of alien life. Many of these rocks came from bodies that were much richer in water, carbon, and nitrogen than Earth, and many had warm and wet interiors that lasted for millions of years. Life apparently did not form in the asteroids. Presumably this is because asteroids did not have the right environments even though they did have the right building materials.
Creation of life apparently needs a richer diversity of disequilibria than can be found inside wet organic-rich interiors of asteroids. Probably what is needed is something akin to environments that occurred on early Earth and hopefully other planets as well.
David Grinspoon: We need to keep an open mind for possible bio-signs in unexpected places as we explore the entire solar system and beyond. If we relax our (understandable) attachment to "life as we know it," other intriguing possibilities become worthy of our consideration.
For a planet to foster the origin of life and maintain the necessary conditions, I believe that the most important requirement is a planet with continuous and vigorous geological activity over billions of years. Watery conditions are needed for our kind of life, but any chemical environment where complexity can flourish might do, and we don't know enough about planets and about chemical evolution to place good limits on these environments.
Although my hunch is that currently Mars is lifeless, I am still holding out for Venus: nice conditions in the clouds, energetic flows, strange UV absorbing pigments, unexplained particle populations, etc., if you don't mind a little acid. Europa, and possibly Titan or Io, also may harbor life.
[Titan is a moon of Saturn; Io is moon of Jupiter.]
Frank Drake: In places like Io and Titan, we may find the first evidence of other biochemistries that are beyond our powers of prediction. I am a little on the pessimistic side with regards to Io -- it has no substantial atmosphere.
But Titan! Wow! A prodigious organic chemical factory, some kind of solvent, even an atmosphere. It sounds better than primitive Earth. Sure, it is very cold there, but chemistry still happens easily if more slowly at Titanian temperatures. Could it be that one creature's arctic clime is another creature's balmy tropical island?
Don Brownlee: My prediction is that the nearest alien neighbors live in feces and food scrap left on the Moon by the six Apollo missions. Even though it’s been three decades, there is a good chance that hearty bacteria live and can reproduce inside encapsulated small damp places and survive the monthly cycles of heat and cold as well as the effects of solar flares, ultraviolet light, and hard vacuum.
If born-on-the-Moon organisms are not living in food scraps (and worse) there are probably dormant terrestrial organisms trapped inside vast numbers of components -- wire harnesses and tape interfaces that are parts of the lunar lander, back packs, surface experiments, rover, etc. Somewhere out there is Allan Shepard’s unsterilized golf ball, which is likely to carry a small zoo of terrestrial microorganisms. Beyond our Moon, my great hope is that microbial life or at least fossil evidence for its prior existence will be found on Mars, Europa, or some other solar system body.
If we find life elsewhere in our solar system, and show that it is not a distant cousin of terrestrial life, this will greatly support the idea that formation of life is easy and commonplace, given the right environmental conditions.
Part 3: Complex Life
TODAY'S PARTICIPANTS
Michael Meyer, the senior scientist for astrobiology at NASA and program scientist on the Mars Odyssey Mission.
Christopher McKay, planetary scientist scientist with the Space Science Division at NASA's Ames Research Center.
Simon Conway Morris, professor of Evolutionary Palaeobiology at the University of Cambridge in England.
David Grinspoon, principal scientist in the Department of Space Studies, Southwest Research Institute in Boulder, Colorado, and author of the forthcoming book "Lonely Planets: The Natural Philosophy of Alien Life"
Peter Ward, co-author of "Rare Earth," and professor of geological sciences at the University of Washington.
Frank Drake, chairman of the board of trustees of the SETI Institute, and professor of astronomy and astrophysics at the University of California at Santa Cruz.
Donald Brownlee, co-author of "Rare Earth," and professor of astronomy of the University of Washington.
In Part 2, the participants discussed how far (or near) alien life might be. Today they examine complex life and the possibility of its occurrence in the universe. Complex life is generally considered any living thing with multiple cells -- as opposed to single celled, microbial life -- and, on Earth anyway, includes everything from the simplest slime molds to human beings. The moderator is Michael Meyer, the NASA senior scientist for astrobiology.
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Michael Meyer: I presume that we are in agreement that microbial life, at least, may be common in our stellar neighborhood and even may be present on other planets in our solar system. That being the premise, the probability of complex life elsewhere is then dependent on the probability of the transition from slime to civilization. It happened here, so why not elsewhere? Do you think that complex life should develop on a sizeable fraction of worlds around other stars?
Christopher McKay: As David Grinspoon pointed out earlier, the Earth is our only example of planetary life. This makes it difficult to unravel what is universal and what is accidental about the nature and history of life. Still, one data point is better than none, and when we look at the question of complex life, our one data point seems to say that complex life arose as a result of the rise of free oxygen. If we take this as being generally true, then we can ask the geophysical question: On what types of planets will free oxygen arise and how long will it take to reach high enough levels?
On Earth it took billions of years for oxygen to rise to present levels. Partly this is because the Earth is efficient at recycling by plate tectonics. This recycling keeps the Earth habitable by cycling the essential elements, but it also would have been a barrier to the buildup of oxygen. Earth probably is not the best possible planet for complex life development, since less plate tectonics would allow a faster rate of oxygen build up.
Mars took this to the extreme. With no plate tectonics, a shallow ocean, and only 38 percent of the Earth’s gravity, Mars might have built up oxygen much faster than the Earth. But the lack of plate tectonics doomed Mars to lose its atmosphere through mineralization. We might find that complex life arose on Mars only to be extinguished later. Perhaps the optimal planet for complex life would be an intermediate between Earth and Mars.
There may be a range of planet types on which oxygen could arise -- and therefore complex life. I would hazard a guess that most -- maybe two-thirds -- of terrestrial planets with life go on to develop complex life at some stage of their history. An optimist’s view.
Simon Conway Morris: The problem in my view is, why did complex life take so long to evolve on Earth? Evidence from oxygen data is frankly equivocal. Maybe the redox state of the Earth's mantle was peculiar in comparison with other similar planets. Alternatively, ocean chemistry may have put the lid on things.
There could be other dimensions that could explain why there was such a brake on the evolution of complex life -- why there were no Meso-Proterozoic dry martinis, but on the other hand, once microbes, then NASA.
David Grinspoon: Planetary biospheres are complex entities whose histories are fraught with contingency, accident, and luck. Therefore, the time it took for complex life to arise on Earth is probably much faster than some and much slower than others.
We can’t stand a mystery without a chief suspect, so we pin the rise of complex life on the rise of oxygen. This may well have factored in, but as Chris pointed out, there is no reason to believe that oxygen rose on Earth as quickly as it might have elsewhere. The rate of plate tectonics is one variable that will change atmospheric history - there are countless others. For example, if Earth had formed less rich in iron, then oxygen would have risen much more quickly because there would not have been as much iron to devour the oxygen.
So in other planetary systems that are less metal-rich, creatures might have evolved to levels far beyond our current state.
Peter Ward: On Earth, evolution has undergone a progressive development of ever more complex and sophisticated forms leading ultimately to human intelligence. Complex life – and even intelligence – could conceivably arise faster than it did on Earth. A planet could go from an abiotic state to a civilization in 100 million years, as compared to the nearly 4 billion years it took on Earth.
Evolution on Earth has been affected by chance events, such as the configuration of the continents produced by continental drift. Furthermore, I believe that the way the solar system was produced, with its characteristic number and planetary positions, may have had a great impact on the history of life here.
It has always been assumed that attaining the evolutionary grade we call animals would be the final and decisive step. Once we are at this level of evolution, a long and continuous progression toward intelligence should occur. However, recent research shows that while attaining the stage of animal life is one thing, maintaining that level is quite another. The geologic record has shown that once evolved, complex life is subject to an unending succession of planetary disasters, creating what are known as mass extinction events. These rare but devastating events can reset the evolutionary timetable and destroy complex life while sparing simpler life forms.
Such discoveries suggest that the conditions allowing the rise and existence of complex life are far more rigorous than are those for life’s formation. On some planets, then, life might arise and animals eventually evolve – only to be soon destroyed by a global catastrophe.
Frank Drake: The Earth’s fossil record is quite clear in showing that the complexity of the central nervous system -- particularly the capabilities of the brain -- has steadily increased in the course of evolution. Even the mass extinctions did not set back this steady increase in brain size. It can be argued that extinction events expedite the development of cognitive abilities, since those creatures with superior brains are better able to save themselves from the sudden change in their environment.
Thus smarter creatures are selected, and the growth of intelligence accelerates.
We see this effect in all varieties of animals -- it is not a fluke that it has occurred in some small sub-set of animal life. This picture suggests strongly that, given enough time, a biota can evolve not just one intelligent species, but many. So complex life should occur abundantly.
There is a claim that "among the millions of species which have developed on Earth, only one became intelligent, so intelligence must be a very, very rare event." This is a textbook example of a wrong logical conclusion. All planets in time may produce one or more intelligent species, but they will not appear simultaneously. One will be first. It will look around and find it is the only intelligent species. Should it be surprised? No! Of course the first one will be alone. Its uniqueness -- in principal temporary -- says nothing about the ability of the biota to produce one or more intelligent species.
If we assume that Earths are common, and that usually there is enough time to evolve an intelligent species before nature tramples on the biota, then the optimistic view is that new systems of intelligent, technology-using creatures appear about once per year. Based on an extrapolation of our own experience, let's make a guess that a civilization's technology is detectable after 10,000 years. In that case, there are at least 10,000 detectable civilizations out there.
This is a heady result, and very encouraging to SETI people.
On the other hand, taking into account the number and distribution of stars in space, it implies that the nearest detectable civilizations are about 1,000 light-years away, and only one in ten million stars may have a detectable civilization. These last numbers create a daunting challenge to those who construct instruments and projects to search for extraterrestrial intelligence. No actual observing program carried out so far has come anywhere close to meeting the requirement of detecting reasonable signals from a distance of 1,000 light years, or of studying 10 million stars with high sensitivity.
Donald Brownlee: But how often are animal-habitable planets located in the habitable zones of solar mass stars? Of the all the stars that have now been shown to have planets, all either have Jupiter-mass planets interior to 5.5 AU [1 AU, or astronomical unit, is the distance from Earth to the Sun] or they have Jupiters on elliptical orbits. It is unlikely that any of these stars could retain habitable zone planets on long-term stable orbits.
On the other hand, many of the stars that do not have currently detectable giant planets could have habitable-zone planets. But even when rocky planets are located in the right place, will they have the "right stuff" for the evolution and long term survival of animal-like life? There are many "Rare Earth" factors (such as planet mass, abundance of water and carbon, plate tectonics, etc.) that may play important and even critical roles in allowing the apparently difficult transition from slime to civilization.
As is the case in the solar system, animal-like life is probably uncommon in the cosmos. This might even be the case for microbes: how can scientists agree that microbial life is common in our celestial neighborhood when there is no data? Even the simplest life is extraordinarily complicated and until we find solid evidence for life elsewhere, the frequency of life will unfortunately be guesswork. We can predict that some planetary bodies will provide life-supporting conditions, but no one can predict that life will form.
Frank Drake: Only about 5 percent of the stars that have been studied sufficiently have hot Jupiters or Jupiters in elliptical orbits. The other 95 percent of the stars studied do not have hot Jupiters, and just what they have is still an open question. The latest discoveries, which depend on observations over a decade or more, are finding solar system analogs. This suggests that 95 percent of the stars- - for which the answers are not yet in -- could be similar to our own system. This is reason for optimism among those who expect solar system analogs to be abundant.
David Grinspoon: I think it is a mistake to look at the many specific peculiarities of Earth's biosphere, and how unlikely such a combination of characteristics seems, and to then conclude that complex life is rare. This argument can only be used to justify the conclusion that planets exactly like Earth, with life exactly like Earth-life, are rare.
My cat "Wookie" survived life as a near-starving alley cat and wound up as a beloved house cat through an unlikely series of biographical accidents, which I won't take up space describing but, trust me, given all of the incredible things that had to happen in just the right way, it is much more likely that there would be no Wookie than Wookie. From this I do not conclude that there are no other cats (The Rare Cat Hypothesis), only that there are no other cats exactly like Wookie.
Life has evolved together with the Earth. Life is opportunistic. The biosphere has taken advantage of the myriad strange idiosyncrasies that our planet has to offer. Not only that, life has created many of Earth’s weird qualities.
So it is easy to look at our biosphere, and the way it so cleverly exploits Earth’s peculiar features, and conclude that this is the best of all possible worlds; that only on such a world could complex life evolve. My bet is that many other worlds, with their own peculiar characteristics and histories, co-evolve their own biospheres. The complex creatures on those worlds, upon first developing intelligence and science, would observe how incredibly well adapted life is to the many unique features of their home world. They might naively assume that these qualities, very different from Earth’s, are the only ones that can breed complexity
This 5-part debate on the Rare Earth hypothesis will run each Monday and Wednesday through July 29. It is produced in cooperation with Astrobiology Magazine, a web-based publication sponsored by the NASA astrobiology program.
Part 1: The Hostile Universe
TODAY'S PARTICIPANTS
Michael Meyer, the senior scientist for astrobiology at NASA and program scientist on the Mars Odyssey Mission.
Peter Ward, co-author of "Rare Earth," and professor of geological sciences at the University of Washington.
Christopher McKay, planetary scientist scientist with the Space Science Division at NASA's Ames Research Center.
David Grinspoon, principal scientist in the Department of Space Studies, Southwest Research Institute in Boulder, Colorado, and author of the forthcoming book "Lonely Planets: The Natural Philosophy of Alien Life"
Frank Drake, chairman of the board of trustees of the SETI Institute, and professor of astronomy and astrophysics at the University of California at Santa Cruz.
Donald Brownlee, co-author of "Rare Earth," and professor of astronomy of the University of Washington.
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When the book "Rare Earth" was published two years ago, it raised a great deal of controversy among astrobiologists. Written by Peter Ward and Donald Brownlee, the book's hypothesis suggests complex life is rare in the universe, and may even be unique to Earth. If life does occur elsewhere, the authors contend, it will only be in the form of single-celled microbial life such as bacteria.
This debate, a 5-part series beginning today, will cover a variety of topics prompted by the Rare Earth hypothesis. The moderator is Michael Meyer, the NASA senior scientist for astrobiology.
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Michael Meyer: Thank you for joining the first in what we hope will be a series of Great Debates. Before delving into the vagaries and specifics of planetary and biological evolution, and into a discussion of whether we are unique or common, it might be useful to set a baseline for at least one prerequisite for complex beings -- life itself. This leads to the first question:
Other than on Earth, is there life in our stellar neighborhood?
Peter Ward: There is a cultural assumption that there are many alien civilizations. This stems in no small way from the famous estimate by Frank Drake -- known as the "Drake Equation" -- that was later amended by Drake and Carl Sagan. They arrived at an estimate that there are perhaps a million intelligent civilizations in the Milky Way Galaxy alone.
The Drake and Sagan estimate was based on their best guess about the number of planets in the galaxy, the percentage of those that might harbor life, and the percentage of planets on which life not only could exist but could have advanced to culture. Since our galaxy is but one of hundreds of billions of galaxies in the universe, the number of intelligent alien species would be numbered in the billions.
Surely, if there are so many intelligent aliens out there, then the number of planets with life must be truly astronomical. But what if the Drake and Sagan estimates are way off? If, as could be the reality, our civilization is unique in the galaxy, does that mean that there might be much less life in general as well?
In my view, life in the form of microbes or their equivalents is very common in the universe, perhaps more common than even Drake and Sagan envisioned. However, complex life -- animals and higher plants -- is likely to be far more rare than commonly assumed. Life on Earth evolved from single celled organisms to multi-cellular creatures with tissues and organs, climaxing in animals and higher plants.
But is Earth’s particular history of life -- one of increasing complexity to an animal grade of evolution -- an inevitable result of evolution, or even a common one? Perhaps life is common, but complex life -- anything that is multi-cellular -- is not.
Chris McKay: There is no solid evidence of life elsewhere, but several factors suggest it is common. Organic material is widespread in the interstellar medium and in our own solar system. We have found planetary systems around other sun-like stars. On Earth, microbial life appeared very quickly -- probably before 3.8 billion years ago. Also, we know that microbial ecosystems can survive in a variety of environments with liquid water and a suitable chemical energy source or sunlight.
These factors suggest that microbial life -- the sort of life the dominated Earth for the first two billion years -- is widespread in the stellar neighborhood.
David Grinspoon: It is always shaky when we generalize from experiments with a sample size of one. So we have to be a bit cautious when we fill the cosmos with creatures based on the time scales of Earth history (it happened so fast here, therefore it must be easy) and the resourcefulness of Earth life (they are everywhere where there is water).
This is one history, and one example of life. When our arguments rest on such shaky grounds, balancing a house of cards on a one-card foundation, we are in danger of erecting structures formed more by our desires than the "evidence."
Frank Drake: I think this is an occasion where that old principal of good science, Occam's Razor, is helpful. Apply Occam's Razor to the question of the origin of life on Earth. We look at the Earth, and with regards to that origin, as best we know, no special or freak circumstances were required. It took water, organics, a source of energy, and a long time. Deep-sea vents are the current favorite and a reasonable place for the origin.
But even if they weren't the culprits, the chemists have found a multitude of other pathways that produce the chemistry of life.
The challenge seems to be not to find the pathway, but the one that was the quickest and most productive. The prime point is that nothing special was required. There will be a pathway that works, on Earth and on similar planets. Then, by Occam's Razor, the origin of life on Earth is nothing more than the result of normal processes on the planet. Furthermore, life should appear very frequently on other Earth-like planets. There will be microbial life nearby the solar system.
Donald Brownlee: While there is hope and even expectation of nearby extraterrestrial life, the goal of "Rare Earth" was to point out that the universe is fundamentally hostile to life. Most planets and other places in the universe clearly could not support any type of Earth-like creatures. The universe is vast, so there may be many Earth-like places, but they will be widely spaced, and if they are too widely spaced they will be isolated from each other.
What fraction of stars harbor Earth-like planets with Earth-like life? Is it one in a hundred, one in a million, or even less? Even the most optimistic have to admit Earth-like environments must be rare.
In our book "Rare Earth," we suggest that extraterrestrial life is likely to be near but that complex animal-like life is rare and will probably not be found close to us in space. A major question about life relates to the environments needed for its formation and long term evolution. Unfortunately Earth is our only successful example. Predictions of life elsewhere are problematic; presently there is no detectable life elsewhere in the solar system.
David Grinspoon: I am not convinced that the Earth’s carbon-in-water example is the only way for the universe to solve the life riddle. I am not talking about silicon, which is a bad idea, but systems of chemical complexity that we have not thought of, which may not manifest themselves at room temperature in our oxygen atmosphere. The universe is consistently more clever than we are, and we learn about complex phenomena, like life, more through exploration than by theorizing and modeling. I think there are probably forms of life out there which use different chemical bases than we, and which we will know about only when we find them, or when they find us.
An obvious rejoinder to this is, "But no one has invented another system that works as well as carbon-in-water." That is true. But to this I would answer, "We did not invent carbon-in-water!" We discovered it. I don’t believe that we are clever enough to have thought of life based on nucleic acids and proteins if we hadn’t had this example handed to us. This makes me wonder what else the universe might be using for its refined, evolving complexity elsewhere, in other conditions that seem hostile to life as we know it.
Frank Drake: All evidence of the most primitive steps in the first 700 million years of chemical evolution on Earth is apparently lost. We grope towards understanding of that profound gap in our knowledge by working backwards, hypothesizing that there once was an RNA world based on self-catalyzing RNA. But this system evolved from something else, and led to the esoteric DNA-protein world.
As David Grinspoon rightly points out, we are not remotely smart enough to hypothesize ab initio the system of the DNA-protein world, or even the RNA world. It was handed to us on a silver platter. This should be a strong warning that we are over our heads when predicting what might have taken place on other worlds.
Give us knowledge of another independent origin of life in space, and the doors to great progress in this field may open.
Part 2: Alien Proximity
TODAY'S PARTICIPANTS
Michael Meyer, the senior scientist for astrobiology at NASA and program scientist on the Mars Odyssey Mission.
Peter Ward, co-author of "Rare Earth," and professor of geological sciences at the University of Washington.
David Grinspoon, principal scientist in the Department of Space Studies, Southwest Research Institute in Boulder, Colorado, and author of the forthcoming book "Lonely Planets: The Natural Philosophy of Alien Life"
Frank Drake, chairman of the board of trustees of the SETI Institute, and professor of astronomy and astrophysics at the University of California at Santa Cruz.
Donald Brownlee, co-author of "Rare Earth," and professor of astronomy of the University of Washington.
On Monday, Part 1 wrangled with the question of whether life could originate and exist anywhere except on Earth. The general consensus was that simple (microbial) life, at least, may be common in the universe. The focus on microbial life continues today in Part 2 as the moderator asks where we can expect to find life in our solar system and beyond. The moderator is Michael Meyer, the NASA senior scientist for astrobiology.
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Michael Meyer: If there is life out there -- either microbial or complex -- where can we expect to find it?
Peter Ward: Life might have originated on Mars and Europa early in the solar system’s history (and may live there still). Many of us think that, at best, we’ll find evidence that life once existed on Mars and may or may not have started on Europa.
My guess is that the Earth is the only place in the solar system where there is existent life -- but we might expect to find a rich fossil record of extinct life on Mars.
Of all planets beyond the Earth, Mars is by far the best known. It has been poked, prodded, examined and measured by a variety of Earth- and space-borne instruments, including those many that have successfully and unsuccessfully either landed or crashed on the surface of the red planet. An enormous amount of information now suggests that early in its history, while our Earth was still a chaotic and uninhabitable world of magma oceans and unceasing asteroidal impacts, Mars may have been a benign world, of equable temperatures and almost planet-spanning oceans. It may, as well, have been a world with an atmosphere that included oxygen.
All of these factors lead to an inescapable conclusion – that the early Martian conditions would have been favorable for the development of life. Some scientists have even suggested that life arose on Mars, and then was transported to Earth.
For several hundred million years or more these benign conditions may have lasted, and in that time span evolution could have worked wonders. Perhaps the first geologists sampling Martian sedimentary rocks older than 4 billion years in age will find not only the fossil remains of bacteria, but also the remains of more complex organisms.
Perhaps the fossils of animals will be found.
What would that scene be like: the swing of a rock hammer against a Martian outcrop, splitting a piece of ancient Martian shale, and the heart-stopping joy of finding a mollusk look-alike or the bones of a fish-equivalent? Yet even if life did attain such a rapid rise in complexity on Mars, it did not last, for Mars as an environment for life died early.
Even as bacteria on Earth were readying for the rush to higher grades of life, Mars was dying or was already long dead – assuming that life originated there at all. On Mars, the oceans seeped back into the planet or were lost to space, the oxygen in the atmosphere bound itself to rocks, and life died out.
David Grinspoon: I agree with the belief that Mars is currently lifeless. My impression that Mars today is dead is derived from the stale atmosphere (no signs of biological disequilibrium yet discerned) and the lack of internally driven geological activity. I think that to support a biosphere over billions of years, a planet needs more than isolated pockets of water.
Don't get me wrong -- I am a big proponent of Mars exploration. No matter what we find there, we will learn a lot. And if Mars is lifeless, this gets us off the hook because there won’t be any difficult ethical choices about human activities there. But all opinions about life elsewhere are just that. We need to go and look.
Donald Brownlee: If no evidence for life is found on Mars, then the formation of life probably is neither easy nor common in the solar system. We already have seriously negative results from asteroidal meteorites.
There are now over 30,000 asteroidal meteorites in captivity, and none of them show compelling evidence of alien life. Many of these rocks came from bodies that were much richer in water, carbon, and nitrogen than Earth, and many had warm and wet interiors that lasted for millions of years. Life apparently did not form in the asteroids. Presumably this is because asteroids did not have the right environments even though they did have the right building materials.
Creation of life apparently needs a richer diversity of disequilibria than can be found inside wet organic-rich interiors of asteroids. Probably what is needed is something akin to environments that occurred on early Earth and hopefully other planets as well.
David Grinspoon: We need to keep an open mind for possible bio-signs in unexpected places as we explore the entire solar system and beyond. If we relax our (understandable) attachment to "life as we know it," other intriguing possibilities become worthy of our consideration.
For a planet to foster the origin of life and maintain the necessary conditions, I believe that the most important requirement is a planet with continuous and vigorous geological activity over billions of years. Watery conditions are needed for our kind of life, but any chemical environment where complexity can flourish might do, and we don't know enough about planets and about chemical evolution to place good limits on these environments.
Although my hunch is that currently Mars is lifeless, I am still holding out for Venus: nice conditions in the clouds, energetic flows, strange UV absorbing pigments, unexplained particle populations, etc., if you don't mind a little acid. Europa, and possibly Titan or Io, also may harbor life.
[Titan is a moon of Saturn; Io is moon of Jupiter.]
Frank Drake: In places like Io and Titan, we may find the first evidence of other biochemistries that are beyond our powers of prediction. I am a little on the pessimistic side with regards to Io -- it has no substantial atmosphere.
But Titan! Wow! A prodigious organic chemical factory, some kind of solvent, even an atmosphere. It sounds better than primitive Earth. Sure, it is very cold there, but chemistry still happens easily if more slowly at Titanian temperatures. Could it be that one creature's arctic clime is another creature's balmy tropical island?
Don Brownlee: My prediction is that the nearest alien neighbors live in feces and food scrap left on the Moon by the six Apollo missions. Even though it’s been three decades, there is a good chance that hearty bacteria live and can reproduce inside encapsulated small damp places and survive the monthly cycles of heat and cold as well as the effects of solar flares, ultraviolet light, and hard vacuum.
If born-on-the-Moon organisms are not living in food scraps (and worse) there are probably dormant terrestrial organisms trapped inside vast numbers of components -- wire harnesses and tape interfaces that are parts of the lunar lander, back packs, surface experiments, rover, etc. Somewhere out there is Allan Shepard’s unsterilized golf ball, which is likely to carry a small zoo of terrestrial microorganisms. Beyond our Moon, my great hope is that microbial life or at least fossil evidence for its prior existence will be found on Mars, Europa, or some other solar system body.
If we find life elsewhere in our solar system, and show that it is not a distant cousin of terrestrial life, this will greatly support the idea that formation of life is easy and commonplace, given the right environmental conditions.
Part 3: Complex Life
TODAY'S PARTICIPANTS
Michael Meyer, the senior scientist for astrobiology at NASA and program scientist on the Mars Odyssey Mission.
Christopher McKay, planetary scientist scientist with the Space Science Division at NASA's Ames Research Center.
Simon Conway Morris, professor of Evolutionary Palaeobiology at the University of Cambridge in England.
David Grinspoon, principal scientist in the Department of Space Studies, Southwest Research Institute in Boulder, Colorado, and author of the forthcoming book "Lonely Planets: The Natural Philosophy of Alien Life"
Peter Ward, co-author of "Rare Earth," and professor of geological sciences at the University of Washington.
Frank Drake, chairman of the board of trustees of the SETI Institute, and professor of astronomy and astrophysics at the University of California at Santa Cruz.
Donald Brownlee, co-author of "Rare Earth," and professor of astronomy of the University of Washington.
In Part 2, the participants discussed how far (or near) alien life might be. Today they examine complex life and the possibility of its occurrence in the universe. Complex life is generally considered any living thing with multiple cells -- as opposed to single celled, microbial life -- and, on Earth anyway, includes everything from the simplest slime molds to human beings. The moderator is Michael Meyer, the NASA senior scientist for astrobiology.
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Michael Meyer: I presume that we are in agreement that microbial life, at least, may be common in our stellar neighborhood and even may be present on other planets in our solar system. That being the premise, the probability of complex life elsewhere is then dependent on the probability of the transition from slime to civilization. It happened here, so why not elsewhere? Do you think that complex life should develop on a sizeable fraction of worlds around other stars?
Christopher McKay: As David Grinspoon pointed out earlier, the Earth is our only example of planetary life. This makes it difficult to unravel what is universal and what is accidental about the nature and history of life. Still, one data point is better than none, and when we look at the question of complex life, our one data point seems to say that complex life arose as a result of the rise of free oxygen. If we take this as being generally true, then we can ask the geophysical question: On what types of planets will free oxygen arise and how long will it take to reach high enough levels?
On Earth it took billions of years for oxygen to rise to present levels. Partly this is because the Earth is efficient at recycling by plate tectonics. This recycling keeps the Earth habitable by cycling the essential elements, but it also would have been a barrier to the buildup of oxygen. Earth probably is not the best possible planet for complex life development, since less plate tectonics would allow a faster rate of oxygen build up.
Mars took this to the extreme. With no plate tectonics, a shallow ocean, and only 38 percent of the Earth’s gravity, Mars might have built up oxygen much faster than the Earth. But the lack of plate tectonics doomed Mars to lose its atmosphere through mineralization. We might find that complex life arose on Mars only to be extinguished later. Perhaps the optimal planet for complex life would be an intermediate between Earth and Mars.
There may be a range of planet types on which oxygen could arise -- and therefore complex life. I would hazard a guess that most -- maybe two-thirds -- of terrestrial planets with life go on to develop complex life at some stage of their history. An optimist’s view.
Simon Conway Morris: The problem in my view is, why did complex life take so long to evolve on Earth? Evidence from oxygen data is frankly equivocal. Maybe the redox state of the Earth's mantle was peculiar in comparison with other similar planets. Alternatively, ocean chemistry may have put the lid on things.
There could be other dimensions that could explain why there was such a brake on the evolution of complex life -- why there were no Meso-Proterozoic dry martinis, but on the other hand, once microbes, then NASA.
David Grinspoon: Planetary biospheres are complex entities whose histories are fraught with contingency, accident, and luck. Therefore, the time it took for complex life to arise on Earth is probably much faster than some and much slower than others.
We can’t stand a mystery without a chief suspect, so we pin the rise of complex life on the rise of oxygen. This may well have factored in, but as Chris pointed out, there is no reason to believe that oxygen rose on Earth as quickly as it might have elsewhere. The rate of plate tectonics is one variable that will change atmospheric history - there are countless others. For example, if Earth had formed less rich in iron, then oxygen would have risen much more quickly because there would not have been as much iron to devour the oxygen.
So in other planetary systems that are less metal-rich, creatures might have evolved to levels far beyond our current state.
Peter Ward: On Earth, evolution has undergone a progressive development of ever more complex and sophisticated forms leading ultimately to human intelligence. Complex life – and even intelligence – could conceivably arise faster than it did on Earth. A planet could go from an abiotic state to a civilization in 100 million years, as compared to the nearly 4 billion years it took on Earth.
Evolution on Earth has been affected by chance events, such as the configuration of the continents produced by continental drift. Furthermore, I believe that the way the solar system was produced, with its characteristic number and planetary positions, may have had a great impact on the history of life here.
It has always been assumed that attaining the evolutionary grade we call animals would be the final and decisive step. Once we are at this level of evolution, a long and continuous progression toward intelligence should occur. However, recent research shows that while attaining the stage of animal life is one thing, maintaining that level is quite another. The geologic record has shown that once evolved, complex life is subject to an unending succession of planetary disasters, creating what are known as mass extinction events. These rare but devastating events can reset the evolutionary timetable and destroy complex life while sparing simpler life forms.
Such discoveries suggest that the conditions allowing the rise and existence of complex life are far more rigorous than are those for life’s formation. On some planets, then, life might arise and animals eventually evolve – only to be soon destroyed by a global catastrophe.
Frank Drake: The Earth’s fossil record is quite clear in showing that the complexity of the central nervous system -- particularly the capabilities of the brain -- has steadily increased in the course of evolution. Even the mass extinctions did not set back this steady increase in brain size. It can be argued that extinction events expedite the development of cognitive abilities, since those creatures with superior brains are better able to save themselves from the sudden change in their environment.
Thus smarter creatures are selected, and the growth of intelligence accelerates.
We see this effect in all varieties of animals -- it is not a fluke that it has occurred in some small sub-set of animal life. This picture suggests strongly that, given enough time, a biota can evolve not just one intelligent species, but many. So complex life should occur abundantly.
There is a claim that "among the millions of species which have developed on Earth, only one became intelligent, so intelligence must be a very, very rare event." This is a textbook example of a wrong logical conclusion. All planets in time may produce one or more intelligent species, but they will not appear simultaneously. One will be first. It will look around and find it is the only intelligent species. Should it be surprised? No! Of course the first one will be alone. Its uniqueness -- in principal temporary -- says nothing about the ability of the biota to produce one or more intelligent species.
If we assume that Earths are common, and that usually there is enough time to evolve an intelligent species before nature tramples on the biota, then the optimistic view is that new systems of intelligent, technology-using creatures appear about once per year. Based on an extrapolation of our own experience, let's make a guess that a civilization's technology is detectable after 10,000 years. In that case, there are at least 10,000 detectable civilizations out there.
This is a heady result, and very encouraging to SETI people.
On the other hand, taking into account the number and distribution of stars in space, it implies that the nearest detectable civilizations are about 1,000 light-years away, and only one in ten million stars may have a detectable civilization. These last numbers create a daunting challenge to those who construct instruments and projects to search for extraterrestrial intelligence. No actual observing program carried out so far has come anywhere close to meeting the requirement of detecting reasonable signals from a distance of 1,000 light years, or of studying 10 million stars with high sensitivity.
Donald Brownlee: But how often are animal-habitable planets located in the habitable zones of solar mass stars? Of the all the stars that have now been shown to have planets, all either have Jupiter-mass planets interior to 5.5 AU [1 AU, or astronomical unit, is the distance from Earth to the Sun] or they have Jupiters on elliptical orbits. It is unlikely that any of these stars could retain habitable zone planets on long-term stable orbits.
On the other hand, many of the stars that do not have currently detectable giant planets could have habitable-zone planets. But even when rocky planets are located in the right place, will they have the "right stuff" for the evolution and long term survival of animal-like life? There are many "Rare Earth" factors (such as planet mass, abundance of water and carbon, plate tectonics, etc.) that may play important and even critical roles in allowing the apparently difficult transition from slime to civilization.
As is the case in the solar system, animal-like life is probably uncommon in the cosmos. This might even be the case for microbes: how can scientists agree that microbial life is common in our celestial neighborhood when there is no data? Even the simplest life is extraordinarily complicated and until we find solid evidence for life elsewhere, the frequency of life will unfortunately be guesswork. We can predict that some planetary bodies will provide life-supporting conditions, but no one can predict that life will form.
Frank Drake: Only about 5 percent of the stars that have been studied sufficiently have hot Jupiters or Jupiters in elliptical orbits. The other 95 percent of the stars studied do not have hot Jupiters, and just what they have is still an open question. The latest discoveries, which depend on observations over a decade or more, are finding solar system analogs. This suggests that 95 percent of the stars- - for which the answers are not yet in -- could be similar to our own system. This is reason for optimism among those who expect solar system analogs to be abundant.
David Grinspoon: I think it is a mistake to look at the many specific peculiarities of Earth's biosphere, and how unlikely such a combination of characteristics seems, and to then conclude that complex life is rare. This argument can only be used to justify the conclusion that planets exactly like Earth, with life exactly like Earth-life, are rare.
My cat "Wookie" survived life as a near-starving alley cat and wound up as a beloved house cat through an unlikely series of biographical accidents, which I won't take up space describing but, trust me, given all of the incredible things that had to happen in just the right way, it is much more likely that there would be no Wookie than Wookie. From this I do not conclude that there are no other cats (The Rare Cat Hypothesis), only that there are no other cats exactly like Wookie.
Life has evolved together with the Earth. Life is opportunistic. The biosphere has taken advantage of the myriad strange idiosyncrasies that our planet has to offer. Not only that, life has created many of Earth’s weird qualities.
So it is easy to look at our biosphere, and the way it so cleverly exploits Earth’s peculiar features, and conclude that this is the best of all possible worlds; that only on such a world could complex life evolve. My bet is that many other worlds, with their own peculiar characteristics and histories, co-evolve their own biospheres. The complex creatures on those worlds, upon first developing intelligence and science, would observe how incredibly well adapted life is to the many unique features of their home world. They might naively assume that these qualities, very different from Earth’s, are the only ones that can breed complexity
