ABCs of Life in the Universe A Web-Based Course Offered through UCSD Extension and the Teacher Education Program W. H. Berger, Professor Scripps Institution of Oceanography W. Baity, Associate Instructor California Space Institute S. Rappoport, Assistant Instructor Center for Astrophysics and Space Sciences Introduction and Course Overview: A Letter To Our Students Welcome to a brand-new course on a brand-new branch of the natural sciences, a field called "astrobiology". This field of science considers the origin of life on Earth (the only planet we know reasonably well) and the possibility of life elsewhere in the Universe, under circumstances sort of similar to those on Earth. Astrobiology is an all-encompassing "Environmental Science", which contains many different disciplines. In this case, the "environment" is the Solar System, the Milky Way galaxy, and even the entire visible Universe. The disciplines called upon to deal with the subject of Life in the Universe are: astrophysics and geophysics (physics applied to stars and planets), cosmochemistry and planetary chemistry (chemistry applied to the origin of elements and to the evolution of the solar system), several branches of biology (the origin of life; metabolic biochemistry; the mechanisms of genetic inheritance and evolution), and the Earth and planetary sciences (evolution of planets; Earth history). Upon completing this course, you should have a good grasp of major questions in astrophysics, planetary physics, Earth history, and the origin and evolution of Life. You will not have an answer to the question of whether our planet is the only one bearing life in the Solar System, the Milky Way galaxy, or the Universe in general. This we don't know, but we can explore what we need to learn to figure it out. We, your instructors, will assume that you have no special knowledge of physics, chemistry, biology, astronomy or earth science as you enter this course. Upon finishing it, we hope you will have an appreciation of these sciences as they apply to one of the most profound questions one can ask about our world: Are we alone in the universe? We worked for about a year on this course, gathering materials, discussing the issues, making and re-making outlines, and attempting to boil down, in part, highly technical information to a level we judged useful for high-school instruction. This judgment is based on the level of preparedness of college freshmen at UCSD in non-science majors. In our experience, these students are sharp and eager to learn, but their background in the hard sciences ranges, on the whole, between spotty and weak. Considering these limitations, we aimed to preserve the important arguments but avoided technical detail. For those better prepared (or more intensely curious) there are links for pursuing many topics in greater depth. Let us know, during and at the end of the course, what you think what we could do better. Future students will benefit from your feedback to us. Let us make a list of the type of questions that are asked in connection with the topic of "Life in the Universe": First, we have various questions that pertain to the one planet known to harbor Life, that is, the third one out from our Sun, where we live: What is this thing we call Life? Precisely which properties set living things apart from non-living things? Why are there so many different forms of life? Are they somehow related and if so, what it the basis of the relationship? When and how did Life first arise on our planet? Did it arise several times? What did it look like when it was still close to its origins? What precisely are the conditions on our planet that made it possible for Life to arise and to persist? Which of these conditions could we do without and still have a chance to have Life arise and persist? Has Life changed the planet in ways that makes it easier for Life to persist? Or worse? Would we recognize, from a distance, that Earth has Life? What makes Life "intelligent"? Was it unavoidable that big brains developed in several lines of organisms on Earth, or is this a matter of chance not normally expected to happen? Second, we have questions that pertain to other planets in the Solar System, aside from than Earth: Which of the planets in the Solar System, other than Earth, might harbor Life, or might have harbored Life at some time in the past? What kinds of clues should we look for, either when sending probes or when using remote sensing? Assuming we find or suspect Life is present on Mars (or some other planet), how should we deal with it? Is it likely to be dangerous to us? Could Earth's Life seed life on other planets in the solar system, or vice versa? Third, we have questions that pertain to the probability of Life outside the Solar System, in the Milky Way Galaxy and in the visible universe in general: What types of stars could conceivably support Life, assuming the right planet is present? Do most stars have planets or is the Sun unusual in being surrounded by so many planets? How do we find out? How many potentially suitable star systems are there in the Milky Way galaxy? How many potentially suitable galaxies are there in the visible universe? How long can Life persist on a given planet, under favorably circumstances? Before you read on to pick up some clues on these various questions, (which are but a sampler of what we shall talk about in this course), pretend that you were asked these questions by a younger sibling, a middle- school student, or your TV repairman. Take a moment to jot down some notes for each question, describing what the answers might be. Done? Then let's proceed. Here are some notes we came up with: What is this thing we call Life? Precisely which properties set living things apart from non-living things? Living things consume and give off material and energy. They grow and reproduce. They have a complicated chemistry involving a myriad of different compounds made from carbon and other elements. They are, on the whole, recognizably "the same" even after many generations of reproduction. They evolve into things (over very long time spans) that are not "the same" at some level. All living things interact with other living things, both of the "same" kind and of a "different" nature. Why are there so many different forms of life? Are they somehow related and if so, what it the basis of the relationship? Living things evolve to fill available opportunities in regard to living space and new ways to make a living. One type of organism can be an ancestor to many different kinds of organisms, this being the fundamental reason for the great diversity of Life. Many seemingly different organisms demonstrably go back to the same ancestor. Many scientists believe that all organisms may go back to the same ancestor. (If this is true, would that make it more or less likely to find life elsewhere in the universe?) When and how did Life first arise on our planet? Did it arise several times? What did Life look like when it was still close to its origins? Life on Earth cannot be older than Earth itself, so it had to arise after 4.6 billion years ago. The first traces of Life (present as chemical fossils) are found in the oldest rocks available, close to 3.8 billion years in age. Thus, Life arose within the first 700 million years or so after the Earth formed. There is plenty of evidence for the presence of primitive life forms for more than 2 billion years. (In contrast, a good fossil record of larger organisms only exists for about the last 700 million years.) Life may have arisen several times (there is no way to know) but presumably there would not have been an equal chance for different life forms to persist. Some would have perished from bad luck, others because resources were monopolized by more advanced forms. The problem is to figure out how the "advanced forms" could hold on to their advantage, by passing on genetic information to the next generation. Replication that was good enough to retain advantages but fuzzy enough to allow for new invention was the crucial element in getting Life started. What precisely are the conditions on our planet that made it possible for Life to arise and to persist? Which of these conditions could we do without and still have a chance to have Life arise and persist? Life as we know it needs fluid water, so the temperature always has to be between freezing and boiling somewhere on the planet. Also, Earth's life is a form of carbon chemistry, so one needs plenty of (mobile) carbon. If water is to be fluid anywhere near the surface of Earth, we also needed an atmosphere (which holds the vapor that would otherwise escape to space). Some scientists think that to organize carbon molecules at the time of Life's origin, there should be solid surfaces forming a template on which the molecules can be lined up. If so, we need solids, fluid water and an atmosphere. Beyond these requirements it is difficult to specify additional needs. We know that 4 billion years ago the Earth looked nothing like it does now, and the composition of ocean and atmosphere was quite different. It is even possible that one could do without the Sun, if enough volcanic energy were available. Has Life changed the planet in ways that makes it easier for Life to persist? Or worse? Could we recognize, from the vantage of space, that a planet has Life? These questions are at the heart of the "Gaia" concept, which states that Life modifies the chemical conditions on Earth for its own benefit. In addition, it states that such modifications in a planet?s chemistry can be detected from very far away. The scientific discipline dealing with Gaia-type questions is called biogeochemistry (established long before the "Gaia hypothesis" was proposed). Through biogeochemistry we know that there were two major changes on Earth brought about by life's processes. One was the extraction of carbon dioxide from the atmosphere. As a consequence, Earth has very little of this gas in the atmosphere (its low concentration is why it is easy for us to double the amount of carbon dioxide in the air by burning coal and oil). The extraction of the carbon dioxide from the atmosphere (by burial of carbon in limestone rocks, coal and oil) has cooled the Earth, which is a good thing because we think the Sun has become hotter through geologic time. The other impact of life processes is the increase of molecular oxygen in the atmosphere, since oxygen is set free when carbon is split from carbon dioxide and buried. The rise of free oxygen in the air (occurring in the last billion years or so) has stimulated the evolution of multi-celled organisms (which have a much smaller surface to take in oxygen with than do the single-celled organisms). The simplest description of this entire process is to say that Life changed the Earth's environment, and new life forms evolved to take advantage of these changes. As long as this last statement is true, the changes induced by living organisms could be interpreted as being "beneficial" to other living organisms. (Though not necessarily beneficial to ancient life forms.) As to the question of whether there is a strong sign of life's processes that could be measured from space, the presence of abundant free oxygen in the atmosphere of a distant planet would be such a measurement. What makes Life "intelligent"? Was it unavoidable that big brains developed in several lines of organisms on Earth, or is this a chance thing, not normally expected to happen? This intriguing question exercises philosophers, artificial intelligence engineers, brain biologists and anthropologists, among others. "Intelligence" may be defined as holding a model of the environment in one's nervous system (or an equivalent structure) and testing possible outcomes of a response to a stimulus without suffering the consequences of bad outcomes (that is, by holding off on the actual experiment). A chief example of this would be the weighing of risk when embarking on appropriating a resource, such as food. As in the case of fishing, we take advantage of the lesser intelligence of our prey when offering a resource (bait) in a trap (a high but unrecognized risk to the less intelligent animal). Intelligence is especially obvious in social animals who warn each other of risks (like crows or monkeys). In contrast, ants tell each other where to find food but not that a horned lizard is eating ants at the food source. The development of large brains goes back at least 40 million years and parallels the evolution of many mammals and birds (whales, elephants, primates, crows, and parrots). The rate of evolution of brain-size in humans is extraordinary. It suggests that positive feedback was at work. (That is, having others around with a large brain puts a premium on being smart and in getting resources and mates). Which of the planets in the Solar System, other than Earth, might harbor Life, or might have harbored Life at some time in the past? This gets back to the question of the presence of liquid water and the temperature range and energy source that goes with it. The innermost planet, Mercury, has no atmosphere (and hence no liquid water) and alternates between utter cold and great heat. Venus is apparently dry, and it is also hot enough there to melt lead. Mars is a reasonable bet for life. From certain landforms it appears that liquid water may have been present on Mars, sometime in the past. However, alternative possible explanations have been offered for each particular landform. If the giant gas ball called Jupiter has Life, it would be unlike anything with which we are familiar. Certain moons of Jupiter may have a chance, although the environment is very strange, when measured against what we have here. Saturn, a gas ball much like Jupiter except smaller and colder, is an unlikely candidate, as are its moons, which are frozen wastelands. Anything farther out is likely to be too cold, although Uranus could conceivably generate sufficient heat, deep inside. Furthermore, forget Earth's moon: it has no atmosphere. So, the most likely candidate is Mars. What kinds of clues should we look for, either when sending probes or when using space probes to perform remote sensing on other planets? We already mentioned the comparatively strange atmospheric composition of the Earth as a clue to life processes. Soil probes, presumably, could pick up organic matter (different types of organic matter and unlikely combinations of it would be of particular interest). On Earth, of course, any soil sample would contain a myriad of bacteria, whether taken on dry (or moist) land or from the floor of the sea. Even solid ice in Greenland has micro-organisms. Remote sensing with satellites is mainly good for detecting water and for exploring the composition of an atmosphere (detecting oxygen and nitrogen, for example). We can find out, by remote sensing, whether Life could exist. But for proving that Life is actually there, we need to have a sample. A "remote" sample from Mars, flung into space as a result of an impact on the Martian surface, made it onto Earth and was found in Antarctica. The rock, singed from its journey through the atmosphere, contained an object that sort of looks like a fossil organism. This generated much discussion (and lots of press releases). Assuming we find or suspect Life is present on Mars (or some other planet), how should we deal with it? Is it likely to be dangerous to us? It seems unlikely that organisms that have no genetic program to deal with Earth life should want to eat us or otherwise harm us. On the other hand, our bodies would not have evolved to deal with an attack from such a source. If rats died from inhaling Mars dust, that would be a bad sign. If they stayed happy, we are probably okay. (Our genetic programs are quite close to those of rats -- probably indistinguishably close from the point of view of any Martian life form). NASA takes no chances in this business: witness the extended quarantine of the space travelers upon their return from the Moon. In the highly unlikely event that we find a Martian outpost of intelligent Life, we have a different problem. If such life forms exist, they are likely to be millions of years ahead of us in philosophy and technology (since we are new to the scene for both, especially the latter) and our guess regarding their intentions would probably be wrong. (Imagine a group of gorillas in the Congo watching a camera crew at one time and people marking trees in the forest at another. Which would they pick out as the more dangerous activity?) Could Earth's Life seed life on other planets in the solar system, or vice versa? The idea of "panspermia" has been kicking around for some time. The concept is that the seeds of living organisms float around in space, over millions of years, and eventually find a suitable planet to propagate. Some organisms can survive surprisingly extreme conditions for surprisingly long times. Whenever Earth gets hit by a big rock coming in from space (for example, the one that made the Meteor Crater in Arizona) debris can be accelerated to such an extent, in the explosion of impact, that it reaches outer space and does not fall back on Earth. If such a piece of debris has bacteria (for example, present inside a rock with hair-thin fractures) it could conceivably carry biological information to other planets. It better land someplace nice there, though. Bacteria need a substrate that is appropriate for their genetic program. Just any place - even of the right temperature and even with some water - is unlikely to be good enough. The same argument holds in reverse. As far as traveling over distances of many light years (as some have speculated should be possible), survival is quite unlikely because the environment in space is highly inhospitable to life, with all sorts of radiation eroding the information contained in a genetic code. In a state of total deep freeze (the only state conceivable when traveling through space in a rock) no repair of such damage would be possible. What types of stars could conceivably support Life, assuming the right planet is present? The main thing is to avoid highly damaging radiation (judging from our own life experience on Earth). Such radiation is common and essentially includes all rays more energetic than blue light. It includes ultraviolet (of which the Sun produces quite a bit, much of it intercepted by our atmosphere), x-rays, and gamma rays. Hot blue stars and strong x-ray sources are thus unfavorable. Reddish stars are probably okay, except when they are "red giants" that expand so far as to swallow any planets they might have in their orbit. Pulsating stars are not good either because they scorch a planet at one time (when large or when bright) and then leave it in the cold at another (when contracted and/or dim). The best kind of star, presumably, is one like our Sun, a relatively unexciting middling-sized star putting out most of its energy in visible light centered near the color yellow. Our sun, which we can call "Sol", has many siblings, and there is no shortage of sun-like stars, as determined from the composition of the light received from stars in the Milky Way galaxy. Do most stars have planets or is the Sun unusual in being surrounded by so many planets? How do we find out? Some time ago it was noted that stars rotate (as does Sol) but that some rotate a lot more than others. The suggestion was made that those that rotate slowly (like Sol), have an entourage of planets which somehow acquired the rotational momentum of the system. If this argument is accepted, something like half of the stars seen in the sky are part of a solar system. In the meantime, the instruments of astronomers have become refined to a point where the peculiar but slight motions of a star disturbed by a planet can be picked up. This ability has spawned a kind of cottage industry for planet hunting, and the number of planets known (or thought) to exist has expanded rapidly in the last few years and is now close to one hundred. Many of these planets may be too close to their central star to provide a good environment for Life to originate. However, just the fact that planets are so common is a welcome sign that we may have company someplace. ("Company" includes, of course, life forms on the level of bacteria.) How many potentially suitable star systems are there in the Milky Way galaxy? The discovery that the Milky Way is in fact the edge of a galaxy, and we are in it, was one of the greatest achievements of human thought. The counting of the stars within that galaxy is another such achievement. One way this was done was to determine the rotation of the galaxy (a measurement not easily done) and from this fact and the galaxy's size calculate its mass. Together with the typical distribution of star mass this calculation yielded a first guess as to the number of stars. This guess can be checked against the observed abundance of stars (number of stars in a given area of the sky) and against the information from near-by sibling galaxies (such as the Andromeda nebula). When all this is done, it turns out that there are more than 100,000,000,000 stars in our galaxy (100 billion or 100E9, where E9 stands for "9 zeroes"). We don't know how many are suitable, but even if we exclude giants, dwarfs, double and triple stars, and everything else that looks even slightly suspicious (pulsing stars, for example), we still come up with more than a billion eligible stars. How many potentially suitable galaxies are there in the visible universe? There are again as many galaxies as there are stars in our galaxy, and the word "billion" becomes commonplace. How exactly galaxies are put together (what is at their center and how the violent processes there may influence the general environment) is a field of active research. Suffice it to say, at this point, that there are plenty of galaxies that are reasonably quiet places - with allowance for the occasional supernova explosion. Thus, as far as a potential habitat for Life, our galaxy does not seem out of the ordinary. How long can Life persist on a given planet, under favorably circumstances? On Earth, this is a matter of the behavior of Sol, whose energy makes Life possible. The Sun will eventually run out of sufficient fuel to keep it going in the accustomed manner, and it will then change to a different fuel cycle and become hostile to the Life it generated and supported. This event is still billions of years into the future. Thus, Life has as long a future as it had a past. If Mars had liquid water at some time, it presumably lost this asset as a function of its own evolution (a decrease in its volcanism and the loss of some its atmosphere). Thus, the history of planetary evolution is also important, in addition to that of our central star. As far as its longevity is concerned, Sol is especially favored since it is sufficiently small (and hence does not burn as quickly as some other stars), yet sufficiently large enough to give off plenty of energy in the visible light spectrum. Since Earth's inner machinery seems to have worked in much the same fashion throughout its history (as far as we can trace this back in mountain-building processes), it appears that both Sol and Earth are optimal in terms of long-term support for Life. Other solar systems are, on the whole, likely to be less favorable. (By the way, contrary to some misconceptions, humans cannot extinguish Life on Earth - this is a canard. What we can do is to mess up the ecology at the Earth's surface. There are bacteria deep within the Earth that will not be much affected by what we do at the surface.) With this letter-to-the-students as background, you will now better understand why the course is organized in the fashion that follows. There is a lot to consider. We start with the Solar System and take a quick look into the vastness of space around it. We shall see that our Solar System is a tiny island in an immense ocean of nothingness. We then zero in on the third planet of Sol, where all the action is (from our point of view, the Center of the Universe.) And we then familiarize ourselves with the central topic of the course: Life itself. These three lessons provide a basis for discussion: 1. ECOLOGY OF THE SOLAR SYSTEM 2. LIFE'S HOME PLANET 3. THE ESSENCE OF LIFE Next, we review how people have looked at the world and how they dealt with the question of origins. How, precisely, does the scientific method differ from making up stories? One important difference between myth and science, as we shall see, is the role of certainty and doubt. If you are certain about what happened, or will happen, you are not doing science. We then turn to what we might learn from the history of Life on Earth, about the mechanisms that sustain and threaten it. This we need to know to assess the probability that it might arise and persist elsewhere. We will conclude this section with a look at the origin of Life, of which we actually know little. One important clue to Life's origin is the amazing similarity between different life forms at the molecular level. These three lessons introduce a method of looking at the world, and a survey of historical facts and of hypotheses useful in gauging the likelihood that Life arises and evolves whenever favorable conditions exist: 4. FROM MYTH TO SCIENCE 5. HISTORY OF LIFE ON EARTH - (MAINLY) FACTS 6. LIFE'S ORIGINS AND THE SOURCE OF UNITY OF LIFE In the last section, we come back to the question of whether there is likely to be Life "out there". We start with the most promising of nearby places: our sibling planet Mars. Did it ever have life forms? To go beyond the Solar System we need a checklist of what Life needs, both Life as we know it and as we do not. Clearly, statements regarding the latter is will seem a lot like science fiction. Finally, once in this "science fiction mode", we will speculate about how abundant Life might be in the galaxy and in the universe in general, and whether (with present understanding of the limits set by physics) intelligent life forms might travel between star systems, across the galaxy, or even between galaxies. Warp speed, anyone? 7. LIFE ON MARS? 8. ECOLOGY OF THE UNIVERSE - A GOOD PLACE TO LIVE? 9. THE SEARCH FOR LIFE OUT THERE What is expected of you? Well, there is the mundane aspect of showing that you learned something, so you get the credit you desire and deserve. This will be assessed through quizzes and also an essay. We would hope that you get interested enough about some favorite subject within all this to produce a little exhibit, on the web, that can be accessed and enjoyed (and criticized) by everyone. A page of text and a couple of well-chosen (or well-crafted) illustrations will be good. The list of questions at the beginning of this letter should stimulate your thinking about what might be interesting to research and communicate. After going through the first 3 lessons, you should choose your favorite topic. Welcome aboard! We start with the tour of the Solar System. Here's to a good journey! Your Crew: Wolf Berger, Bill Baity, Susan Rappoport